patterns in functional diversity

Biodiversity is distributed unevenly across the planet--but do functional diversity and taxonomic diversity show the same patterns? The best-known macroecological pattern is the latitudinal diversity gradient--the tendency for species richness to peak in the tropics. Species richness measures taxonomic diversity, but what about functional diversity? Do species also exhibit more ways of ‘making a living’ in the tropics? Is the diversity of life strategies greater there? Does functional diversity correlate with latitude, temperature, or habitat area the same way taxonomic diveristy does? I am beginning to explore these questions for living bivalves, using our comprehensive database of  bivalve occurrences and genus functional attributes worldwide, in every ocean at every latitude. 

While species diversity measures the number of species in a region, functional diversity measures the variety of life strategies in that region. If we want to fully understand how diversity plays out spatially over the planet, then we need to document macroecological patterns in functional diversity as arduously as we have species diversity, with the ultimate goal of understanding the mechanisms underlying these patterns.

Body size is one of an organism’s most important functional attributes, correlating with virtually every other aspect of its biology and ecology. Global patterns in body size have been noted for many groups of organisms, most famously in Bergmann’s Rule, which posits that body size increases from the tropics towards the poles.

Though Bergmann was referring to terrestrial endotherms, body size clearly shows patterns with latitude--either increasing or decreasing--for a great number of endothermic and ectothermic animals. Nevertheless, our understanding of macroecological patterns in body size is strongly biased towards terrestrial vertebrates. Truly understanding how functional diversity is patterned at global scales requires a more comprehensive understanding of what patterns exist in the marine and invertebrate realms, and how broadly different patterns apply.

I have evaluated body size-latitude patterns for over 60 families of bivalves living at shelf depths worldwide. The taxonomic and geographic scope of this study is unique, in that it (a) covers most families in a major class of marine invertebrates and (b) includes all latitudes in all oceans of the world, thus significantly extending our understanding of size-latitude relationships to the marine realm and allowing us to assess the consistency of trends within the Bivalvia and across the globe. I find that many families do exhibit size-latitude trends, but the direction of these trends is surprisingly variable--some families get bigger with latitude, others get smaller, and others show different trends in different hemispheres!

There are no discernable patterns to these trends--closely related families show inconsistent patterns, as do families with similar life strategies. Size correlates with temeprature and/or productivity for some families, but no consistent patterns emerge among all families. Thus, the processes contributing to size-latitude trends may be different in different regions and/or for different lineages. These findings emphasize that fully understanding macroecological patterns requires global datasets, as regional trends may not extend to larger scales.

J. Derk, public domain

© S. Anderson

E. Haeckel, public domain