Investigating causality of nestedness of insular communities: selective immigrations or extinctions?

Abstract. Most archipelagos are comprised of a nested set of communities, with species on depauperate islands representing proper subsets of those on richer islands. The causality for this common intra-archipelago pattern, however, remains elusive. Here, I present a Monte Carlo approach for investigating whether nestedness results from selective extinctions, selective immigrations, or both. Results for mammal communities of three nearshore archipelagos and two montane forest archipelagos suggested that nestedness may result from both forces. Nestedness was significantly associated with area (extinctions) for all archipelagos studied, and with isolation (immigrations) for islands of Lake Huron and the American Southwest. The degree of nestedness is influenced by the degree of variations among species and islands. In addition, our ability to assess causality of nestedness is influenced by our ability to calculate biologically relevant measures of isolation, and by the potentially confounding effects of selective immigrations and extinctions on community nestedness.     

Geographic gradients of deforestation and mammalian communities in a fragmented, temperate rain forest landscape

Aim We studied the temporal and spatial patterns in deforestation and community structure of mammals in a fragmented old‐growth, temperate rain forest to test the hypothesis that anthropogenic habitat conversion advances in a nonrandom manner across native landscapes, and that its effects on ecological communities are both persistent and predictable. Location The location is the Hood Canal district of Olympic National Forest, Washington, USA. Results Deforestation followed the apparently general pattern observed for deforestation of tropical rain forests and other native landscapes, advancing first along low and relatively level valleys, then to areas at higher elevations and along steeper slopes, and eventually to sites more distant from those of initial land conversion and transportation centres. Mammal surveys within this area indicated that this nonrandom advance of deforestation has created relatively steep geographical and topographic gradients in both local and landscape‐level factors and, ultimately, in the structure of mammalian communities. Conclusion The close and likely causal relationship between anthropogenic habitat loss and the ecological dynamics of mammalian communities and dependent species (e.g. spotted owls) indicates that our abilities to understand and eventually reduce the current extinction crisis may rely heavily on our understanding of, and abilities to modify, the manner in which we expand across and transform native landscapes.     

Trajectories to extinction: spatial dynamics of the contraction of geographical ranges

Aim We examined the range contraction of 309 declining species of animals and plants to determine if the contraction dynamics better matched predictions based on the demographic characteristics of historical populations (demographic hypothesis) or based on the contagion‐like spread of extinction forces (contagion hypothesis). Location Species included in the analysis came from all biogeographic regions. Methods We obtained range maps for 309 species from literature or through personal correspondence with authorities. Hypotheses were contrasted by examining the sequence of changes in the proportion (C) of the remnant range that fell within the central region of the historical range. Monte Carlo simulations and polynomial regressions were employed to examine changes in C during the process of range contraction. Results The results of the Monte Carlo simulations indicated that more species had observed range contractions consistent with the contagion hypothesis than expected by chance (z‐score = 2.922, P = 0.002). The Monte Carlo analysis also indicated that the number of species whose observed range contractions were consistent with the demographic hypothesis was no greater than expected by chance (z‐score = 0.337, P = 0.367). The results of the polynomial regression analysis for the two most common taxonomic groups (mammals and birds) and for all geographical regions (Australia, Africa, Eurasia, and North America) we examined also supported the contagion hypothesis. Main conclusions Most of the examined range contractions are consistent with the contagion hypothesis and that the most likely contagion is human related disturbance. These results have important implications for the conservation of endangered species.     

Elevation gradients of species-density: historical and prospective views

Studies of elevation clines in diversity and composition of ecological communities date back to the origins of biogeography. A modern resurgence of interests in these elevational clines is likely to contribute important insights for developing a more general theory of species diversity. In order to gain a more comprehensive understanding of geographical clines in diversity, the research programme for montane biogeography should include statistically rigorous tests of apparent patterns, comparisons of patterns among regions and taxonomic or ecological groups of species, and analyses of clines in environmental variables concurrent with biogeographical surveys. The conceptual framework for this research programme should be based on the assumption that elevational gradients in species diversity result from a combination of ecological and evolutionary processes, rather than the presumed independent effects of one overriding force. Given that montane ecosystems are hot spots of biological diversity, an expanded and integrated programme for biogeographic surveys in montane regions should provide valuable insights for conservation biologists.     

3,4-Dihydroxyphenylalanine gel diffusion assay for polyphenol oxidase quantification

We have developed a simple, inexpensive plate assay to detect and quantify polyphenol oxidase (PPO) activity from different origins. The logarithm of enzyme activity is linearly correlated with the diameter of the dark, l-3,4-dihydroxyphenylalanine (l-DOPA) oxidized circles produced in the gel, thereby allowing quantification of PPO. Moreover, precision and high reproducibility of the assay were confirmed by statistical analysis.     

Prairie dog towns as islands: applications of island biogeography and landscape ecology for conserving nonvolant terrestrial vertebrates

Aim To evaluate the utility of island biogeography theory as a model for understanding and conserving native communities of nonvolant terrestrial vertebrates at prairie dog towns. Location Oklahoma Panhandle, USA. Methods We surveyed mammal, reptile and amphibian communities on 36 black‐tailed prairie dog (Cynomys ludovicianus) towns during the summers and falls of 1997 to 1999. We used a Geographic Information System (GIS) to characterize the landscape within 10 km of each town. We used Principal Components Analysis (PCA), Correlation Analysis, and Linear Regression to test for patterns in species richness relative to area and isolation of towns, local habitat characteristics, and characteristics of the adjacent landscape. Results Species richness was not significantly correlated with town size, town isolation, or local habitat characteristics. On the other hand, species richness was significantly correlated with characteristics of the landscape within 10 km of the focal town. In addition, species richness of mammals at prairie dog towns during the summer increased in a northerly direction, while richness of mammals at towns during fall increased to the west. Main conclusions These results, albeit contrary to traditional island biogeography theory, are consistent with an emerging view that communities on relatively small islands are strongly influenced by characteristics of the adjacent landscape (or seascape). We recommend that to the extent possible, networks of reserves for prairie dogs and their associated species should include clusters of large towns (i.e. larger than those studied here), as well as large but isolated towns, and that conservation efforts should include management of the intervening matrix of anthropogenic habitats.     

Mammalian island biogeography: effects of area,isolation and vagility

Summary In this paper I reviewed mammalian biogeography for 19 archipelagoes and tested the applicability of the equilibrium theory of island biogeography to mammalian faunas in general. The species-area and species-isolation relationships of terrestrial mammals were consistent with the basic predictions of the equilibrium theory. The z-values for the species-area relationship did not differ significantly from Preston's canonical value of 0.26 (P>0.50), and the modal z-value for non-volant mammals was 0.25. Moreover, z-values increased with isolation and decreased with vagility of the fauna in question. Furthermore, the strength of the species-isolation correlation was negatively correlated with island area and vagility (P<0.001).