Göran Englund

Emergent impacts of multiple predators on prey

Although almost all prey live with many types of predator, most experimental studies of predation have examined the effects of only one predator at a time. Recent work has revealed new insights into the emergent impacts of multiple predators on prey and experimental studies have identified statistical methods for evaluating them. These studies suggest two main types of emergent effect-risk reduction caused by predator-predator interactions and risk enhancement caused by conflicting prey responses to multiple predators. Some theory and generalities are beginning to emerge concerning the conditions that tend to produce these two outcomes.

THE IMPORTANCE OF DATA-SELECTION CRITERIA: META-ANALYSES OF STREAM PREDATION EXPERIMENTS

The value of meta-analysis in ecology hinges on the reproducibility of patterns generated by quantitative synthesis. Meta-analysts will vary in the criteria they use to screen studies and select data within studies, even when addressing exactly the same question. We summarize some of the many decisions that an ecologist must make in deciding what data to include in a synthesis. We then show, using multiple meta-analyses taken from the same literature on stream predation experiments, that meta-analytic conclusions can be colored by selection criteria that are not specifically a function of the relevance of the data. As a consequence, we recommend that meta-analysts perform several meta-analyses using different selection criteria to examine the robustness of reported findings. We also advise ecological meta-analysts to minimize use of selection criteria that are based on judgments of study quality when extracting data from the literature, because of the potential for unconscious bias. The influence of quali...

Scale effects and extrapolation in ecological experiments

Abstract Most ecological experiments are performed on spatial and temporal scales that are much smaller and shorter than the systems and time frames of interest. Available data, however, suggest that experimental results often change with the size of the experimental arena and the duration of the experiment (i.e. are scale-dependent). As a consequence, the interpretation of experimental results often requires extrapolation from the limited spatial and temporal scales of experimental systems to the much larger and longer scales of natural systems. In this paper we discuss the implications of scale-dependence, particularly spatial scale-dependence, for the design and interpretation of experiments. We suggest that the problem of extrapolation across scales should be avoided when possible, either by matching the physical size of experimental units with the size of the system of interest or by designing small-scale experimental systems so that the processes of interest are given a realistic representation. When this is not possible it becomes necessary to translate experimental results to other scales, which requires that the mechanisms that generate scale-dependence are understood and that they can be incorporated into models that make predictions for other scales. We review and classify sources of scale-dependence in ecological responses to perturbations and describe attempts to incorporate these mechanisms into scaling models. Among the mechanisms we describe are exchange processes, nonlinear averaging in heterogeneous systems, and arena artifacts. At present, we do not know if available scaling models can make accurate quantitative extrapolations from experimental to natural scales. Thus, the primary, current value of scale models is the identification of scale ranges with particularly weak or strong scale-dependence. We also note that well-known statistical methods for design, parameter estimation and inference can be used as a framework for extrapolation in field experiments, in the sense that observations from a small number of experimental units can be used to draw conclusions about whole systems. We discuss the value of different statistical designs as tools for extrapolation and note that the choice of scale of an experiment is a critical design decision. The scale of a design is determined by grain (size of experimental units or blocks) and the extent or range covered by the design. The scale range, delimited by grain and extent, determines the scale of the background heterogeneity that can influence the strength of treatment effects. Moreover, both again and extent are related to the variance among experimental units, which means that the choice of scale influences the statistical power of a design as well as the magnitude of the aggregation error, a bias that can arise when the mean value of a set of measurements made in small experimental units are taken to represent a larger, more heterogeneous system.

Habitat specialization, body size, and family identity explain lepidopteran density–area relationships in a cross-continental comparison

Habitat fragmentation may strongly affect species density, species interactions, and the rate of ecosystem processes. It is therefore important to understand the observed variability among species responses to fragmentation and the underlying mechanisms. In this study, we compare density–area relationships (DARs) for 344 lepidopteran species belonging to 22 families (butterflies and moths). This analysis suggested that the DARslope is generally positive for moths and negative for butterflies. The differences are suggested to occur because moths are largely olfactory searchers, whereas most butterflies are visual searchers. The analysis also suggests that DARs vary as a function of habitat specialization and body size. In butterflies, generalist species had a more negative DARslope than specialist species because of a lower patch size threshold. In moths, the differences in DARslope between forest and open habitat species were large for small species but absent for large species. This difference is argued to occur because the DARslope in large species mainly reflects their search mode, which does not necessarily vary between moth groups, whereas the slope in small species reflects population growth rates.

Scaling up the functional response for spatially heterogeneous systems

Scale transition theory is a framework for predicting regional population dynamics from local process functions and estimates of spatial heterogeneity. Using this framework, we estimated regional scale functional responses for a benthic predator-prey system in the Baltic Sea. Functional responses were based on laboratory experiments or field observations of stomach contents, and prey densities measured at a local scale (0.1 m(2)) or a regional scale (300 km(2)). Laboratory data overestimated consumption at high prey densities, whereas predictions based on local scale data tallied closely with consumption observed at the regional scale. The predicted regional functional response was different for increasing and decreasing prey densities, reflecting that predator and prey densities, as well as the covariance between them, exhibit oscillatory dynamics. We conclude that it is important to validate laboratory data with small-scale field observations and that scale transition is a powerful tool for scaling-up process functions in heterogeneous systems.

Fighting and assessment in the net-spinning caddis larva Arctopsyche ladogensis: a test of the sequential assessment game

Eight qualitative predictions from the sequential assessment game were tested using net-spinning caddisfly larvae Arctopsyche ladogensis which fight over silk retreats and catchnets. The results agree with predictions regarding effects of relative size and ownership. Size was crucial for the outcome of contests, large specimens usually winning. Both contest duration and the standard deviation of contest durations were negatively correlated with the relative size of the opponents. Owners were more persistent than intruders, and they won most of the contests. In contests where both opponents regarded themselves as owners, contests lasted longer than owner-intruder ones. Contrary to predictions from the sequential assessment game, resource value in terms of amount of silk invested or amount of food obtained in the net did not influence contest outcome or duration. It is concluded that the sequential assessment game predicted the effects of relative size and ownership but not the effects of resource value.

Effects of fish on the local abundance of crayfish in stream pools

A survey of fish and crayfish distributions in pools in small Kentucky streams suggested that predatory fish have strong effects on the local distribution of crayfish. I tested this hypothesis by experimentally manipulating the density of fish in stream pools. The predator, green sunfish (Lepomis cyanellus), strongly reduced the density of crayfish (Cambarus bartoni). Prey movement rates into and out of pools were low compared to estimated predation rates: consequently. I concluded that observed reductions of crayfish densities by fish predators were due mainly to direct consumption. Estimated rates of predation and prey movements were combined into a model to simulate observed predator impact on crayfish in patches larger and smaller than the scale of the experiment. The simulation suggested that the results were robust to increases in spatial scale. Data from the survey showed that crayfish densities in fish pools and fishless pools only differed for pools deeper than approximately 30-50 cm, probably because shallow pools had fewer and smaller fish. Thus, shallow pools provide a refuge for crayfish against fish. Very shallow pools (10-20 cm) generally had low densities of crayfish. Possible mechanisms that could explain the low density of crayfish in shallow pools include stronger effects of floods and higher predation pressure from terrestrial predators in these pools.

Contrasting effects of anthropogenic and natural acidity in streams: a meta-analysis

Large-scale human activities including the extensive combustion of fossil fuels have caused acidification of freshwater systems on a continental scale, resulting in reduced species diversity and, in some instances, impaired ecological functioning. In regions where acidity is natural, however, species diversity and functioning seem to be less affected. This contrasting response is likely to have more than one explanation including the possibility of adaptation in organisms exposed to natural acidity over evolutionary time scales and differential toxicity due to dissimilarities in water chemistry other than pH. However, empirical evidence supporting these hypotheses is equivocal. Partly, this is because previous research has mainly been conducted at relatively small geographical scales, and information on ecological functioning in this context is generally scarce. Our goal was to test whether anthropogenic acidity has stronger negative effects on species diversity and ecological functioning than natural acidity. Using a meta-analytic approach based on 60 datasets, we show that macroinvertebrate species richness and the decomposition of leaf litter—an important process in small streams—tend to decrease with increasing acidity across regions and across both the acidity categories. Macroinvertebrate species richness, however, declines three times more rapidly with increasing acidity where it is anthropogenic than where it is natural, in agreement with the adaptation hypothesis and the hypothesis of differences in water chemistry. By contrast, the loss in ecological functioning differs little between the categories, probably because increases in the biomass of taxa remaining at low pH compensate for losses in functionality that would otherwise accompany losses of taxa from acidic systems. This example from freshwater acidification illustrates how natural and anthropogenic stressors can differ markedly in their effects on species diversity and one aspect of ecological functioning.

Small-scale spatial structure of Baltic Sea zoobenthos—inferring processes from patterns

In this study, we describe small-scale (6 cm–5 m) spatial patterns of four dominating infaunal taxa in the northern Baltic Sea, with the objective to construct hypotheses about pattern-generating processes. Two neighbouring sites, differing in their physical environment, were extensively sampled, and the distributions of adults and juveniles were analysed with spatial autocorrelograms. Relationships between taxa and size classes were studied with correlations at different scales. As similar spatial patterns and correlations appeared at both sites, this suggests that biotic interactions rather than physical factors were responsible for the patterns. We found that several taxa exhibited patchy distributions at the studied scales. In the case of juvenile Monoporeia affinis (Amphipoda) and juvenile Corophium volutator (Amphipoda), we propose that positive intragroup interactions were responsible for the patterns. The correlation analyses suggested that areas with high densities of adult Macoma balthica (Bivalvia) were avoided by juvenile conspecifics and adults of M. affinis and C. volutator, probably because adult M. balthica reduced the amount of detritus in the surficial sediment.

SCALE DEPENDENCE OF EMIGRATION RATES

In this paper, we examine how per capita emigration rates vary with patch size. Analyses of analytic diffusion models and lattice simulations show that the scale dependence of emigration rates is expected to differ between small and large patches. In large patches, per capita emigration rate (E) is given by E = kA−β. A is the area, k is a constant, and β, which describes the strength of the scale dependence, is given by β = 1−d/2, where d is the fractal dimension of the patch boundary. In small patches, the scale dependence is predicted to level off, and the value of the scaling coefficient (β) is influenced by details about how movement behaviors and mortality rates vary across patch boundaries. However, much of this variation can be explained as effects on the magnitude of per capita emigration rates. Analyses of published empirical studies of the scale dependence of emigration in terrestrial insects show that observed scaling coefficients are within the expected range (0 < β < 0.5), and that scaling coefficients decrease with decreasing scale. Moreover, we find that the fractal dimension of patch boundaries and the magnitude of emigration explain much of the observed variation between different patch networks, species, years, and sexes.