Frontiers of Biogeography

Inclusion of dispersal data in models of species’ distributions in response to environmental change has been advocated for more than 15 years. We investigated whether there has been a shift in recent publications to include dispersal processes and how dispersal estimates explicitly change the conclusions of analyses. To address this question, we conducted a systemic review of the literature to assess what kinds of dispersal data and methods are being included in species distribution models across taxa. We collected metadata on 6,406 publications, 907 of which included dispersal data. The proportion of papers that included dispersal data in estimates of the species’ range increased from 8% to 20% from 1991 to 2017. Evaluation of a subsample of 200 papers showed no evidence for differences in taxa studied between dispersal and non-dispersal publications, with most studies focused on North America or Europe. Dispersal was incorporated at a higher frequency in studies from South America, Africa, and island systems. We found that forecasting models predicting range shifts with climate change rarely used dispersal data, but when they did, range shift projections were greatly affected. Our simulation models, in which a range of dispersal estimates were included, showed that projections were greatly influenced by dispersal distance assumptions. We summarize best practices for future research on distributions, including potential methodologies for dispersal integration and highlight the problems if dispersal is ignored.

The emphasis on vicariance or dispersal has led to alternative and competing approaches to analyze biotic assembly, but both processes should be considered in an evolutionary integrative analysis. I define some relevant concepts (biotas, horobiotas, cenocrons, dispersal, vicariance and extinction) and discuss the differences between the dispersal-vicariance model and the center of origin-dispersal-vicariance (CODA) and vicariance models. I use the philosophical framework of integrative pluralism to justify an integrative evolutionary biogeographic approach, not implying an eclectic or “anything goes” perspective, but that different methods are compatible because they give partial solutions, when answering particular questions. This approach allows for the integration of the results of different analyses to explain biotic assembly.

Islands are ideal research models to study ecological processes, as they vary in size, ecological conditions, and have clearly defined boundaries. Despite great advances in island research, comprehensive understanding of numerous aspects in island ecology is still lacking. Open questions include the effects of spatial scale on island biodiversity, community assembly processes, and the diversity of species forms and functions on islands. Here, I review recent studies investigating species assembly processes and resulting diversity patterns on small islands at local and global scales. I discuss how small-island communities are shaped by environmental, population-level, and species-level processes that differ in strength with island area. Functional trait-based approaches better explained these patterns than measures of species richness on small islands. Detailed ecological understanding of community assembly processes on islands is of paramount importance to conserve biodiversity in an increasingly fragmented natural world.

Marine lakes are emerging ecological and evolutionary natural experimental systems, with genetically isolated resident populations that exhibit extreme population dynamics and rapid phenotypic change. Marine lakes are posited to be marine islands, however, unlike terrestrial islands for which rich models have been developed over the past half-century, we know little of the mechanisms driving changes in marine lakes. This is a critical knowledge gap in efforts to reconcile theory on, or distinguish differences among, island and island-like systems. To reduce this critical knowledge gap, we present a mathematical model describing marine lakes based on a case study of Jellyfish Lake (Ongeim’l Tketau, Mecherchar: OTM), Palau. Empirical data show that marine lakes exhibit delayed and reduced tidal motions, suggesting exchange of a limited amount of water with the neighboring (‘mainland’) ocean. Our model tracks changes in lake level, allowing determination of an exchange rate that is a physical null model for biological colonization and a proxy for colonization distance in island biogeography theory. In addition, we track horizontally averaged in-lake quantities such as salinity and temperature (i.e., marine weather, climate) and stratification (i.e., habitat) — that are known to influence resident species’ distributions and population dynamics — by solving an advection-diffusion equation. We find that weather, ocean conditions, groundwater, and exchanges through tunnels determine the abiotic environment in OTM. By comparing simulations and data, we estimate the difficult-to-measure properties of the surrounding groundwater — the ‘matrix’ in the vernacular of habitat islands — and give a range of realistic values for the effective diffusion coefficient. This coefficient is found to increase in a tropical storm, suggesting that other drivers can be important during perturbations.

Macroecological data are usually structured in space, so taking into account spatial autocorrelation in regression and correlation analyses is essential for a better understanding of patterns and processes. Many methods are available to deal with spatial autocorrelation, but there are some difficulties when one is dealing with huge geographical extents and fine-scale data. So, we propose a relatively simple and fast computer-intensive approach to deal with Principal Coordinate of Neighbor Matrices (PNCM)/Moran’s Eigenvector Mapping (MEM) analyses for large datasets, using global richness pattern of sharks as a model. We performed a variance partitioning approach by regressing species richness against environmental variables and spatial eigenvectors derived from PCNM. Due to the large number of ocean grid cells (> 9000), we ran the analyses 1000 timesby randomly subsampling each time 50 to 4500 cells and compared the distribution of the variance partitioning components, as well as the slopes of the environmental variables. We also estimated Moran’s I coefficients for regression residuals to check if spatial eigenvectors took into account spatial autocorrelation. Comparing statistics of analyses with different sample sizes, we note that although the environmental component increases linearly, other components (unique space and shared) of the most important variables stabilize with about 1000 cells, whereas all other smaller effects tend to stabilize between 2500 and 3000 cells. Besides that, PCNM eigenvectors were able to control spatial autocorrelation very well. We showed that shark richness patterns are strongly and positively correlated with temperature range, according to the well-known pattern of distribution for the taxon, and strong negatively correlated with oxygen supplies, which are higher in polar zones where  ice acts as a barrier to sharks. Our approach clearly shows that it is possible to perform a robust evaluation of global patterns of diversity using eigenvector approaches based on a resampling strategy and allows effective computation of the variance partitioning even when dealing with large datasets.

At the small scales of world, continent and region, centres of local endemism have been hypothesised to be related to refugia and/or distinctive environmental conditions. We consider patterning of local endemics at a large scale to help test the validity of these two hypotheses for centres of local endemism recognised at smaller scales. Our study area was a centre of local endemism on the Tasman and Forestier peninsulas, Tasmania, Australia. We tested the distinctiveness of both the current climatic and edaphic conditions and the potential for refugia during Quaternary climatic fluctuations, using databases, published maps, and direct observation of climate. Inverse rarity analyses at 1 km² showed a pattern of concentration of local endemics near the east and southeast coasts of the peninsulas. However, the ranges of species at a larger scale were largely non-overlapping. Climate did not differ from other coastal areas in southeastern Tasmania that lacked local endemics. Climatically similar areas to the centre of local endemism on and outside the peninsulas also had treeless vegetation on skeletal soils on dolerite and mudstone that were habitat for many of the species in the peninsulas centre of local endemism. The areas with high concentrations of local endemics on the peninsulas were located close to the coastline of the Last Glacial Maximum, unlike other areas with the same climatic and edaphic environments inside and outside the peninsulas. The conclusion that the centre of local endemism relates to a closely adjacent glacial refugium, rather than being a response to a distinctive environment, was reinforced by the non-overlapping distributions of the most locally endemic species at a large scale, and the variability in the habitat of the most locally endemic species. This study illustrates the value of investigating the causes of centres of local endemism at a range of scales and confirms the tight link between centres of endemism and refugia.

Sweet potato dispersal from Americas to French Polynesia predates known human colonization periods, therefore being a long-standing dilemma. According to recent phylogenetic studies, the most likely hypothesis to explain this migration is the sea-drift long-distance dispersal, but no research indicating the response of I. batatas seeds to seawater conditions have been performed so far. The aim of this study was to understand seawater resistance in I. batatas, an essential feature for the sea-drift natural dispersal hypothesis, thus shedding light on the historical biogeography of this species, which also has implications on human civilization history, as the archaeological presence of sweet potato in both continents has been used as an evidence of pre-Columbian contacts between ancient civilizations. The experiment consisted of submitting sweet potato seeds to seawater treatments and observing the respective germination rates after different periods of immersion. Subsequently, one-way ANOVAs were conducted to test for significant differences between groups. All seeds from the seawater immersion treatments germinated, which confirms that I. batatas seeds are resistant to seawater salinity for a period of 120 days. Our results support the sea-drift natural dispersal hypothesis, thus shedding light on part of the logical conditions for one of the major hypotheses on the historical biogeography of this species, which also plays an important role in the discussions related to prehistorical human mobility in Polynesian islands.

The question of which factors determine the geographical change in species composition and abundance (turnover and β diversity) has typically been studied at a single scale, so that, e.g., regional-scale factors are used to explain regional turnover. But cross-scale effects are also important to understand species turnover and the spatial distribution of biodiversity in general. Here, we explored how regional richness, local forest-structure, and regional climatic factors interact to influence local species turnover. We analysed a dataset that includes the distribution of 916 fern species recorded in 1227 plots in the Bolivian Andes, forest-structure variables collected in the field, and climatic variables extracted from global databases. We used path analyses to: (i) select the best models explaining the variation in local species turnover and (ii) identify the factors that have a direct effect on species turnover and those with only indirect effects. We contrasted our results against those obtained from a null model analysis. The most important variable explaining variation in species turnover was regional species richness. We consider that this is the result of interspecific competition resulting in narrower realized ecological niches of species, although further studies are needed to confirm the mechanism. We also found that the relationship between climatic variables and local species turnover is best described by the indirect link between climatic factors and regional species richness. Our results might appear to be in conflict with previous studies finding that climatic and edaphic factors are direct predictors of local and regional variation in fern turnover. However, this is due to the different scales at which turnover was analysed. In contrast to previous studies, ours reflects the cross-scale effect of the variation in regional factors on local species turnover. Our study supports the idea that in regions with high species richness, biotic interactions strongly determine local community composition.

Biological records provide biodiversity information over large spatial and temporal scales.  Our systematic scoping review of biological records from the well-recorded region of the United Kingdom (UK) and Ireland revealed that over half of all studies using biological records were studying species distributions (134 of 253 studies) and/or temporal trends (139 of 253 studies).  A minority of studies (61 of 253) focused on methodological questions, while most studies used biological records with existing methods as tools for answering biological and ecological questions.  However, only 31 of 253 studies tested models using independent data.  Most studies (154 of 253) integrated multiple biological records datasets, showing that biological records hold a largely untapped potential for independently testing conclusions by withholding some of those datasets for use as independent test data.  Our results provide guidance for data providers and researchers interested in more effectively collecting and using biological records.

Quaternary geographic range dynamics of savannah tree species are still not fully understood. The forest refuge hypothesis postulates that climatic and vegetational upheavals during the Pleistocene fragmented the previously continuous ranges of many species into isolated refuges that would have acted as shelters for rainforest taxa and allowed their survival through the Pleistocene cold stages. This hypothesis has recently been applied to studies of taxa in the African savannahs. We here test this hypothesis using the savannah tree species Senegalia senegal (L.) Britton., which is widely distributed across sub-Saharan Africa. Specifically, we ask the following questions: (i) Do we find evidence for savannah refugia during the last 130,000 before-present in sub-Saharan Africa? (ii) Would the climate in West Africa already have been suitable for S. senegal prior to the Last Glacial Maximum (LGM)? Using 1,132 occurrence records,we modeled the geographic distribution of S. senegal and projected the model into the past using climatic conditions from four time slices spanning the last 130,000 years bp. Our analyses show that the projected geographic extent of S. senegal was broader during the Last Interglacial, with a dramatic decline during the LGM and the subsequent recovery through the mid-Holocene to the present day. Our results indicate a range expansion at least from the mid-Holocene to the present and further show that S. senegal had similarly continuous distribution during the LGM as found today in sub-Saharan Africa. We also assessed the regional variation of environmental niche occupancy using a principal components analysis (PCA). The PCA reveals variation in the occupancy of environmental space across sub-Saharan Africa, a key indication of a wide ecological amplitude exhibited by the species. This study provides insights into the ancestral distribution and the temporal dynamics of a key savannah species that have shaped its current areas of occupancy.

Looking south across the island of Mecherchar, Palau.  Four of the archipelago’s >50 marine lakes are visible:  Ongeim’l Tketau (Jellyfish Lake) is in the foreground, with the crocodile and L-shaped lakes complex centre-right.  In this issue, Blanchette & colleagues develop a preliminary physical model to help understand the drivers of community assembly and population dynamics in these insular systems.  This photo used with permission, Copyright 2008 Christoph Gerigk, all rights reserved.