Frontiers of Biogeography

Understanding the full range of consequences of species introductions into island and marine habitats requires quantitative studies of systems that are currently under-represented in the scientific literature. We document the introduction, proliferation and establishment of a non-native sea anemone species in an isolated tropical marine lake, a marine ‘island’. From 2003–2012, we gathered samples to identify the introduced species and used transect and photo-quadrat surveys to describe its abundance, distribution, and any associations with native species or habitats. The non-native sea anemone was first found at the tourist entry into the lake in 2003 and identified as Exaiptasia pallida (Agassiz 1864), a species with zooxanthellae endosymbionts. Temporal patterns of tourism, the spatial extent of the sea anemone in 2003, and genetic analyses of the symbiont were consistent with the early stages of introduction. Subsequent expansion of E. pallida throughout the lake occurred within six years. The native species assemblages that were invaded by E. pallida were heterogeneous among surveys and habitats. Overall, there were few correlations that were significant between percent cover of E. pallida and native species; most significant associations were negative; the majority were on mangrove roots. There was one positive association between E. pallida and a native sponge. No significant relationship was found between the abundance of E. pallida and native species diversity. The rapid expansion of E. pallida but dearth of strong ecosystem effects presents a case study of invasive species in a tropical marine habitat where consequences are not directly proportional to invasive abundance. Whether this outcome is stable and representative of other species introduced into marine lakes, or elsewhere in marine systems, remains to be seen.

The southeastern United States is home to the highest amphibian species richness in North America. An often overlooked component of research on factors that drive amphibian geographic distributions is description of species range shape. Broad-scale range disjunction has implications for phylogeography, ecology, and conservation, but descriptions of fragmentation are usually based on subjective visual assessment of range maps. Here, we describe a method for objectively quantifying range fragmentation and use this method to describe the patterns of amphibian species range shapes in the Southeast. Species ranges varied widely in degree of fragmentation, from completely contiguous to highly fragmented, and degree of isolation of range fragments. Incorporating ecological niche models added information about fine-scale habitat fragmentation. Although 12 species had ranges with either high or moderate fragmentation, the only area where range fragments of more than two of the 12 species overlapped was the Appalachian Highlands, suggesting that range fragmentation in the Southeast is driven by multiple biogeographic processes.

Recent climatic models suggest the late Pleistocene was colder and had different precipitation regimes from the present. If this climatic shift occurred more rapidly than species could adapt, species likely shifted their ranges as populations moved in concert with suitable environmental conditions. We examined changes in altitudinal and horizontal distribution in response to past climate change of amphibian species from different elevational zones and habitat requirements in the Eastern Cordillera of the Colombian Andes. We used environmental information and species occurrence data to model the distribution of 14 amphibian species (seven highland and seven lowland) which we projected to the Last Glacial Maximum (LGM) using two past climatic reconstructions. For these 14 species, we studied the predicted elevational and areal shifts. In agreement with palynological-derived models for Andean flora, we predicted that the elevation of montane amphibians shifted downwards increasing their total altitudinal range. We did not detect any evidence of compression related to drier lowlands. In some cases, the wider distribution areas of high-elevation amphibians during the LGM overlapped with contemporary distributions implying that these areas are present-day refugia for some species. Lowland species showed little or no elevational changes across time, but their areal distributions changed depending on habitat requirements. Four lowland frog species occurring in present-day xeric environments showed substantial range expansion across the lowlands during the LGM, while two species occurring in humid habitats likely expanded their ranges since the LGM. Since the LGM ended, ranges of mid‐ to high-elevation species shrank and shifted to higher elevation. Lowland species in xeric or open habitats have also experienced shrinking ranges, with some evidence that they have been moving upwards. Thus, low- and high-elevation species may be at risk under predicted anthropogenic climate change. Our results generate spatial hypotheses about amphibian responses to climate change that can be tested with phylogeographic data.

Dense vegetation, especially thickets of trees or shrubs, has been associated with actual and perceived crime risk in several parts of the world. In some contexts, invasive alien trees and shrubs can create a habitat structure that is very different from that provided by native vegetation. The role of alien and native plant species at different stages of invasion/densification in mediating criminal activity within a managed landscape remains poorly documented and elaborated. Using the South African city of Cape Town (a rapidly growing metropolitan centre within a global biodiversity hotspot) as a case study, we discuss the role of alien, invasive and native vegetation in mediating criminal activity in urban areas, particularly in a developing-country context. We argue that the incidence of crime may not always be determined by the biogeographical status of dominant plants (i.e., whether vegetation is dominated by native, alien, or invasive alien species), but rather on the structure/habitat they provide. A stronger link between crime and vegetation is likely in areas where tree invasions have drastically altered vegetation structure. This paper draws attention to a novel interaction between humans and vegetation and highlights the need for context-specific approaches when managing plant invasions, particularly in urban areas.

The relationship between an island’s size and the number of species on that island—the island species–area relationship (ISAR)—is one of the most well-known patterns in biogeography and forms the basis for understanding biodiversity loss in response to habitat loss and fragmentation. Nevertheless, there is contention about exactly how to estimate the ISAR and the influence of the three primary ecological mechanisms that drive it — random sampling, disproportionate effects, and heterogeneity. Key to this contention is that estimates of the ISAR are often confounded by sampling and estimates of measures (i.e., island-level species richness) that are not diagnostic of potential mechanisms. Here, we advocate a sampling-explicit approach for disentangling the possible ecological mechanisms underlying the ISAR using parameters derived from individual-based rarefaction curves estimated across spatial scales. If the parameters derived from rarefaction curves at each spatial scale show no relationship with island area, we cannot reject the hypothesis that ISARs result only from random sampling. However, if the derived metrics change with island area, we can reject random sampling as the only operating mechanism and infer that effects beyond sampling (i.e., disproportionate effects and/or heterogeneity) are also operating. Finally, if parameters indicative of within-island spatial variation in species composition (i.e., β-diversity) increase with island area, we can conclude that intra-island compositional heterogeneity plays a role in driving the ISAR. We illustrate this approach using representative case studies, including oceanic islands, natural island-like patches, and habitat fragments from formerly continuous habitat, illustrating several combinations of underlying mechanisms. This approach will offer insight into the role of sampling and other processes that underpin the ISAR, providing a more complete understanding of how, and some indication of why, patterns of biodiversity respond to gradients in island area.

High-latitude marine ecosystems have experienced fewer species invasions than temperate marine ecosystems, a discrepancy that may be attributed to barriers such as low propagule pressure, extreme and seasonal abiotic conditions, and biotic resistance of relatively intact communities. Each of these barriers is being affected by climate change and increasing human activity in high-latitude (>55º N) areas. We reviewed the evidence for each of these barriers limiting species invasion in high-latitude areas in the northern hemisphere. Based on records from government documents of high‐latitude countries, non-native species appear to be increasing in number (in Denmark and the United States) although there remains a paucity of data on invasive species establishment for high-latitude regions. Future study is needed to identify the drivers and impacts of invasions at high latitudes so that managers looking to prevent invasions can focus their efforts on bolstering barriers to invasion in these unique ecosystems.

A view from the shoreline of Sitka, Alaska, with Mount Edgecumbe rising in the distance. Please read the article by Mahanes & Sorte in this issue of Frontiers of Biogeography (11.1, e40527) to learn about how climate change is likely to affect marine species invasions in high latitude areas of the northern hemi-sphere. Photo by Keira Monuki