Increasing Predator Density Research Articles

Primary-Productivity Gradients and Short-Term Population Dynamics in Open Systems

We present three models representing the trophic and behavioral dynamics of a simple food chain (primary producers, grazers, and predators) at temporal scales shorter than the scale of consumer reproduction, and at the spatial scales typically employed in field experiments. These models incorporate flexible behavioral responses of organisms to their predators and resources in spatially heterogeneous environments that are open to immigration and emigration. The basic models include passive immigration at all trophic levels, producer growth rates and losses to grazer consumption, grazer emigration rate as a behavioral response to producer and predator densities, grazer losses to predator consumption, and predator emigration as a function of grazer density. We model this system as: (1) a set of ordinary differential equations (“well-mixed model”); (2) a set of partial differential equations describing a population of discrete grazers foraging on discrete patches of primary producers (“discrete-grazer model”); and (3) a set of simulation rules describing the movement and foraging of individual grazers and the growth of primary producers on discrete patches in explicit space (“individual-based model”). The ordinary differential-equation models produced similar results to individual-based models with well-mixed producers, and the discrete-grazer and individual-based models produced similar results when grazers possessed a long-term memory of patch reward rates. The well-mixed and discrete-grazer models thus represent specific, limiting cases of the general individual-based model. Multiple equilibria and sustained oscillations are possible but are less likely in the discrete-grazer and individual-based models than in the well-mixed model, because localized foraging of discrete grazers leads to the rapid development of spatial heterogeneity in producer biomass and, hence, to a decrease in overall primary production. All models predict that stable equilibrium densities of all trophic levels increase with enrichment, provided grazers increase their emigration rates as predator density increases. If increasing predator density leads to decreasing grazer-emigration rates, predator and grazer densities increase, but producer biomass may increase or decrease with enrichment. These results contrast with predictions from models that assume ideal free distributions of grazers and/or predators with respect to their resources. Our models also predict that densities at all trophic levels will increase with increasing producer immigration, and that producer density will decline with increasing grazer immigration and increase with increasing predator immigration. Our qualitative findings on enrichment are used to interpret an experiment dealing with the short-term dynamics of a stream community open to grazers and predators.

PRIMARY-PRODUCTIVITY GRADIENTS AND SHORT-TERM POPULATION DYNAMICS IN OPEN SYSTEMS

We present three models representing the trophic and behavioral dynamics of a simple food chain (primary producers, grazers, and predators) at temporal scales shorter than the scale of consumer reproduction, and at the spatial scales typically employed in field experiments. These models incorporate flexible behavioral responses of organisms to their predators and resources in spatially heterogeneous environments that are open to immigration and emigration. The basic models include passive immigration at all trophic levels, producer growth rates and losses to grazer consumption, grazer emigration rate as a behavioral response to producer and predator densities, grazer losses to predator consumption, and predator emigration as a function of grazer density. We model this system as: (1) a set of ordinary differential equations (“well-mixed model”); (2) a set of partial differential equations describing a population of discrete grazers foraging on discrete patches of primary producers (“discrete-grazer model”); and (3) a set of simulation rules describing the movement and foraging of individual grazers and the growth of primary producers on discrete patches in explicit space (“individual-based model”). The ordinary differential-equation models produced similar results to individual-based models with well-mixed producers, and the discrete-grazer and individual-based models produced similar results when grazers possessed a long-term memory of patch reward rates. The well-mixed and discrete-grazer models thus represent specific, limiting cases of the general individual-based model. Multiple equilibria and sustained oscillations are possible but are less likely in the discrete-grazer and individual-based models than in the well-mixed model, because localized foraging of discrete grazers leads to the rapid development of spatial heterogeneity in producer biomass and, hence, to a decrease in overall primary production. All models predict that stable equilibrium densities of all trophic levels increase with enrichment, provided grazers increase their emigration rates as predator density increases. If increasing predator density leads to decreasing grazer-emigration rates, predator and grazer densities increase, but producer biomass may increase or decrease with enrichment. These results contrast with predictions from models that assume ideal free distributions of grazers and/or predators with respect to their resources. Our models also predict that densities at all trophic levels will increase with increasing producer immigration, and that producer density will decline with increasing grazer immigration and increase with increasing predator immigration. Our qualitative findings on enrichment are used to interpret an experiment dealing with the short-term dynamics of a stream community open to grazers and predators.

THE EFFECTS OF PREDATION ON THE AGE AND SIZE OF MATURITY OF PREY.

The effects of nonselective predation on the optimal age and size of maturity of their prey are investigated using mathematical models of a simple life history with juvenile and adult stages. Fitness is measured by the product of survival to the adult stage and expected adult reproduction, which is usually an increasing function of size at maturity. Size is determined by both age at maturity and the value of costly traits that increase mean growth rate (growth effort). The analysis includes cases with fixed size but flexible time to maturity, fixed time but flexible size, and adaptively flexible values of both variables. In these analyses, growth effort is flexible. For comparison with previous theory, models with a fixed growth effort are analyzed. In each case, there may be indirect effects of predation on the prey's food supply. The effect of increased predation depends on (1) which variables are flexible; (2) whether increased growth effort requires increased exposure to predators; and (3) how increased predator density affects the abundance of food for juvenile prey. If there is no indirect effect of predators on prey food supply, size at maturity will generally decrease in response to increased predation. However, the indirect effect from increased food has the opposite effect, and the net result of predation is often increased size. Age at maturity may either increase or decrease, depending on functional forms and parameter values; this is true regardless of the presence of indirect effects. The results are compared with those of previous theoretical analyses. Observed shifts in life history in response to predation are reviewed, and the role of size-selective predation is reassessed.

The relative importance of refugia in determining the drift and habitat selection of predaceous stoneflies in a sandy-bottomed stream

Patch structure in sandy, compared to rocky streams, is characterized by isolated snags that can only be colonized by drifting. By measuring drift from patches (snags) of various quality we determined the factors that influence habitat selection and drift of the predaceous stoneflies Acroneuria abnormis and Paragnetina fumosa. The presence of refugia (loose bark and leaf packs) was more important than hunger level and modified the effects of increased predator densities and aggressive interactions. Stoneflies concentrated to 8x natural densities with access to refugia remained longer on snags than a single stonefly without access to refugia. During periods of activity, refugia were defended with larger stoneflies always displacing smaller nymphs. During long periods of inactivity, two and sometimes three nymphs would rest side-by-side sharing the same refuge. Hunger level (starved versus satiated stoneflies), an indirect measure of a predator's response to prey availability, had no significant effect on drift or habitat selection regardless of the presence of refugia. Stonefly predators had a uniform distribution while their prey were clumped. Drift was deliberate and almost always delayed until night, usually at dusk or dawn. An examination of previous research plus the results of this study suggest that non-predatory intra- and interspecific interactions can be an important mechanism causing drift in streams.

Intraspecific competition in immature Amblyseius fallacis, Amblyseius andersoni, Typhlodromus occidentalis and Typhlodromus pyri (Acari: Phytoseiidae)

Intraspecific competition in immature Amblyseius fallacis, Amblyseius andersoni, Typhlodromus occidentalis and Typhlodromus pyri was examined in the laboratory using small cages at five different predator densities (two, four, eight, 16 and 32) in the absence and presence of prey 100 eggs of two-spotted spider mite, Tetranychus urticae (Koch), at 25 ± 1°C, ∼80% RH and 16L:8D photoperiod. In the absence of spider mite prey, some individuals of immature phytoseiids showed increased development and surival with increasing predator densities up to certain limits, but none survived to the adult stage, except for a single male each of A. andersoni and A. fallacis who completed development by cannibalizing on conspecifics at a density of 32 predators per cage. In the absence of spider mite prey, the mean immature survival time was independent of the initial predator density, but the variance of survival time increased with predator density. In the presence of prey, the proportion of immatures surviving to adulthood generally decreased with initial predator density and dropped sharply to almost none at the predator density of 32 for A. fallacis, eight for A. andersoni, 16 for T. occidentalis and four for T. pyri. The number of prey consumed per predator during the first day generally decreased with predator density in all four species, as prey available per predator decreased and the competition for food increased with predator density. Our data indicate that scramble competition is operating in these four species. Although cannibalism was occasionally observed, especially after the exhaustion of prey and in the generalist predators such as A. andersoni, the immatures of these phytoseiids were less influenced by the interference of conspecifics than by the increasing difficulty of finding food at high predator densities. The implications of this study for understanding phytoseiid population dynamics and their use in biological control are discussed.

Prey Aggregation Is Correlated with Increased Predation Pressure in Lake Fish Communities

Aggregations of prey fish, golden shiner (Notemigonus crysoleucas), were examined during 7 yr of predator manipulations in two lakes to determine whether they responded to changes in predation pressure and varied with time-of-day, age, and habitat. Regression analysis was used to examine aggregation in 12 replicate prey densities from two time periods, two ages, two habitats, three sample series, and seven predator densities. Aggregation was assessed as the variance of mean densities for each treatment combination. Multiple regression and ANCOVA analyses indicated that (1) golden shiner aggregated more during day than night, (2) their aggregation was positively related to predator density, (3) young shiner aggregated more than older ones at low predator densities, and (4) aggregation in older shiner was more responsive to increased predator densities than aggregation in younger shiner. These results provide empirical evidence that golden shiner aggregation patterns respond to predation pressure and the response varies with time and age. These results also suggest that variance in net catches can provide an index of fish aggregation and that aggregation observed at the population level is not solely dependent on species and density, but is a behavioural response mediated by several factors including predators.

Predators that Benefit Prey and Prey that Harm Predators: Unusual Effects of Interacting Foraging Adaptation

This article explores models of four-species food chains in which two species adaptively balance food intake and risk o e predation. The purpose is to determine how the interaction of adaptive foraging behaviors in two or more species influences the interactions in the entire food chain. It is shown first that such interacting behaviors will often reach a stable equilibrium. Differential equations are then derived to describe population dynamics when the behaviors remain close to their optima. These equations are very different from Lotka-Volterra-type food web models. The per capita growth rate of each species is affected by the density of every other species in the food chain, and the largest effects may be caused by the species that are most distant in the food chain. Increased predator density may increase the fitness of their prey, while increased prey density may decrease the fitness of their predator. Such unusual interactions are most likely when the adaptive species increase foraging effort in response to increased food availability. The nature of the indirect effects can often be deduced from knowing the sign of the second derivative of foraging cost-and-benefit functions.

A Field Test of Resource Depression by Predatory Stonefly Larvae

Experiments were conducted in two Rocky Mountain streams (Gunnison County, Colorado) to determine the context within which predatory stonefly larvae locally depress their prey populations. In feeding trials where no migration of prey (Baetis bicaudatus, Ephemeroptera) was allowed from flow-through boxes containing natural substrates, Megarcys signata and Kogotus modestus (Perlodidae) significantly depressed prey densities over 14 and 38 h, respectively. However, increasing predator density had no apparent effect on per capita predation rate. In 1984 enclosure experiments allowing prey but not predator migration, Kogotus significantly reduced total prey density and that of two relatively sedentary prey species (Cinygmula mimus, Ephemeroptera, Heptageniidae, and Zapada haysi, Plecoptera, Nemouridae) during three-day trials

Absence of intraspecific interference during feeding by the predatory ladybirds Chilocorus spp. (Coleoptera: Coccinellidae)

Abstract. 1. The hypothesis was tested that intraspecific behavioural interference does not adversely affect the feeding behaviour of adults of three predatory coccinellid species, Chilocorus nigritus (F.), C.bipustulatus (L.) and C.infernalis Mulsant, at densities found under field conditions.2. Feeding rates on mature oleander scale Aspidiotus nerii Bouché were evaluated by two methods at various predator densities. Proportion of the population dispersing was also measured for one of the species.3. Feeding rate did not decrease and dispersal did not increase with increasing predator density. No significant behavioural interference that might have reduced predatory efficiency was observed, counter to assumptions on which published interference models are based.4. Results here help to explain the relative importance of parasitoids and predators in the effective control of red scale Aonidiella aurantii (Mask.). The results also provide guidelines for release of these bio‐control agents.

The functional response of Toxorhynchites rutilus rutilus to changes in the population density of its prey Aedes aegypti.

We present an analysis of the functional response of the predator Toxorhynchites rutilus rutilus (Coquillett) to changes in the density of the larvae of Aedes aegypti (L.) (Diptera: Culicidae). The experiment was replicated for five different ages, and at three different densities of the predator. The data were fitted to Rogers' (1972) random predator equation by non-linear least-squares in order to estimate searching efficiency and handling time for each experimental treatment. The data show that estimated searching efficiencies are highest at intermediate ages of the predator for all predator densities tested. Handling time declines exponentially with increasing predator age. There is a marked interference effect; searching efficiency decreases with increased predator density, and this is most pronounced at intermediate prey ages. Estimated handling times increase with predator density at a rate which declines with increasing predator age.

Predation, Breeding Asynchrony, and the Outcome of Competition Among Treefrog Tadpoles

Densities of a predatory salamander (0, 2, or 4 Notophthalmus viridescens/m3) and a spring—breeding frog tadpole (100, 200, or 400 Hayla crucifer/m3), were controlled in a 3 x 3 factorial experiment replicated three times in 27 artificial ponds. Shortly before all Hyla crucifer metamorphosed from the ponds, 200 hatchlings of a summer—breeding frog, Hyla versicolor, were added to each artificial pond. Differences among ponds in growth and survival of H. versicolor provided a bioassay for the effects of predators and previously established competitors. Increased predator density decreased the survival and increased the mean mass at metamorphosis of H. versicolor, regardless of H. crucifer density. Increased H. crucifer density decreased the survival and mass at metamorphosis of H. versicolor in ponds without predators. Increased H. crucifer density decreased the mass at metamorphosis of H. versicolor adjusted for variation among ponds in the density of surviving H. versicolor, over all artificial ponds. Total H. versicolor biomass was negatively correlated with the biomass of H. crucifer metamorphs, in artificial ponds without predators. This experiment demonstrates strong competition between two species of asynchronously breeding treefrogs in predator—free ponds, and thereby compromises the traditional assumption of weak interspecific competition among temporally separated species. However, predation greatly reduces the intensity of competitive interactions among asynchronously breeding species.

Prey refuges and predator-prey stability

Refuge use can be broadly defined to include any strategy that decreases predation risk, e.g., spatial or temporal refuges, prey aggregations, or reduced search activity by prey. The existence of refuges can clearly have important effects on the coexistence of predators and prey. Previous models have only considered the effects of a constant proportion or constant number of prey using refuges. However, these are only two special cases from among the range of possible patterns of refuge use. We show the general conditions for when refuge use should have a stabilizing effect on Lotka-Volterra-type predator-prey systems. For a stabilizing effect, the proportion of prey in refuges must either: (1) decrease with increasing prey density, or (2) increase with both increasing predator density and increasing predation pressure. We also examine the effect of the cost of refuge use in decreased prey feeding or reproductive rates. Finally, we review some relevant empirical data and present some new data of our own.

Invertebrate Predation and Lotic Prey Communities: Evaluation of In Situ Enclosure/Exclosure Experiments

The influence of the stonefly Kogotus nonus on prey communities was assessed by varying the predator density within small enclosures in a first—order stream in southwestern Alberta. Experimental containers with natural densities of prey, standardized substrate and detritus, and zero, one, two, or three Kogotus were buried in a riffle for 10 during three periods: July 1981, June 1982, and July 1982. Kogotus depressed the densities of Thienemaniella and some other Orthocladiinae, but had no effect on the densities of Corynoneura (Orthocladiinae), Stempellinella (Tanytarsini), and Baetis tricaudatus (Ephemeroptera). Significant reduction of the densities of Thienemaniella and other Orthocladiinae occurred only when these prey items were present at high densities. The extent to which prey densities were depressed was found to be dependent on predator density within the enclosures: increased predator densities resulted in lower prey densities. However, when predator density in the containers exceeded that commonly observed in the riffle, predator efficiency dropped, probably because of interference between predators, the existence of prey refuges, or changes in predator search effort. An experimental design in which containers that excluded predators were compared with controls accessible to predators was used to determine if results would be similar to results from the enclosure experiments. Using this design, observed results could not clearly be attributed to the presence or absence of the predator, and were more likely due to differences in abiotic conditions between the experimental and control containers. It was concluded that observation of predator effects in field manipulative experiments may be strongly dependent on the selection of an experimental design in which predator effects are not confounded with container effects.

The Functional Response of Toxorhynchites Rutilus Rutilus (Diptera: Culicidae), a Predator on Container-Breeding Mosquitoes1

This study approaches mosquito predation by Toxorhynchites r. rutilus in terms of the functional response of the predator. Toxorhynchites r. rutilus displays a Type II functional response, similar to that of most insect predators. Estimates of attack rates within and across its larval instars were not significantly different. The estimates for prey handling time showed no trend within the 4th instar, but decreased with each molt as the predator matured from 1st to 4th instar. At densities of 2 and 4 predators per container, attack rates decreased with increasing predator density, although they were not significantly different. Handling times significantly decreased as predator density increased. The study is summarized in the functional response surfaces for individual and total predator populations.

Egg-care behaviour of Aequidens paraguayensis (Pisces, Cichlidae) in relation to predation pressure and spawning substrate

Aequidens paraguayensis is a cichlid fish with biparental care that spawns on a leaf taken from substrate litter and, after the eggs have hatched, broods the progeny orally. We tested the hypothesis that increasing predator density causes proportional changes in the frequency or duration of parental care activities, including moving the spawn. We predicted that at high predator densities, the parents would pick up the eggs and begin mouthbrooding earlier than usual to prevent egg loss.Increasing predator density led to greater parental sex-role differentiation, the males defending the spawning site while the females tended the eggs. Leaf tugging was performed by the females only and did not increase significantly with predator density. Pairs that spawned on a fixed substrate did not exhibit this sex-role differentiation.Predator density and type of spawning substrate did not affect brood survival or time between spawning and beginning of mouthbrooding. Hatching time is temperature dependent and parents apparently must wait for the egg membranes to break before picking up the hatchlings.

Experimental studies on acarine predator–prey interactions: the effects of predator density on immature survival, adult fecundity and emigration rates, and the numerical response to prey density (Acarina: Phytoseiidae)

Laboratory experiments revealed that increasing the density of the immature instars of Phytoseiulus persimilis had little effect on their survival rates. However, due to the high prey requirements of the immature instars of Amblyseius degenerans and the tendency for underfed individuals of this species to become cannibalistic, their survival rates decreased with increasing predator density. Increasing predator density reduced the fecundity of P. persimilis and caused its numerical response to prey density to reach plateaux at increasingly lower levels of fecundity as the predator density increased. In contrast, increasing predator density did not affect the fecundity of A. degenerans to such an extent and its numerical response appeared eventually to reach maximum levels when sufficient prey were available. The emigration rates of P. persimilis tended to increase with increasing predator density whereas those of A. degenerans increased or decreased depending on the density of the predators and the prey.

Experimental studies on acarine predator–prey interactions: the effects of predator density on prey consumption, predator searching efficiency, and the functional response to prey density (Acarina: Phytoseiidae)

Experiments on the responses of two species of predacious phytoseiid mites, Phytoseiulus persimilis and Amblyseius degenerans, to their own density revealed that, with the exception of adult female P. persimilis, the individual predation rates of all instars of both species increased as predator and prey densities were increased within certain fixed predator to prey ratios. "Interference" between adult female P. persimilis was suggested as a possible reason for this result, and this was confirmed by the application of Hassell and Varley's model to data from an experiment where predator density was increased within various prey densities. Interference did not influence the predation rates for other instars of either species and other factors are considered to account for the results obtained.Increasing predator density caused the functional response of all instars of both species to increase over an increasingly wider range of prey densities. However, due to interference between adult female P. persimilis the functional response curves increased curvilinearly and reached plateaux at progressively lower levels of prey killed per predator as predator density increased. With the other instars of both species the responses became less curvilinear with increasing predator density, and trends in the data suggested that plateaux would be reached at similar levels of prey killed for different predator densities provided sufficient prey are present.

Stability and Prey Behavioural Responses to Predator Density

(1) Earlier models of mutual interference between searching predators or parasites ignored the potentially important effects of prey behavioural responses to predators. (2) A model incorporating simple prey behavioural responses shows that prey availability may either increase or decrease as a function of predator density. Thus both mutual interference and enhancement are possible depending upon the behaviour of the prey. (3) The importance of prey behavioural responses are demonstrated in a laboratory study using an aquatic insect predator with both mobile and immobile prey. Two specific behaviours, spacing and movement, are shown to affect the predation rate. Movement of prey is also related to predator density in a predicted fashion. (4) The stability properties of a Lotka-Volterra system incorporating prey escape responses are discussed. The system is stable if prey responses result in relatively mild interference. Prey responses are destabilizing if they result in mutual enhancement or in interference that is so strong that the attack rate of the entire predator population decreases with increasing predator density. (5) Set in an evolutionary context, the behaviour of notonectids suggests that avoidance of cannibalism may have a larger effect on fitness than maximization of feeding efficiency.

The Feeding of Amoeba proteus on Paramecium aurelia*

SYNOPSIS. Density of prey (Paramecium aurelia) and predator (Amoeba proteus) were varied while volume of inorganic medium was kept constant. Variations in density of prey had little effect on the rates of feeding and reproduction of the amoebae; but with increasing predator density the amoebae captured the paramecia less rapidly and ingested fewer before dividing, altho division size did not change appreciably. Therefore, amoebae of a low density population with a constant food supply carry more nutritional reserves from generation to generation than do those in a denser population.

EFFECTS OF PREDATOR AND PREY DENSITIES ON INTERACTIONS BETWEEN GOLDFISH AND DAPHNIA PULEX (DE GEER)

The effects on predator efficiency of varying predator and prey densities in a confined system were tested by use of the goldfish and Daphnia pulex. Within the limitations of the experiment, the per capita consumption of prey was reduced by increasing predator density. Also, as prey density increased, the effect on per capita consumption of increasing predator density became less.