Frontiers of Biogeography

Diversity-environment relationships are distinct across species pools, and as a result species from different biogeographic pools have different environmental preferences. Regional communities are drawn from available biogeographic pools, subject to environmental and dispersal constraints. Does shared biogeographic history of taxa lead to similar relationships with the environment? We test this idea in the Indian subcontinent, which is at the confluence of multiple biogeographic regions resulting in species from multiple biogeographic pools being distributed here. Species were classified as belonging to four biogeographic affinities based on their geographic distributions: eastern, northern, western and endemic. We investigated spatial patterns of species richness for all mammals (over 1° x 1° grid cells), for each biogeographic group and for 5 major mammalian orders. Generalized Additive Models (GAM) were used to investigate environment-diversity relationship for all mammals, each biogeographic group, and for major mammalian orders in the Indian subcontinent. Species richness of all mammals was found to be highest in the montane regions of the Eastern Himalayas and the Western Ghats. Species richness of each biogeographic group was highest at the border it shared with Asia, in the direction of immigration from Asia. Environment and spatial variables were both correlated with species richness in the Indian subcontinent and each biogeographic group showed a distinct richness-environment relationship. Additionally, biogeographic groups sorted along environmental space, in keeping with our predictions based on their global distributions. Finally, analyses across mammalian orders had low predictive value, suggesting that shared phylogenetic history is relatively less important than biogeographic ancestry in determining relationships to environment. We conclude that historical factors such as immigration and the distinct evolutionary histories of species impact species richness patterns in the Indian subcontinent. Our results provide insights into drivers of regional community assembly in transition zones where multiple biogeographic species pools co-exist.

Statistical analysis and interpretation of biogeographical phenomena in rivers is now possible using a spatially explicit modelling framework, which has seen significant developments in the past decade. I used this approach to identify a spatial extent (geostatistical range) in which the abundance of the parasitic freshwater pearl mussel (Margaritifera margaritifera L.) is spatially autocorrelated in river networks. I show that biomass and abundance of host fish are a likely explanation for the autocorrelation in mussel abundance within a 15-km spatial extent. The application of universal kriging with the empirical model enabled precise prediction of mussel abundance within segments of river networks, something that has the potential to inform conservation biogeography. Although I used a variety of modelling approaches in my thesis, I focus here on the details of this relatively new spatial stream network model, thus advancing the study of biogeographical patterns in river networks.

The current fires raging across Indonesia are emitting more carbon than the annual fossil fuel emissions of Germany or Japan, and the fires are still consuming vast tracts of rainforest and peatlands. The National Interagency Fire Center (www.nifc.gov) notes that 2015 is one worst fire years on record in the U.S., where more than 9 million acres burned -- equivalent to the combined size of Massachusetts and New Jersey. The U.S. and Indonesian fires have already displaced tens of thousands of people, and their impacts on ecosystems are still unclear. In the case of Indonesia, the burning peat is destroying much of the existing soil, with unknown implications for the type of vegetation regrowth. Such large fires result from a combination of fire management practices, increasing anthropogenic land use, and a changing climate.

The expected increase in fire activity in the upcoming decades has led to a surge in research trying to understand their causes, the factors that may have influenced similar times of fire activity in the past, and the implications of such fire activity in the future. Multiple types of complementary data provide information on the impacts of current fires and the extent of past fires. The wide array of data encompasses different spatial and temporal resolutions (Figure 1) and includes fire proxy information such as charcoal and tree ring fire scars, observational records, satellite products, modern emissions data, fire models within global land cover and vegetation models, and sociodemographic data for modeling past human land use and ignition frequency. Any single data type is more powerful when combined with another source of information. Merging model and proxy data enables analyses of how fire activity modifies vegetation distribution, air and water quality, and proximity to cities; these analyses in turn support land management decisions relating to conservation and development.

Analyses of topography show that mountains do not monotonically decrease in area with elevation as is commonly believed and that in reality land area often increases at higher elevations. This finding bodes well for the future of biodiversity since it means that in many parts of the world there are sufficient upslope areas for low- and mid-elevation species to migrate into as temperatures increase. However, more attention needs to be given to determining if migrating species can actually reach these expansive high-elevation areas. Many factors can prevent species from migrating upslope including stable ecotones. Often ecotonal boundaries are not set by mean temperatures alone and thus are not shifting upslope with warming. An example of this are tropical alpine treelines, which are not shifting upslope despite rapid warming potentially due to the stabilizing influences of climatic factors other than mean temperatures (e.g., extreme cold events) or non-climatic factors (e.g., soil or human disturbances). Stable ecotones can potentially prevent species from expanding their ranges into upland areas in which case the amount of land at higher elevations is irrelevant and species may face “mountain top extinctions” long before they reach the actual tops of the mountains.

In their recent paper, Muir et al. (Science, 2015, 348, 1135-1138) demonstrate that the maximum depths of staghorn coral assemblages are shallower at higher latitudes, a trend that correlates with winter light levels. Based on these findings, the authors hypothesize that light availability limits the current latitudinal extent of the group and will constrain future range expansion. Here we reanalyze their data and show that depth-latitude relationships vary substantially among species, and that most species show either no significant pattern or the opposite pattern. In so doing, our reanalysis highlights a common misinterpretation of mixed-effects models: the fallacy of the average. Our findings are also consistent with fossil and contemporary observations of coral range-shifts. The factors that limit the current range extent of corals remain elusive, but they are likely species-specific and will require much further research to elucidate.