Aim Across the planet, grass‐dominated biomes are experiencing shrub encroachment driven by atmospheric CO2 enrichment and land‐use change. By altering resource structure and availability, shrub encroachment may have important impacts on vertebrate communities. We sought to determine the magnitude and variability of these effects across climatic gradients, continents, and taxa, and to learn whether shrub thinning restores the structure of vertebrate communities. Location Worldwide. Time period Contemporary. Major taxa studied Terrestrial vertebrates. Methods We estimated relationships between percentage shrub cover and the structure of terrestrial vertebrate communities (species richness, Shannon diversity and community abundance) in experimentally thinned and unmanipulated shrub‐encroached grass‐dominated biomes using systematic review and meta‐analyses of 43 studies published from 1978 to 2016. We modelled the effects of continent, biome, mean annual precipitation, net primary productivity and the normalized difference vegetation index (NDVI) on the relationship between shrub cover and vertebrate community structure. Results Species richness, Shannon diversity and total abundance had no consistent relationship with shrub encroachment and experimental thinning did not reverse encroachment effects on vertebrate communities. However, some effects of shrub encroachment on vertebrate communities differed with net primary productivity, amongst vertebrate groups, and across continents. Encroachment had negative effects on vertebrate diversity at low net primary productivity. Mammalian and herpetofaunal diversity decreased with shrub encroachment. Shrub encroachment also had negative effects on species richness and total abundance in Africa but positive effects in North America. Main conclusions Biodiversity conservation and mitigation efforts responding to shrub encroachment should focus on low‐productivity locations, on mammals and herpetofauna, and in Africa. However, targeted research in neglected regions such as central Asia and India will be needed to fill important gaps in our knowledge of shrub encroachment effects on vertebrates. Additionally, our findings provide an impetus for determining the mechanisms associated with changes in vertebrate diversity and abundance in shrub‐encroached grass‐dominated biomes.
Connectivity has long played a central role in ecological and evolutionary theory and is increasingly emphasized for conserving biodiversity. Nonetheless, connectivity assessments often focus on individual species even though understanding and preserving connectivity for entire communities is urgently needed. Here we derive and test a framework that harnesses the well-known allometric scaling of animal movement to predict community-level connectivity across protected area networks. We used a field translocation experiment involving 39 species of southern African birds to quantify movement capacity, scaled this relationship to realized dispersal distances determined from ring-and-recovery banding data, and used allometric scaling equations to quantify community-level connectivity based on multilayer network theory. The translocation experiment explained observed dispersal distances from ring-recovery data and emphasized allometric scaling of dispersal based on morphology. Our community-level networks predicted that larger-bodied species had a relatively high potential for connectivity, while small-bodied species had lower connectivity. These community networks explained substantial variation in observed bird diversity across protected areas. Our results highlight that harnessing allometric scaling can be an effective way of determining large-scale community connectivity. We argue that this trait-based framework founded on allometric scaling provides a means to predict connectivity for entire communities, which can foster empirical tests of community theory and contribute to biodiversity conservation strategies aimed at mitigating the effects of environmental change.
Land-cover and land-use change are major drivers of global biodiversity loss. Savannas are experiencing shrub encroachment and land-use changes that affect animal communities, yet how the effects of shrub encroachment vary with land use remains unclear. We also need to determine which species traits explain the effects of shrub encroachment and land-use changes to identify potential drivers of community change and predict species' responses to conservation efforts. We sampled birds across gradients of shrub encroachment and land use (protected savanna, pastures, homestead lands, and sugarcane) in the lowveld savanna of Eswatini to: (1) test for the effects of shrub encroachment, land-use change, and their potential synergies on the occurrence of 48 species of birds; and (2) determine which species traits (those associated with nesting, foraging, or vulnerability to predation) explained species' associations with shrub encroachment and land-use change. We used Bayesian multispecies occupancy models to account for imperfect detection, interpret species responses, and understand the effects of species traits. We found evidence for community-wide synergies between shrub encroachment and land-use effects on birds that varied with species' diets. Agricultural intensification had negative effects on the occurrence of most species examined, while shrub cover typically had positive effects which were stronger in pastures and homestead lands than in protected areas. The negative effects of land-use change were greatest for insectivores and cavity nesters, whereas insectivore occurrence increased with shrub cover, collectively indicating that nesting and foraging traits best explained species responses to land-use and land-cover change in the region. Our results suggest that shrub cover management for bird conservation should vary with land use: shrubs could be thinned in protected areas without reducing bird occupancy but shrubs in homesteads should be retained.
Knowledge about species’ responses to habitat restoration can inform subsequent management and reintroduction planning. We used repeated call‐response surveys to study brown‐headed nuthatch (Sitta pusilla) patch occupancy at the current limits of its apparently expanding range in an area with active habitat restoration. We fit a probit occupancy model that accounted for spatial autocorrelation using restricted spatial regression. Nuthatch occupancy was related to patch‐level vegetation structure and range‐extension context, i.e., latitude, but not prescribed fire history. Latitude and percent tree stocking had a negative relationship with occupancy (coefficients and 95% credible intervals: −1.07 [CI: −1.63, −0.67] and −0.63 [CI: −0.97, −0.350]). The density of recently killed and well‐decayed snags had positive associations with occupancy (coefficients and 95% credible intervals: 0.57 [CI: 0.17, 1.16] and 0.37 [CI: 0.05, 0.72]). Neither grassy herbaceous cover nor percent of stocking in pine were associated with occupancy. We found that restoration efforts created suitable stand structure for brown‐headed nuthatches, but many restored sites in the range‐extension zone appeared to be vacant. Occupied habitats in the range‐extension zone had fewer snags, less frequent fire, and more shrub cover than occupied sites where the species was established. Release from conspecific competition may have permitted nuthatches in the range‐extension zone to exploit habitats that would otherwise have been marginal. Alternatively, nuthatches may be restricted to such sites although there are more suitable sites tens of kilometers away. Experimental translocations and reintroductions could determine how habitat structure and nuthatch density affect the quality of restored sites in the range‐extension zone and enable those sites to achieve their biodiversity potential. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.
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