Effects of algal diversity on the production of biomass in homogeneous and heterogeneous nutrient environments: a microcosm experiment

Abstract

Background: One 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 findings: Experimental 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/significance: Our 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.

Figure 1. Algal biomass through time, estimated by chlorophyll-a.

Chlorophyll was measured in test-tubes inoculated with 5-species polycultures in three N∶P ratios spanning the range used in the experiment. Dark circles show an N∶P ratio of 4∶1, open circles 16∶1, and dark triangles 64∶1.

Figure 2. Mean monoculture biomass (±1SE) for each species at each N∶P ratio in the heterogeneous nutrient environment.

Solid lines show statistically significant linear regressions (P<0.05) where the biomass of a given species decreased with increasing N∶P ratios (also see Table 1).

Figure 3. Effect of algal species richness on algal biomass in homogeneous and heterogeneous nutrient environments.

Each panel shows the mean biomass (±1SE) of species monocultures as well as the 5-species polyculture. Increasing richness from one to five species led to a significant increase in biomass in both environments (see Table 1). However, this was due to the impacts of a single species–Selenastrum (Se)–which came to competitive dominance in polyculture (see Figure 4 & 5).

Figure 4. Proportional deviation of individual algal species (D_i±95% confidence intervals).

(a) Shows the proportional deviation for each species in the homogeneous (dark circles) and heterogeneous (light circles) treatments, and (b) shows proportional deviation of each species across the N∶P gradient in the heterogeneous treatment.

Figure 5. Factors contributing to the net diversity effect.

Here we use Fox's (2005) method to statistically partition the net effect of diversity (circles) into three distinct components: ‘trait-independent complementarity’ (C), ‘dominance effects’ (D), and ‘trait-dependent. complementarity’ (T). Black data points are for analyses using all data. Gray data points give values for a conservative analysis used to adjust for potential contamination of a select few monocultures of Selenastrum by Anabaena (see Methods). Results for the homogeneous environments are given in the left panel, while results for heterogeneous nutrient environments are given at right. Values are the mean±95% confidence intervals for all replicates.