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Disturbance Regimes (disturbance + regime)

Distribution by Scientific Domains

Life Sciences	79%
Humanities and Social Sciences	15%
Earth and Environmental Science	5%

Distribution within Life Sciences

Ecology & Organismal Biology	48%
Conservation Science	15%

Selected Abstracts

The Challenge to Restore Processes in Face of Nonlinear Dynamics,On the Crucial Role of Disturbance Regimes

RESTORATION ECOLOGY, Issue 2 2007
*Article first published online: 14 MAY 200, Anke Jentsch
Abstract Increasingly, restoration ecologists and managers are challenged to restore ecological processes that lead to self-sustaining ecosystem dynamics. Due to changing environmental conditions, however, restoration goals need to include novel regimes beyond prior reference conditions or reference dynamics. In face of these fundamental challenges in process-based restoration ecology, disturbance ecology can offer useful insights. Here, I discuss the contribution of disturbance ecology to understanding assembly rules, ecosystem dynamics, regime shifts, and nonlinear dynamics. Using the patch and multipatch concept, all insights are organized according to two spatial and two temporal categories: "patch,event,""patch,multievent,""multipatch,event," and "multipatch,multievent." This concept implies the consideration of both spatial patterns and temporal rhythms inside and outside of a restoration site. Emerging issues, such as uncoupling of internal and external dynamics, are considered. [source]

Matrix Models as a Tool for Understanding Invasive Plant and Native Plant Interactions

CONSERVATION BIOLOGY, Issue 3 2005
DIANE M. THOMSON
competencia; invasión biológica; plantas invasoras; modelo matricial; perturbación Abstract:,Demographic matrix models are an increasingly standard way to evaluate the effects of different impacts and management approaches on species of concern. Although invasive species are now considered among the greatest threats to biodiversity, matrix methods have been little used to explore and integrate the potentially complicated effects of invasions on native species. I developed stage-structured models to assess the impacts of invasive grasses on population growth and persistence of a federally listed (U.S.A.) endemic plant, the Antioch Dunes evening primrose (Oenothera deltoides subsp. howellii [Munz] W. Klein). I used these models to evaluate two frequently made assumptions: (1) when rare plant populations decline in invaded habitats, invasive species are the cause and (2) invasive plants suppress rare plants primarily through direct resource competition. I compared two control and two removal matrices based on previous experimental work that showed variable effects of invasive grasses on different life-history stages of O. deltoides. Matrix analysis showed that these effects translated into substantial changes in population growth rates and persistence, with control matrices predicting a mean stochastic population growth rate (,) of 0.86 and removal matrices predicting growth rates from 0.92 to 0.93. Yet even the most optimistic invasive removal scenarios predicted rapid decline and a probability of extinction near one in the next 100 years. Competitive suppression of seedlings had much smaller effects on growth rates than did lowered germination, which probably resulted from thatch accumulation and reduced soil disturbance. These results indicate that although invasive grasses have important effects on the population growth of this rare plant, invasion impacts are not solely responsible for observed declines and are likely to be interacting with other factors such as habitat degradation. Further, changes in the disturbance regime may be as important a mechanism creating these impacts as direct resource competition. My results highlight the value of demographic modeling approaches in creating an integrated assessment of the threats posed by invasive species and the need for more mechanistic studies of invasive plant interactions with native plants. Resumen:,Los modelos demográficos matriciales son una forma cada vez más utilizada para evaluar los efectos de diferentes impactos y métodos de gestión sobre las especies en cuestión. Aunque actualmente se considera a las plantas invasoras entre las mayores amenazas a la biodiversidad, los modelos matriciales han sido poco utilizados para explorar e integrar los efectos potencialmente complicados de las invasiones sobre las especies nativas. Desarrollé modelos estructurados por etapas para evaluar los impactos de pastos invasores sobre el crecimiento poblacional y la persistencia de una especie de planta endémica, enlistada federalmente (E.U.A.), Oenothera deltoides ssp. howellii [Munz] W. Klein. Utilicé estos modelos para evaluar dos suposiciones frecuentes: (1) cuando las poblaciones de plantas raras declinan en hábitats invadidos, las especies invasoras son la causa y (2) las plantas invasoras suprimen a las plantas raras principalmente mediante la competencia directa por recursos. Comparé dos matrices de control y dos de remoción con base en trabajo experimental previo que mostró efectos variables de los pastos invasores sobre las diferentes etapas de la historia de vida de O. deltoides. El análisis de la matriz mostró que estos efectos se tradujeron en cambios sustanciales en las tasas de crecimiento y persistencia de la población, las matrices de control predijeron una tasa media de crecimiento poblacional estocástica (,) de 0.86 y las matrices de remoción predijeron tasas de crecimiento de 0.92-0.93. Aun los escenarios más optimistas de remoción de invasores predijeron una rápida declinación y una probabilidad de extinción en 100 años cerca de uno. La supresión competitiva de plántulas tuvo mucho menor efecto sobre las tasas de crecimiento que la disminución en la germinación, que probablemente resultó de la acumulación de paja y reducción en la perturbación del suelo. Estos resultados indican que, aunque los pastos invasores tienen efectos importantes sobre el crecimiento poblacional de esta planta rara, los impactos de la invasión no son los únicos responsables de las declinaciones observadas y probablemente están interactuando con otros factores como la degradación del hábitat. Más aun, los cambios en el régimen de perturbación pueden ser un mecanismo tan importante en la creación de estos impactos como la competencia directa por recursos. Mis resultados resaltan el valor del enfoque de los modelos demográficos para la evaluación integral de las amenazas de especies invasoras y la necesidad de estudios más mecanicistas de las interacciones de plantas invasoras con plantas nativas. [source]

Metapopulation Extinction Risk under Spatially Autocorrelated Disturbance

CONSERVATION BIOLOGY, Issue 2 2005
A. S. KALLIMANIS
patrón espacial de perturbación; simulaciones espacialmente explícitas; SLOSS; umbral de extinción Abstract:,Recent extinction models generally show that spatial aggregation of habitat reduces overall extinction risk because sites emptied by local extinction are more rapidly recolonized. We extended such an investigation to include spatial structure in the disturbance regime. A spatially explicit metapopulation model was developed with a wide range of dispersal distances. The degree of aggregation of both habitat and disturbance pattern could be varied from a random distribution, through the intermediate case of a fractal distribution, all the way to complete aggregation (single block). Increasing spatial aggregation of disturbance generally increased extinction risk. The relative risk faced by populations in different landscapes varied greatly, depending on the disturbance regime. With random disturbance, the spatial aggregation of habitat reduced extinction risk, as in earlier studies. Where disturbance was spatially autocorrelated, however, this advantage was eliminated or reversed because populations in aggregated habitats are at risk of mass extinction from coarse-scale disturbance events. The effects of spatial patterns on extinction risk tended to be reduced by long-distance dispersal. Given the high levels of spatial correlation in natural and anthropogenic disturbance processes, population vulnerability may be greatly underestimated both by classical (nonspatial) models and by those that consider spatial structure in habitat alone. Resumen:,Los modelos recientes de extinción generalmente muestran que la agregación espacial de hábitat reduce el riesgo de extinción debido a una recolonización más rápida de sitios vacíos por extinción local. Extendimos la investigación para incluir la estructura espacial en el régimen de perturbación. Desarrollamos un modelo metapoblacional espacialmente explícito en el que el patrón espacial tanto del hábitat como de los regímenes de perturbación podía variar aleatoriamente de fractal a completamente agregado (bloque) y con una amplia gama de distancias de dispersión. El incremento de la agregación espacial de la perturbación generalmente incrementó el riesgo de extinción. El riesgo relativo que enfrentan poblaciones en paisajes diferentes fue muy variable, dependiendo del régimen de perturbación. Con perturbación aleatoria, la agregación espacial de hábitat redujo el riesgo de extinción, como en estudios anteriores. Sin embargo, cuando la perturbación estaba autocorrelacionada espacialmente, esta ventaja se eliminaba o invertía debido a que las poblaciones en hábitats agregados están en riesgo de extinción masiva por eventos perturbadores a escala gruesa. Los efectos de patrones espaciales sobre el riesgo de extinción tendieron a reducirse por la dispersión de larga distancia. Debido a los altos niveles de correlación espacial en los procesos naturales y humanos de perturbación, la vulnerabilidad puede estar enormemente subestimada tanto por modelos clásicos (no espaciales) como por los que sólo consideran la estructura espacial del habitat. Los modelos que consideran la estructura espacial del hábitat solo subestiman el riesgo en comparación con modelos que consideran la estructura especial de la perturbación. [source]

Detecting the Effects of Fishing on Seabed Community Diversity: Importance of Scale and Sample Size

CONSERVATION BIOLOGY, Issue 2 2003
Michel J. Kaiser
I investigated the importance of the extent of area sampled to the observed outcome of comparisons of the diversity of seabed assemblages in different areas of the seabed that experience either low or high levels of fishing disturbance. Using a finite data set within each disturbance regime, I pooled samples of the benthic communities at random. Thus, although individual sample size increased with each additional level of pooled data, the number of samples decreased accordingly. Detecting the effects of disturbance on species diversity was strongly scale-dependent. Despite increased replication at smaller scales, disturbance effects were more apparent when larger but less numerous samples were collected. The detection of disturbance effects was also affected by the choice of sampling device. Disturbance effects were apparent with pooled anchor-dredge samples but were not apparent with pooled beam-trawl samples. A more detailed examination of the beam-trawl data emphasized that a whole-community approach to the investigation of changes in diversity can miss responses in particular components of the community ( e.g., decapod crustacea ). The latter may be more adversely affected by disturbance than the majority of the taxa found within the benthic assemblage. Further, the diversity of some groups ( e.g., echinoderms ) actually increased with disturbance. Experimental designs and sampling regimes that focus on diversity at only one scale may miss important disturbance effects that occur at larger or smaller scales. Resumen: Las perturbaciones antropogénicas de ambientes terrestres y marinos, tales como la tala y la pesca, se identifican generalmente con impactos negativos sobre la diversidad de especies. Sin embargo, observaciones empíricas a menudo no apoyan este supuesto. Investigué la importancia de la extensión del área muestreada sobre los resultados observados de comparaciones de la diversidad de ensamblajes de fondos marinos en diferentes áreas que experimentaron niveles bajos o altos de perturbación por pesca. Usando un juego finito de datos dentro de cada régimen de perturbación, se combinaron las muestras de comunidades bénticas de manera aleatoria. Por lo tanto, a pesar de que el tamaño de muestra individual incrementó con cada nivel adicional de datos combinados, el número de muestras disminuyó en consecuencia. La detección de los efectos de la perturbación sobre la diversidad de especies dependió en gran medida de la escala. A pesar del incremento en replicación de las escalas pequeñas, los efectos de la perturbación fueron más visibles cuando las muestras recolectadas fueron más grandes pero menos numerosas. La detección de los efectos de la perturbación también fueron afectados por la selección del equipo de muestreo. Los efectos de la perturbación eran evidentes cuando se usaron muestras mezcladas de dragas de ancla, pero no fueron evidentes para muestras mezcladas de redes de arrastre con vigas. Un análisis más detallado de los datos de las redes de arrastre muestran que una aproximación a nivel de toda la comunidad para investigar los cambios de diversidad puede resultar en la pérdida de información a nivel de componentes específicos ( por ejemplo crustáceos decápodos ) de la comunidad. Estos pueden ser adversamente afectados en mayor medida por la perturbación que la mayoría de los taxones que integran el ensamblaje béntico. Además, la diversidad de algunos grupos ( por ejemplo los equinodermos ) de hecho aumentó con la perturbación. Los diseños experimentales y los regímenes de muestreo que se enfocan en la diversidad a una sola escala pueden no detectar los efectos importantes de la perturbación que ocurren a mayores o menores escalas. [source]

Management strategies for plant invasions: manipulating productivity, disturbance, and competition

DIVERSITY AND DISTRIBUTIONS, Issue 3 2004
Michael A. Huston
ABSTRACT The traditional approach to understanding invasions has focused on properties of the invasive species and of the communities that are invaded. A well-established concept is that communities with higher species diversity should be more resistant to invaders. However, most recently published field data contradict this theory, finding instead that areas with high native plant diversity also have high exotic plant diversity. An alternative environment-based approach to understanding patterns of invasions assumes that native and exotic species respond similarly to environmental conditions, and thus predicts that they should have similar patterns of abundance and diversity. Establishment and growth of native and exotic species are predicted to vary in response to the interaction of plant growth rates with the frequency and intensity of mortality-causing disturbances. This theory distinguishes between the probability of establishment and the probability of dominance, predicting that establishment should be highest under unproductive and undisturbed conditions and also disturbed productive conditions. However, the probability of dominance by exotic species, and thus of potential negative impacts on diversity, is highest under productive conditions. The theory predicts that a change in disturbance regime can have opposite effects in environments with contrasting levels of productivity. Manipulation of productivity and disturbance provides opportunities for resource managers to influence the interactions among species, offering the potential to reduce or eliminate some types of invasive species. [source]

Linking ecological theory with stream restoration

FRESHWATER BIOLOGY, Issue 4 2007
P. S. LAKE
Summary 1. Faced with widespread degradation of riverine ecosystems, stream restoration has greatly increased. Such restoration is rarely planned and executed with inputs from ecological theory. In this paper, we seek to identify principles from ecological theory that have been, or could be, used to guide stream restoration. 2. In attempts to re-establish populations, knowledge of the species' life history, habitat template and spatio-temporal scope is critical. In many cases dispersal will be a critical process in maintaining viable populations at the landscape scale, and special attention should be given to the unique geometry of stream systems 3. One way by which organisms survive natural disturbances is by the use of refugia, many forms of which may have been lost with degradation. Restoring refugia may therefore be critical to survival of target populations, particularly in facilitating resilience to ongoing anthropogenic disturbance regimes. 4. Restoring connectivity, especially longitudinal connectivity, has been a major restoration goal. In restoring lateral connectivity there has been an increasing awareness of the riparian zone as a critical transition zone between streams and their catchments. 5. Increased knowledge of food web structure , bottom-up versus top-down control, trophic cascades and subsidies , are yet to be applied to stream restoration efforts. 6. In restoration, species are drawn from the regional species pool. Having overcome dispersal and environmental constraints (filters), species persistence may be governed by local internal dynamics, which are referred to as assembly rules. 7. While restoration projects often define goals and endpoints, the succession pathways and mechanisms (e.g. facilitation) by which these may be achieved are rarely considered. This occurs in spite of a large of body of general theory on which to draw. 8. Stream restoration has neglected ecosystem processes. The concept that increasing biodiversity increases ecosystem functioning is very relevant to stream restoration. Whether biodiversity affects ecosystem processes, such as decomposition, in streams is equivocal. 9. Considering the spatial scale of restoration projects is critical to success. Success is more likely with large-scale projects, but they will often be infeasible in terms of the available resources and conflicts of interest. Small-scale restoration may remedy specific problems. In general, restoration should occur at the appropriate spatial scale such that restoration is not reversed by the prevailing disturbance regime. 10. The effectiveness and predictability of stream ecosystem restoration will improve with an increased understanding of the processes by which ecosystems develop and are maintained. Ideas from general ecological theory can clearly be better incorporated into stream restoration projects. This will provide a twofold benefit in providing an opportunity both to improve restoration outcomes and to test ecological theory. [source]

Plant palatability and disturbance level in aquatic habitats: an experimental approach using the snail Lymnaea stagnalis (L.)

FRESHWATER BIOLOGY, Issue 5 2002
ARNAUD ELGER
1.,The palatability of aquatic macrophytes to the snail Lymnaea stagnalis was investigated in the laboratory. Eight species of macrophyte were selected from habitats that differed in either flood disturbance regime or nutrient status. 2.,In a non-choice test, single macrophyte species were offered to individual snails. The average amount of plant dry mass consumed per Lymnaea dry mass ranged from 3.6 ± 1.4 (±SE) to 63.6 ± 13.9 mg g,1 day,1 across plant species. In a choice test, all eight plant species were presented simultaneously to sets of five snails. The average total consumption was 66.1 ± 3.8 mg g,1 day,1 and the maximum average consumption for a single plant was 26.2 ± 3.6 mg g,1 day,1. 3.,In both tests, the amount consumed by snails differed significantly between the plant species. The species growing in undisturbed habitats were the least consumed. Habitat nutrient status was unrelated to plant palatability. 4.,These results suggest that macrophyte species growing in habitats that are rarely disturbed by floods allocate a greater proportion of their resources to resisting herbivory. [source]

Relationship between post-fire regeneration and leaf economics spectrum in Mediterranean woody species

FUNCTIONAL ECOLOGY, Issue 1 2009
S. Saura-Mas
Summary 1Recent work has identified global-scale relationships between key leaf traits (leaf economics spectrum). However, it is important to determine whether this approach can be applied at local scale with smaller subsets of species facing similar environments. Since fire is a key process in Mediterranean shrubland dynamics we analyze whether fire-related life-history traits influence the pattern of correlation between the leaf economic spectrum and leaf moisture traits. 2Using structural equation modelling and exploratory path analysis, we developed alternative models to test how interspecific leaf traits are related to the seasonal variation of water content (leaves and shoots) and to the type of post-fire regeneration of Mediterranean woody species. 3This study demonstrates that for these species seasonal variation in water content and fuel moisture would be better predicted by the presence or absence of a trait describing post-fire seedling establishment than by the leaf economic spectrum traits. However, leaf dry matter content (LDMC) is influenced by both the leaf economic spectrum and the post-fire regenerative type. 4Seeder species (those that recruit via seeds immediately after fire) present lower LDMC and higher relative seasonal variation of relative water content (RWCrsv) than non-seeders. We hypothesize that since seeder species mostly evolved under the Mediterranean climate, they developed a particular strategy of drought tolerance (without causing an effect to the relation between the volume occupied by cytoplasm relative to the volume occupied by cell walls), which is the cause of the observed relation between LDMC and RWCrsv. 5This study suggests that the leaves of Mediterranean woody species would follow the general leaf economics spectrum (Wright et al. 2004) but that specific selective forces, such as disturbance regime, acting at regional scale also play a relevant role to explain leaf traits related to water content. [source]

Offshore renewable energy: ecological implications of generating electricity in the coastal zone

JOURNAL OF APPLIED ECOLOGY, Issue 4 2005
ANDREW B. GILL
Summary 1Global-scale environmental degradation and its links with non-renewable fossil fuels have led to an increasing interest in generating electricity from renewable energy resources. Much of this interest centres on offshore renewable energy developments (ORED). The large scale of proposed ORED will add to the existing human pressures on coastal ecosystems, therefore any ecological costs and benefits must be determined. 2The current pressures on coastal ecology set the context within which the potential impacts (both positive and negative) of offshore renewable energy generation are discussed. 3The number of published peer-review articles relating to renewable energy has increased dramatically since 1991. Significantly, only a small proportion of these articles relate to environmental impacts and none considers coastal ecology. 4Actual or potential environmental impact can occur during construction, operation and/or decommissioning of ORED. 5Construction and decommissioning are likely to cause significant physical disturbance to the local environment. There are both short- and long-term implications for the local biological communities. The significance of any effects is likely to depend on the natural disturbance regime and the stability and resilience of the communities. 6During day-to-day operation, underwater noise, emission of electromagnetic fields and collision or avoidance with the energy structures represent further potential impacts on coastal species, particularly large predators. The wider ecological implications of any direct and indirect effects are discussed. 7Synthesis and applications. This review demonstrates that offshore renewable energy developments will have direct and, potentially, indirect consequences for coastal ecology, with these effects occurring at different scales. Ecologists should be involved throughout all the phases of an ORED to ensure that appropriate assessments of the interaction of single and multiple developments with the coastal environment are undertaken. [source]

Interactions of multiple disturbances in shaping boreal forest dynamics: a spatially explicit analysis using multi-temporal lidar data and high-resolution imagery

JOURNAL OF ECOLOGY, Issue 3 2010
Udayalakshmi Vepakomma
Summary 1.,Mixed-wood boreal forests are often considered to undergo directional succession from shade-intolerant to shade-tolerant species. It is thus expected that overstorey gaps should lead to the recruitment of shade-tolerant conifers into the canopy in all stand development stages and that the recruitment of shade-intolerant hardwoods would be minimal except in the largest gaps. 2.,We analysed short-term gap dynamics over a large 6-km2 spatial area of mixed-wood boreal forest across a gradient of stands in different developmental stages with different times of origin since fire (expressed as stand ,age') that were affected differentially by the last spruce budworm (SBW) outbreak. Structural measurements of the canopy from lidar data were combined with spectral classification of broad species groups to characterize the gap disturbance regime and to evaluate the effect of gap openings on forest dynamics. 3.,Estimated annual gap opening rates increased from 0.16% for 84-year-old stands to 0.88% for 248-year-old stands. Trees on gap peripheries in all stands were more vulnerable to mortality than interior canopy trees. 4.,Due to recovery from the last SBW outbreak 16 years previously, gap closure rates were higher than opening rates, ranging from 0.44% to 2.05% annually, but did not show any relationship with stand age. There was, however, a continuing legacy of the last SBW outbreak in old-conifer stands in terms of a continued high mortality of conifers. In all stands, the majority of the openings were filled from below, although a smaller but significant proportion filled from lateral growth of gap edge trees. 5.,Synthesis. The forest response to moderate- to small-scale disturbances in old-growth boreal forest counters the earlier assumption that the transition from one forest state to the next is slow and directional with time since the last fire. Overall, a small 6% increase in hardwoods was observed over 5 years, largely due to regeneration in-filling of hardwoods in gaps instead of successional transition to more shade-tolerant conifers. Gaps are vital for hardwood maintenance while transition to softwoods can occur without perceived gap-formation as overstorey trees die, releasing understorey trees. [source]

Geographic patterns of diversity in streams are predicted by a multivariate model of disturbance and productivity

JOURNAL OF ECOLOGY, Issue 3 2006
BRADLEY J. CARDINALE
Summary 1Univariate explanations of biodiversity have often failed to account for broad-scale patterns in species richness. As a result, increased attention has been paid to the development and testing of more synthetic multivariate hypotheses. One class of multivariate hypotheses, founded in successional diversity theory, predict that species richness is jointly influenced by periodic disturbances that create new niche opportunities in space or time, and the production of community biomass that speeds displacement of inferior by superior competitors. 2While the joint response of diversity to disturbance and productivity has gained support from theoretical and small-scale experimental studies, evidence that corresponding patterns of biodiversity occur broadly across natural systems is scarce. 3Using a data set that employed standardized methods to sample 85 streams throughout the mid-Atlantic United States of America, we show that biogeographical patterns of primary producer diversity in stream ecosystems are consistent with the predictions of a multivariate model that incorporates disturbance frequency and community biomass production as independent variables. Periphyton species richness is a concave-down function of disturbance frequency (mean no. floods year,1) and of biomass production (µg of biomass accrual cm,2 day,1), and an increasing function of their interaction. 4Changes in richness across the disturbance × productivity response surface can be related to several predicted life-history traits of the dominant species. 5Our findings complement prior studies by showing that multivariate models which consider interactive effects of community production and ecosystem disturbance are, in fact, candidate explanations of much broader patterns of richness in natural systems. Because multivariate models predict synergistic effects of ecological variables on species diversity, human activities , which are simultaneously altering both the disturbance regime and productivity of streams , could be influencing biodiversity more than previously anticipated. [source]

Litter effects on plant regeneration in arid lands: a complex balance between seed retention, seed longevity and soil,seed contact

JOURNAL OF ECOLOGY, Issue 4 2005
JOSÉ L. ROTUNDO
Summary 1The recovery of rare, threatened plant populations can sometimes be achieved through modification of the disturbance regime. Accumulation of litter is likely to follow a reduction in grazing pressure and we therefore examine its effects on seedling recruitment of a threatened species in a temperate arid rangeland. 2We studied the effects of litter on seed longevity in the field and performed glasshouse and field experiments with natural and plastic litter to assess their physical and biological/chemical nature. Seeds were sown on the soil surface, buried or within the litter layer. Published data on spatial distribution of seeds were used to calculate the net effect of litter on seedling recruitment. 3Litter increased seed longevity. In the glasshouse, litter increased seedling emergence and growth for surface, but not for buried, seeds. Seeds within the litter layer (no seed-soil contact) showed reduced seedling emergence and growth. In the field, litter did not have a direct effect but emergence was promoted by burial. 4Integrating the effects of microsite quality and seed density showed that litter microsites recruited c. 50% of Bromus pictus seedlings, double that expected from its cover (25%). The positive effect of litter on seed density and on seed longevity outweighed the negative effect of litter acting as a mechanical barrier to burial. 5Inconsistent effects of litter on plant recruitment in arid environments may be due to responses being dependent on seed size and shape, and thus may represent indirect effects acting via the probability of burial. Alternatively, studies may report effects only on seed retention or emergence and growth rather than net effects on establishment. [source]

Plastic changes in seed dispersal along ecological succession: theoretical predictions from an evolutionary model

JOURNAL OF ECOLOGY, Issue 2 2005
OPHÉLIE RONCE
Summary 1We use a deterministic model to explore theoretically the ecological and evolutionary relevance of plastic changes in seed dispersal along ecological succession. Our model describes the effect of changing disturbance regime, age structure, density and interspecific competition as the habitat matures, enabling us to seek the evolutionarily stable reaction norm for seed dispersal rate as a function of time elapsed since population foundation. 2Our model predicts that, in the context of ecological succession, selection should generally favour plastic strategies allowing plants to increase their dispersal rate with population age, contrary to previous predictions of models that have assumed genetically fixed dispersal strategies. 3More complex patterns can evolve showing periods with high production of dispersing seeds separated by periods of intense local recruitment. These patterns are due to the interaction of individual senescence with change in ecological conditions within sites. 4Evolution of plastic dispersal strategies affects the patterns of density variation with time since foundation and accelerates successional replacement. An interesting parallel can be drawn between the evolution of age-specific dispersal rates in successional systems and the evolution of senescence in age-structured populations. 5Seed dispersal plasticity could be a potential mechanism for habitat selection in plants and have implications for range expansion in invasive species because recently founded populations at the advancing front may show different patterns to those in the established range. [source]

Natural avalanche disturbance shapes plant diversity and species composition in subalpine forest belt

JOURNAL OF VEGETATION SCIENCE, Issue 5 2007
Christian Rixen
Abstract Background: Disturbances by avalanches have created unique habitats for animals and plants in subalpine ecosystems worldwide, but at the same time avalanches can pose a major threat to humans. Thus, avalanches are suppressed by means of avalanche barriers to protect settlements and infrastructures in populated areas of the European Alps. As a consequence, the disturbance regime in avalanche tracks has fundamentally changed. Methods: In the present study we address ecological consequences of avalanche suppression on plant diversity. We analysed plant diversity and species composition in recent and old avalanche tracks with and without avalanche suppression and in undisturbed adjacent forests at high and low elevations. Results: The number of species was higher in both active and inactive avalanche tracks as compared to undisturbed subalpine forest. The species composition indicated a wider range of ecological niches in active than in inactive avalanche tracks. The vegetation from active tracks showed lower indicator values for temperature and nitrogen availability. The proportion of alpine species was lower in formerly active tracks. Conclusions: The conditions that exist in active avalanche tracks increase plant diversity in relation to undisturbed forest. In the few decades following avalanche suppression, species composition changes in tracks from which avalanches have been excluded. Continued suppression of avalanche disturbance may lead to a decline in plant and habitat diversity. Avalanche disturbance can exert an important influence on the biodiversity of subalpine forests and provide important habitats. Anthropogenic changes in the natural regime of avalanche disturbance are likely to contribute significantly to future landscape changes in subalpine forests. [source]

Temporal variability of disturbances: is this important for diversity and structure of marine fouling assemblages?

MARINE ECOLOGY, Issue 3 2007
Heather Sugden
Abstract Natural communities are constantly changing due to a variety of interacting external processes and the temporal occurrence and intensity of these processes can have important implications for the diversity and structure of marine sessile assemblages. In this study, we investigated the effects of temporal variation in a disturbance regime, as well as the specific timing of events within different regimes, on the composition and diversity of marine subtidal fouling assemblages. We did this in a multi-factorial experiment using artificial settlement tiles deployed at two sites on the North East coast of England. We found that although there were significant effects of disturbances on the composition of assemblages, there were no effects of either the variation in the disturbance regime or the specific timing of events on the diversity or assemblage composition at either site. In contrast to recent implications we conclude that in marine fouling assemblages, the variability in disturbance regimes (as a driving force) is unimportant, while disturbance itself is an important force for structuring robust ecosystems. [source]

Trawl-induced bottom disturbances off the south coast of Portugal: direct observations by the ,Delta' manned-submersible on the Submarine Canyon of Portimão

MARINE ECOLOGY, Issue 2007
Paulo Morais
Abstract The effects of marine fishing activities on benthic habitat and communities have become an important environmental issue. In addition to the direct removal of target species, effects include by-catch, damage to benthic organisms and alteration of habitat structure. The growing number of studies on the impact of fishing on bottom habitats indicates that the effects vary with the physical nature of the seabed and with the local natural disturbance regime. Several studies have been conducted on fisheries by-catch and discards off the south coast of Portugal since 1996. The results provide an idea of the impact on biodiversity: more than 60% of the species caught by the trawl fishery are discarded. The crustacean trawl captures the greatest number of species, probably due to the greater fishing depth range. In April 2004, the manned submersible ,Delta' (from DELTA Oceanographics, USA) conducted a sea campaign integrated in the SEMAPP programme (Science, Education, and Marine Archaeology Program in Portugal) to directly observe and assess the biological, geological, and archaeological aspects. A total of 15 dives were conducted mainly near the head and in the flanks inside Portimão canyon, down to 300 m. Observations showed variations in bottom type, the sedimentary framework and biological communities. These dives (150,300 m depth) also revealed a heavily trawled canyon bottom and erosion structures leading to changes in habitat structure and biodiversity. Of special interest was the occurrence of low-relief boulder substrates with relatively high densities of demersal finfish and shellfish. These sites served as a refuge for several species, whose abundance here was greater than on the surrounding areas of fine-grained substrates. Strong marks on the bottom are apparently caused by the doors of the trawl nets. Their impact will be examined in greater detail in future studies. [source]

Zonation Patterns of Belizean Offshore Mangrove Forests 41 Years After a Catastrophic Hurricane,

BIOTROPICA, Issue 3 2006
Cyril Piou
ABSTRACT Mangroves are prone to bearing frequently the full brunt of hurricanes and tropical storms. The extent of destruction and early regeneration are widely studied. The purpose of this study was to add a long-term view of mangrove regeneration and assess the potential effects on mangrove horizontal zonation patterns of catastrophic destruction. Hattie, a category five hurricane, hit the Belizean coast in 1961. It passed directly over the Turneffe Atoll where our study area, Calabash Cay, is located. At four sites on this island, we analyzed mangrove forest structure along transects parallel to the shoreline within zones delineated by species dominance and tree height. We propose an index based on the Simpson index of diversity to express changes in the heterogeneity of the species dominance. Physical,chemical parameters and nutrient availability were also measured. The destruction levels were estimated by analysis of the distribution of diameter at breast heights of the bigger trees in the inland zones. Variations in species dominance among sites and zones could be explained by interactions of various factors. Further, different levels of destruction between the two sides of the island had a significant effect on current patterns of species and structural zonation at Calabash. We conclude that disturbance regime in general should be considered as a factor potentially influencing mangrove horizontal zonation patterns. [source]

SYNTHESIS: Evolutionary history of Pacific salmon in dynamic environments

EVOLUTIONARY APPLICATIONS (ELECTRONIC), Issue 2 2008
Robin S. Waples
Abstract Contemporary evolution of Pacific salmon (Oncorhynchus spp.) is best viewed in the context of the evolutionary history of the species and the dynamic ecosystems they inhabit. Speciation was complete by the late Miocene, leaving c. six million years for intraspecific diversification. Following the most recent glacial maximum, large areas became available for recolonization. Current intraspecific diversity is thus the product of recent evolution overlaid onto divergent historical lineages forged during recurrent episodes of Pleistocene glaciation. In northwestern North America, dominant habitat features have been relatively stable for the past 5000 years, but salmon ecosystems remain dynamic because of disturbance regimes (volcanic eruptions, landslides, wildfires, floods, variations in marine and freshwater productivity) that occur on a variety of temporal and spatial scales. These disturbances both create selective pressures for adaptive responses by salmon and inhibit long-term divergence by periodically extirpating local populations and creating episodic dispersal events that erode emerging differences. Recent anthropogenic changes are replicated pervasively across the landscape and interrupt processes that allow natural habitat recovery. If anthropogenic changes can be shaped to produce disturbance regimes that more closely mimic (in both space and time) those under which the species evolved, Pacific salmon should be well-equipped to deal with future challenges, just as they have throughout their evolutionary history. [source]

Linking ecological theory with stream restoration

Geomorphology and fish assemblages in a Piedmont river basin, U.S.A.

FRESHWATER BIOLOGY, Issue 11 2003
D. M. Walters
Summary 1.,We investigated linkages between fishes and fluvial geomorphology in 31 wadeable streams in the Etowah River basin in northern Georgia, U.S.A. Streams were stratified into three catchment sizes of approximately 15, 50 and 100 km2, and fishes and geomorphology were sampled at the reach scale (i.e. 20,40 times stream width). 2.,Non-metric multidimensional scaling (NMDS) identified 85% of the among-site variation in fish assemblage structure and identified strong patterns in species composition across sites. Assemblages shifted from domination by centrarchids, and other pool species that spawn in fine sediments and have generalised food preferences, to darter-cyprinid-redhorse sucker complexes that inhabit riffles and runs, feed primarily on invertebrates, and spawn on coarser stream beds. 3.,Richness and density were correlated with basin area, a measure of stream size, but species composition was best predicted (i.e. |r| between 0.60,0.82) by reach-level geomorphic variables (stream slope, bed texture, bed mobility and tractive force) that were unrelated to stream size. Stream slope was the dominant factor controlling stream habitat. Low slope streams had smaller bed particles, more fines in riffles, lower tractive force and greater bed mobility compared with high slope streams. 4.,Our results contrast with the ,River Continuum Concept' which argues that stream assemblages vary predictably along stream size gradients. Our findings support the ,Process Domains Concept', which argues that local-scale geomorphic processes determine the stream habitat and disturbance regimes that influence stream communities. [source]

The Deciduous Forest , Boreal Forest Ecotone

GEOGRAPHY COMPASS (ELECTRONIC), Issue 7 2010
David Goldblum
Ecotones have been subject to significant attention over the past 25 years as a consensus emerged that they might be uniquely sensitive to the effects of climate change. Most ecotone field studies and modeling efforts have focused on transitions between forest and non-forest biomes (e.g. boreal forest to Arctic tundra, forest to prairie, subalpine forests to alpine tundra) while little effort has been made to evaluate or simply understand forest,forest ecotones, specifically the deciduous forest , boreal forest ecotone. Geographical shifts and changes at this ecotone because of anthropogenic factors are tied to the broader survival of both the boreal and deciduous forest communities as well as global factors such as biodiversity loss and dynamics of the carbon cycle. This review summarizes what is known about the location, controlling mechanisms, disturbance regimes, anthropogenic impacts, and sensitivity to climate change of the deciduous forest , boreal forest ecotone. [source]

Linking Spatial Pattern and Ecological Responses in Human-Modified Landscapes: The Effects of Deforestation and Forest Fragmentation on Biodiversity

GEOGRAPHY COMPASS (ELECTRONIC), Issue 4 2009
John A. Kupfer
Studies of forest loss and fragmentation provide clear examples of the linkages between ecological pattern and process. Reductions in forest area lead to higher within-patch extinction rates, the eventual loss of area-sensitive species, and declines in species richness and diversity. Forest loss also results in increased isolation of remnants, lower among-patch immigration rates, and less ,rescue' from surrounding populations. Specific responses, however, are sometimes counterintuitive because they depend on life-history tradeoffs that influence population dynamics and species co-existence in heterogeneous landscapes, not just forest remnants. Thus, while fragmentation generally favours r-selected, generalist strategies, such as high dispersal and a wide niche breadth, ecological outcomes may be confounded by species-specific responses to conditions in the human-dominated matrix and the ways in which forest edges shape cross-landscape movements. Given that pressures on global forestlands continue to intensify due to growing population sizes, economic pressures, and needs for space and resources, successfully maintaining or restoring species will necessitate a combination of short- and long-term actions that address both habitat protection and restoration. Doing so will require an interdisciplinary approach that gives adequate attention to the manners by which forest loss and fragmentation affect population dynamics through changes in forest area, isolation, habitat quality, matrix properties, and edge effects as well as the synergistic interactions of fragmentation with climate change, human-altered disturbance regimes, species interactions and other drivers of species population declines. [source]

Will increased storm disturbance affect the biodiversity of intertidal, nonscleractinian sessile fauna on coral reefs?

GLOBAL CHANGE BIOLOGY, Issue 11 2008
S. J. WALKER
Abstract Relatively little is known about how the future effects of climatic change, including increases in sea level, temperature and storm severity and frequency, will impact on patterns of biodiversity on coral reefs, with the notable exception of recent work on corals and fish in tropical reef ecosystems. Sessile invertebrates such as ascidians, sponges and bryozoans occupying intertidal rubble habitats on coral reefs contribute significantly to the overall biodiversity and ecosystem function, but there is little or no information available on the likely impacts on these species from climate change. The existing strong physical gradients in these intertidal habitats will be exacerbated under predicted climatic change. By examining the distribution and abundance of nonscleractinian, sessile invertebrate assemblages exposed to different levels of wave action and at different heights on the shore around a coral reef, we show that coral reef intertidal biodiversity is particularly sensitive to physical disturbance. As physical disturbance regimes increase due to more intense storms and wave action associated with global warming, we can expect to see a corresponding decrease in the diversity of these cryptic sessile assemblages. This could impact negatively on the future health and productivity of coral reef ecosystems, given the ecosystem services these organisms provide. [source]

Not seeing the ocean for the islands: the mediating influence of matrix-based processes on forest fragmentation effects

GLOBAL ECOLOGY, Issue 1 2006
John A. Kupfer
ABSTRACT The pervasive influence of island biogeography theory on forest fragmentation research has often led to a misleading conceptualization of landscapes as areas of forest/habitat and ,non-forest/non-habitat' and an overriding focus on processes within forest remnants at the expense of research in the human-modified matrix. The matrix, however, may be neither uniformly unsuitable as habitat nor serve as a fully,absorbing barrier to the dispersal of forest taxa. In this paper, we present a conceptual model that addresses how forest habitat loss and fragmentation affect biodiversity through reduction of the resource base, subdivision of populations, alterations of species interactions and disturbance regimes, modifications of microclimate and increases in the presence of invasive species and human pressures on remnants. While we acknowledge the importance of changes associated with the forest remnants themselves (e.g. decreased forest area and increased isolation of forest patches), we stress that the extent, intensity and permanence of alterations to the matrix will have an overriding influence on area and isolation effects and emphasize the potential roles of the matrix as not only a barrier but also as habitat, source and conduit. Our intention is to argue for shifting the examination of forest fragmentation effects away from a patch-based perspective focused on factors such as patch area and distance metrics to a landscape mosaic perspective that recognizes the importance of gradients in habitat conditions. [source]

Predator size, prey size and threshold food densities of diving ducks: does a common prey base support fewer large animals?

JOURNAL OF ANIMAL ECOLOGY, Issue 5 2009
Samantha E. Richman
Summary 1. Allometry predicts that a given habitat area or common prey biomass supports fewer numbers of larger than smaller predators; however, birds from related taxa or the same feeding guild often deviate from this pattern. In particular, foraging costs of birds may differ among locomotor modes, while intake rates vary with accessibility, handling times and energy content of different-sized prey. Such mechanisms might affect threshold prey densities needed for energy balance, and thus relative numbers of different-sized predators in habitats with varying prey patches. 2. We compared the foraging profitability (energy gain minus cost) of two diving ducks: smaller lesser scaup (Aythya affinis, 450,1090 g) and larger white-winged scoters (Melanitta fusca, 950,1800 g). Calculations were based on past measurements of dive costs with respirometry, and of intake rates of a common bivalve prey ranging in size, energy content and burial depth in sediments. 3. For scaup feeding on small prey <12 mm long, all clams buried deeper than 5 cm were unprofitable at realistic prey densities. For clams buried in the top 5 cm, the profitability threshold decreased from 216 to 34 clams m,2 as energy content increased from 50 to 300 J clam,1. 4. For larger scoters feeding on larger prey 18,24 mm long, foraging was profitable for clams buried deeper than 5 cm, with a threshold density of 147 m,2 for clams containing 380 J clam,1. For clams <5 cm deep, the threshold density decreased from 86 to 36 clams m,2 as energy content increased from 380 to 850 J clam,1. If scoters decreased dive costs by swimming with wings as well as feet (not an option for scaup), threshold prey densities were 11,12% lower. 5. Our results show that threshold densities of total prey numbers for different-sized ducks depend on prey size structure and depth in the sediments. Thus, heterogeneity in disturbance regimes and prey population dynamics can create a mosaic of patches favouring large or small predators. Whether a given area or total prey biomass will support greater numbers of larger or smaller predators will vary with these effects. [source]

The resilience of calcareous and mesotrophic grasslands following disturbance

JOURNAL OF APPLIED ECOLOGY, Issue 3 2005
RACHEL A. HIRST
Summary 1Understanding habitat disturbance and recovery is vital for successful conservation management and restoration, particularly of subseral communities with high nature conservation interest and sites subject to unavoidable disturbance pressures, such as that arising from access and recreational activities. 2Grassland resilience was investigated on the Salisbury Plain Training Area (SPTA) in southern England, the largest of the UK military training areas. SPTA contains the greatest expanse of unimproved chalk grassland in north-west Europe, a habitat of particular nature conservation interest. 3Historical aerial photographs were used to identify 82 calcareous and mesotrophic grassland sites disturbed over a 50-year time period. Vegetation, soils and seed bank data were collected from each old disturbance site. Revegetation time periods following disturbance were compared, and habitat resilience following disturbance investigated using the succession of surface vegetation along the chronosequence, the combined changes of vegetation and soil chemistry, and finally vegetation and seed bank composition. 4The sampled calcareous grasslands were less resilient following disturbance than the mesotrophic grasslands, with slower colonization of bare ground and target species re-assembly. The mesotrophic grasslands typically took between 30 and 40 years to re-establish following disturbance, whereas calcareous grasslands took at least 50 years. 5Even after such long time periods, there remained subtle but significant differences between the vegetation composition of the disturbed and undisturbed swards. Perennial forb species, particularly hemicryptophytes, persisted at higher frequencies in swards disturbed 50 years ago than in undisturbed swards. 6Synthesis and applications. Prediction of habitat resilience following disturbance is dependent on which components of the system are investigated. However, data such as that presented here can help land managers understand how palimpsests of current habitat characteristics may have evolved, and how disturbance regimes may be managed in the future. It is likely that the resilience of grasslands such as those on SPTA may have been overestimated, and perceptions of habitat carrying capacity for disturbance events may require re-evaluation. [source]

Reciprocal distribution of two congeneric trees, Betula platyphylla var. japonica and Betula maximowicziana, in a landscape dominated by anthropogenic disturbances in northeastern Japan

JOURNAL OF BIOGEOGRAPHY, Issue 12 2005
Katsuhiro Osumi
Abstract Aim, Information has been compiled on disturbance regimes and the life-history characteristics of Betula platyphylla var. japonica (Miq.) Hara and B. maximowicziana Regel to investigate the impact of humans on the present distribution patterns of these two congeneric tree species. Location, The study area is in the central region of the northern Kitakami Mountain Range, located in the northeast of Honshu Island, Japan. Methods, First, the present distributions of B. maximowicziana and B. platyphylla var. japonica were mapped at the landscape scale. To examine the factors affecting the distribution patterns, topographic features and past land use were taken into account. Second, life-history traits of both species were clarified at various growth stages either by fieldwork or through a literature search. Previous studies have provided some information on seed production, seed dispersal, seedling dynamics, sprouting, and growth. In the present study, field observations and experiments were conducted regarding seed-bank formation, size and age at reproduction, and the life span of canopy trees. Results,Betula platyphylla var. japonica and B. maximowicziana were spatially segregated in the studied landscape (14,000 ha). The distribution of each species was correlated more strongly with land use at the beginning of the twentieth century than with site environmental factors such as altitude or slope angle. Betula platyphylla var. japonica was distributed more frequently on former grasslands, whereas B. maximowicziana was almost exclusively limited to past old-growth forests. As typical pioneers, the two species showed similar life-history traits but differed in several critical points. Betula platyphylla var. japonica has a vigorous sprouting ability, which might increase its resistance to burning and logging, whereas B. maximowicziana forms a persistent seed bank in the soil, indicating an advantage in regenerating in disturbances found in forest communities. Other critical differences were detected in age-related characteristics such as minimum age of reproduction and life span. Main conclusions, The earlier reproductive maturity and the shorter life span of B. platyphylla var. japonica indicate a shorter population cycle than that for B. maximowicziana. The latter would be excluded from grasslands that were burned frequently, as a result of the longer time span before initial reproduction and its poor sprouting ability. In contrast, B. platyphylla var. japonica would be excluded from old-growth forests, where disturbance was infrequent, owing to its shorter life span and lack of a persistent seed bank. [source]

Using historical ecology to understand patterns of biodiversity in fragmented agricultural landscapes

JOURNAL OF BIOGEOGRAPHY, Issue 11 2005
Ian D. Lunt
Abstract Aim, To enhance current attempts to understand biodiversity patterns by using an historical ecology approach to highlight the over-riding influence of land-use history in creating past, current and future patterns of biodiversity in fragmented agricultural landscapes. Methods, We develop an integrative conceptual framework for understanding spatial and temporal variations in landscape patterns in fragmented agricultural landscapes by presenting five postulates (hypotheses) which highlight the important role of historical, anthropogenic disturbance regimes. We then illustrate each of these postulates with examples drawn from fragmented woodlands in agricultural areas of south-eastern Australia, and discuss these findings in an international context. Location examples are drawn from agricultural areas in south-eastern Australia. Results, We conclude that there is limited potential to refine our understanding of patterns of biodiversity in human-modified landscapes based on traditional concepts of island biogeography, or simple assumptions of ongoing destruction and degradation. Instead, we propose that in agricultural landscapes that were largely cleared over a century ago: (1) present-day remnant vegetation patterns are not accidental, but are logically arrayed due to historic land-use decisions, (2) historic anthropogenic disturbances have a major influence on current ecosystem conditions and diversity patterns, and (3) the condition of remnant ecosystems is not necessarily deteriorating rapidly. Main conclusions, An historical ecology approach can enhance our understanding of why different species and ecosystem states occur where they do, and can explain internal variations in ecological conditions within remnant ecosystems, too often casually attributed to the ,mess of history'. This framework emphasizes temporal changes (both past and future) in biotic patterns and processes in fragmented agricultural landscapes. Integration of spatially and temporally explicit historical land-use information into ecological studies can prove extremely useful to test hypotheses of the effects of changes in landscape processes, and to enhance future research, restoration and conservation management activities. [source]

Effects of natural disturbance and selective logging on Nothofagus forests in south-central Chile

JOURNAL OF BIOGEOGRAPHY, Issue 7 2002
William Pollmann
Aim Changes in stand structure, floristic composition and tree population dynamics during the last four centuries were described in southern temperate rain forests. The impacts of natural and anthropogenic disturbance since the late 1560s were examined for old-growth and logged forests. Location The study was conducted in montane Nothofagus alpina forests in the Andean Range of south-central Chile. Study sites were located at a range of altitudes between 1000 and 1250 m a.s.l. Methods Temporal variation in species recruitment and annual dendroecological data were used to determine the historical development and disturbance history of three old-growth forests, and three stands after selective logging in the late 1880s to early 1900s. Considering the spatial structure of evergreen vs. deciduous elements, the forests are a mixture of deciduous and evergreen tree species. Results Stem density, maximum stem diameter and basal area differed between the old-growth and logged stands, but species composition did not. At lower altitudes, N. alpina was the dominant canopy species in both old-growth and logged stands, but regeneration of N. alpina was significantly different in these stands. At higher altitudes, N. alpina and N. dombeyi were the dominant canopy species in both old-growth and logged stands, and here regeneration patterns of these Nothofagus species were similar. After selective logging, in mixed forests of shade-intolerant Nothofagus and more shade-tolerant trees (such as Laurelia philippiana) there has been a shift in regeneration from more shade-tolerant resprouting species towards Nothofagus. Major and moderate releases in radial growth, indicative of disturbance, occurred in most of the older trees during the last four centuries, and especially during the last 100 years. Growth rates of N. alpina are higher than those of associated shade-tolerant trees, and apparently increase after disturbances. Main conclusions Results suggest that under disturbance regimes dominated by treefall gaps, and additionally canopy openings by selective logging, maintenance of Nothofagus species appears to be associated with complementary differences in growth rate, sprouting capacity, canopy residence time, and longevity. Such small-scale canopy openings may help explain the relative abundance of N. alpina in montane mixed rain forests in the Andean Range, where the maintenance of Nothofagus species in many stands has been attributed to a high frequency of coarse-scale disturbance. My results serve to emphasize that understanding the species coexistence and forest dynamics in Nothofagus forests may require attention to interspecific differences in life-history characteristics. [source]

Cordia millenii: on the risk of local extinction?

AFRICAN JOURNAL OF ECOLOGY, Issue 3 2009
Fred Babweteera
Abstract Selective logging of valuable tropical timber trees is a conservation concern because it threatens the long-term sustainability of forests. However, there is insufficient information regarding the postlogging recovery of harvested species. Here, I assessed the seed dispersal patterns, recruitment and abundance of Cordia millenii, a valuable timber tree in two Ugandan tropical rain forests that have been subjected to varying disturbance regimes. The aim was to determine the vulnerability of Cordia in these forests. The rate of seed dispersal was lower in the heavily disturbed Mabira Forest compared with the less disturbed Budongo Forest. Frugivores in Mabira were small-bodied individuals that spat seeds beneath fruiting trees, whereas 90% of the fruit in Budongo was consumed by large-bodied chimpanzees that disperse seeds over long distances. Juveniles of Cordia were not found in the closed forest, although they were found in forest gaps in Budongo but not Mabira. Mature tree density was higher in Budongo compared with Mabira. Lack of effective seed dispersal coupled with the inability of seedlings of Cordia to establish under closed canopy account for the arrested recruitment in Mabira. Enrichment planting in felling gaps is necessary to avoid local extinction of Cordia in forests without large vertebrates. Résumé L'abattage sélectif des espèces précieuses d'arbres tropicaux inquiète le milieu de la conservation parce qu'il menace la durabilité des forêts à long terme. Cependant, il n'y a pas assez d'informations au sujet de la restauration des espèces prélevées après l'abattage. Ici, j'ai évalué les schémas de dispersion des semences, le recrutement et l'abondance de Cordia millenii, une espèce d'arbre prisée, dans deux forêts tropicales pluviales d'Ouganda qui ont été soumises à des régimes de perturbation variables. Le but était de déterminer la vulnérabilité de Cordia dans ces forêts. Le taux de dispersion des graines était plus faible dans la forêt très perturbée de Mabira que dans la forêt de Budongo, moins dérangée. Les frugivores de Mabira étaient des individus de petite taille qui recrachaient les graines sous les arbres producteurs des fruits alors que 90% des fruits de Budongo étaient consommés par des chimpanzés qui peuvent disperser les semences sur le longues distances. On n'a pas trouvé de juvénile de Cordia dans la forêt fermée; on en trouvait dans des clairières dans la forêt de Budongo, mais pas dans celle de Mabira. La densité des arbres matures était plus haute à Budongo qu'à Mabira. Le manque de dispersion efficace des graines, coupléà l'incapacité des semences de Cordia de s'établir sous une canopée fermée, intervient dans l'arrêt du recrutement à Mabira. Il est nécessaire de pratiquer un enrichissement des plantations dans les clairières déboisées pour éviter l'extinction locale des Cordia dans les forêts qui n'abritent pas de grands vertébrés. [source]