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Studies of microbial communities from aquatic ecosystems provide important insights into relations between various aspects of ecosystem functioning and changes in biodiversity. Aquatic microbial systems provide a valuable counterpoint to studies of terrestrial systems, because patterns reflect consequences of interactions occurring over many generations of community development, and are unlikely to represent artifacts of the initial conditions established in experimental communities. In this paper we re-analyse our previously published data to separate the contributions of temporal and spatial variation to overall variation in ecosystem functioning. A new analysis based on re-sampling confirms a negative relationship between richness and the variability of one ecosystem process, carbon dioxide flux. The negative relationship reflects high variation among communities of low species richness, rather than high temporal variation within communities of low richness. We also review the various transformations and summary statistics proposed as alternate measures of variability in ecosystem functioning, to point out that different measures are often appropriate for different kinds of data. Finally, we conclude that arguments about the cosmopolitan distribution of microbes do not preclude the existence of important relations between microbial species richness and ecosystem functioning.

Summary 1 Theory predicts that communities comprising many species connected randomly by strong interactions are unlikely to exhibit stable, feasible equilibria (i.e. positive equilibria to which all species will return following perturbation), making the observed complexity of natural communities very surprising. This prediction has never been tested experimentally in speciose communities. 2 We tested this prediction using long-term data from experimental communities of bacteria, protists and small metazoans in laboratory microcosms. Initial communities varied in species richness ( S ), composition and connectance ( C ; the fraction of possible interspecific interactions that are actually realized). 3 Many initial communities lost species to extinction (unstable and/or infeasible communities), but population densities and species compositions subsequently stabilized. 4 We used logistic regression to describe the probability that an initial community would be stable and feasible as a function of complexity [( SC ) 1/2 ]. Probability of a stable, feasible equilibrium declined with complexity, as predicted by theory. 5 We also found that communities converged in connectance over time, so that at the end of the experiment connectance was independent of species richness. Monte Carlo simulations indicated that this result was highly improbable if extinctions occurred at random with respect to connectedness. Connectance is also independent of species richness in well-resolved natural food webs, a pattern usually ascribed to phylogenetic and/or physiological constraints on diet breadth. However, such constraints cannot explain our results. Trophic interactions causing non-random extinctions may provide a novel explanation for patterns in the connectance and species richness of whole food webs.

Species richness can affect both the size and stability of populations. Many simple theories predict that complex speciose communities should be less stable than simple ones. In contrast, stability of collective community attributes, such as functional groups, may be greater in more complex communities. We used aquatic microcosms containing microbial food webs to generate an experimental gradient of species richness. Biweekly counts of population samples permitted estimation and comparison of density, persistence time, and temporal variation in density for populations and trophic functional groups embedded in communities of varying richness. Densities of nearly half of the species declined as species richness increased, demonstrating broad community-wide density compensation that resulted from more abundant competitors and predators in species-rich communities. Persistence time, a measure of population stability, also declined with increasing species richness. Another measure of population stability, temporal variation in density, indicated that abundances of most species embedded in diverse communities varied no more than in less diverse communities. In contrast to results for individual species, temporal variation of entire functional groups composed of multiple species decreased as species richness increased. The stability of individual populations was not predicted by that of functional groups. Interspecific interactions and statistical averaging may both contribute to the reduced temporal variability of functional groups created by aggregating multiple species.

Links between biodiversity and ecosystem function provide compelling reasons for conserving maximal numbers of species in ecosystems1,2,3,4,5,6. Here we describe a previously unrecognized effect of biodiversity on ecosystem predictability, where predictability is inversely related to temporal and spatial variation in ecosystem properties. By manipulating biodiversity in aquatic microbial communities, we show that one process, ecosystem respiration, becomes more predictable as biodiversity increases. Analysis of similar patterns extracted from other studies2,3,6 indicates that biodiversity also enhances predictability in terrestrial ecosystems. Biodiversity can also affect average levels of ecosystem performance, but the extent to which different species make unique or redundant contributions to ecosystem processes remains controversial3,7,8,9,10. Nonlinear effects of biodiversity on the decomposition of particulate organic matter and resistance of communities to invasion indicate that different species have redundant functions in our system. The consequences of biodiversity are also not restricted to early successional situations as described in previous studies1,2,3,4,6, because strong effects persist even after ecosystems develop for periods corresponding to 40–80 generations of dominant organisms.

This chapter reviews the concept of functional diversity as it applies to single trophic levels and then extends the concept to food webs. One component of biodiversity that measures the extent of resource use differences among species is functional diversity. It may also explain and predict ecosystem functioning more accurately than does species richness. Experiments, mostly in plant communities, show that variation in functional diversity can have substantial effects on ecosystem functioning, though effects appear contingent upon factors such as spatial scale and environmental conditions. Functional diversity explained more variance in decomposition rate than in respiration rate, suggesting that a single measure of functional diversity will not be able to explain or predict all ecosystem processes. Different information about the species and/or different measures of diversity may be required for different functions, or diversity may have less functional significance for some ecosystem processes compared to others. Decades of research show that multiple factors determine the distribution of biomass and productivity in a food web. More research demonstrates the importance of the identity of trophic groups for ecosystem processes, perhaps more so than the diversity of trophic strategies. Another example of why functional diversity may not predict food web functioning is provided by studies of interactions among different trophic levels.

Ephemeral rain pools contain diverse assemblages of protists and small metazoans. Rain pool organisms survive periods of desiccation as dormant cysts or resting stages, and become active shortly after the pools fill with water. Non-equilibrium mechanisms related to the frequent drying of pools could promote the observed coexistence of over 15 species of ecologically similar microorganisms. We used dormant organisms from the substrate of a well-characterized ephemeral pool to create 20 microcosms that experienced two different disturbance regimes. Undisturbed microcosms held a constant volume of medium over the entire 30-d study period. Disturbed microcosms dried periodically, at approximately five-d intervals, and were then refilled with medium to initiate new rounds of community development. Daily monitoring of the presence or absence of active organisms provided information about the number and identity of species in disturbed and undisturbed microcosms. Undisturbed microcosms contained a greater number of active species, primarily because some predatory species only became active after at least five to ten d of uninterrupted community development. Species composition diverged rapidly after the first drying episode as communities in undisturbed and disturbed microcosms developed along different successional trajectories. If non-equilibrium mechanisms contribute to diversity in this system, their contribution must occur over more episodes of disturbance than we observed.