Frontiers of Biogeography

cover: Schooling bannerfish. Picture by Jon Hanson, creative commons license; modified from Wikipedia Commons.

Approximately 0.5–2.0 million eukaryotic species inhabit the seas, whereas 2.0–10.0 million inhabit freshwater or the land. Much has been made of this several-fold difference in species richness but there is little consensus about the causes. Here, I ask a related question: what is the relative density of species in marine and non-marine realms? I use recent estimates of global eukaryotic species richness and published estimates of the areal coverage and depth of habitat for freshwater, marine, and terrestrial biomes. I find that the marine realm harbors ~99.83% of the habitable volume on this planet. Eukaryotic species density of the marine realm is ~3600-fold (i.e., 3-4 orders of magnitude) less than that of non-marine environments. Species–volume relationships (SVRs) help reconcile actinopterygian fish diversity with global primary productivity and emphasize the interacting roles of abiotic and biotic complexity in shaping patterns of biodiversity in freshwater, the sea, and on land. Comparing SVRs of habitats within and across realms may help resolve the factors and interactions that influence species density.

The keystone concept has been widely applied in the ecological literature since the idea was introduced in 1969. While it has been useful in framing biodiversity research and garnering support in conservation policy circles, the terminology surrounding the concept has been expanded to the extent that there is considerable confusion over what exactly a keystone species is. Several authors have argued that the term is too broadly applied, while others have pointed out the technical and theoretical limitations of the concept. Here, we chart the history of the keystone concept’s evolution and summarise the plethora of different terms and definitions currently in use. In reviewing these terms, we also analyse the value of the keystone concept and highlight some promising areas of recent work.

Phylogenetic and phylogeographic approaches have become widespread in evolutionary biology, ecology, and biogeography.  However, analyses that incorporate inferences from historical biogeography (e.g. timing of colonization of a region) may be essential to answer the most important large-scale questions in these fields, but they remain infrequently used.  I focus on two examples here.  First, I argue that understanding the origins of biodiversity hotspots (and other high-diversity regions) requires comparing the timing of biogeographic colonization and diversification rates among regions.  In contrast, phylogeographic studies (analyses within species within a region) may themselves say little about why a region is diverse relative to others.  Second, incorporating historical biogeograpy can help address the processes that determine community species richness and structure, such as dispersal, in-situ trait evolution, and in-situ speciation.  In contrast, the widespread “community phylogenetics” approach (focusing on relatedness of species in communities) may have limited ability to explain community richness and structure.

Memorial University of Newfoundland, Department of Geography, Assistant Professor in Biogeography

University of California, Merced, Associate/Full Professor in Natural Resource Management of Public Lands and Protected Areas