Frontiers of Biogeography (FoB) is the scientific journal of the International Biogeography Society (IBS, www.biogeography.org), a not-for-profit organization dedicated to promotion of and public understanding of the biogeographical sciences. IBS launched FoB to provide an independent forum for biogeographical science, with the academic standards expected of a journal operated by and for an academic society.
Volume 15, Issue 3, 2023
Opinions, Perspectives & Reviews
The two major rainforests of the neotropics, the Amazon and Atlantic forests, show maximum expansion during the warm and wet conditions of interglacial periods, including the current Holocene. They are connected by a network of gallery forests through the Cerrado biome. However, the extent of their expansion during glacial periods, when they were more disjunct, is unknown. During glacial periods, a pollen assemblage comprising Podocarpus–Ilex–Hedyosmum–Myrsine displays higher frequencies in marine, continental and coastal Brazilian pollen records. This assemblage is observed today in the high-elevation grasslands of the Cerrado and Atlantic Forest biomes and in the coastal vegetation, the restinga, of southern Brazil. We therefore reviewed the possible migration routes for these species by tracking glacial period Podocarpus–Ilex–Hedyosmum–Myrsine assemblages in published pollen records. The marine pollen records provide evidence of a glacial expansion of restinga, its floristic composition being continuous with the dominant regional vegetation, a cold type of shrubby grassland. There appear to be two migration routes, one involving the expansion of high-elevation grassland taxa within the lowlands, and the other low-sea level stands of coastal restinga. We conclude that the Cerrado was a node of migration between the Amazon and Atlantic rainforests, linking the Andes to the central and coastal mountains of Brazil. The Brazilian mountain ranges represent refugia of ancient taxa that colonized the continent up to the Andes and modified the floristic composition of the two rainforests during the Pleistocene glacial periods.
Towards a holistic perspective on the development of island syndrome by examining its occurrence patterns in insular plants
The acquisition of evidence pertaining to island syndrome often relies on opportunistic observations, yet prior researchers have gradually compiled a body of examples that collectively shed light on its occurrence patterns and dynamics. Our comprehensive literature review revealed that island syndrome dominantly occurs in angiosperms on oceanic islands, with a notable abundance of taxa exhibiting high endemism and possessing functional traits associated with facultative and generalized biotic interactions. While acknowledging the influence of unequal research interest and sampling efforts on the observed patterns, deviations from prevailing sampling biases evident in global plant databases and herbarium collections lend credence to genuine differences in the occurrence of island syndrome. The disproportionate incidence of island syndrome, delineated by taxonomic groups, traits, and specific islands, can be ascribed to the distinct biogeography of oceanic islands and the presence of idiosyncratic ecological and evolutionary processes that contribute to its development. Within an evolutionary framework, our overarching hypothesis posits island syndrome as a transformative trajectory away from the diverse strategies adopted by mainland plants to the alternative strategies exhibited on islands due to their isolation and ecological simplicity. This perspective fosters a more holistic perspective, encompassing the myriad and graded responses of plants to evolutionary pressures encountered on islands. Rather than dismissing the biased occurrence patterns in the examples of island syndrome, we contend that their underlying insights hold substantial value in formulating a general, mechanistic model that enhances our understanding of the development of island syndrome and its evolutionary implications.
- 1 supplemental ZIP
Oceanic islands are global hotspots of biodiversity – many of them harboring marvels of evolution in isolation. Unfortunately, insular biotas are also highly susceptible to extinction, especially following colonization by humans. Here, we assess the influence of humanity on the diversity and biological distinctiveness of mammals inhabiting 37 oceanic islands. We compiled lists of mammals inhabitingthese islands prior to and then following colonization by hominids (including Homo erectus and H. sapiens). We then quantified the dynamics in diversity (as measured by species richness) and distinctiveness (measured as beta-diversity) among islands. We compared mammalian assemblages on islands prior to humanity, following colonization by early hominids and then following colonization by H. sapiens (in the latter case, separating assemblage dynamics resulting from extinctions of native species from the effects of species introductions). As expected, early hominids hardly influenced mammalian diversity or distinctiveness. In contrast, colonization by H. sapiens was initially followed by numerous extinctions and substantial declines in species richness, which then however rebounded to exceed pre-humanity levels. These post-humanity increases in species richness were paradoxically accompanied by substantial declines in distinctiveness among islands. This paradox of anthropogenic enrichment is readily resolved by observing that species introductions to the islands (the sources of the post-humanity surges in species richness) were comprised of a highly redundant set of small species (primarily rats and house mice), resulting in the homogenization and downsizing of island life.
- 4 supplemental PDFs
The dynamics of species range borders may be difficult to study due to the local rarity of populations and individuals. This hindrance applies less when range borders are parapatric contact zones. Analysis of species in parapatry has the further advantages of no (inferred) absence data and that shortcomings in data gathering such as uneven sampling apply to the counterpart species about equally. The large-bodied newts Triturus cristatus and T. marmaratus are spatially segregated within a wide area of range overlap in the west of France. They locally show abutting or slightly overlapping distributions with many isolated occurrences (here called ‘patches’) of either species within the continuous range section of the other. Historical and genetic data suggest that T. cristatus has been superseding T. marmoratus. Species replacement should be also discernible from local species distributions, with more, larger and more distantly positioned patches in the receding than the advancing species. Atlas data for France and from the French region Pays de la Loire largely confirm these predictions. The data also indicate that T. marmoratus patches may be void of T. cristatus, suggesting that they are persisting strongholds, whereas the more admixed T. cristatus patches are in flux. The species’ differential ecological signature is that of forested, hilly terrain for T. marmoratus and open, flat terrain for T. cristatus. Accordingly, the main dispersal route for T. cristatus towards the Atlantic coast has been through the valley of the Loire River, with a secondary, intraspecific contact zone at the Normandy coast. A literature survey revealed several other species pairs of European herpetofauna that may be amenable to patch analysis, as will be species pairs in other groups of organisms characterized by limited, habitat dependent dispersal. In such efforts the availability of species occurrence data from atlases and digital databases is an indispensable asset.
- 1 supplemental PDF
Ecological niche comparison among closely related tree species of Lauraceae using climatic and edaphic data
Edaphic specialization is considered to promote ecological differentiation among closely related species of Damburneya (Lauraceae) occurring in sympatry. However, little is known about the effects of soil and other key environmental factors like climate on the ecological niche and distributionof these tree species. Here, we assessed the role of climate and soil on niche divergence and potential distribution of four Damburneya species whose distributions span Central America and Mexico. We performed ecological niche modeling with MaxEnt using three sets of environmental data: climatic-only, edaphic-only, and a combination of both, to characterize species niches and suitable distribution areas. Niche overlap was quantified, and niche similarity was tested to assess niche differentiation among species. Climate and soil determined species’ potential distribution. While climatic niches were mostly similar, edaphic niches tended to differ. Warm and moist tropical forests with no water deficit and low seasonality in precipitation are the most suitable environments for the four species. This study supports previous reports of wide ecological plasticity of Damburneya salicifolia based on its distribution and leaf trait variation, as it occurred in drier environments with wider temperature and soil pH ranges than the other species. The observed patterns of niche similarity did not reflect the phylogenetic relationships between species, suggesting that the modeled niches do not necessarily reflect past evolutionary processes but rather the current environmental variation across the geographical ranges of the species. The results suggest that the studied species are similarly constrained by climate and toleratewide edaphic variation, supporting a potential role for soils on ecological divergence within the genus. On the other hand, performance and predictions varied between models built with different datasets. This research supports the utility of including climate and soil data in ecological niche models to comprehensively analyze the niche and distribution of plant species.
- 3 supplemental PDFs
- 1 supplemental ZIP
Climatic niche differentiation and paleodistribution of the longtail alpine garter snake complex Thamnophis scalaris (Squamata: Colubridae): Holocene refugia in the Mexican highlands
Mexico’s topographic and environmental heterogeneity, in combination with environmental fluctuations of the Neogene-Quaternary, has uniquely influenced the evolutionary history and distribution patterns of the region’s flora and fauna, sometimes causing closely related species to exhibit distinct climatic niches. Our study aimed to characterize the climatic niches of Thamnophis scalaris and Thamnophis scaliger, as well as evaluate the impact of the Pleistocene-Holocene transition on their paleodistributions. We generated 357 models per species, each with three sets of distinct combinations of climatic variables, based on 108 occurrence records for T. scalaris and 62 for T. scaliger. We evaluated the niche overlap, equivalency, and similarity between both species and transferred the present-day models to eight distinct historical periods, with the goal of encompassing the distinctive climatic variation of the Pleistocene-Holocene (P-H) transition. Both species showed significant differences in their respective climatic regimes and did not display climatic niche conservatism (the tendency of species to retain ancestral ecological characteristics), despite their previously reported ecological, morphological, and biogeographic similarities. Likewise, they seem to have responded similarly to the environmental changes in the P-H, with both paleodistributions experiencing expansion phases during glacial periods and contraction phases during interglacial periods. Possible areas of refugia that remained climatically stable and viable for both species throughout this period were identified. These refugia could potentially harbor a greater genetic diversity with respect to regions that recently acquired suitable conditions for the establishment of these populations. As such, this work offers a methodological procedure that may be used as an early inference for identifying specific regions of interest in phylogeographic studies and conservation planning.
- 1 supplemental PDF
The Maple-leaf oak, Quercus acerifolia (E.J.Palmer) Stoynoff & Hess, is listed as Critically Imperiled by the State of Arkansas and considered endangered in the IUCN Red List of Threatened Oak Species. It is endemic to the interior highlands of the Ouachita Mountains in west-central Arkansas, where it is reported to occur in only four isolated locations. No specific research exists regarding predicted climate change impacts on the Q. acerifolia, but given its small range and habitat specificity, such climate change-driven impacts will likely pose significant risks to remaining populations. We used an ensemble species distribution modeling (SDM) approach to predict climatically suitable habitat for Q. acerifolia within its native range. We investigate how future changes in climate may impact habitat suitability. Currently, the estimated area of climatically suitable habitat area for Q. acerifolia is 2,523 km2. By 2050, the predicted climatically suitable habitat area is 749 km2, a 70% reduction in habitat extent. By 2100, the model ensemble predicts a suitable habitat of only 285 km2 or an 89% loss of present suitable habitat. The model ensemble also predicted climatically suitable habitat area in 20 counties (14 in Arkansas and six in Oklahoma), including the currently known four locations in Arkansas. Although Q. acerifolia is rare and is at risk of extinction due to potential climate-change driven habitat reduction, the SDM ensemble identified several new habitat areas for the species. New habitat information can be used to search for existing Q. acerifolia populations or guide reintroduction efforts, leading to enhanced focus on long-term management, conservation, and restoration of this critically-imperiled species.
- 1 supplemental PDF