Frontiers of Biogeography

cover: Northern spruce engraver (Ips perturbatus) gallery. Picture by Edward H. Holsten, USDA Forest Service, Bugwood.org. Image Number 0805014 (CC BY license) at Forestry Images, a source for forest health, natural resources and silviculture images. See more on the effects of bark beetle infestations through time at Jesse L. Morris’ thesis abstract in this issue of Frontiers of Biogeography.

The recent outbreak of native bark beetles in western North America is unprecedented in severity and scale, at least during the historical period. The aim of this work is to develop a proxy-based methodology to understand how bark beetle disturbances are recorded in lake sediments. Three hypotheses are tested to determine how the ecological impacts of severe spruce beetle (Dendroctonus rufipennis) disturbances are recorded following mortality of Engelmann spruce (Picea engelmannii). Outbreaks are hypothesized to: (1) decrease the ratio of spruce to fir pollen; (2) increase soil erosion and mobilize terrestrial C; and (3) leach foliar N, enhancing algal productivity. To test these hypotheses, sediment cores from spruce beetle-affected basins were analyzed for pollen, insect remains, organic and minerogenic content, and isotopic and elemental concentrations. The dataset was tested statistically using generalized linear mixed models (GLMMs) to determine if the response variables differed significantly between outbreak and non-outbreak periods.

Keystone plant groups can be used to infer the evolution of biomes and biogeographical change of communities and taxa. In this thesis I investigated whether lineages in Trachycarpeae palms could be used to track different forest types through time and whether change in biome or biogeographic region had an effect on species diversification. These questions were approached using genetic data integrated with fossil record, species distribution, and speciation models. Although the three chapters of my thesis had additional foci outside of the main goal of inferring biogeographic change and diversification through time, they come together to paint a clear picture of how fine-scale and interdisciplinary studies can lead to more robust hypothesis testing and conclusions. I found that outside of tracking tropical forests through time, palms are useful for understanding island biogeography and the formation of other types of biomes.

The role of physiological parameters as determinants of macroscale patterns is still widely disputed. Using amphibians as a model, I revisited three fundamental themes in macroecology from a physiological perspective: the global diversity gradient, ecogeographical rules and the evolution of the climatic niche at physiological and macroecological scales. To do this, I used a variety of data types, performed a number of spatial and phylogenetic analyses and proposed novel applications for some methods. I also provided support for some well-established hypotheses regarding the physiological determinants of species diversity and trait variation across space, while revealing other lesser known patterns and the possible processes underlying species’ distributions and niche evolution. I emphasize the need for a novel integration of theoretical and methodological approaches to improve the analysis of broad-scale ecological processes, in particular those related to the fundamental features (e.g. physiology) of species. I also highlight the strategic role of macroecology in this quest, especially in the face of ongoing environmental changes.

Central America is an ideal region for comparative phylogeographic studies because of its intricate geologic and biogeographic history, diversity of habitats and dynamic climatic and tectonic history. The aim of this work was to assess the phylogeography of two rodents codistributed throughout Central America, in order to identify if they show concordant genetic and phylogeographic patterns. The synopsis includes four parts: (1) an overview of the field of comparative phylogeography; (2) a detailed review that describes how genetic and geologic studies can be combined to elucidate general patterns of the biogeographic and evolutionary history of Central America; and a phylogeographic analysis of two species at both the (3) intraspecific and (4) comparative phylogeographic levels. The last incorporates specific ecological features and evaluates their influence on the species’ genetic patterns. Results showed a concordant genetic structure influenced by geographic distance for both rodents, but dissimilar dispersal patterns due to ecological features and life history.

Historical biogeography has been characterized by a large diversity of methods and unresolved debates about which processes, such as dispersal or vicariance, are most important for explaining distributions. A new R package, BioGeoBEARS, implements many models in a common likelihood framework, so that standard statistical model selection procedures can be applied to let the data choose the best model. Available models include a likelihood version of DIVA (“DIVALIKE”), LAGRANGE’s DEC model, and BAYAREA, as well as “+J” versions of these models which include founder-event speciation, an important process left out of most inference methods. I use BioGeoBEARS on a large sample of island and non-island clades (including two fossil clades) to show that founder-event speciation is a crucial process in almost every clade, and that most published datasets reject the non-J models currently in widespread use. BioGeoBEARS is open-source and freely available for installation at the Comprehensive R Archive Network at http://CRAN.R-project.org/package=BioGeoBEARS. A step-by-step tutorial is available at http://phylo.wikidot.com/biogeobears.

Models of biogeographic processes can both enhance and inhibit our ability to ask questions that guide our understanding of patterns and processes. The two ‘traditional’ models of island biogeography, the Equilibrium Model and the Vicariance Model, raise important and insightful questions about relevant processes, but both fail to raise many crucial questions. An example involving the non-volant mammals of the Philippine archipelago shows that both models highlight some, but not all, relevant patterns and processes. The more recently proposed General Dynamic Model successfully combines many of the positive aspects of the two traditional models, but leaves some important questions unasked. We pose a number of questions here that may help guide further development of models of island biogeography.