Frontiers of Biogeography

Detailed information on species distributions is crucial for answering central questions in biogeography, ecology, evolutionary biology and conservation. Millions of species occurrence records have been mobilized via international data-sharing networks, but inherent biases, gaps and uncertainties hamper broader application. In my PhD thesis, I presented the first comprehensive analyses of global patterns and drivers of these limitations across different taxonomic groups and spatial scales. Integrating 300 million occurrence records for terrestrial vertebrates and plants with comprehensive taxonomic databases, expert range maps and regional checklists, I demonstrated extensive taxonomic, geographical and temporal biases, gaps and uncertainties. I identified key socio-economic drivers of data bias across different taxonomic groups and spatial scales. The results of my dissertation provide an empirical baseline for effectively accounting for data limitations in distribution models, as well as for prioritizing and monitoring efforts to collate additional occurrence information.

A species pool is the set of species that could potentially colonize and establish within a community. It has been a commonly used concept in biogeography since the early days of MacArthur and Wilson’s work on Island Biogeography. Despite their simple and appealing definition, an operational application of species pools is bundled with a multitude of problems, which have often resulted in arbitrary decisions and workarounds when defining species pools. Two recently published papers address the operational problems of species pool delineations, and show ways of delineating them in a probabilistic fashion. In both papers, species pools were delineated using a process-based, mechanistical approach, which opens the door for a multitude of new applications in biogeography. Such applications include detecting the hidden signature of biotic interactions, disentangling the geographical structure of community assembly processes, and incorporating a temporal extent into species pools. Although similar in their conclusions, both ‘probabilistic approaches’ differ in their implementation and definitions. Here I give a brief overview of the differences and similarities of both approaches, and identify the challenges and advantages in their application.

Island biogeographical research is becoming more and more fashionable, with the continuous identification of new challenges that are critical for the advancement of science. In this contribution we identify biases and limitations associated with island biogeographical studies, and also describe recent advances and propose new perspectives. The main proposals include: 1) downscaling island biogeographical studies to local/plot scale; 2) investigating geographical patterns of intra-specific genetic variation to infer dispersal processes among and within islands; 3) using applied biogeographical research to respond to the current island biodiversity crisis; and 4) applying new computer-intensive methods such as artificial intelligence (AI) approaches.

Since the contributions of Charles Darwin and Alfred Russel Wallace, oceanic archipelagos have played a central role in the development of biogeography. However, despite the critical influence of oceanic islands on ecological and evolutionary theory, our focus has remained limited to either the island-level of specific archipelagos or single archipelagos. Recently, it was proposed that oceanic archipelagos qualify as biotic provinces, with diversity primarily reflecting a balance between speciation and extinction, with colonization having a minor role. Here we focus on major attributes of the archipelagic geological dynamics that can affect diversity at both the island and the archipelagic level. We also re-affirm that oceanic archipelagos are appropriate spatiotemporal units to frame analyses in order to understand large scale patterns of biodiversity.

The development of meaningful models of species richness dynamics in island ecosystems requires accurate measurement of existing biodiversity. To test the assumption that mammalian diversity on tropical oceanic islands is well documented, we conducted a 12-year intensive survey of the native mammal fauna on Luzon Island, a large (ca. 103,000 km²), mostly volcanic island in the Philippines, which was thought to be well known. Prior to the start of our study in 2000, 28 native, non-flying mammals had been documented, and extrapolation from prior discoveries indicated that the rate of discovery of new species was steady but low. From 2000 to 2012, we surveyed non-flying mammals at 17 locations and discovered at least 28 additional species, doubling the number known. Nearly all of the new species are restricted to a single mountain or mountain range, most of which had not been sampled previously, thus also doubling the number of local centers of endemism within Luzon from four to eight. The number of species on a mountain is strongly correlated with the elevation of the peak, and the number of endemic species on a mountain range is strongly correlated with the maximum elevation of the range. All 28 of the new species, and 20 of the species discovered prior to 2000, are members of two morphologically and ecologically diverse endemic clades (“cloud rats” and “earthworm mice”), which strongly implies that species richness has primarily been the product of speciation within the island. We reject the general assumption that mammals on tropical oceanic islands are sufficiently well known that analysis and modeling of the dynamics of species richness may be conducted with precision. In the development of conceptual biogeographic models and implementation of effective conservation strategies, existing estimates of species richness, levels of endemism, and the number of subcenters of endemism should be actively reassessed and verified through robust field, museum, and laboratory studies.

I describe the set of fundamental principles of biogeography that can serve as an integrative, conceptual framework for unifying and advancing our abilities to explain the geography of life – generally.  I assert that patterns of variation of biotas among regions and across geographic gradients result from the very regular patterns of variation in environmental conditions across the geographic template. This happens through the influence of that variation on the fundamental biogeographic processes (immigration, extinction and evolution), the influence of those fundamental processes on each other, and ecological feedback in the form of interspecific interactions, which influence the fundamental capacities of other species to immigrate, survive and evolve.  I then summarize principal patterns and current theory in island biogeography within the context of the fundamental, unifying principles and show how they can inform a more integrative, conceptual framework for explaining a genuinely comprehensive set of ecological and evolutionary phenomena for insular biotas.

The most geologically recent island of Azores, Pico Island, seen from Faial island (Porto Pim Beach). Picture by Paulo A. V. Borges.