Ecological realism and mechanisms by which diversity begets stability

We investigated how ecological realism might impact the outcome of three experimental manipulations of species richness to determine whether the patterns and the mechanisms underlying richness–variability relationships differ as ecological communities are increasingly exposed to external forces that may drive richness–variability patterns in nature. To test for such an effect, we conducted experiments using rock pool meio‐invertebrate communities housed in three experimental venues: controlled laboratory microcosms, artificially constructed rock pools in the field, and naturally occurring rock pools in the field. Our results showed that experimental venue can have a strong effect on the outcome of richness manipulation experiments. As ecological realism increased, the strength of the relationship between species richness and community variability declined from 32.9% in the laboratory microcosms to 16.8% in the artificial pools to no effect of species richness on community variability in the natural rock pools. The determinants of community variability also differed as ecological realism increased. In laboratory microcosms, community variability was driven solely by mechanisms related to increasing species richness. In artificial rock pools, community variability was driven by a combination of direct and indirect environmental factors as well as mechanisms related to increasing species richness. In the natural rock pools community variability was independent of species richness and was only related to environmental factors. In summary, we found that stabilizing mechanisms associated with species interactions were influential in establishing species richness–variability relations only in the less realistic experimental venues (the laboratory microcosms and the artificial rock pools in the field), and that these mechanisms diminished in importance as ecological realism and complexity of the experimental venue increased. Our results suggest that the effects of diversity might be more difficult to detect in natural systems due to the combined effects of biotic and abiotic forcing, which can mask our ability to detect richness effects.     

Tadpole mortality varies across experimental venues: do laboratory populations predict responses in nature?

Laboratory experiments are widely used to study how populations in nature might respond to various biological interactions, but the relevance of experiments in artificial venues is not known. We compiled mortality and growth data from 424 anuran populations carried out under laboratory, mesocosm, field enclosure, and field settings to determine if major differences exist amongst experimental venues and how this might influence experimental responses of tadpoles amongst venues. Our results show that there are fundamental differences in survival amongst venues, with the highest mortality occurring in field populations and the lowest in laboratory populations. Separation of mesocosm and field enclosure data based on the possibility of predatory interactions indicates that predation is an important factor leading to increased mortality in natural populations. Comparisons of size distributions across venues (although size data were limited for field populations) suggest that variation in tadpole size is low in natural populations compared to populations in artificial venues. We infer from this that mortality has a homogenizing effect on size in nature, resulting in natural populations that are not a random sample of hatched individuals. This finding suggests that populations reared under controlled laboratory conditions in the absence of predation (and other selective pressures) may not be representative of natural populations.     

Evaluating a predator–prey interaction in the field: the interaction between beetle larvae (predator) and tadpoles (prey)

The larval amphibian community of temporary pond ecosystems has served as a model for studies in community ecology, with a majority of this work being conducted in mesocosms. Recent research has suggested that mesocosms may overestimate ecological effects; therefore, experimental studies conducted under field conditions are required to gauge the results of mesocosm studies. To assess a species interaction under more natural conditions, we conducted a series of field experiments examining the predator–prey interaction between beetle larvae ( Dytiscus sp.; predator) and larval wood frogs Rana sylvatica (prey) in central Pennsylvania, USA. Quantitative sampling of woodland ponds indicated that beetle larvae of the genus Dytiscus were the most common predator of tadpoles. In a field enclosure experiment, dytiscids were effective predators of tadpoles in the pond environment. Moreover, tadpoles avoided areas in a pond containing caged dytiscids, demonstrating that tadpoles recognize the chemical stimuli of predators in complex environments. The results of this study are consistent with data from prior laboratory and mesocosm studies and suggest that these venues can produce reliable interpretations of predator–prey dynamics in this community.     

Evaluating condition-specific and asymmetric competition in a species-distribution context

Mechanisms resulting in parapatric distributions of closely related taxa have long interested ecologists. If two species are distributed across an environmental gradient, and differ in their ability to cope with environmental conditions, the outcome of competitive interactions may be dependent on prevailing abiotic conditions. Two closely related species of poison frogs in north–central Peru were observed to occupy parapatric distributions across an elevation gradient. Ameerega bassleri is a highland endemic restricted to a small region of Peru, and A. trivittata is distributed throughout lowland Amazonia. The goal of this study was to examine the effect of an abiotic factor (elevation) on two biotic factors (intraspecific and interspecific competition), by measuring growth and survival in the larvae of A. trivittata and A. bassleri . Using mesocosm experiments arranged in a fractional factorial design, we found that (1) A. bassleri had a strong negative effect on the growth and survival of A. trivittata regardless of elevation, (2) A. trivittata had no effect on the growth of A. bassleri at either elevation, but did appear to reduce the survival of A. bassleri more strongly in the lowlands than highlands, (3) lowland conditions uniformly reduced survival in all treatments and in both species, and (4) competition was strongly asymmetric between A. bassleri and A. trivittata . We conclude that the perceived low density of A. trivittata in highland sites may be influenced by the presence of A. bassleri, but the lower limit to the distribution of A. bassleri cannot be explained by competition with A. trivittata and may be due to physiological constraints imposed by lowland conditions.     

Competition between the nymphs of two regionally co-occurring species of Notonecta (Hemiptera: Notonectidae)

1. Two species of freshwater invertebrate predator, Notonecta maculata and N. obliqua , showed a negative association in a series of small, man-made ponds in the Peak National Park, Derbyshire, U.K. The present study examines the potential role of interspecific interactions among nymphs on this regional distribution pattern.
2. The survival, development and feeding efficiency of nymphs were examined in laboratory and field mesocosm experiments with intra- and interspecific competition and contrasting environmental complexity.
3. Survival to adulthood and mean lifespan varied significantly in interspecific competition treatments in both laboratory and field experiments, with N . maculata showing higher survival in the simple environment and N. obliqua higher survival in the complex environment.
4. Variations in feeding efficiency were consistent with the survival trends: N. maculata had a higher efficiency in the simple environment, whereas N. obliqua had greater efficiency in the complex environment. There was evidence of a developmental response in feeding efficiency, with differences between species increasing with age.
5. These results suggest that the relative competitive abilities of the two species are affected by habitat complexity, and that competition between species may modify the species distribution where they co-occur.     

Understanding of the impact of chemicals on amphibians: a meta-analytic review

Many studies have assessed the impact of different pollutants on amphibians across a variety of experimental venues (laboratory, mesocosm, and enclosure conditions). Past reviews, using vote-counting methods, have described pollution as one of the major threats faced by amphibians. However, vote-counting methods lack strong statistical power, do not permit one to determine the magnitudes of effects, and do not compare responses among predefined groups. To address these challenges, we conducted a meta-analysis of experimental studies that measured the effects of different chemical pollutants (nitrogenous and phosphorous compounds, pesticides, road deicers, heavy metals, and other wastewater contaminants) at environmentally relevant concentrations on amphibian survival, mass, time to hatching, time to metamorphosis, and frequency of abnormalities. The overall effect size of pollutant exposure was a medium decrease in amphibian survival and mass and a large increase in abnormality frequency. This translates to a 14.3% decrease in survival, a 7.5% decrease in mass, and a 535% increase in abnormality frequency across all studies. In contrast, we found no overall effect of pollutants on time to hatching and time to metamorphosis. We also found that effect sizes differed among experimental venues and among types of pollutants, but we only detected weak differences among amphibian families. These results suggest that variation in sensitivity to contaminants is generally independent of phylogeny. Some publication bias (i.e., selective reporting) was detected, but only for mass and the interaction effect size among stressors. We conclude that the overall impact of pollution on amphibians is moderately to largely negative. This implies that pollutants at environmentally relevant concentrations pose an important threat to amphibians and may play a role in their present global decline.     

Competition, predation and species responses to environmental change

Despite much effort over the past decade on the ecological consequences of global warming, ecologists still have little understanding of the importance of interspecific interactions in species responses to environmental change. Models predict that predation should mitigate species responses to environmental change, and that interspecific competition should aggravate species responses to environmental change. To test this prediction, we studied how predation and competition affected the responses of two ciliates, Colpidium striatum and Paramecium tetraurelia , to temperature change in laboratory microcosms. We found that neither predation nor competition altered the responses of Colpidium striatum to temperature change, and that competition but not predation altered the responses of Paramecium tetraurelia to temperature change. Asymmetric interactions and temperature-dependent interactions may have contributed to the disparity between model predictions and experimental results. Our results suggest that models ignoring inherent complexities in ecological communities may be inadequate in forecasting species responses to environmental change.     

Intra- and interspecific competition among coexisting lotic snails

The competitive interactions of two lotic snails, Elimia cahawbensis and Elimia carinifera , were examined in a second-order spring-fed stream. We first demonstrated food limitation in laboratory microcosms where snails grew faster when exposed to enhanced periphyton levels. We then tested the magnitude and relative strengths of intra- and interspecific competition in similar stream and laboratory mesocosm experiments. Treatments were maintained in Plexiglas enclosures over a 7-week period with 0, 1×, 2× and 4× ambient biomass of each species alone, as well as mixed species treatments at 2× and 4× ambient. Snail responses to treatments were almost identical in field and laboratory experiments. Growth rates of both species were reduced by increased density of snails indicating strong intra- and interspecific competition among E. cahawbensis and E. carinifera . An analysis of the strengths of intra- and interspecific competition indicated minimal differences for either species, implying a lack of competitive dominance. Although periphyton biomass was generally highest without snails, there was little difference in periphyton biomass and snail production over the four-fold density range, regardless of species composition. These results suggest that E. cahawbensis and E. carinifera are functionally redundant with density-dependent responses in growth rate resulting in similar grazing pressure across a density gradient. This clearly demonstrates that species impact is not necessarily reflected by measures of abundance or biomass, and that secondary production should be considered.     

Quantitative Genetics of Response to Competitors in Nemophila Menziesii: A Field Experiment

Recent investigations of evolution in heterogeneous environments have begun to accommodate genetic and environmental complexity typical of natural populations. Theoretical studies demonstrate that evolution of polygenic characters depends heavily on the genetic interdependence of the expression of traits in the different environments in which selection occurs, but information concerning this issue is scarce. We conducted a field experiment to assess the genetic variability of the annual plant Nemophila menziesii in five biotic regimes differing in plant density and composition. Significant, though modest, additive genetic variance in plant size was expressed in particular treatments. Evidence of additive genetic tradeoffs between interspecific and intraspecific competitive performance was found, but this result was not consistent throughout the experiment. Two aspects of experimental design may tend to obscure genetically based tradeoffs across environments in many previously published experiments: (1) inability to isolate additive genetic from other sources of variation and (2) use of novel (e.g., laboratory) environments.     

Experimental examination of the effects of ultraviolet-B radiation in combination with other stressors on frog larvae

Ultraviolet-B radiation (UVB) is a ubiquitous stressor with negative effects on many aquatic organisms. In amphibians, ambient levels of UVB can result in impaired growth, slowed development, malformations, altered behavior and mortality. UVB can also interact with other environmental stressors to amplify these negative effects on individuals. In outdoor mesocosm and laboratory experiments we studied potential synergistic effects of UVB, a pathogenic fungus, Batrachochytrium dendrobatidis (Bd), and varying temperatures on larval Cascades frogs (Rana cascadae). First, we compared survivorship, growth and development in two mesocosm experiments with UVB- and Bd-exposure treatments. We then investigated the effects of UVB on larvae in the laboratory under two temperature regimes, monitoring survival and behavior. We found reduced survival of R. cascadae larvae with exposure to UVB radiation in all experiments. In the mesocosm experiments, growth and development were not affected in either treatment, and no effect of Bd was found. In the laboratory experiment, larvae exposed to UVB demonstrated decreased activity levels. We also found a trend towards reduced survival when UVB and cold temperatures were combined. Our results show that amphibian larvae can suffer both lethal and sublethal effects when exposed to UVB radiation.     

Field experimentation in meiofaunal ecology

Field manipulations with meiofauna have become popular research tools recently. Over 40 experiments manipulating meiofauna have been conducted including natural experiments. Investigator induced experiments include pollution experiments, colonization (non-pollution) experiments, trophic-interaction experiments, organism-substrate experiments and others. We review the contributions of these investigations.To adequately interpret manipulative experiments we provide a protocol and procedures section which emphasizes what is necessary for experimentation with meiofauna. These include: establishing an hypothesis, experimental design, controls, time scale of the experiment, taxon in question, replicating and coupling laboratory and field experiments.We suggest several areas where we feel meiofauna manipulations will provide fruitful research answers in the future and urge meiofauna experimentation in the following areas: food for higher trophic levels; habitat complexity: prey refuges; pollution ecology; competition; suspension: drift of meiofaunal taxa and nutrient remineralization. None of these research areas are easy, but with ingenuity should provide exciting and provocative areas for research.     

Species interactions and thermal constraints on ant community structure

Patterns of species occurrence and abundance are influenced by abiotic factors and biotic interactions, but these factors are difficult to disentangle without experimental manipulations. In this study, we used observational and experimental approaches to investigate the role of temperature and interspecific competition in controlling the structure of ground‐foraging ant communities in forests of the Siskiyou Mountains of southwestern Oregon. To assess the potential role of competition, we first used null model analyses to ask whether species partition temporal and/or spatial environments. To understand how thermal tolerances influence the structure of communities, we conducted a laboratory experiment to estimate the maximum thermal tolerance of workers and a field experiment in which we added shaded microhabitats and monitored the response of foragers. Finally, to evaluate the roles of temperature and interspecific competition in the field, we simultaneously manipulated shading and the presence of a dominant competitor (Formica moki). The foraging activity of species broadly overlapped during the diurnal range of temperatures. Species co‐occurrence patterns varied across the diurnal temperature range: species were spatially segregated at bait stations at low temperatures, but co‐occurred randomly at high temperatures. The decreased abundance of the co‐occurring thermophilic Temnothorax nevadensis in shaded plots was a direct effect of shading and not an indirect effect of competitive interactions. Thermal tolerance predicted the response of ant species to the shading experiment: species with the lowest tolerances to high temperatures showed the greatest increase in abundance in the shaded plots. Moreover, species with more similar thermal tolerance values segregated more frequently on baits than did species that differed in their thermal tolerances. Collectively, our results suggest that thermal tolerances of ants may mediate competitive effects in habitats that experience strong diurnal temperature fluctuations.     

Implications of scale for patterns and processes in stream ecology

Abstract Although the scale-dependence of ecological patterns and processes is recognized by freshwater ecologists, current knowledge of scale effects is rudimentary and non-quantitative. We review issues of spatial and temporal scale in this paper to highlight conceptual problems relating to scale and some potential solutions. We present examples of how the spatial scale of a study influences observed patterns and their interpretation, and discuss how the size of an experimental arena influences the degree to which the dynamics of studied populations are influenced by exchange processes (immigration and emigration). The results of small-scale field experiments in streams will often be strongly influenced by the per capita exchange rates of organisms and differences in exchange rates may explain differences in the perceived effects of stream manipulations across scales. Spatial extent also influences the amount of spatial heterogeneity within a study site or arena, with important consequences for the outcome of predator-prey interactions. We suggest that changes in the availability of prey refuges may help explain why predator manipulations in streams appear to weaken as arena size increases. We also recommend that new techniques for decomposing and quantifying spatial heterogeneity be applied to characterize scale-dependent variation in freshwater systems. Lastly, we discuss the pitfalls of mismatching the temporal scale of experiments and models. Models incorporating spatial heterogeneity and the behaviour of organisms are needed to predict the short-term outcome of perturbations in streams, whereas models predicting long-term dynamics will need to integrate the impacts of episodic disturbance and all life history stages of organisms. In general, we recommend that freshwater ecologists undertake more multi-scale sampling and experimentation to examine patterns and processes at multiple scales, and make greater attempts to match the scales of their observations and experiments to the characteristic scales of the phenomena that they investigate.     

Defensive tradeoffs are not prerequisites to plant diversity in a two species model

Trait‐based theories of biodiversity consider interspecific tradeoffs among species‐specific traits as prerequisites to maintaining community evenness, a component of species diversity. Such tradeoffs are commonly observed in plant communities, particularly in relation to traits associated with resistance to herbivory. Indeed, global experiments show that interspecific tradeoffs are common between plant defense and growth or competitive ability; however, the positive effects of herbivory on plant diversity predicted by theories with trait‐based tradeoffs are far less commonly observed. Moreover, both the overall and relative importance of these tradeoffs in promoting plant diversity are not well known. To disentangle the relationships among growth, competition, and defense in relation to plant community evenness, we built a model that describes the effects of a shared herbivore on two plant species with the potential to differ in each of these traits. While tradeoffs between plant defense and growth or competitive ability can increase plant diversity via evenness, this is not always the case nor is it a requirement for increased diversity. Herbivores may increase plant diversity even in the absence of defensive tradeoffs, preferentially consuming apparently maladapted species, by limiting the negative effects of interspecific interactions. Therefore, the importance of defensive tradeoffs in increasing diversity may not be as important, or as straightforward, as previously hypothesized.     

Separating the effects of intra- and interspecific age-structured interactions in an experimental fish assemblage

We documented patterns of age-structured biotic interactions in four mesocosm experiments with an assemblage of three species of co-occurring fishes from the Florida Everglades, the eastern mosquitofish (Gambusia holbrooki), sailfin molly (Poecilia latipinna), and bluefin killifish (Lucania goodei). These species were chosen based on their high abundance and overlapping diets. Juvenile mosquitofish and sailfin mollies, at a range of densities matching field estimates, were maintained in the presence of adult mosquitofish, sailfin mollies, and bluefin killifish to test for effects of competition and predation on juvenile survival and growth. The mesocosms held 1,200 l of water and all conditions were set to simulate those in Shark River Slough, Everglades National Park (ENP), USA. We placed floating mats of periphyton and bladderwort in each tank in standard volumes that matched field values to provide cover and to introduce invertebrate prey. Of 15 possible intra- and interspecific age-structured interactions, we found 7 to be present at the densities of these fish found in Shark River Slough marshes. Predation by adult mosquitofish on juvenile fish, including conspecifics, was the strongest effect observed. We also observed growth limitation in mosquitofish and sailfin molly juveniles from intra- and interspecific competition. When maintained at high densities, juvenile mosquitofish changed their diets to include more cladocerans and fewer chironomid larvae relative to low densities. We estimated size-specific gape limitation by adult mosquitofish when consuming juvenile mosquitofish and sailfin mollies. At high field densities, intraspecific competition might prolong the time period when juveniles are vulnerable to predation by adult mosquitofish. These results suggest that path analysis, or other techniques used to document food-web interactions, must include age-specific roles of these fishes.     

Having the guts to compete: how intestinal plasticity explains costs of inducible defences

Predators commonly induce phenotypic changes that make prey better at surviving predation at the cost of reduced growth. While we have a good understanding of how trait changes affect predation risk, we lack a mechanistic understanding of why predator‐induced phenotypes differ in growth. Using two mesocosm experiments, we combined phenotypic plasticity theory with predictions from optimal digestion theory to demonstrate that intra‐ and interspecific competition induced relatively long guts while predators induced relatively short guts. The longer guts induced by competition appear to be an adaptive response that allows more efficient digestion and more rapid growth whereas the shorter guts induced by predators appear to result from a tradeoff of building larger tails in predator environments at the cost of smaller bodies. By combining these two bodies of theory, we now have a much better understanding of the mechanisms that cause the phenotypic trade‐offs that select for inducible defences.     

Competition with stone crabs drives juvenile spiny lobster abundance and distribution

Interspecific competition is assumed to have a strong influence on the population dynamics of competing species, but is not easily demonstrated for mobile species in the wild. In the Florida Keys (USA), anecdotal observations have long pointed to an inverse relationship in abundance of two large decapod crustaceans found co-occurring in hard-bottom habitat, the stone crab Menippe mercenaria and the Caribbean spiny lobster Panulirus argus. We used them to explicitly test whether competition for a renewable resource (shelter) can drive the abundance and distribution of the inferior competitor. We first explored this relationship in shelter competition mesocosm experiments to determine the competitively dominant species. Results showed that stone crabs are clearly the dominant competitors regardless of the number of lobsters present, the presence of co-sheltering species such as the spider crab, Damithrax spinosissimus, or the order of introduction of competitors into the mesocosm. We also found that lobsters use chemical cues from stone crabs to detect and avoid them. We then tested the ramifications of this competitive dominance in the field by manipulating stone crab abundance and then tracking the abundance and distribution of spiny lobsters through time. Increased stone crab abundance immediately resulted in decreased lobster abundance and increased aggregation. The opposite occurred on sites where stone crabs were removed. When we stopped removing stone crabs from these sites, they soon returned and lobster abundance decreased. This study explicitly demonstrated that interspecific competition can drive population dynamics between these species, and ultimately, community composition in these shallow water habitats.     

Evidence for competition between mudsnails (Hydrodiidae): a field experiment

The paper describes an experimental investigation of competition between Hydrobia ulvae and H. ventrosa using enclosed populations at a site at which the species coexist naturally. lntraspecific competition is more intense than interspecific competition and may have a regulatory influence on snail densities. Other experimental studies of competition between mudsnails are reviewed and found to infer strongly the occurrence of competition in natural field populations. However, features of the species' field distributions previously attributed to interspecific competition can be more convincingly explained by other processes.     

Understanding the consequences of changing biodiversity on rocky shores: How much have we learned from past experiments?

Manipulative experiments are increasingly used to establish causal connections between biodiversity and productivity or other measures describing the functioning of ecological systems. Many studies have detected significant effects of changing the number of species in experimental treatments on productivity and other response variables, but interpretation of these results is complicated by difficulties in separating effects of number, identity and relative density of species. A variety of experimental designs has been proposed to circumvent these problems, but no approach has been developed to address all these problems simultaneously. Rocky shore ecology has contributed relatively little to this debate, despite a long tradition of experimental manipulations of species in this environment. Here, I illustrate exemplars of biodiversity and biodiversity-like experiments in rocky shore habitats, to derive lessons for future experimental analyses of biodiversity in marine coastal systems. It is found that the principles underlying modern experiments on biodiversity have been available for a long time, being developed in studies of competition among intertidal gastropods more than 20 years ago, but have been largely overlooked by aquatic and terrestrial ecologists. Building on these principles, I propose a design of experiment that can discriminate among effects of number, identity and density of species in highly diverse assemblages. Possible applications of this approach to the analysis of biodiversity and its effects on the ability of assemblages to withstand disturbances are discussed.