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Have human activities altered productivity-diversity relationships in California?
Williams, John1, Seabloom, Eric1, Slayback, Daniel2, Stoms, David1, Shafer, Sarah3, Bartlein, Patrick4, 1 University of California Santa Barbara, Santa Barbara, CA2 University of Maryland, College Park, MD3 US Geological Survey, Corwallis, OR4 University of Oregon, Eugene, OR
ABSTRACT- Previous attempts to model the biogeographic factors of species richness generally focus upon energetic, ecophysiological, and/or historical controls on species distributions, ignoring the substantial impacts of humans on species distributions and ecosystem functioning at regional to continental scales. In this study we seek to understand how the relationship between vascular plant richness, net primary productivity, and various abiotic indices of environmental favorability in California has been modified by anthropogenic activities. The Californian vegetation is diverse (comprising over 20% of US plant species), includes strong gradients in productivity (>three orders of magnitude from the southeastern deserts to the coastal mountains), and has been widely and variably impacted by human activities. Using plant species richness data from the CalFlora database, NPP estimates for 1982-1999 from GLO-PEM, IPCC CRU 1961-1990 climate data, StatsGo soil textural properties, and remotely sensed indices of land cover conversion, we constructed and tested path models representing alternate hypotheses about a) the relationship between NPP, richness, and exogenous environmental variables and b) the direct and indirect impacts of human activity. The distribution of NPP and species richness was estimated for two alternate states representing pre-settlement and present-day conditions; all other variables were assumed to be unchanged. Path models that exclude anthropogenic influence are able to explain a high proportion of the variance in both pre-settlement and present-day plant richness. However, additional variance in the present-day distribution of plant richness corresponds to indices of land cover use.
Key words: NPP, California, richness, biogeography