Frontiers of Biogeography

Topography influences evolutionary and ecological processes by isolating populations and enhancing habitat diversity. While the effects of large-scale topography on patterns of species richness and endemism are increasingly well documented, the direct effect of local topography on endemism is less understood. This study compares different aspects of topographic isolation, namely the isolating effect of deep barrancos (ravines) and the effect of increasing isolation with elevation in influencing patterns of plant endemism within a topographically diverse oceanic island (La Palma, Canary Islands, Spain). We collected plant presence–absence data from 75 plots in 8 barrancos on the northern coast of La Palma, spanning an elevation gradient from 95 to 674m a.s.l. Using mixed-effects models, we assessed the effect of barranco depth and elevation on the percentage of single-island endemics, multi-island endemics, and archipelago endemics. We found that percent endemism was not significantly correlated with barranco depth and correlated negatively with elevation within barrancos (rather than the expected positive relationship). The topographic barriers associated with the deep island barrancos thus appear insufficient to drive speciation through isolation in oceanic island plants. The decrease in endemism with elevation contradicts findings by previous broader-scale studies and it may reflect local influences, such as high habitat heterogeneity at low elevations.

Arctic and alpine species are expected to be particularly vulnerable to climate change as they inhabit areas of extreme climates. To understand how such species may respond, we compared two groups of bumblebees that specialise in arctic (Alpinobombus) and alpine (Mendacibombus) biomes. These bumblebee species are all extreme cold specialists with similar ecological niches, making them good candidate species for comparison of how groups inhabiting different biomes may respond to climate change. Using an ensemble of species distribution models for eighteen bumblebee species (ten Mendacibombus; eight Alpinobombus), we estimated their current distributions using selected climate variables. The models were used to predict future distributions based on two future climate change scenarios for 2040-2060 and three dispersal scenarios. We found significant differences between the predicted relative area changes of the two groups under all combinations of climate change and dispersal scenarios. Alpinobombus species were consistently projected to have larger distribution declines, while the responses of Mendacibombus species were much more varied, with some Mendacibombus species projected to have distribution expansions provided that they are able to disperse to occupy new territory. From these results, we show that arctic species would be much more likely than alpine species to experience distribution declines under climate change.

As climate changes, species’ ranges may shift poleward. However, habitat loss in intervening areas has been hypothesized potentially to impede the movements of these species. Populations near range margins offer opportunities to study how marginal species have reacted to habitat loss. We examined the presence/absence of bird species in landscapes that were historically mainly forested (natural land covers) in Southern Ontario, Canada. We used logistic regression to determine each bird species’ probability of occupancy (pocc) as a function of natural cover in 991 landscapes, each 100-km2. We distinguished three groups of species: i) southerly species whose northern range limits fall in the study area (n=37), ii) northerly species whose southern range limits fall in the study area (n=35), and iii) mid-range species (n=106). We compared pocc for these three groups of species in six different habitat guilds. We found that species near their southern range edges are less likely to occur in landscapes where forest amount is reduced, while species near their northern range edge are more likely to occur in landscapes with reduced forest. This result is independent of habitat guild. Our results are inconsistent with the hypothesis in the climate change literature proposing that loss of natural land cover near poleward range margins would inhibit range expansion in response to climatic warming. Rather, we hypothesize that, at southern range edges, the dual stresses of climatic warming and forest conversion both reduce species’ ability to occupy a landscape. However, near northern (potentially expanding) range edges, partially disturbed landscapes are more readily invaded than undisturbed landscapes.

Although the distribution patterns of major amphibian lineages are mainly explained by a Pangean origin with subsequent vicariant diversification, dispersal events have exerted a strong influence on present-day distributions. Long-distance dispersal (LDD) involves movements outside the standard geographic limits and outside the genetic neighbourhood area of individuals. Although considered ‘rare’, LDD is disproportionately important to amphibian populations, species and communities. To understand the role of LDD in shaping current biogeographic patterns in these tetrapods, we reviewed the cases reported in the literature. A systematic bibliographic search was performed to obtain information on how many studies have dealt with LDD in amphibians, which methods they used, which taxa and distances were involved, and when/where events had apparently occurred. In 41 studies, we recovered at least 90 LDD events (3 active, 87 passive) involving at least 56 extant species and 38 genera. Most events (73) involved the colonization of islands, with rafting being suggested as the most conceivable means of overwater passive dispersal for these vertebrates. In this review, we show that LDD events have played an important role in shaping current amphibian biogeographic patterns, especially the occurrence of disjunct distributions and the colonization of islands.

Being snapshots in time, species ranges may fall short of representing all of the geographic or environmental space that they are able to occupy. This has important implications for niche studies yet most comparative studies overlook the transient nature of species distributions and assume that they are at equilibrium. We review the methods most widely used for niche comparisons today and suggest a modified framework to describe and compare niches based on snapshot species range data. First, we introduce a new environmental space-based Niche Equivalence Statistic to test niche similarity between two species, which explicitly incorporates the spatial distribution of environments and their availability into statistical tests. We also introduce a new Background Statistic to measure the ability of this Niche Equivalence Statistic to detect differences based on the available environmental-space. These metrics enable fair comparisons between different geographies when the ranges of species are out of equilibrium. Based on distinct parameterizations of the new Equivalence and Background statistics, we then propose a Niche Divergence Test and a Niche Overlap Test, which allow assessment of whether differences between species emerge from true niche divergences. These methods are implemented in a new R package, ‘humboldt’ and applied to simulated species with pre-defined niches. The new methods improve accuracy of niche similarity and associated tests – consistently outperforming other tests. We show that the quantification of niche similarity should be performed only in environmental space, which is less sensitive than geographic space to the spatial abundance of key environmental variables. Further, our methods characterize the relationships between non-analogous and analogous climates in the species’ distributions, something not available previously. These improvements allow assessment of whether the different environmental spaces occupied by two taxa emerge from true niche evolution, as opposed to differences in life history and biological interactors, or differences in the variety and configuration of environments accessible to them.