Relative Importance Of Predation Research Articles

Context-dependence of abiotic and biotic factors influencing performance of juvenile clams

Post-settlement survival and growth of bivalves can be limited by abiotic and biotic factors, both of which are spatially variable. Rarely has the importance of these factors been tested concurrently in the field. Our study spanned three spatial scales in estuarine waters of Washington state (Region: north vs. south; Within-region: fresher vs. saline; Within-site: mid- vs. low-tidal elevation). Predator access and sediment conditions were manipulated in a crossed experimental design, with juvenile (3-mm and 6-mm) Manila clams (Ruditapes philippinarum) outplanted in open- or closed-top mesh tubes. We found differences between treatments that appeared only at sites with cancrid crabs, suggesting that predators, rather than emigration, likely reduced numbers of clams in open tubes. We had hypothesized that clams at lower tidal elevations, which experience longer immersion times, would show improved growth but reduced survivorship because of greater exposure to marine predators. However, these patterns were evident at only one of three sites (lower-elevation treatments were lost at the fourth). The larger size class of clams was more tolerant of abiotic stressors at all sites, but the magnitude of difference in survival between size classes was sometimes dependent on other treatments. The maximum predator effect on survival was 74% (north, high salinity, low-intertidal site), whereas the maximum abiotic effect appeared as 62% lower survival and 59% slower growth for 3-mm clams at another site (north, fresher). In laboratory trials, high water temperatures (28–32 °C) and low salinity (5–15) acted synergistically to cause juvenile clam (6–12 mm) mortality, whereas clams tolerated each of these stressors alone. Context-dependence in the relative importance of predation and abiotic stressors was apparent in our results, but contrary to expectations, abiotic stressors did not characterize southern or fresher sites. Instead, extreme high temperatures occurred at the site with the wide tidal flat rather than in the south, and the within-region salinity differences appeared not to exceed tolerances of juvenile clams.

Influences of Lake Trout (Salvelinus namaycush) and Mysis diluviana on kokanee (Oncorhynchus nerka) in Lake Pend Oreille, Idaho

Research on Lake Pend Oreille, Idaho, has focused on the influence of two potential limiting factors for kokanee Oncorhynchus nerka (Walbaum, 1792): competition for food with Mysis diluviana (Loven, 1862, hereafter Mysis) and predation by lake trout Salvelinus namaycush (Walbaum, 1792). Population fluctuations of Mysis and lake trout have resulted in substantial heterogeneity in food web conditions, apparently altering both bottom-up and top-down dynamics. Therefore, relative importance of predation and competition were evaluated as drivers of kokanee abundance, biomass, and production. A series of general linear models was used to evaluate relative influences of Mysis and lake trout on kokanee. Kokanee production was a density-dependent process and the collapse of Mysis corresponded to an increase in the modeled maximum annual production of kokanee from 224 tonnes to 408 tonnes. Lake trout also negatively influenced kokanee biomass. A Mysis-mediated, predator-induced kokanee biomass collapse occurred when lake trout and Mysis abundances were both high. Sustainable management of this fishery requires recognition that competition with Mysis will define the scope of kokanee production and therefore the scope of sustainable predation.

How will sea-level rise affect threats to nesting success for Seaside Sparrows?

ABSTRACT Sea-level rise (SLR) threatens the nesting success of salt marsh breeding birds, including Seaside Sparrows (Ammodramus maritimus), by increasing the magnitude and frequency of extreme high tides that flood nests. However, the threat to nesting success from tidal flooding is intertwined with that of predation because the threats are connected through a trade-off along a nest height gradient. Therefore, to understand the risk to nesting success from SLR, it is necessary to consider predation threats simultaneously. I used an individual-based model of Seaside Sparrow nesting behavior, calibrated using empirical data on nest success rates and nest-site selection behaviors, to project the effects of SLR conditions on the relative importance of predation and flooding threats in affecting nesting success, and to investigate whether nest-site selection along a gradient of nest height can modulate the risk of SLR. Outputs from the model revealed that present-day levels of predation risk pose as great a r...

Predation or Scavenging? Prey Body Condition Influences Decision‐Making in a Facultative Predator, the Wolverine

The interaction between predators and their prey is a key factor driving population dynamics and shaping wildlife communities. Most predators will scavenge in addition to killing their own prey, which alters predation effects and implies that one cannot treat these as independent processes. However, the relative importance of predation vs. scavenging and the mechanisms driving variation of such are relatively unstudied in ecological research on predator-prey relationships. Foraging decisions in facultative predators are likely to respond to environmental conditions (e.g., seasonality) and inter- or intraspecific interactions (e.g., prey availability, presence of top predators, scavenging competition). Using data on 41 GPS-collared wolverines (Gulo gulo) during 2401 monitoring days, in four study sites in Scandinavia, we studied variation in diet and feeding strategies (predation vs. scavenging), along a gradient of environmental productivity, seasonality, density, and body mass of their main prey, semidomestic reindeer (Rangifer tarandus). The most important factor affecting the relative extent of predation and scavenging was mean prey body mass. Predation was more pronounced in summer, when vulnerable reindeer calves are abundant, and individual kill rates were negatively related to local reindeer body mass. This relationship was absent in winter. The probability of scavenging was higher in winter and increased with decreasing local reindeer body mass, likely as a response to increased carrion supply. Wolverine feeding strategy was further influenced by predictable anthropogenic food resources (e.g., slaughter remains from hunted ungulates) and the presence of a top predator, Eurasian lynx (Lynx lynx), which provided a continuous carrion supply promoting scavenging. Our results suggest that wolverine feeding strategies are flexible and strongly influenced by seasonally dependent responses to prey body condition in combination with carrion supply. This study demonstrates that large-scale environmental variation can result in contrasting predator feeding strategies, strongly affecting trophic interactions and potentially shaping the dynamics of ecological communities.

Predation or scavenging? Prey body condition influences decision‐making in a facultative predator, the wolverine

AbstractThe interaction between predators and their prey is a key factor driving population dynamics and shaping wildlife communities. Most predators will scavenge in addition to killing their own prey, which alters predation effects and implies that one cannot treat these as independent processes. However, the relative importance of predation vs. scavenging and the mechanisms driving variation of such are relatively unstudied in ecological research on predator–prey relationships. Foraging decisions in facultative predators are likely to respond to environmental conditions (e.g., seasonality) and inter‐ or intraspecific interactions (e.g., prey availability, presence of top predators, scavenging competition). Using data on 41 GPS‐collared wolverines (Gulo gulo) during 2401 monitoring days, in four study sites in Scandinavia, we studied variation in diet and feeding strategies (predation vs. scavenging), along a gradient of environmental productivity, seasonality, density, and body mass of their main prey, semidomestic reindeer (Rangifer tarandus). The most important factor affecting the relative extent of predation and scavenging was mean prey body mass. Predation was more pronounced in summer, when vulnerable reindeer calves are abundant, and individual kill rates were negatively related to local reindeer body mass. This relationship was absent in winter. The probability of scavenging was higher in winter and increased with decreasing local reindeer body mass, likely as a response to increased carrion supply. Wolverine feeding strategy was further influenced by predictable anthropogenic food resources (e.g., slaughter remains from hunted ungulates) and the presence of a top predator, Eurasian lynx (Lynx lynx), which provided a continuous carrion supply promoting scavenging. Our results suggest that wolverine feeding strategies are flexible and strongly influenced by seasonally dependent responses to prey body condition in combination with carrion supply. This study demonstrates that large‐scale environmental variation can result in contrasting predator feeding strategies, strongly affecting trophic interactions and potentially shaping the dynamics of ecological communities.

Parallel and nonparallel behavioural evolution in response to parasitism and predation in Trinidadian guppies.

Natural enemies such as predators and parasites are known to shape intraspecific variability of behaviour and personality in natural populations, yet several key questions remain: (i) What is the relative importance of predation vs. parasitism in shaping intraspecific variation of behaviour across generations? (ii) What are the contributions of genetic and plastic effects to this behavioural divergence? (iii) And to what extent are responses to predation and parasitism repeatable across independent evolutionary lineages? We addressed these questions using Trinidadian guppies (Poecilia reticulata) (i) varying in their exposure to dangerous fish predators and Gyrodactylus ectoparasites for (ii) both wild-caught F0 and laboratory-reared F2 individuals and coming from (iii) multiple independent evolutionary lineages (i.e. independent drainages). Several key findings emerged. First, a population's history of predation and parasitism influenced behavioural profiles, but to different extent depending on the behaviour considered (activity, shoaling or boldness). Second, we had evidence for some genetic effects of predation regime on behaviour, with differences in activity of F2 laboratory-reared individuals, but not for parasitism, which had only plastic effects on the boldness of wild-caught F0 individuals. Third, the two lineages showed a mixture of parallel and nonparallel responses to predation/parasitism, with parallel responses being stronger for predation than for parasitism and for activity and boldness than for shoaling. These findings suggest that different sets of behaviours provide different pay-offs in alternative predation/parasitism environments and that parasitism has more transient effects in shaping intraspecific variation of behaviour than does predation.

Foraging and predation risk for larval cisco (Coregonus artedi) in Lake Superior: A modelling synthesis of empirical survey data

The relative importance of predation and food availability as contributors to larval cisco (Coregonus artedi) mortality in Lake Superior were investigated using a visual foraging model to evaluate potential predation pressure by rainbow smelt (Osmerus mordax) and a bioenergetic model to evaluate potential starvation risk. The models were informed by observations of rainbow smelt, larval cisco, and zooplankton abundance at three Lake Superior locations during the period of spring larval cisco emergence and surface-oriented foraging. Predation risk was highest at Black Bay, ON, where average rainbow smelt densities in the uppermost 10 m of the water column were >1000ha−1. Turbid conditions at the Twin Ports, WI-MN, affected larval cisco predation risk because rainbow smelt remained suspended in the upper water column during daylight, placing them alongside larval cisco during both day and night hours. Predation risk was low at Cornucopia, WI, owing to low smelt densities (<400ha−1) and deep light penetration, which kept rainbow smelt near the lakebed and far from larvae during daylight. In situ zooplankton density estimates were low compared to the values used to develop the larval coregonid bioenergetics model, leading to predictions of negative growth rates for 10mm larvae at all three locations. The model predicted that 15mm larvae were capable of attaining positive growth at Cornucopia and the Twin Ports where low water temperatures (2–6°C) decreased their metabolic costs. Larval prey resources were highest at Black Bay but warmer water temperatures there offset the benefit of increased prey availability. A sensitivity analysis performed on the rainbow smelt visual foraging model showed that it was relatively insensitive, while the coregonid bioenergetics model showed that the absolute growth rate predictions were highly sensitive to input parameters (i.e., 20% parameter perturbation led to order of magnitude differences in model estimates). Our modelling indicated that rainbow smelt predation may limit larval cisco survival at Black Bay and to a lesser extent at Twin Ports, and that starvation may be a major source of mortality at all three locations. The framework we describe has the potential to further our understanding of the relative importance of starvation and predation on larval fish survivorship, provided information on prey resources available to larvae are measured at sufficiently fine spatial scales and the models provide a realistic depiction of the dynamic processes that the larvae experience.

Diversity, Functional Similarity, and Top-Down Control Drive Synchronization and the Reliability of Ecosystem Function

The concept that diversity promotes reliability of ecosystem function depends on the pattern that community-level biomass shows lower temporal variability than species-level biomasses. However, this pattern is not universal, as it relies on compensatory or independent species dynamics. When in contrast within-trophic level synchronization occurs, variability of community biomass will approach population-level variability. Current knowledge fails to integrate how species richness, functional distance between species, and the relative importance of predation and competition combine to drive synchronization at different trophic levels. Here we clarify these mechanisms. Intense competition promotes compensatory dynamics in prey, but predators may at the same time increasingly synchronize, under increasing species richness and functional similarity. In contrast, predators and prey both show perfect synchronization under strong top-down control, which is promoted by a combination of low functional distance and high net growth potential of predators. Under such conditions, community-level biomass variability peaks, with major negative consequences for reliability of ecosystem function.

The Relative Importance of Predators and People in Structuring and Conserving Ecosystems

The Relative Importance of Predators and People in Structuring and Conserving Ecosystems

Predator effects on prey population dynamics in open systems

Animal population dynamics in open systems are affected not only by agents of mortality and the influence of species interactions on behavior and life histories, but also by dispersal and recruitment. We used an extensive data set to compare natural loss rates of two mayfly species that co-occur in high-elevation streams varying in predation risk, and experience different abiotic conditions during larval development. Our goals were to generate hypotheses relating predation to variation in prey population dynamics and to evaluate alternative mechanisms to explain such variation. While neither loss rates nor abundance of the species that develops during snowmelt (Baetis bicaudatus) varied systematically with fish, loss rates of the species that develops during baseflow (Baetis B) were higher in streams containing brook trout than streams without fish; and surprisingly, larvae of this species were most abundant in trout streams. This counter-intuitive pattern could not be explained by a trophic cascade, because densities of intermediate predators (stoneflies) did not differ between fish and fishless streams and predation by trout on stoneflies was negligible. A statistical model estimated that higher recruitment and accelerated development enables Baetis B to maintain larger populations in trout streams despite higher mortality from predation. Experimental estimates suggested that predation by trout potentially accounts for natural losses of Baetis B, but not Baetis bicaudatus. Predation by stoneflies on Baetis is negligible in fish streams, but could make an important contribution to observed losses of both species in fishless streams. Non-predatory sources of loss were higher for B. bicaudatus in trout streams, and for Baetis B in fishless streams. We conclude that predation alone cannot explain variation in population dynamics of either species; and the relative importance of predation is species- and environment-specific compared to non-predatory losses, such as other agents of mortality and non-consumptive effects of predators.

Survivorship of Immature Stages of &lt;I&gt;Anopheles gambiae&lt;/I&gt; s.l. (Diptera: Culicidae) in Natural Habitats in Western Kenya Highlands

We examined the survivorship of Anopheles gambiae s.l. Giles (Diptera: Culicidae) larvae and habitat productivity in three major habitat types in the western Kenya highlands. The age-specific distribution was determined for larvae and pupae, and survivorship curves were constructed. Larval-to-pupal survivorship was 6.8% in drainage ditches, 4.3% in cow hoofprints, and 1.8% in disused goldmines, respectively. High mortality rates were observed in all developmental stages. The estimated daily survival rate was highest in drainage ditches (0.74), followed by cow hoofprints (0.71), and it was lowest in disused goldmines (0.62). Productivity of emerging An. gambiae adults was generally low in these larval habitats (1.35, 1.55, and 1.84 mosquitoes per m2 per wk in drainage ditches, disused goldmines, and cow hoofprints, respectively). In total, seven families of larval mosquito predators were identified from the larval habitats, including Hydrophilidae, Dytiscidae, Corixidae, Nepidae, Notonectidae, Belostomatidae, and Cordulidae. Predator density in disused goldmines was significantly higher than that of other habitat types. Determination of the relative importance of predation, habitat stability and food contents on natural mosquito habitat productivity would help to design cost-effective vector control methods specifically targeted at the productive habitats.

Relative importance of predation and intraspecific competition in regulating growth and survival of juveniles of the soft-shell clam, Mya arenaria L., at several spatial scales

Predation appears to be the single most important biotic factor regulating populations of bivalves in estuarine and coastal soft sediments. However, the relative roles of predation and intraspecific competition are rarely investigated simultaneously over different spatial scales, making generalities about these mechanisms difficult. Using juveniles of the soft-shell clam, Mya arenaria (initial mean shell length [SL] ± 95% CI = 12.4 ± 0.13 mm), I tested the interactive effects of predator deterrence and intraspecific density (660 vs. 1320 individuals m − 2 ) on growth and survival responses over a 185-day period from May to November 2003 at spatial scales that spanned four orders of magnitude: embayments, sites within embayments, tidal gradients, and blocks that were 10,000's, 1000's, 100's, and 5 m apart, respectively. Replicate field experiments were conducted from May to November 2003 at the upper and lower tidal heights at each of two intertidal mud flats (sites) within each of two embayments (Passamaquoddy Bay [PB] and Cobscook Bay [CB]) in eastern Maine. Mean survival, relative growth, and the abundance of wild recruits each varied significantly over all spatial scales. Predation was the most important factor affecting clam survival, explaining 45% of the total variability, whereas embayment, sites within embayments, tidal gradient, and intraspecific density collectively accounted for less than 10% of the variation. At all four intertidal sites, clam survival in experimental units designed to deter predators averaged 72%, but the degree of enhancement varied between embayments (PB = 61%; CB = 267%). Average survival rate was higher (by 12–16%), but growth was slower (by ca. 50%) in upper vs. lower intertidal blocks; however, the patterns differed for both variables between sites within each embayment. The effect of increasing intraspecific clam density was to lower survival by ca. 17% (56% [660 m − 2 ] vs. 48% [1320 m − 2 ]) in both embayments, but growth was unaffected. Overall, clams doubled in SL, although mean relative growth was 15% greater in CB than PB. Tidal gradient, sites within embayments, and blocks were the three most important factors explaining 35%, 19%, and 22% of total variation in relative clam growth, respectively. In Maine and the northeast US, juveniles of Mya reach their highest abundance above mean low tide levels. Experimental evidence presented here suggests that differential predation along the tidal gradient is the dominant factor controlling clam abundance and distribution patterns in the intertidal zone.

Honey‐Making Bee Colony Abundance and Predation by Apes and Humans in a Uganda Forest Reserve1

ABSTRACTHoney‐making bee colonies in Bwindi Impenetrable National Park were investigated with Batwa Pygmies locating 228 nests of Apis and five stingless bees (Meliponini). The relative importance of predation, food supply, nesting site, and elevation affecting abundance were studied for meliponines in particular. Nest predation and overall nest abundance had no correlation with elevation along a 1400 m gradient, nor did flowering phenology or pollen collection. Many suitable, large trees were unoccupied by bee nests. In 174 ha of forest plots, 2 Meliponula lendliana, 13 M. nebulata, 16 M. ferruginea, 16 M. bocandei, and 20 Apis mellifera adansonii nests occurred, suggesting a habitat‐wide density of 39 nests/km2. Compared to other studies, Ugandan Meliponini were uncommon (0.27 colonies/ha, tropical mean = 1.9/ha), while Apis mellifera was numerous (0.12 nests/ha, tropical mean = 0.06/ha), despite park policy allowing humans to exploit Apis. Meliponine colony mortality from predators averaged 12 percent/yr and those near ground were most affected. Tool‐using humans and chimpanzees caused 82 percent of stingless bee nest predation. Selective factors affecting nest heights and habit may include auditory hunting by predators for buzzing bees, and indirect mutualists such as termites that leave potential nesting cavities. Mobility and free‐nesting by honey bee colonies should enable rapid community recovery after mortality, especially in parks where human honey hunting is frequent, compared to sedentary and nest‐site‐bound Meliponini.

EFFECTS OF PREDATION, FLOODING, AND CONTAMINATION ON REPRODUCTIVE SUCCESS OF CALIFORNIA CLAPPER RAILS (RALLUS LONGIROSTRIS OBSOLETUS) IN SAN FRANCISCO BAY

Abstract We assessed the reproductive success of the California Clapper Rail (Rallus longirostris obsoletus), an endangered subspecies restricted to San Francisco Bay, and the relative importance of predation, flooding, and contaminants as factors affecting that success. Our study was conducted in six tidal marshes in the northern and southern reaches of San Francisco Bay. This assessment, conducted in four breeding seasons (1991, 1992, 1998, 1999), determined that productivity of California Clapper Rails was much reduced over the natural potential. Only 69% of clapper rail eggs whose viability could be assessed were viable. Hatchability of eggs in North Bay and South Bay marshes was 65% and 70%, respectively. Only 45% of the nests successfully hatched at least one egg. Despite mean clutch sizes of 6.7 and 6.9 in the North and South bays, respectively, clapper rails produced only 1.9 and 2.5 young per nesting attempt. Flooding was a minor factor, reducing the number of eggs available to hatch by only 2.3%...

Effects of Predation, Flooding, and Contamination on Reproductive Success of California Clapper Rails (Rallus Longirostris Obsoletus) in San Francisco Bay

Abstract We assessed the reproductive success of the California Clapper Rail (Rallus longirostris obsoletus), an endangered subspecies restricted to San Francisco Bay, and the relative importance of predation, flooding, and contaminants as factors affecting that success. Our study was conducted in six tidal marshes in the northern and southern reaches of San Francisco Bay. This assessment, conducted in four breeding seasons (1991, 1992, 1998, 1999), determined that productivity of California Clapper Rails was much reduced over the natural potential. Only 69% of clapper rail eggs whose viability could be assessed were viable. Hatchability of eggs in North Bay and South Bay marshes was 65% and 70%, respectively. Only 45% of the nests successfully hatched at least one egg. Despite mean clutch sizes of 6.7 and 6.9 in the North and South bays, respectively, clapper rails produced only 1.9 and 2.5 young per nesting attempt. Flooding was a minor factor, reducing the number of eggs available to hatch by only 2.3%. Predation on eggs was a major factor affecting nest success, reducing productivity by a third. Failed eggs were examined for abnormal development and contaminant concentrations. Contamination appeared to adversely influence California Clapper Rail reproductive success, as evidenced by deformities; embryo hemorrhaging; embryo malpositions; a depressed rate of hatchability; excess concentrations of mercury, barium, and chromium over known avian embryotoxic thresholds; and a correlation of deformities with elevated concentrations of some trace elements in eggs that failed to hatch. Mercury was the only significant contaminant common to all marshes. Efectos de la Depredación, las Inundaciones y la Contaminación sobre el Éxito Reproductivo de Rallus longirostris obsoletus en la Bahía de San Francisco

Estimating multivariate selection gradients in the fossil record: a naticid gastropod case study

The purpose of this paper is to explore the possibility and utility of estimating multi- variate selection in fossil assemblages, using naticid gastropods as a case study. We used the pres- ence or the absence of a naticid borehole as an index of survival with respect to drilling attacks, enabling us to estimate the multivariate selection gradient exerted by this predator on the shell length and shell thickness of two bivalve genera, Astarte and Spisula. We hypothesized that naticid selection pressure would favor the survival of large, thick-shelled bivalve prey throughout the Ce- nozoic. Differential survival of prey was recorded over geologic time using processed bulk assemblages from the Miocene, Pliocene, and Pleistocene Epochs. Multivariate logistic regressions were per- formed by time period to determine if length and thickness were important factors affecting sur- vival. The direction and magnitude of selection on length and thickness for the two genera ranged from zero (no selection) to large positive or negative values. Only two selection coefficients were significant after sequential Bonferroni corrections: thin-shelled Astarte survived substantially bet- ter than thick-shelled Astarte during the Pleistocene ( b avggrad 52 1.23) and large Spisula survived slightly better than small Spisula during the Miocene Epoch (bavggrad 5 0.05). This is the first study using fossils to calculate multivariate selection gradients. It suggests that naticids were not necessarily strong agents of selection on two traits previously thought to be im- portant to survival of drilling attacks for two of their common prey species. We also show that multivariate selection gradient estimates differ from traditional predation intensity estimates but are superior for estimating the magnitude and direction of natural selection because they use dif- ferential mortality of different prey phenotypes rather than just absolute mortality from predation. This work is especially significant for research that involves estimating the relative importance of predation (naticid or otherwise) as an evolutionary force and will be useful for fossil studies where differential survival can be recorded.

Changes in soil arthropod functional group in a wheat crop under conventional and no tillage systems in Argentina

Different functional groups of soil arthropodofauna present in the agro-ecosystem can be severely modified by tillage practices. The abundance of different trophic groups subject to conventional tillage (CT) and no tillage (NT) practices were evaluated compared to a natural field boundary (FB) in a wheat crop. Arthropods were captured using pitfall traps and collected every 20 days during 10 months, and grouped according to their habits in predators, phytophagous and detritivorous. Tillage systems affected the abundance of arthropod fauna and the proportion between different functional groups as regards a FB. Predators constituted the most abundant group of all arthropods captured, and their number was higher under NT than under CT. In CT, an increase in predators was observed only in spring, probably associated with a recolonisation from the adjacent plots of NT. Phytophagous was the least representative group in the three evaluated systems, it was higher under cropped plots (NT and CT) than in the FB, and was not affected by tillage practices. Under NT, the number of predators, remained higher along the crop development. Phytophagous activity in no till plots remains constant. In FB, the number of captured individuals was lower and relatively stable along the whole sampling period than in the cultivated plots. Ploughing action (CT plots) provoked a decrease in the relative importance of predators and an increase in phytophagous as regards those in NT and FB. Total biomass of captured arthropod fauna showed significant differences between treatments, being higher in NT and lower in CT plots. It is concluded that tillage systems affect not only the abundance of arthropod fauna but also the proportion between different functional groups. The consequences of these changes for soil quality are discussed later on.

Effects of removing duck-nest predators on nesting success of grassland songbirds

During 1995 and 1996 we monitored natural and simulated nests of grassland songbirds following removal of duck-nest predators, primarily raccoons (Procyon lotor), striped skunks (Mephitis mephitis), and red foxes (Vulpes vulpes), in North Dakota, U.S.A. We were unable to detect an effect of predator removal on survival of natural (n = 153 nests) or simulated (n = 2264 nests) nests of grassland songbirds in either year. However, the assemblage of predators destroying simulated nests differed between removal and non-removal sites in 1995, when medium-sized carnivores and small mammals were the most important predators on non-removal sites, whereas ground squirrels were most important on removal sites. Our study indicates that grassland songbirds were not negatively affected, at least in the short term, by removal of duck nest predators. However, changes in the relative importance of predators on removal sites in 1995 suggest that smaller predators that were not targeted by removal efforts (i.e., ground squirrels) may have caused compensatory predation. Long-term studies are necessary to determine if the removal of duck-nest predators affects the numerical or functional response of predators at a lower trophic level, such as ground squirrels and other small mammals.

The relative importance of predation and competition in two reef fishes.

Competition and predation may both strongly influence populations of reef fishes, but the importance of these processes relative to one another is poorly understood. I quantified the effects of predation and competition on the growth and survival of two temperate reef fishes, Lythrypnus dalli and Coryphopterus nicholsii, in field experiments in which I manipulated the densities of the two species and the abundance of predators (using exclosure cages) on small replicate patch reefs. I also evaluated the influence of predators on the behavior of the two species to help interpret the mechanisms of any predatory influences on growth or survival. Predation was much more important than competition (inter- or intraspecific) in Lythrypnus. For Coryphopterus, neither competition nor predation were particularly important. Behaviorally, both species responded to predators by reducing foraging rate and hiding. This altered behavior, however, had no repercussions for growth or survival of Coryphopterus. In contrast, Lythrypnus grew more slowly and suffered greater mortality when exposed to predators. Interspecific competition did not significantly influence either species. Intraspecific competition did not affect the growth of Coryphopterus, but survival tended to be lower at high densities. Growth of Lythrypnus was depressed by intraspecific competition, but survival was not, except that, in the presence of predators, survival was density dependent. In contrast to the historical emphasis placed on the role of competition, this study indicates that predation can be more important than competition in determining patterns of abundance of some reef fishes. For example, predators not only influenced foraging of both Lythrypnus and Coryphopterus, but they also reduced growth and survival of Lythrypnus, and therefore appear to help maintain the marked habitat segregation between the two species.

Applications of the Menge‐Sutherland Model to Acid‐Stressed Lake Communities

The predictions of the Menge‐Sutherland model with respect to the effects of environmental stress on species richness, food web complexity, food chain length, and the relative importance of predation, competition, and disturbance in structuring communities were examined using midsummer zooplankton communities of 46 lakes near Sudbury, Ontario with pHs of 3.8‐7.2. The lakes were sampled in the 1970s and in 1990 yielding a total of 92 food webs. Species richness increased linearly with pH rather than peaking at intermediate levels of stress as predicted by the model. Food web complexity and food chain length increased with pH, as predicted. The importance of competition in food web structure was variable at low pH and consistently high at moderate pH, a result that does not support model predictions that disturbance, competition, and predation should sequentially predominate with increasing pH. High variability in the relative importance of competition in acidic lakes was explained by fish presence or absence; in lakes without fish, a larger proportion of links in the zooplankton food web was due to predation by invertebrates. Persistence of predators (fish) in some acidic lakes reduces the applicability of the Menge‐Sutherland model.