The relative effects of inter- and intra-specific diversity on community productivity: A niche-based theoretical approach

Background/Question/Methods

Ecologists recognize that both species diversity and diversity within a species can have strong impacts on community biomass production and resource uptake. The primary mechanistic hypothesis for a positive relationship between diversity and productivity is that greater genetic diversity, at the inter- or intra-specific level, will result in broader phenotypic variation in traits that enhance the uptake and assimilation of resources in a heterogeneous environment. In some communities, such as species monocultures, it is clear that intra-specific diversity has a strong influence on productivity relative to interspecific variation. However, in species rich communities, even those showing strong dominance by one or a few species, it is less clear how the relative contributions of inter- and intra-specific diversity to total community overyielding compare. Here I use a Lotka-Volterra resource uptake model to directly compare the impacts of inter- and intra-specific diversity community biomass production in a series of standard monoculture-versus-polyculture design experiments. Using this niche-based meta-community framework, I mimicked experimental diversity manipulations at both the inter- and intra-specific levels. I assume a haploid model for intra-specific diversity where increased diversity results in multiple “populations” within a species that vary in maximum resource uptake rate across an environmental gradient.

Results/Conclusions

In this model, non-transgressive overyielding is always predicted at equilibrium for any set of populations. As the intrinsic niche breadth of populations decreases relative to the environmental gradient, transgressive overyielding increases. If patches are evenly distributed across the environmental gradient, community composition is even and intra-specific diversity has low influence relative to inter-specific diversity. As patches are progressively skewed in distribution, communities become less even. When niche breadth is high and transgressive overyielding of communities is low at the inter-specific level, overyielding due to intra-specific diversity is low for both dominant and rare species compared to overyielding due to inter-specific diversity. In contrast, when niche breadth is low and transgressive overyielding at the inter-specific level is high, intra-specific variation of the dominant species can have similarly high overyielding effects. The general considerations that emerge for researchers and managers from this study are (i) that intra-specific diversity may have relatively little impact on overyielding in communities with high evenness, and (ii) that both intra- and inter-specific diversity may be similarly influential in communities characterized by higher levels of dominance.