Natural systems are experiencing increased levels of nutrient pollution and eutrophication. Increased inputs of limiting nutrients can act to simplify habitats and lead to a loss of biodiversity (Harpole & Tilman 2007, Nature). Recently I have shown how changes in the supply of water and nitrogen (both important global change factors in Southern California) can have synergistic effects on ecosystem gas exchange by affecting community composition and species’ phenologies (Harpole, Potts & Suding 2007, Global Change Biology).
How ecosystems will respond to changes in resource supply will depend on which resources are limiting to plant growth as well as the degree to which they are limited by multiple resources. Using meta-analysis, I found evidence that suggests that multiple resource limitation may be more ubiquitous than commonly assumed (Harpole, Goldstein & Aicher 2007).
Co-limitation by multiple nutrients makes California grassland systems ideal for using ecological stoichiometry as a conceptual framework to gain insight into community processes (Moe, et al. 2004, Oikos). In a separate meta-analysis of global patterns of N and P limitation, we found a global pattern of strong synergistic effects of N and P in freshwater, marine and terrestrial ecosystems, challenging the paradigm that freshwater systems are primarily P-limited whereas terrestrial systems are primarily N-limited (Elser et al. 2007, Ecology Letters). Because N and P are two of the most common nutrient pollutants to natural systems, their increased availability should lead to increased limitation by other factors, impacts of which are very poorly understood.
One important implication of limitation by multiple resources relates directly to Hutchinson's (1957) prediction that species diversity should scale with the diversity of limiting resources. This “Niche Dimension” hypothesis predicts that each limiting resource potentially provides a unique niche axis that can allow additional species to coexist. I found that plant species diversity decreased with decreasing numbers of limiting resources (i.e., reduced niche dimension, see figure) in experimental and observational studies (Harpole & Tilman 2007, Nature). Simplification of habitats due to eutrophication can lead to long-term biodiversity loss by decreasing niche dimensionality.