Does productivity drive diversity or vice versa? A test of the multivariate productivity-diversity hypothesis in streams

Effects of total resources, resource ratios, and species richness on algal productivity and evenness at both metacommunity and local scales

The study of the interrelationship between productivity and biodiversity is a major research field in ecology. Theory predicts that if essential resources are heterogeneously distributed across a metacommunity, single species may dominate productivity in individual metacommunity patches, but a mixture of species will maximize productivity across the whole metacommunity. It also predicts that a balanced supply of resources within local patches should favor species coexistence, whereas resource imbalance would favor the dominance of one species. We performed an experiment with five freshwater algal species to study the effects of total supply of resources, their ratios, and species richness on biovolume production and evenness at the scale of both local patches and metacommunities. Generally, algal biovolume increased, whereas algal resource use efficiency (RUE) and evenness decreased with increasing total supply of resources in mixed communities containing all five species. In contrast to predictions for biovolume production, the species mixtures did not outperform all monocultures at the scale of metacommunities. In other words, we observed no general transgressive overyielding. However, RUE was always higher in mixtures than predicted from monocultures, and analyses indicate that resource partitioning or facilitation in mixtures resulted in higher-than-expected productivity at high resource supply. Contrasting our predictions for the local scale, balanced supply of resources did not generally favor higher local evenness, however lowest evenness was confined to patches with the most imbalanced supply. Thus, our study provides mixed support for recent theoretical advancements to understand biodiversity-productivity relationships.

Biodiversity improves water quality through niche partitioning

Excessive nutrient loading of water bodies is a leading cause of water pollution worldwide, and controlling nutrient levels in watersheds is a primary objective of most environmental policy. Over the past two decades, much research has shown that ecosystems with more species are more efficient at removing nutrients from soil and water than are ecosystems with fewer species. This has led some to suggest that conservation of biodiversity might be a useful tool for managing nutrient uptake and storage, but this suggestion has been controversial, in part because the specific biological mechanisms by which species diversity influences nutrient uptake have not been identified. Here I use a model system of stream biofilms to show that niche partitioning among species of algae can increase the uptake and storage of nitrate, a nutrient pollutant of global concern. I manipulated the number of species of algae growing in the biofilms of 150 stream mesocosms that had been set up to mimic the variety of flow habitats and disturbance regimes that are typical of natural streams. Nitrogen uptake rates, as measured by using (15)N-labelled nitrate, increased linearly with species richness and were driven by niche differences among species. As different forms of algae came to dominate each unique habitat in a stream, the more diverse communities achieved a higher biomass and greater (15)N uptake. When these niche opportunities were experimentally removed by making all of the habitats in a stream uniform, diversity did not influence nitrogen uptake, and biofilms collapsed to a single dominant species. These results provide direct evidence that communities with more species take greater advantage of the niche opportunities in an environment, and this allows diverse systems to capture a greater proportion of biologically available resources such as nitrogen. One implication is that biodiversity may help to buffer natural ecosystems against the ecological impacts of nutrient pollution.

The functional role of producer diversity in ecosystems

Over the past several decades, a rapidly expanding field of research known as biodiversity and ecosystem functioning has begun to quantify how the world's biological diversity can, as an independent variable, control ecological processes that are both essential for, and fundamental to, the functioning of ecosystems. Research in this area has often been justified on grounds that (1) loss of biological diversity ranks among the most pronounced changes to the global environment and that (2) reductions in diversity, and corresponding changes in species composition, could alter important services that ecosystems provide to humanity (e.g., food production, pest/disease control, water purification). Here we review over two decades of experiments that have examined how species richness of primary producers influences the suite of ecological processes that are controlled by plants and algae in terrestrial, marine, and freshwater ecosystems. Using formal meta-analyses, we assess the balance of evidence for eight fundamental questions and corresponding hypotheses about the functional role of producer diversity in ecosystems. These include questions about how primary producer diversity influences the efficiency of resource use and biomass production in ecosystems, how primary producer diversity influences the transfer and recycling of biomass to other trophic groups in a food web, and the number of species and spatial /temporal scales at which diversity effects are most apparent. After summarizing the balance of evidence and stating our own confidence in the conclusions, we outline several new questions that must now be addressed if this field is going to evolve into a predictive science that can help conserve and manage ecological processes in ecosystems.

Indirect effects of species interactions on habitat provisioning

Species that shelter in a biogenic habitat can influence their refugia and, in turn, play an essential role in shaping local patterns of biodiversity. Here we explore a positive feedback loop between the provisioning rate of habitat-forming branching corals and their associated fishes and show how interactions between two groups of fish—the planktivorous damselfish and predatory hawkfish—altered the feedback. A field experiment confirmed that skeletal growth of branching coral (genus Pocillopora) increased substantially with increasing numbers (biomass) of resident fishes, likely because they greatly increased the interstitial concentrations of nutrients. Because there is a positive relationship between colony size and number (biomass) of associated fishes (primarily damselfishes in the Family Pomacentridae), a structure–function feedback loop exists in which increasing numbers of damselfish enhance coral growth and larger corals host greater abundances (and species richness) of fish. However, interactions between damselfishes and arc-eye hawkfish, Paracirrhites arcatus, a largely solitary resident, can disrupt this positive feedback loop. Field surveys revealed a marked pattern of fish occupancy related to coral size: Pocillopora colonies of sufficient size to host fish (>40 cm circumference) had either groups of damselfish or an arc-eye hawkfish; only larger colonies (>75 cm) were occupied by both the damselfish and hawkfish. Subsequent short- and long-term experiments revealed that on intermediate-sized Pocillopora colonies, arc-eye hawkfish prevented the establishment of damselfish by suppressing their recruitment. The demographic consequences to the host coral were substantial; in a 1-year-long experiment, intermediate-size Pocillopora occupied by hawkfish grew at half the rate of corals that hosted groups of damselfish. These findings indicate that: (1) species which occupy a biogenic habitat can enhance the provisioning rate of their habitat; (2) such positive feedbacks between community structure and ecosystem function can be disrupted by a strong interactor; (3) even substantial consequences on ecosystem processes that arise can be difficult to discern.

The Humpbacked Species Richness-Curve: A Contingent Rule for Community Ecology

Functional relationships involving species richness may be unimodal, monotonically increasing, monotonically decreasing, bimodal, multimodal, U-shaped, or with no discernable pattern. The unimodal relationships are the most interesting because they suggest dynamic, nonequilibrium community processes. For that reason, they are also contentious. In this paper, we provide a wide-ranging review of the literature on unimodal (humpbacked) species richness-relationships. Though not as widespread as previously thought, unimodal patterns of species richness are often associated with disturbance, predation and herbivory, productivity, spatial heterogeneity, environmental gradients, time, and latitude. These unimodal patterns are contingent on organism and environment; we examine unimodal species richness-curves involving plants, invertebrates, vertebrates, plankton, and microbes in marine, lacustrine, and terrestrial habitats. A goal of future research is to understand the contingent patterns and the complex, interacting processes that generate them.

Comment on "Climate, critters, and cetaceans: Cenozoic drivers of the evolution of modern whales"

Marx and Uhen (Reports, 19 February 2010, p. 993) suggested that correlated diversity changes in the fossil record of whales and diatoms reflects secular evolutionary signals of underlying ecological drivers. We question the meaning of this association and outline avenues for more complete testing of correlations between productivity and marine consumers through geologic time.

Separating the influence of resource 'availability' from resource 'imbalance' on productivity-diversity relationships

One of the oldest and richest questions in biology is that of how species diversity is related to the availability of resources that limit the productivity of ecosystems. Researchers from a variety of disciplines have pursued this question from at least three different theoretical perspectives. Species energy theory has argued that the summed quantities of all resources influence species richness by controlling population sizes and the probability of stochastic extinction. Resource ratio theory has argued that the imbalance in the supply of two or more resources, relative to the stoichiometric needs of the competitors, can dictate the strength of competition and, in turn, the diversity of coexisting species. In contrast to these, the field of Biodiversity and Ecosystem Functioning has argued that species diversity acts as an independent variable that controls how efficiently limited resources are utilized and converted into new tissue. Here we propose that all three of these fields give necessary, but not sufficient, conditions to explain productivity-diversity relationships (PDR) in nature. However, when taken collectively, these three paradigms suggest that PDR can be explained by interactions among four distinct, non-interchangeable variables: (i) the overall quantity of limiting resources, (ii) the stoichiometric ratios of different limiting resources, (iii) the summed biomass produced by a group of potential competitors and (iv) the richness of co-occurring species in a local competitive community. We detail a new multivariate hypothesis that outlines one way in which these four variables are directly and indirectly related to one another. We show how the predictions of this model can be fit to patterns of covariation relating the richness and biomass of lake phytoplankton to three biologically essential resources (N, P and light) in a large number of Norwegian lakes.