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Although ecological theory exists to predict dynamics in communities with intraguild predation (IGP), few empirical tests have examined this theory. IGP theory, in particular, predicts that when two competitors interact via IGP, with increasing resource productivity: (1) the IG predator will increase in abundance as the IG prey declines, and (2) increasing dominance of the IG predator will cause resource density to increase. Here, we provide a first test of these predictions in a field community consisting of a scale insect and its two specialist parasitoids, Aphytis melinus (the IG predator) and Encarsia perniciosi (the IG prey). The shared resource, California red scale, is a pest of citrus, and its productivity varies across a threefold range among citrus cultivars. We examined both absolute and relative densities of parasitoids along this natural gradient of scale productivity in three citrus cultivars (orange, grapefruit and lemon). Although both parasitoid species were found in all three cultivars, their abundances reflected those predicted by IGP theory: the IG prey species dominated at low productivity and the IG predator dominated at high productivity. This relationship was caused by an increase in Aphytis density with productivity. In addition, the density of scale increased with the dominance of the IG predator. These results from a field system demonstrate the important dynamic outcomes for food webs with IGP.

California red scale is suppressed to very low densities by the parasitoid Aphytis melinus. The system also appears stable. We report on an experimental test of the hypothesis that stability is caused by a refuge for scale. In a grapefruit grove in southern California in 1984-1985, the bark in the interior part of the tree provided a partial refuge from parasitism. Scale were -100 times denser there than in the exterior of trees. In a field experiment, we removed Argentine ants from some blocks of trees to test whether (1) ants caused the refuge by interfering with Aphytis and (2) the expected reduction in scale density in the refuge would lead to an unstable interaction in the exterior. We also tested for density- dependent parasitism, host mutilation, and predation by analyzing data from samples and from scale placed in the field. The temporal variability of the scale was at the low end of the range recorded in field populations. The experiment provided some evidence in support of the refuge hypothesis. The population in the refuge fluctuated much less than that in the exterior. Ant exclusion led to increased parasitism and lower scale density in the interior, and to increased fluc- tuations in abundance in the refuge and exterior. However, these changes were relatively small and perhaps temporary, suggesting that (1) ants are not the main cause of the refuge and that (2) we did not reduce the refuge density enough to determine whether the system would go unstable in the absence of the refuge population. Parasitism, host mutilation, and predation rates on scale showed no temporal density dependence, either direct or delayed, though detection of such patterns is difficult. Possible alternative stabilizing mechanisms include size-dependent interactions between red scale and Aphytis.

We analyze a metapopulation model of the interactions between Lotka-Volterra-type prey and predators that occur in two environmentally distinguishable patches and are linked by migration. Environmental differences between the patches tend to stabilize the otherwise neutrally stable model by causing the per capita immigration rate on a patch to be temporally density-dependent, partly as a consequence of out-of-phase fluctuations in density. However, the environmental differences can also lead to indirect effects on the temporal dependence of per capita prey death rate on prey density in each patch and on temporal dependence of per capita predator birthrate on predator density in each patch. Spatially density-dependent movement by the prey can be either uniformly destabilizing or initially stabilizing and then destabilizing as the degree of density dependence increases, depending on the overall rate of prey movement. Aggregation by the predator to the patch with more prey modifies one or more of the three processes listed above. Typically, weak aggregation is stabilizing, and strong aggregation is destabilizing. Aggregation can also render unstable an initially stable model. We conclude that metapopulation and single-population models are not good analogues of each other and that predator aggregation affects the two types of models via different mechanisms.

Patterns of growth, development, and reproduction have been observed in many Daphnia species, and there have been some attempts to explain them using models that take into account rates of intake, assimilation, maintenance, and energy allocation rules. We show, however, that existing models cannot capture some essential features of individual growth, especially under conditions of low food supply that are typical of field conditions. These features include: (1) a sigmoid growth curve, and (2) the time to starvation or the performance of individuals during periods of low food availability. We propose and test a new hypothesis based on the idea that allometric relationship for physiological rates are stage dependent. We show that ingestion rates increase much faster with juvenile body size than with adult body size for several Daphnia species. Existing data suggest that allometric relationships for respiration are not stage dependent, and we derive a maintenance function that takes into account overheads associated with growth and basal metabolic rates. The new allometric relationships for ingestion and maintenance, along with an accurate description of the onset of maturity and partitioning of energy between growth and reproduction, can account for the sigmoid growth pattern displayed by Daphnia. Existing models cannot explain Daphnia's performance when food availability is low, and this led us to examine how Daphnia stores energy and uses reserves. Our review synthesizes disparate observations on the structure and dynamics of reserves, and forms the basis for a new model of Daphnia pulex.

In a population convergence experiment, the initial densities of adults of the predatory backswimming bug Notonecta hoflmanni were set above and below a putative equilibrium density in stock tanks. The experiment was done at two constant rates of food supply (wingless Drosophila) for the larger instars (in natural pools Notonecta feed mainly on terrestrial arthropods that fall on the water surface). It was predicted that the densities of the resulting populations would converge on an equilibrium set by the rate of food supply for the larger instars. The tanks also contained zooplankton (mainly Daphnia), which were the main food supply of the smaller instars of Notonecta. The resulting over- wintering populations converged towards the appropriate equilibrium densities, via density- dependent and food-dependent fecundity and then cannibalism. However, the populations overshot their equilibria, producing overconvergence. In natural populations such over- convergence might tend to produce 2-yr cycles in abundance. Overconvergence resulted from the insensitivity of the survivorship of the original adults (at least over the short term) to differences in food supply between treatments, allowing them to continue to affect (via cannibalism and reproduction) the eventual density of the new overwintering popu- lation. Because Notonecta population density was determined by the externally supplied, locally uncoupled food supply, even though the early instars depended for food largely upon dynamic populations of zooplankton, the dynamics of the Notonecta population were simpler than a description of the food web might suggest.

Understanding spatial population dynamics is fundamental for many questions in ecology and conservation1,2,3,4. Many theoretical mechanisms have been proposed whereby spatial structure can promote population persistence, in particular for exploiter–victim systems (host–parasite/pathogen, predator–prey) whose interactions are inherently oscillatory and therefore prone to extinction of local populations5,6,7,8,9,10,11. Experiments have confirmed that spatial structure can extend persistence11,12,13,14,15,16, but it has rarely been possible to identify the specific mechanisms involved. Here we use a model-based approach to identify the effects of spatial population processes in experimental systems of bean plants (Phaseolus lunatus), herbivorous mites (Tetranychus urticae) and predatory mites (Phytoseiulus persimilis). On isolated plants, and in a spatially undivided experimental system of 90 plants, prey and predator populations collapsed; however, introducing habitat structure allowed long-term persistence. Using mechanistic models, we determine that spatial population structure did not contribute to persistence, and spatially explicit models are not needed. Rather, habitat structure reduced the success of predators at locating prey outbreaks, allowing between-plant asynchrony of local population cycles due to random colonization events.

STABLE predator–prey models with a tension between the stabilizing effect of prey density-dependence and the destabilizing effect of predators generally become unstable when the nutrient environment of the prey is enriched1–3. This 'paradox of enrichment' occurs in a wide range of models incorporating realistic features4–6. Enriched models quickly produce cycles with enormous amplitude and increased periods2,3,6–8. In real systems, however, there have been few tests for 'paradox' behaviour, and the few results are equivocal9–12. We have now tested for paradox behaviour in populations of the freshwater zooplankter Daphnia and their algal prey. In lakes and ponds these populations show both stable and cyclic dynamics13,14 caused by the interaction between the populations. The cycles are not driven by external forces14,15, but do not seem to be of the paradox-of-enrichment type: their amplitude is small, their period (equal to a Daphnia generation) is short, and there is good evidence that they are caused by the developmental delay of Daphnia14,15. Stage-structured models of the Daphnia–algae interaction indicate that such single-generation 'time-lag' cycles occur in the progression of instability from stable equilibrium to paradox cycles6,7, and that this progression may be driven by increasing nutrient levels7. We show here, however, that enrichment does not lead to paradox cycles, either in field or experimental Daphnia–algae systems. We suggest that concomitant changes in the predator–prey interaction accompanying enrichment could account for the absence of paradox behaviour.

Growth of the backswimmer Notonecta hoffmani is better on a diet of Daphnia pulex than on a diet of Ceriodaphnia reticulata. Offered a choice of these two zooplankton species, Notonecta selects Daphnia or large individuals of either species. To test whether this preference affects prey species and size-classes differently, we measured population changes when the predator and one prey species or the other were placed together in plastic tubs for 7-8 wk. Roughly half of the replicated laboratory populations of Daphnia pulex that were exposed to predation by a constant number and age-class of Notonecta were driven extinct within 8 wk. The remainder of these populations maintained densities similar to those of control populations without predators. Populations surviving at each sampling period partially compensated for mortality caused by predators by small increases in individual fecundity, which maintained their population birth rates at the same level as those of control populations. Fecundity was inversely related to the density of adult Daphnia and to the total biomass of Daphnia. In populations exposed to predation, the other compensatory mechanism was a decrease in the mortality of young from factors (probably starvation) other than predation. Predation reduced the average size of individuals in Daphnia populations and consequently markedly reduced the average biomass. This presumably increased the food per capita available to juveniles and small adults in predation treatments over the level available to their coun- terparts in controls, and hence both reduced mortality from starvation and increased size-related fecundity. Populations exposed to predation appeared to have stable dynamics for varying periods. However, as the experiment progressed, apparently random events in some populations caused the predation rate to exceed Daphnia's ability to compensate, and in each successive intersampling period a few populations were driven extinct. Extinction was probabilistic. Thus both a deterministic and a stochastic framework are required to explain the dynamics of these laboratory populations. Only two of 18 Ceriodaphnia populations were driven extinct. Notonecta instar II had the largest effect on the density of this species and also caused its temporal variability to increase. We have less information on the dynamics of this zooplankter than on D. pulex.

A general predator is assumed to divide its hunting time between two sub-habitats with different prey species, spending a larger fraction (φ) of search time in an area as the relative prey abundance there increases. This always causes switching in the model, and changes a functional response from one that imposes a risk on the average prey that decreases with prey density in the direction of one that imposes an increasing risk. I discuss the conditions for a response that is density dependent, and those predatory attributes that make such a response more likely. Transit time between subhabitats always increases the density dependent effect, and is necessary for “system stability” in a Lotka-Volterra model with two prey species. Experiments have confirmed the model's basic assumption. General predators do not fit easily into classical predator-prey models of simple “closed” communities, and then the degree of density dependence of the functional response becomes a useful measure of a predator's short-term stabilizing effect on a prey species. The model demonstrates how spatial heterogeneity can be stabilizing.

Predators and insect parasites (parasitoids) sometimes aggregate in patches containing high prey (host) densities because they search longer there, causing higher death (parasitism) rates in patches with more prey (hosts). This behavior can stabilize the otherwise unstable, discrete-time, Nicholson-Bailey model, typically at the cost of increasing the host equilibrium density. The key feature that produces both effects in this model appears to be the absence of within-generation dynamics; in particular, parasitoids are not able to reaggregate in response to changes in local host density within the generation. We examine the effects of aggregation on the otherwise neutrally stable Lotka-Volterra model. We allow the parasitoid to reaggregate continually as local host density changes. Aggregation appears as positive covariance between the parasitoid and host density across patches. It affects the model by altering the initially linear functional response. Aggregation always increases parasitoid efficiency an...

We consider a model for the functional response of a predator feeding on two species of prey. We assume that the predator searches randomly, at constant speed, for randomly distributed prey, but th...

To explore the idea that the dynamics of the zooplankter Daphnia and its algal food supply can be accounted for by simple predator-prey models, we tested the ability of such models to predict how the average densities change along a gradient of nutrient enrichment. Total algal carrying capacity is known to increase from nutrient-poor to nutrient-rich lakes. Here we show that this is true also for edible algae alone. We also provide evidence that the realized per capita growth rate of edible algae increases with trophy (trend 1). A synthesis of laboratory and field data suggests two other trends associated with increasing nutrient status: (2) Daphnia's attack rate decreases in response to increasing concentrations of inedible algae; and (3) its death rate increases in response to greater predation pressure. In models of the Lotka-Volterra type, either trend 2 or a combination of trends 1 and 3 leads to the prediction that the abundance of both Daphnia and their algal prey should increase along a nutrient g...

We investigated size—selective predation upon Daphnia pulex and D. magna by the mosquitofish (Gambusia). The independent variables were the relative size of predator and prey, prey density, and predator satiation. Gambusia actively chose to attack small prey (which we showed to be more profitable). Gambusia's preference increased monotonically, or decreased, or peaked at an intermediate prey size, depending upon fish size, prey size, and fish satiation level. These results contrast with the hypothesis the "gape—limited predators" select the largest prey available, and "size—limited predators" select small or intermediate—sized prey. Optimal diet theory predicts that a predator should drop less profitable prey items from the diet at higher prey densities. In contrast, Gambusia maintained a mixed diet and its preferences did not change in response to changes in either the absolute or the relative abundance of prey. Moreover, the ingestion rate would have been higher if less profitable (larger) prey types had been ignored. Gambusia did sharply alter its size—selective feeding behavior in response to its own satiation level, and we expect that this factor is correlated with prey density in the field. Well—fed fish concentrated their attacks, to a greater degree than starved fish, on small, more profitable prey. They also ingested less biomass per unit time. This last result suggests that maximizing food intake per unit time may not be the goal of well—fed fish, and that different criteria may determine the profitability of prey at different prey densities.

The conventional wisdom of biological control of insect pests, and its related ecological theory, is that successful natural enemies in long-lived ecosystems (1) impose a low, stable pest equilibrium, and (2) share the following properties: (a) host-specific; (b) synchronous with the pest; (c) can increase in density rapidly when the pest does; (d) need only one pest individual to complete their life cycle; (e) have a high search rate for the pest; (f) aggregate at areas of high pest density, which is thought to stabilize the interaction. These features are more characteristic of parasitoids than predators. We suggest that a stable pest equilibrium is neither a necessary nor a sufficient condition for control. We show that satisfactory control in model systems is compatible with both local extinction of the pest and polyphagy in the natural enemy. Only one of nine real examples of successful control is convincingly a stable interaction; the remainder show either strong evidence for instability and local e...

In laboratory experiments Notonecta hoffmani was size-selective, although prey size alone did not explain its preference between prey species. When Notonecta was fed mosquito larvae, its preference first increased and then decreased with increasing prey size. Its consistent preference for the larger size class of a given species of zooplankton presumably reflects the fact that all zooplankton were small relative to the predator. Preference was determined mainly by differences in the relative attack rates on the different prey, but in two instances relative success rates of attacks were equally or more important. Although differences in the frequency of encounter influenced attack rates, the main determinant appeared to be the differences in the predator’s response to different prey once they were encountered. The results suggest that the behavior underlying Notonecta’s preferences is stereotyped and inflexible. The results help explain the effects of Notonecta on field populations of prey.

We propose two measures of how stabilizing a functional response is. We suggest that predation be regarded as stabilizing at a prey density of H if the predation rate is increasing then-i.e., if f(H)/H is increasing where f is the functional response. This is equivalent to asking that an increase in prey density results in an increased chance of a given prey being killed by the predator. With Hm as the maximum value of H for which this criterion holds, our measures are Hm and f(Hm). We relate this criterion and these measures to local stability and also to structural stability in a modified Lotka-Volterra model and a general multispecies model. The criteria will be used in detailed models of switching in predators (in the following paper in this issue) and of patchiness.

If a predator eats more prey at higher prey densities, grows more as a consequence, and then kills more prey because of its larger size, this produces a developmental response to prey density. In its functional response a predator sea shore snail caused a decreasing percentage mortality as prey (barnacle) density increased (type 2 response). This causes instability in prey numbers. However, when account was taken of the developmental response in a hypothetical pseudosnail, the prey mortality caused over a year by a single—predator as a function of prey density, was different from that expected on the basis of the functional response alone. There is a tendency for the mortality to be a fairly constant percentage of prey density over a rather large range of prey densities. This results in the reduction of the destabilizing effect. Problems of integrating these responses with the numerical response and of generalizing the results are discussed. See full-text article at JSTOR

The predatory behavior of a ladybird, Coccinella, was examined as it preyed upon varying ratios of two aphid species, Acrythosiphon pisum and Aphis fabae. Experiments were done in petri dishes and on bean plants. The ladybirds attacked the two prey species in direct proportion to their relative abundance. There was little variability among predators in a given treatment. Training had no effect on the subsequent diets of the predators, and all treatments provided a very good fit to a simple no—switching" model. Predation on each species was density independent. This simple type of predation contrasts markedly with the results of earlier experiments on predatory snails in which variability among predators and a strong training effect produce switching. See full-text article at JSTOR

"Switching" in predators which attack several prey species potentially can stabilize the numbers in prey populations. In switching, the number of attacks upon a species is disproportionately large when the species is abundant relative to other prey, and disproportionately small when the species is relatively rare. The null case for two prey species can be written: P1/P2 = cN1/N2, where P1/P2 is the ratio of the two prey expected in the diet, N1/N2 is the ratio given and c is a proportionality constant. Predators were sea—shore snails and prey were mussels and barnacles. Experiments in the laboratory modelled aspects of various natural situations. When the predator had a strong preference (c) between prey the data and the "null case" model were in good agreement. Preference could not altered by subjecting predators to training regimens. When preference was weak the data did not fit the model replicates were variable. Predators could be trained easily to one or other prey species. From a number of experiments it was concluded that in the weak—preference case no switch would occur in nature except where there is an opportunity for predators to become trained to the abundant species. A patchy distribution of the abundant prey could provide this opportunity. Given one prey species, snails caused a decreasing percentage mortality as prey numbers increased. This occurred also with 2 prey species present when preference was strong. When preference was weak the form of the response was unclear. When switching occurred the percentage prey mortality increased with prey density, giving potentially stabilizing mortality. The consequences of these conclusions for prey population regulation and for diversity are discussed.

We examine how spatial subdivision of predator–prey systems affects colonization processes in metapopulations. Dynamics of the herbivorous spider mite Tetranychus urticae (prey) and the predatory mite Phytoseiulus persimilis are highly unstable on isolated bean plants (Phaseolus lunatus) and ultimately result in extinction of prey and predators. Assembling a collection of 90 plants without any dispersal barriers (a super-island experiment) does not modify the persistence of the predator–prey system. Subdividing the system into a metapopulation with barriers for dispersal (a collection of eight islands with 10 plants per island) leads to persistence of the predator–prey dynamics for many generations. In this paper, we use the time series of colonization events and prey and predator densities from the super-island and metapopulation experiments to understand how colonization processes of prey and predatory mites are altered by spatial subdivision. Using survival analysis, we estimate how prey and predator colonization probability is affected by densities of the colonist pool at different distances from the target plant. Contrasting the results from the super-island and metapopulation experiments reveals that spatial subdivision affects the discovery rate of prey outbreaks by predatory mites and differentially affects colonization by prey and predators. Prey colonization is primarily determined by local densities of prey in spatially subdivided systems, whereas predator colonization retains primarily “global” influences. Our analysis of colonization processes suggests mechanisms accounting for stability in the metapopulation experiments and provides the quantitative basis for the development of colonization functions to explore these mechanisms in predator–prey models of acarine systems.

SUMMARY (1) In laboratory experiments we estimated the attack rates and preferences of several instars of the predatory backswimmer, Notonecta hoffmani. The prey included mosquito larvae, surface prey, and three species of zooplankton: Ceriodaphnia reticulata, Daphnia pulex and Moina sp. Notonecta's predatory behaviour was shown to be stereotyped: neither level of hunger nor previous diet significantly influenced preference. Attack rate increased exponentially, and handling time decreased exponentially, with temperature. (2) These laboratory results and others reported elsewhere were used to explain the results of field experiments in which treatment prey populations in stock tanks on ranches in Southern California were exposed to predation by Notonecta populations at natural densities while controls were not. (3) In these field experiments, Notonecta strongly influenced the structure and dynamics of the community. It drove mosquito larvae and D. pulex extinct and reduced the density of Moina sp., C. reticulata, and several other species. Notonecta either destabilized its prey populations, by driving them extinct, or did not affect the degree of temporal variability. (4) We discuss the relationship of these results to prevailing views on the importance of predation by invertebrates in aquatic communities, extinction in natural communities, and the question of whether predators in general act to stabilize or destabilize prey populations. The qualitative results obtained in the field were well-predicted by the information obtained in the laboratory, but caution is needed in extrapolating estimates of absolute attack rates from the laboratory to the field.

SUMMARY (1) Growth rate and size at moult increased with prey density for Notonecta hoffmanni Hungerford nymphs raised at various densities of their mosquito larvae prey. (2) The functional response of different sized instar IV nymphs was type-2, and large predators ate more at high prey densities than did small predators. (3) The feeding rate of individual predators did not vary in any regular way during the instar. Predator size at the start of the instar was a good predictor of the predators' functional responses for both long- and short-term experiments. Short-term experiments were good predictors of the long-term functional responses: the relationship between these two responses remains linear over a wide range of prey densities. These facts allow us to predict the functional response over the whole instar from a knowledge of predator weight. (4) Variation in predator size within one instar (IV) was as important as differences in the predators' size in different instars in determining the predator's feeding rate.