David Wooster

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.

Meta-analysis: synthesizing research findings in ecology and evolution

The growing number of empirical studies performed in ecology and evolution creates a need for quantitative summaries of research domains to generate higher-order conclusions about general trends and patterns. Recent developments In meta-analysis (the area of statistics that is designed for summarizing and analyzing multiple independent studies) have opened up new and exciting possibilities. Unlike more traditional qualitative and narrative reviews, meta-analysis allows powerful quantitative analyses of the magnitude of effects and has a high degree of objectivity because it is based on a standardized set of statistical procedures. The first pioneering applications in ecology and evolution demonstrate that meta-analysis is both tractable and powerful.

A review of the drift and activity responses of stream prey to predator presence

In streams the impact that predators have on prey density is unclear. A recent review suggests that predators have on average a small to moderate negative effect on prey density and that predatory invertebrates have a stronger impact than predatory vertebrates. We suggest that a reason for this difference between predatory invertebrates and vertebrates is differences in prey emigration responses to the two types of predators. If prey increase their emigration rates in the presence of predators, predator impacts on local prey density will be strong; conversely, if prey decrease their emigration rates in the presence of predators, prey density might actually build up in patches containing predators and predator impact on prey density will be weak or even positive. We reviewed 22 studies that examined the impact that predatory vertebrates and invertebrates have on prey drift and activity (crawling and rates of emergence from refuge). Our review revealed that predatory invertebrates cause prey to increase drift rates more often than expected by chance. Predatory vertebrates had variable effects on prey drift rates (sometimes increasing and sometimes decreasing prey drift rates), but they caused prey to decrease activity more often than expected by chance. These results provide a potential mechanism for the relatively large impact that predatory invertebrates have on prey density as compared to the impact that predatory vertebrates have. We suggest directions for future research that include examining the impact that both types of predators have on prey emigration behavior and benthic density in the same system

Predator impacts on stream benthic prey

The impact that predators have on benthic, macroinvertebrate prey density in streams is unclear. While some studies show a strong effect of predators on prey density, others show little or no effect. Two factors appear to influence the detection of predator impact on prey density in streams. First, many field studies have small sample sizes and thus might be unable to detect treatment effects. Second, streams contain two broad classes of predators, invertebrates and vertebrates, which might have different impacts on prey density for a variety of reasons, including availability of refuge for prey and prey emigration responses to the two types of predators. In addition, predatory vertebrates have more complex prey communities than predatory invertebrates; this complexity might reduce the impact that predatory vertebrates have on prey because of indirect effects. I conducted a meta-analysis on the results of field studies that manipulate predator density in enclosures to determine (1) if predators have a significant impact on benthic prey density in streams, (2) if the impacts that predatory invertebrates and vertebrates have differ, and (3) if predatory vertebrates have different impacts on predatory prey versus herbivorous prey. The results of the meta-analysis suggest that on average predators have a significant negative effect on prey density, predatory invertebrates have a significantly stronger impact than predatory vertebrates, and predatory vertebrates do not differ in their impact on predatory versus herbivorous invertebrate prey. Three methodological variables (mesh size of enclosures, size of enclosures, and experimental duration) were examined to determine if cross correlations exist that may explain the differences in impact between predatory invertebrates and vertebrates. No correlation exists between mesh size and predator impact. Over all predators, no correlation exists between experimental duration and predator impact; however, within predatory invertebrates a correlation does exist between these variables. Also, a correlation was found between enclosure size and predator impact. This correlation potentially explains the difference in impact between predatory invertebrates and predatory vertebrates. Results of the meta-analysis suggest two important areas for future research: (1) manipulate both types of predators within the same system, and (2) examine their impacts on the same spatial scale.

Amphipod (Gammarus minus} responses to predators and predator impact on amphipod density

Abstract Recent theoretical work suggests that predator impact on local prey density will be the result of interactions between prey emigration responses to predators and predator consumption of prey. Whether prey increase or decrease their movement rates in response to predators will greatly influence the impact that predators have on prey density. In stream systems the type of predator, benthic versus water-column, is expected to influence whether prey increase or decrease their movement rates. Experiments were conducted to examine the response of amphipods (Gammarus minus) to benthic and water-column predators and to examine the interplay between amphipod response to predators and predator consumption of prey in determining prey density. Amphipods did not respond to nor were they consumed by the benthic predator. Thus, this predator had no impact on amphipod density. In contrast, amphipods did respond to two species of water-column predators (the predatory fish bluegills, Lepomis macrochirus, and striped shiners, Luxilus chrysocephalus) by decreasing their activity rates. This response led to similar positive effects on amphipod density at night by both species of predatory fish. However, striped shiners did not consume many amphipods, suggesting their impact on the whole amphipod “population” was zero. In contrast, bluegills consumed a significant number of amphipods, and thus had a negative impact on the amphipod “population”. These results lend support to theoretical work which suggests that prey behavioral responses to predators can mask the true impact that predators have on prey populations when experiments are conducted at small scales.

Chironomid responses to spatially structured, dynamic, streambed landscapes

Abstract We used a repeated-measures factorial field experiment to ask whether the arrangement and stability of leafy debris affect the density of chironomids in leaf packs in dynamic, streambed landscapes. We replaced natural debris in 24 streambed landscapes with leaf packs suspended from wires that snagged organic debris as it moved downstream. We arranged 64 wires (each with 3 leaf packs) in each landscape so that they were distributed among 8 large (aggregated) or 64 small (fragmented) patches. We manipulated landscape stability by removing accumulated debris from 0% (stable), 50% (semi-stable), or 100% (unstable) of the patches in each landscape 3 d after a flood. We determined chironomid density in leaf packs 2 d before the flood, 3 d after the flood, and 5 d after manipulating stability (8 d after the flood). We mapped the debris in each landscape and used multiple regression to evaluate the effect of landscape characteristics on density. Mean patch area was significantly smaller, and the number of patches, landscape perimeter, and landscape perimeter to area (P:A) ratio were significantly greater in fragmented than in aggregated landscapes. Chironomid density was significantly higher in fragmented than in aggregated landscapes and significantly lower in all landscapes after the flood. Density was ∼30% higher in unstable landscapes than in stable or semi-stable landscapes after debris removal. Density was negatively correlated with patch area, and debris accumulation may have caused patches to lose their value as refugia by making patches too large for chironomids to enter. Thus, disturbances that maintain fragmented patch arrangements with small patches and high landscape P:A ratios may be critically important for retention of individuals and to population persistence in dynamic landscapes.

Habitat partitioning by chironomid larvae in arrays of leaf patches in streams

Abstract We used 2 approaches to investigate possible mechanisms underlying positive responses of chironomids to fragmentation of resources in streambeds. We manipulated arrays of leaf patches on a sandy streambed and asked whether responses of chironomids to patch arrangement and leaf species in patches were genus or instar specific. We manipulated densities of Chironomus riparius instars I and III in laboratory microcosms and assessed density dependent dispersal and settlement behaviors. In streambed arrays, chironomid densities in leaf packs were ∼2× higher in fragmented than in aggregated arrays, but density was not affected by leaf species. Density of larvae in sand cores did not differ with respect to patch arrangement or leaf species. Genus and instar composition differed between sand cores and leaf packs, but did not differ with respect to leaf patch arrangement or leaf species in arrays. Resource partitioning in streambed arrays appeared to occur among congeneric instars rather than among genera. In laboratory microcosms, instar III larvae dispersed from high-quality patches into poor-quality patches when larval density was high, and the presence of instar III larvae strongly inhibited settlement of instar I larvae into high-quality patches. We concluded that recruitment of larvae to leaf patches from the regional species pool (drift) probably was random, but intraspecific competitive interactions and density dependent dispersal may be important mechanisms structuring chironomid assemblages in leaf patches. Therefore, larval abundances should be high in streambeds where resources such as leaf patches have spatial arrangements that reduce density dependent dispersal and recruitment inhibition, mitigate the effects of dispersal, and enable congeneric instars to partition the streambed habitat.

Prey dispersal and predator impacts on stream benthic prey

The impact that predators have on local invertebrate prey density in streams varies between different types of predators as well as across different types of prey. Insights into mechanisms generating this variation can be gained by considering dispersal responses of prey to predators. If predators induce increases in prey dispersal rates then local prey density will decline. In contrast, if predators suppress prey dispersal rates then this will tend to increase prey density. In this chapter we review evidence revealing that prey dispersal responses to predators are important in driving patterns in predator impact on local prey density in streams. Future studies on predator impact on local prey density should examine the dispersal responses of prey to determine the degree to which dispersal generates patterns of predator impact on local prey density. Finally, we show that the spatial scale at which a study is conducted will have a strong influence on the degree to which prey dispersal influences predator impact on prey density. Small-scale studies will reflect mostly prey dispersal effects. In contrast, large-scale studies will reflect mostly direct (consumptive) effects of predators on prey density.

Impact of season-long water abstraction on invertebrate drift composition and concentration

Surface water abstraction from rivers for irrigated agriculture is one of the largest uses of freshwater resources in the world. Water abstraction has important impacts on the structure of riverine assemblages. However, little work has examined the chronic, season-long impacts on ecosystem functions. Invertebrate drift is an important ecosystem function of river systems influencing nutrient cycling, food webs, and invertebrate population dynamics. We examined the season-long impact of reduced discharge resulting from multiple points of abstraction on drift assemblage composition, concentration, and total drift load. Early in the season, water abstraction had little impact on drift assemblage composition. However, later in the irrigation season, the drift assemblage at sites impacted by water abstraction diverged from upstream, control sites. The degree of change in assemblage composition at impacted sites was related to the amount of water abstracted such that sites with the lowest discharge also had assemblages that differed most strongly from control sites. Drift assemblages at impacted sites became dominated by tolerant microcrustaceans. In addition, water abstraction resulted in an increase in drift concentration (ind./m3). However, despite this increase in concentration at impacted sites, total drift load (# of invertebrates drifting in the river) decreased with decreasing discharge.

Effects of Temperature on Growth Rate and Behavior of Epeorus albertae (Ephemeroptera: Heptageniidae) Nymphs

ABSTRACT Anthropogenic disturbances affect temperature in river systems. Temperature potentially affects life histories of macroinvertebrates and alters behavior and biological functions. Temperature preferences and tolerance ranges for key taxa are therefore critical for understanding impacts of human-induced changes to water temperatures on river ecosystems. The objective of this study was to examine the effect of water temperature on growth rate and behavior of Epeorus albertae (McDunnough) nymphs. Nymphs were collected from the Umatilla River in eastern Oregon, and exposed to temperatures of 18, 22, and 28°C. Nymphs held at 28°C exhibited increased growth rates compared with individuals held at 18 and 22°C. However, at 28°C the accumulation of nymphal tissues was not consistent with that of nymphs held in lower temperatures; ratios of head capsule width to total body length were significantly lower in individuals at 28°C compared with those held at the lower temperatures. This indicates that the nymphs held at the high temperature had longer total body length relative to the developmental stage, represented by head capsule width, when compared with insects in cooler temperatures. To examine the effect of water temperature on behavior, active drift of mayflies was examined in experimental chambers held at 12, 18, 22, and 28°C. The number of drifting insects observed was significantly higher at 28°C compared with 22, 18, and 12°C. These results indicate that temperature is a factor influencing growth and behavior of E. albertae and is likely to lead to limitations in habitat use of this mayfly.