Plant diversity and ecosystem productivity: Theoretical considerations

Abstract

Ecosystem processes are thought to depend on both the number and identity of the species present in an ecosystem, but mathematical theory predicting this has been lacking. Here we present three simple models of interspecific competitive interactions in communities containing various numbers of randomly chosen species. All three models predict that, on average, productivity increases asymptotically with the original biodiversity of a community. The two models that address plant nutrient competition also predict that ecosystem nutrient retention increases with biodiversity and that the effects of biodiversity on productivity and nutrient retention increase with interspecific differences in resource requirements. All three models show that both species identity and biodiversity simultaneously inf luence ecosystem functioning, but their relative importance varies greatly among the models. This theory reinforces recent experimental results and shows that effects of biodiversity on ecosystem functioning are predicted by well-known ecological processes.Recent studies have shown that several community and ecosystem processes are correlated with species diversity (1-12), but why this should occur remains mathematically largely unexplained. Here we report that three different ecological models predict that ecosystem productivity, standing crop, and resource use depend on species diversity, much as has been experimentally observed (3, 4, 6). Our models provide simple mechanisms that explain how such dependencies can arise and help resolve the controversy over the importance for ecosystem functioning of species identity versus species diversity.The functioning of ecosystems has long been known to depend on the identities of the species the ecosystems contain (13-19), and hypothesized to depend on the number of species. However, recent work, plus early observations by Darwin (20) Because the functioning of an ecosystem may depend both on the identities and the numbers of its species, it is necessary to distinguish between these two dependencies in both experimental and theoretical studies. This requires, first, that the group of all potential species, called the ''species pool,'' be defined. Then, to attribute effects to species diversity, effects must occur in comparisons of the average responses of two or more levels of diversity. At each level of diversity, there must be numerous replicate ecosystems, each with a random and independent combination of species chosen from the species pool. By having many random species combinations drawn from a large species pool, the mean response among replicate ecosystems at a given level of diversity becomes independent of particular species combinations. The differences among mean responses for different levels of diversity then measure the effect of diversity. The variance among the various species combinations at a given diversity level measures the effects of alternative species compositions.We apply this approach to two models of interspecific plant competition for nutrient...

Complex facilitation and competition in a temperate grassland: loss of plant diversity and elevated CO2 have divergent and opposite effects on oak establishment

“…This fits well with evidence that herbaceous species richness is positively related to biomass production in grasslands (Tilman et al 2001;Reich et al 2001a;van Ruijven and Berendse 2003;Roscher et al 2005;Isbell et al 2011) and that this often leads to decreased availability of resources at higher levels of plant diversity (Tilman et al 1997b;Dijkstra et al 2005;Reich 2009). Further, work on woody encroachment patterns in the semi-arid southwestern USA demonstrates a similar pattern between woody growth limitation and belowground competition for resources.…”

“…As increasing numbers of species co-occur in an assemblage, their complementary resource acquisition strategies use overall resource pools more completely. This complementary resource use results in greater community-level biomass (Tilman et al 1997b) and, consequently, the community itself becomes less susceptible to colonization (Kennedy et al 2002).…”

The mammalian communities in coffee plantations around a protected area in the Western Ghats, India

“…Lantana is exceedingly efficient at nutrient uptake and use, enabling it to grow on highly impoverished soils Rawat et al 1994). Such an ability to extract and use nutrients efficiently would give it a competitive advantage over other species in low-fertility environments (Tilman et al 1997), for example, in forests already degraded by frequent burning.…”

Managing helminths of ruminants in organic farming

“…Increased helminth diversity might be a result of lower pressure from anthelmintic treatments and it could possibly seed the predictability of the intensity of helminth infection. Several researchers of free-living organisms or plants have linked diversity to the predictability [26] or sustainability of populations [59], in populations that do not increase or decrease enormously. Predictable or sustainable parasitic helminth communities should have low or medium intensity, depending on pathogenicity, so that helminth infections do not alter the host populations.…”