AVIAN COMMUNITY DYNAMICS IN A FRAGMENTED TROPICAL LANDSCAPE

Estimates of community attributes such as species richness, local extinction, and turnover are critical when evaluating ecological restoration efforts. Estimates of species richness based on counts can be biased by variation in the probability of detection among different species. We quantified the effects of livestock exclusion on riparian bird communities using mark-recapture models to account for variation in species detection rates. Specifically, we estimated species richness and other community parameters for fenced and grazed sites with robust design models where closed-captures were treated as mixtures, and then used transition rates to calculate derived vital rates for avian communities. Estimates of species richness based on unadjusted counts were correlated with estimates from robust design models, but counts failed to detect important temporal changes in species richness. Estimates of species richness from robust design models increased at fenced and grazed sites over an 8-year period, but community vital rates were unaffected by cattle exclusion. We examined qualitative changes in abundance of birds in four nesting guilds, and concluded that temporal changes may have been driven by regional dynamics in avian communities. Our mark-recapture analysis allowed us to compare standardized estimates of community parameters between habitats, observers, and time periods after accounting for variation in detection rates. Robust design models are a useful tool that will facilitate accurate assessments of community dynamics following future restoration efforts.

Agricultural intensification has been shown to reduce biodiversity through processes such as habitat degradation and fragmentation. We tested whether several small or single large habitat fragments (re-visiting the 'single large or several small' debate) support more species across a wide range of taxonomic groups (plants, leafhoppers, true bugs, snails). Our study comprised 14 small (<1 ha) and 14 large (1.5-8 ha) fragments of calcareous grassland in Central Germany along orthogonal gradients of landscape complexity and habitat connectivity. Each taxon was sampled on six plots per fragment. Across taxa, species richness did not differ between large and small fragments, whereas species-area accumulation curves showed that both overall and specialist species richness was much higher on several small fragments of calcareous grassland than on few large fragments. On average, 85% of the overall species richness was recorded on all small fragments taken together (4.6 ha), whereas the two largest ones (15.1 ha) only accounted for 37% of the species. This could be due to the greater geographic extent covered by many small fragments. However, community composition differed strongly between large and small fragments, and some of the rarest specialist species appeared to be confined to large fragments. The surrounding landscape did not show any consistent effects on species richness and community composition. Our results show that both single large and many small fragments are needed to promote landscape-wide biodiversity across taxa. We therefore question the focus on large fragments only and call for a new diversified habitat fragmentation strategy for biodiversity conservation.

On the use of automated cameras to estimate species richness for large‐ and medium‐sized rainforest mammals

AimIn urbanized areas, exotic invasions, native extinctions, and the alteration of habitats and natural processes drive homogenization, which is a form of biotic impoverishment. This study examines whether urbanization and flooding induce homogenization of herbaceous communities in riparian forests and quantifies the relationships between taxonomic and functional β‐diversity.LocationMontréal, Québec, Canada.MethodsInventories were conducted in 56 riparian forests. Taxonomic and functional β‐diversity were calculated as between‐site similarities in species or trait composition for three levels of urbanization and flooding. Differences among the disturbance levels were compared using tests for homogeneity in multivariate dispersions. We quantified the correlation between local species richness, exotic proportion, taxonomic and functional β‐diversity. We also partitioned taxonomic β‐diversity into species turnover and richness difference.ResultsUrbanization led to taxonomic and functional differentiation, while increased flooding led to taxonomic and functional homogenization. We found a significant correlation between taxonomic and functional β‐diversity. Changes in β‐diversity were associated with species and trait turnover among both urbanization and flood levels, and with changes in species richness. Differentiation was associated with low species richness, and homogenization with high species richness. Exotic invasions tended to favour differentiation, but only at a low urbanization level.Main ConclusionsThe effect of urbanization on plant diversity in riparian forests was twofold: first, it directly induced taxonomic and functional differentiation through its effect on species loss and turnover (higher β‐diversity at high urbanization level); second, differentiation was indirectly favoured through the reduction in flooding (higher β‐diversity at low flood level). Taxonomic and functional β‐diversity followed similar patterns, likely because species invasions and extinctions are not random, but are related to species traits. Our results underline the need to move our focus from exotic species to the true underlying factors of biodiversity loss and homogenization, notably land use changes and human disturbances.

This study characterized rainforest economic tree species composition and indices of diversity in Delta State, South-southern Nigeria: Implications for biodiversity conservation. The study area was stratified into 25 divisions based on the existing Local Government Areas (LGA). From each LGA, one larger fragment (&gt;15,000m²) and one smaller (≤15,000m²) fragment were selected, making a total of 50 fragments from which data were collected on the size of rainforest fragment, population of individual tree species, and the species of trees of the rainforest origin using standard approaches in quadrats of 15m×15m. Data analysis involved the descriptive and inferential statistics, as well as Simpson’s index of tree species diversity. Results revealed that larger fragments have higher population of trees than smaller fragments. The mean population values of tree species were 306.40 and 80.88 for the larger and smaller fragments; with paired mean, standard deviation, and standard error of mean values of 225.52, 103.89 and ±20.78 respectively. With P (0.000) &lt; 0.05, and t-value of 10.85, the observed mean differences in the population of trees between the larger and smaller rainforest fragments was significant at 0.05 level of confidence. The mean population density values of 0.013 and 0.006 for the larger and smaller fragments were observed; with paired mean, standard deviation, and standard error of mean values of 0.0069, 0.003 and ±0.0005 respectively. With P (0.000) &lt; 0.05, and t-value of 14.11, the observed mean differences in the population density of trees between the larger and smaller rainforest fragments was significant at 0.05 level of confidence. Piptadeniastrum africanum was the most dominant tree species in the study area. Effective biodiversity conservation is recommended to prevent the extinction of rainforest tree species of economic importance in the study area.

Local extinction of specialist species due to fragmentation is one of the major causes of biodiversity loss. Increased extinction rates in smaller fragments are expected to result from both smaller local population sizes, which increase the effect of environmental or demographic stochasticity, and increased edge effects. The relative effect sizes of these two factors are still poorly investigated, however. We attempt to disentangle these effects on ground beetle communities of temperate broadleaved woodland fragments situated in one of the most urbanised regions in Belgium. Assemblages were sampled along transects that extended from 30 m outside to 100 m inside both small and large historical forest fragments. Although species assemblages within the forest were highly distinct compared to those sampled outside the forest, species turnover along these transects was less pronounced within forest fragments indicating only weak edge effects. The magnitude of edge effects did not differ significantly between large and small fragments. Nevertheless, larger differences in species composition were observed with respect to fragment size, wherein highly specialised species persisted only in the largest fragment. In summary, increased local extinction processes in smaller fragments, which led to a strong reduction in specialised and wingless forest species, appeared to be the most important factor that drives changes in species composition in this historical and fragmented woodland complex.

Habitat fragmentation is one of the most important threats to biodiversity. Decreasing patch size may lead to a reduction in the size of populations and to an increased extinction risk of remnant populations. Furthermore, colonization rates may be reduced in isolated patches. To investigate the effects of isolation and patch size on extinction and colonization rates of plant species, calcareous grasslands at three sites in the Swiss Jura Mountains were experimentally fragmented into patches of 0.25, 2.25, and 20.25 m2 by frequent mowing of the surrounding area from 1993 to 1999. Species richness in the fragment plots and adjacent control plots of the same sizes was recorded during these 7 years. In agreement with the theory of island biogeography, colonization rate was reduced by 30% in fragments versus non-isolated controls, and extinction increased in small versus large plots. Habitat specialists, in contrast to generalists, were less likely to invade fragments. In the last 4 years of the experiment, extinction rates tended to be higher in fragment than in control plots at two of the three sites. Despite reduced colonization rates and a tendency of increased extinction rates in fragments, fragmented plots had only marginally fewer species than control plots after 7 years. Hence, rates were a more sensitive measure for community change than changes in species richness per se. From a conservation point of view, the detected reduced colonization rates are particularly problematic in small fragments, which are more likely to suffer from high extinction rates in the long run.

Background and AimsIn many systems, postfire vegetation recovery is characterized by temporal changes in plant species composition and richness. We attribute this to changes in resource availability with time since fire, with the magnitude of species turnover determined by the degree of resource limitation. Here, we test the hypothesis that postfire species turnover in South African fynbos heathland is powered by fire-modulated changes in nutrient availability, with the magnitude of turnover in nutrient-constrained fynbos being greater than in fertile renosterveld shrubland. We also test the hypothesis that floristic overlaps between fynbos and renosterveld are attributable to nutritional augmentation of fynbos soils immediately after fire.MethodsWe use vegetation survey data from two sites on the Cape Peninsula to compare changes in species richness and composition with time since fire.Key ResultsFynbos communities display a clear decline in species richness with time since fire, whereas no such decline is apparent in renosterveld. In fynbos, declining species richness is associated with declines in the richness of plant families having high foliar concentrations of nitrogen, phosphorus and potassium and possessing attributes that are nutritionally costly. In contrast, families that dominate late-succession fynbos possess adaptations for the acquisition and retention of sparse nutrients. At the family level, recently burnt fynbos is compositionally more similar to renosterveld than is mature fynbos.ConclusionsOur data suggest that nutritionally driven species turnover contributes significantly to fynbos community richness. We propose that the extremely low baseline fertility of fynbos soils serves to lengthen the nutritional resource axis along which species can differentiate and coexist, thereby providing the opportunity for low-nutrient extremophiles to coexist spatially with species adapted to more fertile soil. This mechanism has the potential to operate in any resource-constrained system in which episodic disturbance affects resource availability.

A decline in species richness moving from equatorial regions to polar regions is a common, but not universal, macroecological pattern. Many studies have focused on this pattern, but few have focused on how the vital rates responsible for species richness patterns, local rates of species extinction and turnover, vary with latitude. We examine patterns of richness, turnover and extinction in North American avian communities inhabiting three ecoregions, using methods that account for failure to detect all species present. We use breeding bird point count data from &gt; 1000 routes in the Breeding Bird Survey collected from 1982 to 2001 to estimate richness, extinction probability and turnover rates. Our analyses differ from others in 1) the use of annual estimates derived at specific locations rather than index data accumulated over numbers of years, 2) the use of estimators that incorporated detection probabilities and 3) a focus on dynamical processes (colonization, extinction) in addition to static patterns (species richness). We find average species richness estimates (48 to 135 species) increasing with latitude for all three regions, contradicting predictions based on the latitudinal diversity gradient. The estimated rates of extinction and turnover declined with latitude across the three ecoregions. We speculate that higher richness might be linked to periods of superabundant food supply in northern areas that support greater numbers of resident and migrant species. Our primary ecological conclusions are that the latitudinal gradient in species richness is reversed for North American birds in the studied ecoregions, and that both local extinction and turnover decrease from southern to northern latitudes. Thus, the vital rates that determine richness show evidence of greater stability and reduced dynamics in northern areas of higher richness. We recommend additional studies examining patterns of colonization, extinction and turnover in communities, that use clearly defined estimators that deal with detection probability.

Dictated by limited resource availability for land acquisition, a central question in conservation biology is the ability of areas of different size to maintain species diversity. The selected reserves should not only be species rich at the moment, but should also maintain species diversity in the long run. We used two sets of data on vascular plant species in boreal lakes collected in 1933/34 and 1996 to test the relationships between lake area and the extinction, immigration and turnover rates of the species. Moreover, we investigated, whether the number of species in 1933/34 or water connection between lakes was related to extinction, immigration and turnover rates of species. We found that lake area or shoreline length was not correlated with immigration or turnover rate. But extinction rate was slightly negatively correlated with shoreline length. The original number of species was positively related to the number of species extinctions and to the absolute turnover rate in the lakes, which indicates that species richness does not create stability in these communities. Species number was not correlated with immigration rate. Upstream water connections in the lakes did not affect immigration, extinction or turnover rates. We conclude that length of the shoreline is a better measure of suitable area for water plants than the lake area, and that because the correlation between shoreline length and extinction rate was slight, also small lakes can be valuable for conservation.

Experimental warming in situ suggests that warming could lead to a loss of biodiversity. However, species that remain in situ and experience climate change will interact with species tracking climate change, which could also affect patterns of biodiversity. The relative contribution of species gains and losses to net changes in species richness is still unclear. We use transplanted plant communities to test the hypothesis that both the change in climate and ecological communities tracking climate change will influence how species richness responds to climate change. Three intact alpine plant communities were reciprocally transplanted to create scenarios in which species experienced warmer and wetter conditions (transferred to lower elevations) and cooler and drier conditions (transferred to higher elevations) over 10 years on the Tibetan Plateau. Communities transplanted into the same elevation as controls represent species tracking climate change. Transferring to lower elevations generally caused a net increase in richness and a higher rate of gains relative to the control plots; the magnitude of this effect depended on the specific elevation. Transferring to higher elevations lead to either net increases or decreases in richness and gains, depending on elevation. Species gains predicted much more variation in changes in species richness (50%) than did species loss (9%). Species richness at the receptor site and the donor site were both important predictors of variation in species richness, and the abiotic environment did not explain additional variation. Changes in cover of dominant plant species in response to transfers did not predict changes in species richness, species gain or species loss. Our results suggest that species gains from species tracking climate change at the receptor sites, rather than species loss from the donor sites, predicted changes in species richness. Synthesis. Warming experiments with physical barriers to dispersal may overestimate the negative effect of warming on plant diversity by not accounting for species gains. Our study highlights the importance of biotic factors in addition to the abiotic environment, when considering how climate change will affect plant diversity.

Understanding the causes underlying changes in species diversity is a fundamental pursuit of ecology. Animal species richness and composition often change with decreased forest structural complexity associated with logging. Yet differences in latitude and forest type may strongly influence how species diversity responds to logging. We performed a meta-analysis of logging effects on local species richness and composition of birds across the world and assessed responses by different guilds (nesting strata, foraging strata, diet, and body size). This approach allowed identification of species attributes that might underlie responses to this anthropogenic disturbance. We only examined studies that allowed forests to regrow naturally following logging, and accounted for logging intensity, spatial extent, successional regrowth after logging, and the change in species composition expected due to random assembly from regional species pools. Selective logging in the tropics and clearcut logging in temperate latitudes caused loss of species from nearly all forest strata (ground to canopy), leading to substantial declines in species richness (up to 27% of species). Few species were lost or gained following any intensity of logging in lower-latitude temperate forests, but the relative abundances of these species changed substantially. Selective logging at higher-temperate latitudes generally replaced late-successional specialists with early-successional specialists, leading to no net changes in species richness but large changes in species composition. Removing less basal area during logging mitigated the loss of avian species from all forests and, in some cases, increased diversity in temperate forests. This meta-analysis provides insights into the important role of habitat specialization in determining differential responses of animal communities to logging across tropical and temperate latitudes.

Turnover-driven loss of forest-dependent species changes avian species richness, functional diversity, and community composition in Andean forest fragments

Landscape structure, habitat fragmentation, and the ecology of insects

Summary We fitted species‐area curves to the power function and examined changes in the parameters to quantify changes in species richness of all plants together, trees only and non‐trees over five scales of magnitude (0.01 m2 to 400 m2) after a wildfire in the Linville Gorge Wilderness Area, North Carolina, USA. Increases in species richness of all plants together occurred after the fire at all scales and increased in magnitude as scale increased. However, a lack of change in the slopes (z‐values) of species‐area curves indicates that proportional changes were independent of scale of observation below 400 m2. Changes in species richness were predominantly driven by immigration, which was significantly related to fire severity. Survival of species present pre‐fire was greater than local extinction, but neither was related to severity. Species richness of trees increased at all scales but proportional increases were smaller at larger scales and slopes of species‐area curves decreased after the fire. Local seedling recruitment increased species richness at small scales, but low rates of immigration due to dispersal limitation in most species limited increases at larger scales. Directional changes in species richness of non‐trees were not always consistent at fine scales but both absolute and relative changes were positive at scales ≥ 1 m2 and increased with increasing scale. Slopes of species‐area curves increased post‐fire because localized patterns of immigration within plots resulted in little mixing of species at small scales but large changes in species richness at larger scales. Fire in the southern Appalachians increases plant species richness within local communities, but rates of species turnover and patterns of beta diversity are maintained by local recruitment of tree seedlings at small scales and immigration of herb, shrub and vine species at larger scales. Although decreased levels of competition after disturbance promote species coexistence at small scales, changes in species richness at larger scales are determined by the degree that the local community is linked to the species pool of the surrounding landscape through processes related to dispersal, particularly mass effects.