A study finds that human activity has led to a severe decrease in Caribbean shark abundance.
Image credit: Sean Mattson (International Center for Tropical Agriculture, Cali, Colombia).
Contributed by Andrea Rinaldo, December 23, 2015 (sent for review October 7, 2015; reviewed by Evan P. Economo and Pablo A. Marquet)
Significance
How biodiversity changes with elevation has long attracted the interest of researchers because it provides key clues to how biota respond to geophysical drivers. Experimental evidence reveals that biodiversity often peaks at intermediate elevations, and yet a clear explanation is still elusive. Here, we investigate a factor that has been overlooked to date: Mountainous landscapes hold fractal properties with elevational bands forming habitat patches that are characterized by different areal extent and connectivity, well-known drivers of biodiversity. When the geometry of real landscapes is taken into account, ecological dynamics naturally produce mid-peak elevational gradients of species richness. These results further biodiversity theory and our understanding of the distribution of life on Earth.
Abstract
Elevational gradients of biodiversity have been widely investigated, and yet a clear interpretation of the biotic and abiotic factors that determine how species richness varies with elevation is still elusive. In mountainous landscapes, habitats at different elevations are characterized by different areal extent and connectivity properties, key drivers of biodiversity, as predicted by metacommunity theory. However, most previous studies directly correlated species richness to elevational gradients of potential drivers, thus neglecting the interplay between such gradients and the environmental matrix. Here, we investigate the role of geomorphology in shaping patterns of species richness. We develop a spatially explicit zero-sum metacommunity model where species have an elevation-dependent fitness and otherwise neutral traits. Results show that ecological dynamics over complex terrains lead to the null expectation of a hump-shaped elevational gradient of species richness, a pattern widely observed empirically. Local species richness is found to be related to the landscape elevational connectivity, as quantified by a newly proposed metric that applies tools of complex network theory to measure the closeness of a site to others with similar habitat. Our theoretical results suggest clear geomorphic controls on elevational gradients of species richness and support the use of the landscape elevational connectivity as a null model for the analysis of the distribution of biodiversity.
Footnotes
- ↵1To whom correspondence may be addressed. Email: enrico.bertuzzo{at}epfl.ch or andrea.rinaldo{at}epfl.ch.
Author contributions: E.B., F.C., L.M., F.A., I.R.-I., and A.R. designed research; E.B. and F.C. performed research; E.B. and F.C. analyzed data; and E.B., F.C., L.M., F.A., I.R.-I., and A.R. wrote the paper.
Reviewers: E.P.E., Okinawa Institute of Science and Technology Graduate University; and P.A.M., Catholic University of Chile.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1518922113/-/DCSupplemental.
Freely available online through the PNAS open access option.
Geomorphic controls on species richness
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