摘要:

A recent model, designed with stream systems in mind, suggested that prey exchange (movement of prey among patches) tends to reduce predator impacts on prey density; that is, rapid prey immigration into a patch with predators can swamp local effects of predators on prey density. The previous model, however, included two assumptions that influence the model's qualitative predictions. First, it assumed that the system's focal patches are surrounded by an environment that has no predators and a constant prey density. More importantly, it assumed that prey do not alter their per capita emigration rates in response to the presence of predators. We extended the earlier model by relaxing these assumptions. Specifically, we: (1) addressed predator impacts in patches surrounded by background environments that do not have predators, (2) allowed the background environment to have a constant or decreasing prey density (i.e., we examined situations in which predators deplete prey), and (3) accounted for the fact that prey per capita emigration rates are often altered by the presence of predators. Our most interesting results concerned the profound effects of prey emigration behavior on the relationship between prey exchange and predator impact. If prey do not alter their emigration rates in response to predators, then, as predicted by the earlier model, high prey exchange should result in very low predator impact. If, however, prey disperse out of patches in response to the presence of predators (i.e., if prey per capita emigration rates are higher out of patches with predators than out of predator—free patches), then even very high prey exchange rates cannot swamp predator impact; instead, prey emigration adds to predator impact. Thus, depending on prey emigration behavior, increased overall prey exchange can either decrease or increase predator impact. Predators can also suppress prey emigration (e.g., if prey hide in refuges so effectively that they disperse at low rates from predator patches). In that case, high prey exchange rates tend to result in “negative predator impacts” (i.e., higher prey density in patches with predators). The details of the above relationships are influenced by whether or not the background environment has predators. If the background has predators, but predators do not deplete prey (e.g., if predation is offset by recruitment), then predator impact depends only on the ratio of per capita emigration rates out of predator and predator—free patches; that is, attack rates do not influence impact. In contrast, if predators can deplete prey, then attack rates influence predator impact. In that situation, if attack rates are highly relative to prey emigration rates out of predator—free patches, then predator impact steadily increases over time. A literature review suggested that prey alterations in per capita emigration rates in response to the presence of predators can potentially explain some surprising natural phenonomena, including the existence of “negative predator impacts,” and the apparent tendency for invertebrate predators (primarily, stoneflies) to have stronger impacts, and the apparent tendency for vertebrate predators (primarily, fish). Finally, we discussed possible adaptive links between prey escape success, refuge use, dispersal behavior, and predator impacts. This discussion raised a “paradox of danger” that due to their effects on prey exchange, more dangerous predators might often have unexpectedly weak effects on local prey density. In this context, we suggest a framework for studying relationships between prey behavior, prey exchange, and predator impacts.

ISSN:0012-9658    

DOI:10.2307/1937446

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Researchers have recently suggested that frequent prey dispersal into and out of an area can swamp the local effect on predation. Where prey are mobile, the extent of prey movement can therefore explain variation in the apparent effect of predators on local prey density. I compared the effect of the dominant predator in a temperate stream (brook charr, Salvelinus fontinalis Mitchill) on five insect prey taxa (mayflies: Ephemeroptera), to test the prediction that the extinct of density reduction should be less for prey taxa dispersing more frequently. The propensity of the mayflies to disperse by drifting downstream in the water column was measured in unmanipulated areas of the stream. Relative propensity to drift of the five mayflies was, from greatest to least: Baetis, Paraleptophlebia, Ephemerella, Eurylophella, Stenonema. I then tested effects of charr on the mayflies by manipulating charr density in fenced 35—m sections of the stream. Charr densities were adjusted to zero, medium, and high levels relative to natural densities. Rates of predation by charr in stream sections did not vary among the five mayfly taxa. Charr caused a large reduction in the density of Baetis, had a smaller effect on Paraleptophlebia, but had no detectable effect on the density of the other mayflies. Drift dispersal into the stream sections did not differ among charr densities. Effects of charr on prey densities could thus have been caused by direct predation or by increases in emigration from areas containing charr. Charr caused increased drift dispersal of Baetis and of Paraleptophlebia, but had no influence on drift of the other three mayflies. The reduction in density of Baetis by charr was due more to the charr—induced increase in drifting of Baetis than to direct predation on Baetis. The hypothesis that frequent prey dispersal swamps the effects of predators assumes that predators influence prey density primarily by consuming prey. In this system charr also influenced prey densities by causing increases in prey drift rates, affecting the mayflies that drifted most frequently. For this reason, the mayflies drifting less frequently were not, as predicted, the ones most strongly affected by charr. Variation in the effect of predators on prey density may thus be partially explained by both (1) changes in prey dispersal not related to predators, as proposed in the initial hypothesis, and (2) influences of predators on prey dispersal.

ISSN:0012-9658    

DOI:10.2307/1937447

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We quantified production dynamics, determined the trophic basis of production, and assessed the overlap in resource utilization within a diverse assemblage of mayfly larvae from the submerged woody (snag) habitat of a Coastal Plain blackwater river (Ogeechee River). Total annual production (as dry mass) was among the highest reported for mayflies, ranging from 20.8 to 42.2 g/m2of snag surface (7.4 to 12.3 g/m2of channel bottom) in two consecutive years. Relatively little change occurred between years for four out of six families. Biomass turnover rates (annual production/biomass) were generally high, ranging from 8 (single generation per year) to 96 (multiple generations per year) for individual taxa. Distinct types of temporal production patterns occurred: (1) concentration in winter (Ephemerella argo, E. dorothea, Eurylophella sp., and Isonychia), (2) concentration in summer (Baetis ephippiatus, Tricorythodes sp., Caenis spp.) or (3) spread throughout the year and peaking in summer (Baetis intercalaris, Stenonema modestum, S. integrum, S. exiguum, and Hexagenia sp.). Temporal overlap of production, using the proportional similarity index, was highest among closely related taxa (>0.60), with the least overlap (<0.15) among taxa in different families. Overlap based upon the amount that various food types contribute to production was high among all species, suggesting even less resource partitioning for food than for time. Of the annual food consumption (as dry mass) by mayflies per unit snag surface area (423 g°m—2°yr—1), °87% consisted of amorphous detritus that is rapidly replenished on snags from floodplain—derived seston. This food source accounted for °70% of total mayfly production. We suggest that a fluctuating habitat (snags), a continuously replenished food supply, high biomass turnover, and high drift densities help perpetuate a persistent, resilient, and diverse mayfly assemblage with little resource partitioning.

ISSN:0012-9658    

DOI:10.2307/1937448

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Along a gradient of increasing productivity in lakes, the abundance of perch (Perca fluviatilis) first increases and then decreases, whereas the abundance of ruffe (Gymnocephalus cernuus) and that of roach (Rutilus rutilus) continue to increase. Perch, unlike roach and ruffe, undergoes dramatic ontogenetic diet shifts. The decrease in the abundance of perch along the productivity gradient may be due to competition from roach at its plankton—feeding stage and from ruffe at its benthic—macroinvertebrate—feeding stage. Previous studies of these three species have focused on the effects of roach on the other species. To further examine the competitive relationship among these species, we studied the effect of increasing the density of ruffe, while maintaining a relatively high, constant density of roach, on the diet and growth of perch. Food resources were also monitored during this 2—mo experiment, which was conducted in 10 enclosures in a pond in southern Sweden. Based on prior experiments we expected roach, an efficient planktivore, to reduce the biomass of zooplankton in all enclosures. Moreover, we expected that the increased density of benthic—feeding ruffe would force perch to increase its consumption of zooplankton, resulting in reduced growth. We found, as predicted, that the biomass of cladocerans of copepods decreased over time but was not affected by ruffe density. The biomass of the commonly consumed macroinvertebrates Sialis (a megalopteran), trichopterans, and ephemeropterans was lower in enclosures with ruffe than in enclosures without ruffe. Moreover, the biomass of Sialis decreased and that of chironomids and the clam Pisidium increased over time. High ruffe densities were related to an increased proportion of zooplankton in the diet of perch in August, but not in September. The diet of ruffe changed little along the ruffe gradient. Only the proportion of trichopterans (in August) and Pisidium (in September) in the diet of ruffe increased along the ruffe gradient. The growth of both perch and ruffe decreased with increasing ruffe density, whereas the growth of roach was constant. These data suggest that one reason perch decrease in abundance along a productivity gradient may be because they are competitively sandwiched between plantivore and benthivore specialists.

ISSN:0012-9658    

DOI:10.2307/1937449

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Development of low oxygen conditions (hypoxia) in bottom waters of stratified lakes has a profound effect on the distribution of aquatic organisms. Fish avoid hypoxic waters by moving into oxygenated water and suffer massive die—offs when migration is not possible. However, central mudminnows (Umbridae: Umbra limi) were captured regularly in severely hypoxic bottom waters of a lake in northern Wisconsin, USA. Caging experiments confirmed the lethality of this environment to mudminnows. The presence of freshly ingested phantom midge larvae (Diptera: Chaoborus) in the stomachs of mudminnows during the day coupled with the observation that Chaoborus are restricted to hypoxic waters during the day indicate that mudminnows routinely ventured into a lethal environment to forage. This conclusion was supported by caging experiments whereby mudminnows denied access to hypoxic water during the day did not contain Chaoborus in their stomachs. In laboratory experiments, ingestion of air bubbles enhanced the ability of mudminnows to forage in hypoxic water, but even in the absence of surface access, mudminnows readily foraged in hypoxic waters. These data help explain reports of other fish venturing into hypoxic waters and suggest that foraging in hypoxic waters may be common among some fishes when food abundance is low in surface waters.

ISSN:0012-9658    

DOI:10.2307/1937450

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Fits of nonlinear difference equations to time series with observation errors were examined by stochastic simulation and analysis of plankton time series from two lakes. Even modest observation errors (e.g., coefficient of variation among replicate samples °10%) cause errors in model identification and bias in parameter estimates. The latter problem can be corrected by estimation techniques that account for observation error, but model identification is difficult unless the state variables are manipulated. Without manipulation, statistical criteria tend to favor linear models, even when data are simulated by nonlinear processes. Methods that account for observation error produced satisfactory fits to time series of edible algae from two lakes over 7 yr. In Paul Lake, which has not been manipulated, the best fitting model included linear growth, a linear functional response for grazing loss, and an autoregressive moving average model for the errors. In manipulated Tuesday Lake, the best fitting model included linear growth and a nonlinear functional response. Experimental manipulations, or other substantial perturbations, may be essential for detection of nonlinearities in ecological interactions.

ISSN:0012-9658    

DOI:10.2307/1937451

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Cascading trophic interactions are important in many freshwater pelagic food webs, but their importance in more complex, omnivore—rich littoral—zone food webs is less well known. We tested the existence of a trophic cascade involving omnivorous crayfish (Orconectes rusticus), macroinvertebrates, periphyton, and macrophytes using 9—m2cages in the littoral zone of Plum Lake, Wisconsin, USA. Treatments in the replicated (N = 4) experiment were crayfish enclosures, crayfish exclosures, and cageless references. During June—September, we measured macrophyte shoot numbers, macroinvertebrate numbers, and periphyton (on plastic strips) chlorophyll a, and dry mass (DM). We expected that crayfish foraging would directly reduce abundance and change species composition of macrophytes and macroinvertebrates and would indirectly enhance periphyton abundance by reducing the abundance of grazing snails. In enclosures, macrophyte and snail (but not nonsnail macroinvertebrate) densities declined significantly throughout the experiment, whereas densities of macrophytes, snails, and nonsnail macroinvertebrates increased in exclosures and cageless references. Some of the reduction in macrophytes resulted from nonconsumptive fragmentation of macrophytes by crayfish. Consistent with the cascading trophic interactions model, periphyton chlorophyll a per unit surface area increased in enclosures, but declined in exclosures. Periphyton quality (as indexed by chlorophyll a/DM) also increased in enclosures relative to exclosures and cageless references. However, because of large reductions in macrophyte surface area (which periphyton colonizes) in enclosures, total amount of periphyton chlorophyll a in enclosures (relative to exclosures) probably declined while periphyton quality per unit surface area and periphyton quality increased. Thus, the impacts of crayfish omnivory on periphyton, expressed in two conflicting indirect effects, confirm the possibility that omnivory can complicate cascading trophic predictions. Overall, results support the existence of strong trophic interactions in the littoral zone, in which omnivorous crayfish control abundance of macrophytes, snails, and periphyton.

ISSN:0012-9658    

DOI:10.2307/1937452

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Habitat differences in variables describing food web structure were examined using data from two food web compilations, taking into account differences between webs in species richness and using independent data points only. Log—log regressions of connectance, linkage density, and food chain length on species number were calculated. In most cases these relationships were significant. The residuals from the regression lines were used to analyze differences between habitats in connectance, linkage density, and food chain length. In one compilation of insect—dominated webs, aquatic habitats had comparatively high connectance and linkage density when species number had been accounted for. In the other compilation, marine pelagic webs had higher relative connectance, relative linkage density, and mean chain length. In addition, desert webs had higher relative connectance. Several of the results when taking species richness into account do not agree with previous studies. Therefore it is concluded that it is important in examinations of food web patterns to account for web species number and to use independent observations as far as possible.

ISSN:0012-9658    

DOI:10.2307/1937453

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Ecological processes often depend upon the patterning, as well as the absolute density, of resources. In this paper, we develop methods for describing pattern from the perspective of the organism encountering and exploiting the resources, and for reconstructing pattern from the description. The essence of our description is the “structure function,” which is the probability that a point r units away from the current point contains resources, conditional on the resource state of the current point. We first show how the structure function is determined from pattern and then describe an algorithm (the method of the “Force to be Full” for constructing pattern in any number of dimensions from a given structure function. We illustrate our ideas with empirical data for krill surveys and with simulated but complex three—dimensional patterns.

ISSN:0012-9658    

DOI:10.2307/1937454

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Periodic matrix models are used to describe the effects of cyclic environmental variation, both seasonal and interannual, on population dynamics. If the environmental cycle is of length m, with matrices B(1), B(2),...., B(m)describing population growth during the m phases of the cycle, then population growth over the whole cycle is given by the product matrix A = B(m)B(m—1)...B(1). The sensitivity analysis of such models is complicated because the entries in A are complicated combinations of the entries in the matrices B(i), and thus do not correspond to easily interpreted life history parameters. In this paper we show how to calculate the sensitivity and elasticity of population growth rate to changes in the entries in the individual matrices B(i)making up a periodic matrix product. These calculations reveal seasonal patterns in sensitivity that are impossible to detect with sensitivity analysis based on the matrix A. We also show that the vital rates interact in important ways: the sensitivity to changes in a rate at one point in the cycle may depend strongly on changes in other rates at other points in the cycle.

ISSN:0012-9658    

DOI:10.2307/1937455

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY