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Open Grassland (open + grassland)
Selected AbstractsInvasive alien plants infiltrate bird-mediated shrub nucleation processes in arid savannaJOURNAL OF ECOLOGY, Issue 4 2007S. J. MILTON Summary 1,The cultivation and dissemination of alien ornamental plants increases their potential to invade. More specifically, species with bird-dispersed seeds can potentially infiltrate natural nucleation processes in savannas. 2To test (i) whether invasion depends on facilitation by host trees, (ii) whether propagule pressure determines invasion probability, and (iii) whether alien host plants are better facilitators of alien fleshy-fruited species than indigenous species, we mapped the distribution of alien fleshy-fruited species planted inside a military base, and compared this with the distribution of alien and native fleshy-fruited species established in the surrounding natural vegetation. 3Abundance and diversity of fleshy-fruited plant species was much greater beneath tree canopies than in open grassland and, although some native fleshy-fruited plants were found both beneath host trees and in the open, alien fleshy-fruited plants were found only beneath trees. 4Abundance of fleshy-fruited alien species in the natural savanna was positively correlated with the number of individuals of those species planted in the grounds of the military base, while the species richness of alien fleshy-fruited taxa decreased with distance from the military base, supporting the notion that propagule pressure is a fundamental driver of invasions. 5There were more fleshy-fruited species beneath native Acacia tortilis than beneath alien Prosopis sp. trees of the equivalent size. Although there were significant differences in native plant assemblages beneath these hosts, the proportion of alien to native fleshy-fruited species did not differ with host. 6Synthesis. Birds facilitate invasion of a semi-arid African savanna by alien fleshy-fruited plants, and this process does not require disturbance. Instead, propagule pressure and a few simple biological observations define the probability that a plant will invade, with alien species planted in gardens being a major source of propagules. Some invading species have the potential to transform this savanna by overtopping native trees, leading to ecosystem-level impacts. Likewise, the invasion of the open savanna by alien host trees (such as Prosopis sp.) may change the diversity, abundance and species composition of the fleshy-fruited understorey. These results illustrate the complex interplay between propagule pressure, facilitation, and a range of other factors in biological invasions. [source] Holocene coastal environmental changes on the periphery of an area of glacio-isostatic uplift: an example from Scapa Bay, Orkney, UK,JOURNAL OF QUATERNARY SCIENCE, Issue 8 2007A. C. de la Vega-Leinert Abstract The first detailed investigation of a deep, coastal, sedimentary basin in Orkney reveals a complex Holocene history of back-barrier morphodynamics. At Scapa Bay, the sea flooded a freshwater marsh after ca. 9400,yr BP at ca. ,5.4,m OD. Before ca. 7800 BP, abundant sediment from nearby cliffs was mobilised inland into a series of gravel barriers across the valley mouth. By ca. 7500 BP, direct marine influence was restricted in the back-barrier area, although saltmarsh persisted until ca. 5900 BP. By then, at least four gravel ridges had enclosed the backing lagoon, where freshwater inputs became dominant. As terrestrial sediments filled the basin, another freshwater marsh developed. The multiple barrier complex demonstrates progradation resulting from continuous sediment supply in a sheltered embayment. The progressively rising height of the barrier crests seawards probably resulted from a combination of factors such as barrier morphodynamics, increased storminess and long-term rising relative sea levels. The dominant vegetation surrounding Scapa Bay changed from open grassland to scrub ca. 9400 BP, then to deciduous woodland ca. 7800 BP, and to dwarf-shrub heath ca. 2600 BP, the latter probably a response to a combination of climate change and human activity. Copyright © 2007 John Wiley & Sons, Ltd. [source] Consequences of shrub expansion in mesic grassland: Resource alterations and graminoid responsesJOURNAL OF VEGETATION SCIENCE, Issue 4 2003Michelle S. Lett Anon. (1986) Abstract. In the mesic grasslands of the central United States, the shrub Cornus drummondii has undergone widespread expansion in the absence of recurrent fire. We quantified alterations in light, water and N caused by C. drummondii expansion in tall-grass prairie and assessed the hypothesis that these alterations are consistent with models of resource enrichment by woody plants. Responses in graminoid species, particularly the dominant C4 grass Andropogon gerardii, were concurrently evaluated. We also removed established shrub islands to quantify their legacy effect on resource availability and assess the capability of this grassland to recover in sites formerly dominated by woody plants. The primary effect of shrub expansion on resource availability was an 87% reduction in light available to the herbaceous understorey. This reduced C uptake and N use efficiency in A. gerardii and lowered graminoid cover and ANPP at the grass-shrub ecotone relative to undisturbed grassland. Shrub removal created a pulse in light and N availability, eliciting high C gain in A. gerardii in the first year after removal. By year two, light and N availability within shrub removal areas returned to levels typical of grassland, as had graminoid cover and ANPP were similar to those in open grassland. Recovery within central areas of shrub removal sites lagged behind that at the former grass-shrub ecotone. These results indicate that the apparent alternative stable state of C. drummondii dominance in tall-grass prairie is biotically maintained and driven by reductions in light, rather than resource enrichment. Within areas of shrub removal, the legacy effect of C. drummondii dominance is manifest primarily through the loss of rhizomes of the dominant grasses, rather than any long-term changes in resource availability. C. drummondii removal facilitates grassland recovery, but the effort required to initiate this transition is a significant cost of woody plant expansion in mesic grasslands. Prevention of woody plant expansion in remnant tall-grass prairies is, therefore, a preferred management option. [source] Natural and human dimensions of environmental change in the proximal reaches of Botswana's Okavango DeltaTHE GEOGRAPHICAL JOURNAL, Issue 1 2010HAMISAI HAMANDAWANA Though wetlands are vital for the proper functioning of terrestrial ecosystems and provisioning of a wide range of goods and services, their sustainability is being threatened by inappropriate human resource use practices due to our limited understanding of how these systems operate and lack of appropriately informed interventions. We attempt to address these limitations by using historical CORONA photographs of 1967, Landsat imagery of 1989, 1994 and 2001 and information from the literature to investigate the role of natural and human factors in influencing the direction of environmental change in the proximal reaches of Botswana's Okavango Delta. Results of this investigation point to fragmentation of natural habitats, localised degradation of areas close to perennial water supplies, significant increase in woody cover, significant decrease in open grassland, increase in scrub and shrubs, deterioration in the quality of grazing and depletion of specific woody species. With the direction of change pointing to persistent decrease in the environment's supporting potentials, there is urgent need to adopt intervention strategies potentially capable of enhancing sustainable utilisation of natural resources in this sub-region. [source] Carnivore biodiversity in Tanzania: revealing the distribution patterns of secretive mammals using camera trapsANIMAL CONSERVATION, Issue 2 2010N. Pettorelli Abstract Biodiversity monitoring is critical to assess the effectiveness of management activities and policy change, particularly in the light of accelerating impacts of environmental change, and for compiling national responses to international obligations and agreements. Monitoring methods able to identify species most likely to be affected by environmental change, and pinpoint those changes with the strongest impacts, will enable managers to target efforts towards vulnerable species and significant threats. Here we take a new approach to carnivore monitoring, combining camera-trap surveys with ecological niche factor analysis to assess distribution and patterns of habitat use of mammalian carnivore assemblages across northern Tanzania. We conducted 11 surveys over 430 camera-trap stations and 11 355 trap-days. We recorded 23 out of 35 carnivore species known to occur in Tanzania and report major extensions to the known distribution of the bushy-tailed mongoose Bdeogale crassicauda, previously thought to be rare. Carnivore biodiversity tended to be higher in national parks than in game reserves and forest reserves. We explored habitat use for seven species for which we had sufficient information. All species tended to be found near rivers and southern Acacia commiphora woodlands (except one mongoose species), and avoided deciduous shrubland, favouring deciduous woodland and/or open grassland. All species tended to avoid croplands suggesting that habitat conversion to agriculture could have serious implications for carnivore distribution. Our study provides a first example where camera-trap data are combined with niche analyses to reveal patterns in habitat use and spatial distribution of otherwise elusive and poorly known species and to inform reserve design and land-use planning. Our methodology represents a potentially powerful tool that can inform national and site-based wildlife managers and policy makers as well as international agreements on conservation. [source] Conundrums in mixed woody,herbaceous plant systemsJOURNAL OF BIOGEOGRAPHY, Issue 11 2003Joanna I. House Abstract Aims To identify approaches to improve our understanding of, and predictive capability for, mixed tree,grass systems. Elucidation of the interactions, dynamics and determinants, and identification of robust generalizations that can be broadly applied to tree,grass systems would benefit ecological theory, modelling and land management. Methods A series of workshops brought together scientific expertise to review theory, data availability, modelling approaches and key questions. Location Ecosystems characterized by mixtures of herbaceous and woody plant life-forms, often termed ,savannas', range from open grasslands with few woody plants, to woodlands or forests with a grass layer. These ecosystems represent a substantial portion of the terrestrial biosphere, an important wildlife habitat, and a major resource for provision of livestock, fuel wood and other products. Results Although many concepts and principles developed for grassland and forest systems are relevant to these dual life-form communities, the novel, complex, nonlinear behaviour of mixed tree,grass systems cannot be accounted for by simply studying or modelling woody and herbaceous components independently. A more robust understanding requires addressing three fundamental conundrums: (1) The ,treeness' conundrum. What controls the relative abundance of woody and herbaceous plants for a given set of conditions at given site? (2) The coexistence conundrum. How do the life-forms interact with each other? Is a given woody,herbaceous ratio dynamically stable and persistent under a particular set of conditions? (3) The net primary productivity (NPP) conundrum. How does NPP of the woody vegetation, the herbaceous vegetation, and the total ecosystem (woody + herbaceous) change with changes in the tree,grass ratio? Tests of the theory and conceptual models of determinants of mixed woody,herbaceous systems have been largely site- or region-specific and have seldom been broadly or quantitatively evaluated. Cross-site syntheses based on data and modelling are required to address the conundrums and identify emerging patterns, yet, there are very few data sets for which either biomass or NPP have been quantified for both the woody and the herbaceous components of tree,grass systems. Furthermore, there are few cross-site comparisons spanning the diverse array of woody,herbaceous mixtures. Hence, initial synthesis studies should focus on compiling and standardizing a global data base which could be (1) explored to ascertain if robust generalizations and consistent patterns exist; and (2) used to evaluate the performance of savanna simulation models over a range of woody,herbaceous mixtures. Savanna structure and productivity are the result of complex and dynamic interactions between climate, soils and disturbances, notably fire and herbivory. Such factors are difficult to isolate or experimentally manipulate in order to evaluate their impacts at spatial and temporal scales appropriate for assessing ecosystem dynamics. These factors can, however, be evaluated with simulation models. Existing savanna models vary markedly with respect to their conceptual approach, their data requirements and the extent to which they incorporate mechanistic processes. Model intercomparisons can elucidate those approaches most suitable for various research questions and management applications. Conclusion Theoretical and conceptual advances could be achieved by considering a broad continuum of grass,shrub,tree combinations using data meta-analysis techniques and modelling. [source] Status of the Mara Woodlands in KenyaAFRICAN JOURNAL OF ECOLOGY, Issue 3 2004Matthew J. Walpole Abstract The woodlands of the Masai Mara National Reserve in Kenya have suffered dramatic declines over four decades as a result of elephant and fire pressure. This study examined the current status of woody resources in the Reserve and browse pressure thereon, using both classification (TWINSPAN) and ordination (DCA) techniques. From 333 widespread regular plots used to survey the vegetation, a total of 62 woody species were identified. Thirteen woody habitats were identified on the basis of species composition, varying from species-rich closed thickets and forest to less diverse open grasslands. A NW/SE stratification of the more open habitats was observed, possibly as a result of differences in soils, rainfall and drainage. Both plant density and diversity were lower than in communal and privately managed areas outside the Reserve. Moreover, browser pressure was substantially higher than that observed previously in the ecosystem, and suggests increased competition for scarcer woody resources within the Reserve. This has implications for the management of the ecosystem as a whole. As woodland and thickets continue to decline, long-term monitoring should expand to encompass the wider habitat diversity of the open grasslands and unprotected areas where much of the regeneration potential resides. Résumé Les forêts de la Réserve Nationale de Masai Mara, au Kenya, ont subi des réductions dramatiques depuis quatre décennies, suite à la pression des éléphants et des feux. Cette étude a examiné le statut actuel des ressources en bois dans la Réserve et la pression du pâturage qui s'y ajoute, en utilisant les techniques de classification (TWINSPAN) et d'ordination (DCA). Dans 333 plots régulièrement dispersés, utilisés pour étudier la végétation, on a identifié 62 espèces ligneuses. On a identifié aussi 13 habitats forestiers sur la base de la composition des espèces, depuis les buissons et les forêts fermés riches en espèces jusqu'aux prairies ouvertes moins diverses. On a observé une stratification NO/SE dans les habitats plus ouverts, résultat peut-être de différences de sols, de chutes de pluies et de drainage. La densité et la diversité des plantes étaient plus faibles que dans les aires gérées en commun ou de façon privée en dehors de la Réserve. Qui plus est, la pression des animaux qui mangent les buissons y était substantiellement plus élevée que celle qu'on observait jadis dans cet écosystème, et elle laisse penser qu'il y a une compétition plus forte pour des ressources ligneuses plus rares dans la Réserve. Ceci a des implications pour la gestion de l'écosystème dans son ensemble. Si les forêts et les buissons continuent à se rèduire, le monitoring à long terme devrait s'élargir pour englober la plus grande diversité d,habitat des prairies ouvertes et des aires non protégées où réside l'essentiel du potentiel de régénération. [source] Spatial density-dependent parasitism and specificity in the robber fly Mallophora ruficauda (Diptera: Asilidae)AUSTRAL ECOLOGY, Issue 1 2010MARCELA K. CASTELO Abstract The density-dependence in parasitism by the robber fly Mallophora ruficauda (Diptera: Asilidae) on scarab beetle larvae (Coleoptera: Scarabaeidae) populations was studied in the present research. Mallophora ruficauda is a pestiferous species common in the open grasslands of the Pampas region of South America. Adults are predators of insects and larvae are solitary parasitoids of third instar larvae of several species of scarab beetle (Coleoptera: Scarabaeidae). In contrast with most studied host-parasitoid interactions, host searching by M. ruficauda is carried out by both larvae and adults. Typically, robber fly females lay eggs on tall grasses from where larvae drop to the ground, and attack hosts which are buried in the soil. We carried out our study at two spatial scales close to 14 apiaries located in the provinces of Buenos Aires and Entre Ríos (Argentina). We found that parasitism is density-independent at the larger spatial scale and inversely density-dependent at the smaller one. We also found that M. ruficauda selects Cyclocephala signaticollis among several scarab beetle species. Specificity is observed both at large and small spatial scales. We discuss the implications of both host specificity and host searching behaviour on the observed parasitism patterns. [source] | |||||||||||||