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Habitat Fragments (habitat + fragment)
Selected AbstractsSusceptibility of Common and Rare Plant Species to the Genetic Consequences of Habitat FragmentationCONSERVATION BIOLOGY, Issue 3 2007OLIVIER HONNAY diversidad genética; endogamia; fragmentación de hábitat; sistema reproductivo; tamaño poblacional Abstract:,Small plant populations are more prone to extinction due to the loss of genetic variation through random genetic drift, increased selfing, and mating among related individuals. To date, most researchers dealing with genetic erosion in fragmented plant populations have focused on threatened or rare species. We raise the question whether common plant species are as susceptible to habitat fragmentation as rare species. We conducted a formal meta-analysis of habitat fragmentation studies that reported both population size and population genetic diversity. We estimated the overall weighted mean and variance of the correlation coefficients among four different measures of genetic diversity and plant population size. We then tested whether rarity, mating system, and plant longevity are potential moderators of the relationship between population size and genetic diversity. Mean gene diversity, percent polymorphic loci, and allelic richness across studies were positively and highly significantly correlated with population size, whereas no significant relationship was found between population size and the inbreeding coefficient. Genetic diversity of self-compatible species was less affected by decreasing population size than that of obligate outcrossing and self-compatible but mainly outcrossing species. Longevity did not affect the population genetic response to fragmentation. Our most important finding, however, was that common species were as, or more, susceptible to the population genetic consequences of habitat fragmentation than rare species, even when historically or naturally rare species were excluded from the analysis. These results are dramatic in that many more plant species than previously assumed may be vulnerable to genetic erosion and loss of genetic diversity as a result of ongoing fragmentation processes. This implies that many fragmented habitats have become unable to support plant populations that are large enough to maintain a mutation-drift balance and that occupied habitat fragments have become too isolated to allow sufficient gene flow to enable replenishment of lost alleles. Resumen:,Las poblaciones pequeñas de plantas son más propensas a la extinción debido a la pérdida de variación genética por medio de la deriva génica aleatoria, el incremento de autogamia y la reproducción entre individuos emparentados. A la fecha, la mayoría de los investigadores que trabajan con erosión genética en poblaciones fragmentadas de plantas se han enfocado en las especies amenazadas o raras. Cuestionamos si las especies de plantas comunes son tan susceptibles a la fragmentación del hábitat como las especies raras. Realizamos un meta análisis formal de estudios de fragmentación que reportaron tanto tamaño poblacional como diversidad genética. Estimamos la media general ponderada y la varianza de los coeficientes de correlación entre cuatro medidas de diversidad genética y de tamaño poblacional de las plantas. Posteriormente probamos si la rareza, el sistema reproductivo y la longevidad de la planta son moderadores potenciales de la relación entre el tamaño poblacional y la diversidad genética. La diversidad genética promedio, el porcentaje de loci polimórficos y la riqueza alélica en los estudios tuvieron una correlación positiva y altamente significativa con el tamaño poblacional, mientras que no encontramos relación significativa entre el tamaño poblacional y el coeficiente de endogamia. La diversidad genética de especies auto compatibles fue menos afectada por la reducción en el tamaño poblacional que la de especies exogámicas obligadas y especies auto compatibles, pero principalmente exogámicas. La longevidad no afectó la respuesta genética de la población a la fragmentación. Sin embargo, nuestro hallazgo más importante fue que las especies comunes fueron tan, o más, susceptibles a las consecuencias genéticas de la fragmentación del hábitat que las especies raras, aun cuando las especies histórica o naturalmente raras fueron excluidas del análisis. Estos resultados son dramáticos porque muchas especies más pueden ser vulnerables a la erosión genética y a la pérdida de diversidad genética como consecuencia de los procesos de fragmentación que lo se asumía previamente. Esto implica que muchos hábitats fragmentados han perdido la capacidad para soportar poblaciones de plantas lo suficientemente grandes para mantener un equilibrio mutación-deriva y que los fragmentos de hábitat ocupados están tan aislados que el flujo génico es insuficiente para permitir la reposición de alelos perdidos. [source] Asymmetric dispersal and survival indicate population sources for grassland butterflies in agricultural landscapesECOGRAPHY, Issue 2 2007Erik Öckinger We tested the hypothesis that populations in small habitat fragments remaining in agricultural landscapes are maintained by repeated immigration, using three grassland butterflies (Aphantopus hyperantus, Coenonympha pamphilus and Maniola jurtina). Transect counts in 12 matched sets of semi-natural pastures, and linear habitat elements proximate and isolated from the pastures showed that population densities of M.,jurtina and C.,pamphilus were significantly higher in pastures and in linear habitats adjacent to these than in isolated linear elements. A mark-recapture study in a 2×2 km landscape indicated that individuals of all three species are able to reach even the isolated linear elements situated at least 1 km from the grasslands. For two of the species, A.,hyperantus and C.,pamphilus, analysis of the mark-recapture data revealed higher daily local survival rates in the semi-natural pastures and more individuals dispersing from pastures to linear habitat elements. The proportion of old compared to young individuals of C.,pamphilus and M.,jurtina were significantly higher in linear elements than in semi-natural pastures, which suggests that butterflies emerging in pastures subsequently dispersed to the linear elements. In combination, these results suggest that semi-natural pastures act as population sources, from which adult butterflies disperse to surrounding linear elements. Hence, preservation of the remaining fragments of semi-natural grassland is necessary to keep the present butterfly abundance in the surrounding agricultural landscape. [source] Computer-generated null models as an approach to detect perceptual range in mark,re-sight studies , an example with grasshoppersECOLOGICAL ENTOMOLOGY, Issue 2 2005Silke Hein Abstract., 1. Dispersal and habitat detection are key factors for the colonisation of habitat fragments in heterogeneous landscapes. The ability to recognise a habitat from a certain distance should increase the survival chances of a dispersing individual; however, due to methodological problems there is little information on the perceptual range of most species. 2. In a field experiment, 44 individually marked grasshoppers of the species Oedipoda caerulescens (Orthoptera: Acrididae: Locustinae) were released into an unfamiliar, hostile environment at various distances from a patch of preferred habitat. 3. Whether individuals reached the habitat or not was measured, as well as the daily movement distances. The number of individuals that reached the habitat was tested against computer-generated predictions based on different underlying rules for the movement behaviour of individuals but not accounting for the ability to detect habitat from distance. 4. On the first day a significantly higher proportion of grasshoppers arrived in the habitat than predicted by any of the null models. 5. It was concluded that individuals of O. caerulescens are able to detect their preferred habitat from a distance. 6. Edge permeability was very low as none of the individuals left the habitat once they had reached it. 7. Additional analyses showed that individuals changed movement behaviour from a directed walk with great daily distances in unsuitable habitat to a walk with significantly shorter daily distances within the preferred habitat. 8. The problems that arose in the field experiment are discussed and recommendations are given for further studies. [source] Spillover edge effects: the dispersal of agriculturally subsidized insect natural enemies into adjacent natural habitatsECOLOGY LETTERS, Issue 5 2006Tatyana A. Rand Abstract The cross-edge spillover of subsidized predators from anthropogenic to natural habitats is an important process affecting wildlife, especially bird, populations in fragmented landscapes. However, the importance of the spillover of insect natural enemies from agricultural to natural habitats is unknown, despite the abundance of studies examining movement in the opposite direction. Here, we synthesize studies from various ecological sub-disciplines to suggest that spillover of agriculturally subsidized insect natural enemies may be an important process affecting prey populations in natural habitat fragments. This contention is based on (1) the ubiquity of agricultural,natural edges in human dominated landscapes; (2) the substantial literature illustrating that crop and natural habitats share important insect predators; and (3) the clear importance of the landscape matrix, specifically distance to ecological edges, in influencing predator impacts in agroecosystems. Further support emerges from theory on the importance of cross-boundary subsidies for within site consumer,resource dynamics. In particular, high productivity and temporally variable resource abundance in agricultural systems are predicted to result in strong spillover effects. More empirical work examining the prevalence and significance of such natural enemy spillover will be critical to a broader understanding of fragmentation impacts on insect predator,prey interactions. [source] Subsidized Island Biogeography Hypothesis: another new twist on an old theoryECOLOGY LETTERS, Issue 4 2001W.B. Anderson We present a new hypothesis for predicting and describing patterns of species diversity on small islands and habitat fragments. We have modified the traditional island biogeography equilibrium theory to incorporate the influence of spatial subsidies from the surrounding matrix, which vary among islands and habitat fragments, on species diversities. The modification indicates three possible directions for the effects of spatial subsidies on diversity, which depend on where the focal community falls on the hypothesized unimodal curve of the productivity,diversity relationship. The idea is novel because no recent syntheses of productivity,diversity,area relationships examine the role of allochthonous resources on recipient communities' diversity patterns. [source] Factors determining mammal species richness on habitat islands and isolates: habitat diversity, disturbance, species interactions and guild assembly rulesGLOBAL ECOLOGY, Issue 1 2000Barry J. Fox Abstract 1For over three decades the equilibrium theory of island biogeography has galvanized studies in ecological biogeography. Studies of oceanic islands and of natural habitat islands share some similarities to continental studies, particularly in developed regions where habitat fragmentation results from many land uses. Increasingly, remnant habitat is in the form of isolates created by the clearing and destruction of natural areas. Future evolution of a theory to predict patterns of species abundance may well come from the application of island biogeography to habitat fragments or isolates. 2In this paper we consider four factors other than area and isolation that influence the number and type of mammal species coexisting in one place: habitat diversity, habitat disturbance, species interactions and guild assembly rules. In all examples our data derive from mainland habitat, fragmented to differing degrees, with different levels of isolation. 3Habitat diversity is seen to be a good predictor of species richness. Increased levels of disturbance produce a relatively greater decrease in species richness on smaller than on larger isolates. Species interactions in the recolonization of highly disturbed sites, such as regenerating mined sites, is analogous to island colonization. Species replacement sequences in secondary successions indicate not just how many, but which species are included. Lastly, the complement of species established on islands, or in insular habitats, may be governed by guild assembly rules. These contributions may assist in taking a renewed theory into the new millennium. [source] Spatial relationships between intensive land cover and residual plant species diversity in temperate farmed landscapesJOURNAL OF APPLIED ECOLOGY, Issue 6 2006SIMON M. SMART Summary 1In temperate farmed landscapes conservation policies increasingly emphasize large-scale reductions in land-use intensity. Yet despite a managed reversion to more favourable abiotic conditions, depleted regional species pools may prevent the re-assembly of target communities. 2Using national-scale survey data recorded across Great Britain in 1998, we investigated the extent to which grassland indicator plant species persisted on potential refuge habitats across a spatial gradient of intensive land cover in lowland 1-km squares. These habitats comprised road verges, field boundaries, watercourse banks and small biotope fragments. Intensive land cover comprised built land, arable and improved grassland. 3The rate of reduction in indicator species richness across the intensive land cover gradient was significantly lower in all potential refuge features than in surrounding fields and larger areas of habitat. 4The best refuge locations were watercourse banks and small biotopes. In both cases, indicator species richness was higher than adjacent fields at the lowest intensive land cover and stayed higher as intensive land cover increased. 5However, as intensive land cover increased, plant traits associated with higher nutrient availability were more prominently represented among indicator species. 6Although richer assemblages of indicator species persisted on refuge features, population sizes are likely to be small, because of species,area effects, and also vulnerable to nutrient surpluses and reduced or inappropriate disturbance. 7Synthesis and applications. Across the British lowlands, linear landscape features and small habitat fragments can provide limited safe havens for unimproved grassland plant species. However, the identity of refuge features and their species richness and composition are likely to vary with local conditions. Three activities are therefore paramount in assessing their role in larger scale extensification schemes: (i) development of rapid ways of assessing the plant diversity and distribution of refuge features in local areas; (ii) quantification of the risks posed to the viability of residual source populations through implementation of different options for incorporating them into extensification schemes; (iii) maximization of scheme performance by targeting landscapes with sufficient residual diversity to enable increases in population size of the target species in the medium term. [source] The tails of two geckos tell the story of dispersal in a fragmented landscapeMOLECULAR ECOLOGY, Issue 16 2007KENDI F. DAVIES The fragmentation of habitat is a major cause of biodiversity loss. However, while numerous studies have suggested that reducing the size of populations and isolating them on fragments leads ultimately to the extinction of a species (small isolated populations are extinction prone), the evidence has been rather conjectural. This is because dispersal is so difficult to measure and isolation difficult to confirm. In past studies, evidence that populations become small and isolated on fragments, leading to declines, has relied on spatial patterns of distribution and abundance. Thus, a species not trapped in the matrix in which fragments are embedded might be assumed isolated on fragments, and if low in abundance on fragments compared to continuous habitat is assumed to have declined on fragments due to this isolation. However, without accurately measuring the degree of isolation, it is difficult to distinguish the role of isolation from other important causes of population decline that are correlated with fragment and population size, such as habitat degradation. Developments in molecular techniques and statistical methods now make it possible to measure isolation. Refreshingly, in this issue Hoehn et al. analyse microsatellite DNA with a suite of statistical methods to show convincingly that a declining species of gecko suffers from greater isolation on habitat fragments than a contrasting gecko that is able to disperse between fragments and hence persist in the severely fragmented wheatbelt of Western Australia. [source] Female-biased dispersal, low female recruitment, unpaired males, and the extinction of small and isolated bird populationsOIKOS, Issue 2 2001Svein Dale Small and isolated populations are usually assumed to be at a high risk of extinction due to environmental or demographic stochasticity, genetic problems, or too little immigration. In birds, natal dispersal is usually female-biased, but the consequences of such a pattern on vulnerability to extinction of isolated populations has not received much attention before. In this paper I derive predictions as to how female-biased natal dispersal may differentially affect the extinction risk of populations and species with contrasting distributions, migratory behaviours, life histories and mating systems. Female-biased dispersal will lead to male-biased sex ratios in small, isolated or fragmented populations, in particular because recent research has shown that females often have a limited ability to search for mates and may therefore effectively be lost from the breeding population if they disperse into areas empty of conspecifics. I reviewed published studies on birds and found that a high proportion of unpaired males is common in isolated populations or populations in small habitat fragments. Dispersal of females may therefore increase the vulnerability to extinction of small or isolated populations, or populations at the periphery of a species' distribution range. I also predict that vulnerability to extinction should be greater for migratory than for resident species and greater for short-lived than for long-lived species because of differences in the time available for females to locate unpaired males. Further, extinction risk may also be greater for birds than for mammals due to differences in which sex disperses and patterns of parental care. Finally, mating system will also affect vulnerability to extinction when natal dispersal leads to biased sex ratios. I review available evidence for these predictions (e.g. songbird declines in North America) and discuss implications for conservation. [source] Investigating the distribution of prairie dogs in an urban landscapeANIMAL CONSERVATION, Issue 3 2009S. B. Magle Abstract Habitat fragmentation is a prevalent threat to biological diversity, and urbanization is a primary agent of fragmentation and a leading cause of species endangerment. Landscape biogeographic and local habitat characteristics can be important determinants of the distribution of species in habitat patches in urban landscapes. However, the specifics of which characteristics are most critical to maintaining biological diversity are not fully known for prairie ecosystems, especially in fragmented urban habitat. This study focuses on black-tailed prairie dogs along an urban gradient in Denver, CO. Prairie dogs have declined precipitously throughout the region and are an essential part of the prairie ecosystem, making them excellent study subjects. We identified a series of habitat fragments along a gradient of urbanization in the fully urbanized areas and south suburbs of Denver, CO, both containing and not containing prairie dogs. Local characteristics, including fragment slope and vegetative cover, and landscape characteristics, including fragment size, age and connectivity, were measured on each fragment. We used likelihood-based methods to explore which variables most accurately predicted prairie dog occurrence within our study area. Multiple factors influenced the distribution of prairie dogs in urban settings, with colony connectivity the strongest predictor of occupancy. Large and recently isolated fragments near other prairie dog colonies, flat areas and those with high graminoid cover were most likely to support prairie dog populations. Our study provides the first attempt to model prairie dog occurrence in highly fragmented urban habitat and has important implications for the management and conservation of prairie dogs. [source] What happens if density increases?ANIMAL CONSERVATION, Issue 4 2007Conservation implications of population influx into refuges Abstract Sudden catastrophic events like fires, hurricanes, tsunamis, landslides and deforestation increase population densities in habitat fragments, as fleeing animals encroach into these refuges. Such sudden overcrowding will trigger transient fluctuations in population size in the refuges, which may expose refuge populations to an increased risk of extinction. Until recently, detailed information about the operation of density dependence in stage-structured populations, and tools for quantifying the effects of transient dynamics, have not been available, so that exploring the extinction risk of such transient fluctuations has been intractable. Here, we use such recently developed tools to show that extinction triggered by overcrowding can threaten populations in refuges. Apart from situations where density dependence acts on survival, our results indicate that short-lived species may be more at risk than longer-lived species. Because dynamics in local populations may be critical for the preservation of metapopulations and rare species, we argue that this aspect warrants further attention from conservation biologists. [source] Dispersal ability determines the genetic effects of habitat fragmentation in three species of aquatic insectAQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, Issue 5 2010Kozo Watanabe Abstract 1.The dispersal ability of species and the geographic scale of habitat fragmentation both may influence the extent of gene flow between fragments, but their interactions have rarely been tested, particularly among co-occurring species. 2.Population genetic structures of three species of aquatic insect were compared in streams fragmented by reservoirs and in unfragmented streams in north-eastern Japan, using 52, 37, and 58 RAPD markers. The three species studied included a strong disperser Cincticostella elongatula (Ephemeroptera: Ephemerellidae), an intermediate disperser Stenopsyche marmorata (Trichoptera: Stenopsychidae), and a weak disperser Hydropsyche orientalis (Trichoptera: Hydropsychidae). 3.The patterns of genetic isolation by distance (IBD) supported a priori hypotheses of dispersal ability. The strong disperser (C. elongatula) exhibited significant IBD only at the largest spatial scale studied (among drainages, r=0.50, P<0.01). The intermediate disperser (S. marmorata) showed IBD both within (r=0.22, P<0.01) and among (r=0.45, P<0.01) drainages. The weak disperser (H. orientalis) did not exhibit significant IBD at any scale. 4.Pairwise genetic differentiation (,) indicated that neither the weak disperser nor the strong disperser were genetically differentiated above and below reservoirs when compared with reference sites. This was in contrast to previous results for S. marmorata, for which subpopulations were genetically fragmented across larger (>4.1,km), but not smaller (<2.9,km) reservoirs. 5.We suggest that intermediate dispersers, i.e. those at equilibrium between migration and genetic drift within drainages, are more likely to be affected by fragmentation than either strong or weak dispersers. Intermediate dispersers could therefore be used as indicator species in studies aimed at detecting the effects of distance between habitat fragments (e.g. reservoir size) for conservation planning. Copyright © 2010 John Wiley & Sons, Ltd. [source] Densities of polychaetes in habitat fragments depend on the surrounding matrix but not the complexity of the remaining fragmentAUSTRAL ECOLOGY, Issue 4 2009VICTORIA J. COLE Abstract Biogenically engineered habitats are constantly changing in space and time, resulting in changes to the landscape and the ecology of associated taxa. Using patchily distributed biogenic habitats on intertidal rocky shores the influences of different aspects of the landscape were investigated, that is, the surrounding matrix and the habitat itself. Experiments to test the effect of the matrix were carried out by transplanting assemblages in artificial habitats from one type of biogenically engineered matrix (created by the tubeworm Galeolaria caespitosa) to another (created by the oyster Saccostrea glomerata). Change to the surrounding matrix resulted in the densities of polychaete worms in central fragments of habitat changing to become more similar to their new surroundings. To test how the habitat influenced predator,prey interactions, polychaetes were transplanted from a complex habitat (created by the turfing alga Corallina officinalis) to less complex habitat (created by the tubeworm G. caespitosa), with or without the presence of the predatory polychaete, Perinereis amblyodonta. This experiment also tested the influence of habitat replacement on densities of polychaetes, regardless of any new interactions between species. Despite coralline turf being a much more complex habitat than Galeolaria, P. amblyodonta successfully preyed on other species of polychaetes in both habitats. Furthermore, in the absence of P. amblyodonta, survival of polychaetes did not differ between the habitats. These types of experiments will be useful for testing similar hypotheses in different landscapes and will assist in gaining a more general understanding of habitat modification. [source] | ||||||||||||||||||||||