Effects of Algal Diversity on the Production of Biomass in Homogeneous and Heterogeneous Nutrient Environments: A Microcosm Experiment

Abstract

BackgroundOne of the most common questions addressed by ecologists over the past decade has been-how does species richness impact the production of community biomass? Recent summaries of experiments have shown that species richness tends to enhance the production of biomass across a wide range of trophic groups and ecosystems; however, the biomass of diverse polycultures only rarely exceeds that of the single most productive species in a community (a phenomenon called ‘transgressive overyielding’). Some have hypothesized that the lack of transgressive overyielding is because experiments have generally been performed in overly-simplified, homogeneous environments where species have little opportunity to express the niche differences that lead to ‘complementary’ use of resources that can enhance biomass production. We tested this hypothesis in a laboratory experiment where we manipulated the richness of freshwater algae in homogeneous and heterogeneous nutrient environments.Methodology/Principal FindingsExperimental units were comprised of patches containing either homogeneous nutrient ratios (16∶1 nitrogen to phosphorus (N∶P) in all patches) or heterogeneous nutrient ratios (ranging from 4∶1 to 64∶1 N∶P across patches). After allowing 6–10 generations of algal growth, we found that algal species richness had similar impacts on biomass production in both homo- and heterogeneous environments. Although four of the five algal species showed a strong response to nutrient heterogeneity, a single species dominated algal communities in both types of environments. As a result, a ‘selection effect’–where diversity maximizes the chance that a competitively superior species will be included in, and dominate the biomass of a community–was the primary mechanism by which richness influenced biomass in both homo- and heterogeneous environments.Conclusions/SignificanceOur study suggests that spatial heterogeneity, by itself, is not sufficient to generate strong effects of biodiversity on productivity. Rather, heterogeneity must be coupled with variation in the relative fitness of species across patches in order for spatial niche differentiation to generate complementary resource use.

Habitat structure mediates biodiversity effects on ecosystem properties

“…Indeed, the inclusion of one or more components of habitat structure in all of our models confirms the importance of environmental complexity in mediating species interactions (e.g. [56,57]) and shaping biodiversity -function relations [21] (although see [58] for a counterexample).…”

Dispersal scales up the biodiversity–productivity relationship in an experimental source-sink metacommunity

“…As expected, we found a positive BEF relationship only at the regional scale where niche differentiation occurs. Under the complementarity hypothesis (Tilman et al 1997;Loreau 1998), multiple resources are required for a positive biodiversity-productivity relationship; and the more potential niches available, the more the positive effects of diversity on productivity should be expressed (Cardinale et al 2000;Wacker et al 2008;Weis et al 2008;Replansky & Bell 2009). In our experiment, it is at the regional scale that most diversity has evolved and that niches have differentiated.…”

Species traits and species diversity affect community stability in a multiple stressor framework

“…However, the importance of species identity effects in biodiversity−production relationships is frequently found and often related to trait combinations (for a discussion see Hillebrand & Matthiessen 2009). The presence of such 'super-species' that show high performance for multiple situations is a frequent phenomenon in experiments with a small species pool and low complexity of physical or biological structure (Weis et al 2008, Ptacnik et al 2010.…”