Predicting community structure in snakes on Eastern Nearctic islands using ecological neutral theory and phylogenetic methods

Predicting species presence and richness on islands is important for understanding the origins of communities and how likely it is that species will disperse and resist extinction. The equilibrium theory of island biogeography (ETIB) and, as a simple model of sampling abundances, the unified neutral theory of biodiversity (UNTB), predict that in situations where mainland to island migration is high, species-abundance relationships explain the presence of taxa on islands. Thus, more abundant mainland species should have a higher probability of occurring on adjacent islands. In contrast to UNTB, if certain groups have traits that permit them to disperse to islands better than other taxa, then phylogeny may be more predictive of which taxa will occur on islands. Taking surveys of 54 island snake communities in the Eastern Nearctic along with mainland communities that have abundance data for each species, we use phylogenetic assembly methods and UNTB estimates to predict island communities. Species richness is predicted by island area, whereas turnover from the mainland to island communities is random with respect to phylogeny. Community structure appears to be ecologically neutral and abundance on the mainland is the best predictor of presence on islands. With regard to young and proximate islands, where allopatric or cladogenetic speciation is not a factor, we find that simple neutral models following UNTB and ETIB predict the structure of island communities.     

Microbial colonization dynamics in temporary aquatic systems

Most studies of community development in insular systems have investigated the colonization dynamics of only a portion of the biotic community using islands that have the benefit of prior biotic modification. Two experiments assessed the predictive ability of equilibrium island theory with regard to the development of the protistan community in temporary aquatic islands (100-1 plastic swimming pools) void of any prior biotic modification. The first experiment ran for 170 d (March–September 1985) and manipulated the access of islands to animals that represent potentially important dispersal pathways for microbes. A second experiment (May–June 1986) investigated in greater detail the early stages of species accrual in the absence of dispersal pathways other than via the atmosphere. Polyurethane foam substrates were used as sampling devices to increase the accuracy and replicability of sampling and provide habitat for colonists. Sampling was determined to be asymptotic. Species accrual was asymptotic in both experiments, although it initially lagged behind that predicted by theory. Autotrophs approached equilibrium faster than heterotrophs, but at a lower species richness. The predicted number of autotroph species at equilibrium was lower in islands that were excluded from contact with birds compared to less exclosed islands. The colonization dynamics of the entire community was not significantly different among islands having different degrees of exclosure. In both experiments, rates of species immigration were nonmonotonic with respect to time and species richness. This relationship appeared to reflect the importance of species interactions during the initial accrual phase before equilibrium. Rates of extinction were positively correlated with both these parameters, although they tended to decrease with time during the equilibrium period in less exclosed islands. Turnover at equilibrium was significant and resulted in directional changes in species composition over time. Assortative processes appeared to be important since later colonists exhibited greater persistence. Colonizing species generally fall into three autecological categories: 1) those that were ubiquitous and had temporally predictable patterns of immigration (successional species); 2) those possessing temporally predictable distributions but not spatially ubiquitous distributions (dispersal limited species); 3) those that showed little temporal or spatial predictability in immigration (transient or allochthonous species). Individual islands exhibited various degrees of fluctuation in species number during the predicted equilibrium which were poorly correlated with exogenous environmental variables and physicochemical habitat parameters. The presence of predacious mosquito larvae(Culex spp.) invariably resulted in a sharp decreases in microbial species richness, while documented contact with rodents was followed by an increase in species number in an island so contaminated. Several aspects of microbial colonization of temporary waters that contradict equilibrium predictions appear to be strongly influenced by microbe-microbe as well as macrobe-microbe interactions.     

When do localized natural enemies increase species richness?

The Janzen–Connell hypothesis states that local species‐specific density dependence, mediated through specialist enemies of offspring such as fungal pathogens and insect seed predators, can facilitate coexistence of species by preventing recruitment near conspecific adults. We use spatially explicit simulation models and analytical approximations to evaluate how spatial scales of offspring and enemy dispersal affect species richness. In comparison with model communities in which both offspring and enemies disperse long distances, species richness is substantially decreased when offspring disperse long distances and enemies disperse short distances. In contrast, when both offspring and enemies disperse short distances species richness more than doubles and adults of each species are highly spatially clumped. For the range of conditions typical of tropical forests, locally dispersing specialist enemies may decrease species richness relative to enemies that disperse long distances. In communities where dispersal distances of both offspring and enemies are short, local effects may enhance species richness.     

Assemblage-level responses of phyllostomid bats to tropical forest fragmentation: land-bridge islands as a model system

Aim Working within a system of high structural contrast between fragments and the surrounding matrix, we assessed patterns of species loss and changes in species composition of phyllostomid bats on artificial land‐bridge islands relative to mainland assemblages, and evaluated the responses of bats to forest edges. We further examined the relative influence of local‐scale characteristics (e.g. vegetation structure, island area) versus landscape attributes (e.g. forest cover, patch density) and the importance of spatial scale in determining phyllostomid species richness and composition on islands. Location Islands in Gatún Lake and adjacent mainland peninsulas in the Barro Colorado Nature Monument, Panama. Methods Bats were sampled over a 2‐year period on 11 islands as well as at forest‐edge and interior sites on adjacent mainland, resulting in > 8400 captures. Results The islands harboured a less diverse and structurally simplified phyllostomid bat fauna. Islands far from the mainland were especially species‐poor. This decline in species richness was associated with compositional shifts towards assemblages strongly dominated by frugivores with good dispersal abilities. Members of other ensembles, most importantly gleaning animalivores, were much less common or absent. Although overall species composition was not significantly altered, species richness at continuous forest‐edge sites was significantly lower compared with that at interior sites. Distance from the mainland and amount of forest cover in the landscape were the best predictors of species richness and assemblage composition. Responses were scale‐dependent. At the local scale, species richness was independent of island area but was correlated positively with distance from the mainland. In contrast, area effects became more important at larger spatial scales, suggesting that many species use multiple fragments. Main conclusions Our results underline the conservation value of small habitat remnants, which, even when embedded in a hostile matrix, can support a relatively diverse bat fauna, provided that there is a low degree of patch isolation and spatial proximity to larger tracts of continuous forest. Although the results at the assemblage level were inconclusive, we demonstrate that certain bat species and ensembles, particularly gleaning animalivores, exhibit high edge‐sensitivity. Our results point to habitat loss rather than changes in landscape configuration as the main process after isolation underlying phyllostomid bat responses, suggesting that conservation efforts should focus on habitat preservation instead of trying to minimize fragmentation per se at the expense of habitat amount.     

Adventive flora of Israel — Phytogeographical,ecological and agricultural aspects

73 adventive wild species occur in the flora of Israel: 39 are of tropical origin, 20 are North American. The geographical distribution pattern suggests that about two thirds of the species reached Israel through neighbouring countries, while only one third arrived directly from their countries of origin. — Three patterns of population dynamics can be recognized: accidental species (7), colonizing species (30), and species penetrating into natural habitats (3). There is insufficient information about the remaining species. — About 20 species are widespread noxious weeds which now have economic significance in Israel, 20 others are known to have similar ecological tendencies in other countries. These species soon may become aggressive weeds in Israel.