Prey Population Density Research Articles

DENSITY DEPENDENCE, PREY DEPENDENCE, AND POPULATION DYNAMICS OF MARTENS IN ONTARIO

Ecological factors influencing demographic parameters of mammalian carnivores are poorly understood, due to the difficulty of simultaneously measuring predator and prey populations over an extended period. We used cohort analysis based on age-specific harvest data to estimate population densities over 20 yr for martens (Martes americana). Marten abundance increased threefold over the study period, probably due to relaxation in harvest intensity at the beginning of the study interval. Changes in rates of population growth by martens were positively correlated with population densities of three species of small mammals recorded over the same time span: deer mice (Peromyscus maniculatus), red squirrels (Tamiasciuris hudsonicus), and flying squirrels (Glaucomys sabrinus). We found no evidence of short-term cycles in marten abundance, nor were there significant cross-correlations at lags greater than 1 with prey population densities. Martens also showed evidence of density-dependent population growth. Such density dependence beyond the demographic effect of variation in prey density was possibly due to agonistic interactions among territory holders. Such mixtures of prey dependence and density dependence often have a stabilizing influence in theoretical models, which could contribute to the observed stability of deer mouse, red squirrel, and marten populations in Algonquin Park. Harvest intensity was negatively related to yearly variation in marten population growth. Mortality due to trapping averaged 37.9% over two decades, with no detectable relationship to changes in marten population density. Hence, harvesting acted as a stochastic external variable that was additive to density-dependent and prey-dependent effects.

The effect of predator and prey density on the induced defence of a ciliate

1. The level of antipredator defence should be proportional to the actual attack probability to minimize the cost of defence and maximize the net benefit. 2. The hypothesis that the induced antipredator morphology of Euplotes octocarinatus is a graded response to the actual risk of predation by Stylonychia mytilus was tested by manipulating the density of both prey and predator populations. 3. The magnitude of the response was graded according to both predator and prey density. A dense prey population may be protective since a prey is more exposed to a predator's attack as a solitary individual. 4. The results suggest that Euplotes is able to ‘estimate’ the real risk of predation and respond appropriately, without mobilizing more resources than needed. 5. Separation of the prey and predator with a nylon net revealed that the response was not induced by a water‐transmitted factor but that direct cell‐to‐cell contacts were important. This finding departs from those of other studies.

The evolution of warning signals

Warning coloration signals are a familiar and conspicuous phenomenon in nature. However, the fundamental question of how warning signals initially evolved remains unanswered. For an unpalatable prey to evolve a signal to indicate its unprofitability, a rare and conspicuous mutant in a population of unpalatable cryptic prey must overcome a double disadvantage: a greater risk of being detected (as a result of being more conspicuous) and of being attacked (because its rarity results in a decreased association with aversion) by a predator1,2. Although the prior evolution of prey gregariousness may help warning signals to evolve3,4,5,6,7,8, such an evolutionary order may not always be the case4,9,10,11. Here we present a theoretical model that describes a mechanism for the evolution of warning signals without having to invoke gregariousness. Specifically, a predator's generalization of stimulus in associative learning, with a ‘peak shift’ towards greater conspicuousness5,12,13,14,15, allows a warning signal to evolve when the prey population density exceeds a threshold. Once a warning signal starts to evolve, it continues to grow; the resulting, evolutionarily stable16 conspicuousness of prey is discontinuously greater than that of the original cryptic prey, drawing an unambiguous distinction in their appearance.

Population dynamics of a dense assemblage of marine detritivores

Accumulations of macrophyte detritus provide food and refuge for a dense (up to 3.5·10 6 ind. m −2) assemblage of amphipod and leptostracan crustaceans in a Southern California submarine canyon. The objective of this work was to determine how the physical environment and biological interactions limit these populations, that rarely, if ever, appear to be food limited. Preliminary observations suggested that the detritus-crustaceans would incur significant losses during the Winter, resulting from wave disturbance. Predation by dense schools of fishes associated with this habitat also appeared to be important. Laboratory and field experiments demonstrated that the detritus provided an effective refuge for the crustaceans. Predation rate of fishes upon detritus-associated crustaceans increased as the density of prey populations increased and/or as the thickness of the detritus mat was reduced (increasing number per m 3 but not per m 2). Density (m −2) of the mat crustaceans fluctuated seasonally, being greatest in the Winter and Spring following storms that eliminated large portions of the habitat. Few animals appeared to be directly eliminated by the storm disturbance. The important effect of storms was to reduce the quantity and quality of the detritus refuge by concentrating the crustaceans into smaller patches with less detrital cover. During the calm summer months the bacterium Beggiatoa sp. spread out over large portions of the detritus mat. Beneath the bacteria oxygen concentration was reduced, and infaunal density was two orders of magnitude lower than in unaffected portions of the mat. The summer increase in bacterial cover constituted a biological disturbance that functionally reduced the habitat area available to the mat fauna and left them more vulnerable to predation. Secondary production in the detrital mats is among the highest reported from natural environments. This is possible because the mat crustaceans are unable to graze down their food supply (detritus and associated microbes). The refuge provided by the detritus for large populations of invertebrates also is crucial; larger refugia translated into increased carrying capacity which allowed both greater population sizes and production, much of which became available to fishes. Most marine ecosystems are supported by allochthonous material that enters food webs via detritivores. Seafloor features that collect detritus constitute an important source of patchiness over a wide range of scales because detrital delivery is governed by physical processes. The La Jolla/Scripps Canyon system, by accumulating organic debris, provides a large food source from shallow to continental slope depths. This resource supports large numbers of fishes and presumably increases local production in higher trophic levels.

Effects of habitat fragmentation on provisioning rates, diet and breeding success in two species of tit (great tit and blue tit).

The aim of this study was to examine the effects of forest fragmentation on the ability of parent birds to provide their young with an adequate food supply. To examine whether prey population densities of the great tit (Parus major L.) and the blue tit (P. caeruleus L.) vary between study areas in different forest size classes we compared provisioning rates and chick diet and related these parameters to breeding success. We filmed 217 nests over two breeding seasons and collected data on frass fall as a general estimate of caterpillar availability. Nests which were attended by none or one parent only during filming (n = 46) were excluded from the analyses. In both years and for both species feeding rates were highest in the smallest fragments and lowest in the large forest. There was also a suggestion that differences in feeding rates between areas vary between years. We found no consistent tendency for prey size to change with forest size, although both species brought slightly smaller prey items to the nest in the smallest forest fragments and feeding rates correlated negatively with prey size. Caterpillars were the main item fed to nestlings, in both species. We found no evidence to suggest that either frass fall or the proportion of caterpillars in the diet varied with forest size. There was also no correlation between mean frass fall and the total number of caterpillars brought to the nests, in either species. Breeding success, as measured by clutch size, brood size, fledging weight and fledging success, did not differ between the small fragments and the large forest, in either species. There was also no relationship between provisioning rate (as concerns volume of prey fed to nestlings and the quality of chick diet) and breeding success parameters. In conclusion, this study does not suggest suboptimal foraging or breeding conditions in small fragments compared to a nearby large forest, for either species.

Effects of Predators on Prey Growth Rate: Relative Contributions of Thinning and Reduced Activity

Predators affect individual growth rates of surviving prey in two conflicting ways. First, predation acts to increase growth rate by thinning the density of prey populations, which releases survivors from competition. At the same time, predators intimidate prey into decreasing their feeding activity and increasing refuge use, causing prey to grow more slowly. Both processes are known to affect individual growth rates in many systems, but their relative importances and interactive effects have not been measured. We designed an experiment to estimate the separate and joint effects of thinning and activity suppression, using Rana sylvatica tadpoles reared in 1100-L outdoor artificial ponds The experiment manipulated the perceived risk of predation (using caged Anax dragonfly larvae) independently from the loss rate (by manually removing tadpoles every other day according to a predetermined mortality schedule). The presence of predators caused tadpoles to decrease time spent feeding and swimming, verifying that the conditions for behaviorally-mediated growth suppression were satisfied. During the first half of the experiment, when tadpoles were small and not yet competing for food, growth declined sharply with predation risk but was unaffected by thinning. During the second half of the experiment, when tadpoles were much larger and had presumably depleted food resources, growth rate increased under thinning but was unaffected by predation risk. Overall, there was an interaction among treatments because activity suppression was only important at low density, while thinning was especially important in the absence of predation risk. Our results suggest that the numerical effects of predators on prey will predominate in communities composed of strongly interacting species with resource depletion (e.g., communities with clear keystone predator effects), whereas growth costs of predator avoidance may quantitatively affect species interactions in communities with less severe exploitative competition.

Global existence of periodic solutions in a class of delayed Gause-type predator-prey systems

where x(t), y(t) denote the population density of prey and predator at time t, respectively. g(e), p(e) and /z(s) are assumed to satisfy appropriate conditions. v, a positive constant, stands for the death rate of predator y in the absence of prey x. We may think of this system as herbivores (y) grazing upon vegetation (x), which take time r to recover. In view of the fact that many predator-prey systems display sustained fluctuations (for examples, see Freedman [l]), it is thus desirable to construct predator-prey models capable of producing nonconstant periodic solutions. When r = 0, (1.1) reduces to the well-known undelayed Gause-type or Kolmogorov predator-prey systems (again, see Freedman [l] and the references cited theorein). Under some assumptions, the resulting systems can have a unique limit cycle in the positive cone (see Kuang and Freedman [2] and the references cited therein). To the best of our knowledge about delayed population interaction models, the existing literature establishes the existence of a periodic solution by Hopf bifurcation argument, or reduces the delayed systems to undelayed systems of higher dimensions. Therefore, these existence results are generally local. Most of the existing results on the global existence of nonconstant periodic solutions in delayed equations deal with scalar equations with single discrete delay, such as 13-101

Indicator Plants for Monitoring Pest Population Growth

Some plants are more attractive to agricultural pests than the crop to which they are a pest. The use of these plants as indicator plants to monitor pest populations is examined. Bean plants were found to be effective indicators for forecasting the growth of carmine spider mite, Tetranychus cinnabarinus (Boisduval), on greenhouse tomatoes, Lycopersicum esculentum L. Spider mite population growth rates were the same on both host plants, but because they became established up to 5 wk earlier on beans, Phaseolus vulgaris L., monitoring carmine spider mite population growth on beans provides enough time to order and distribute natural enemies for their control on tomatoes. The oviposition and developmental rates of mites did not differ on the 2 host plants. However, the temperature threshold for oviposition was 7°C lower on beans than tomatoes, resulting in earlier establishment and onset of population growth in the spring on the indicator plants. The potential for using indicator plants to establish biological control agents was investigated with the predatory mite Phytoseiulus persimilis Athias-Henriot. P. persimilis became established equally well when broadcast on the crop (with its much lower prey population density) as when released only on infested indicator plants. Best control of carmine spider mites by Phytoseiulus was obtained when the predators were released when spider mite density reached 12 per tomato leaf, which occurred when the mite density on beans was 100 per leaf, 5 wk after reaching 12 mites per bean leaf. Acting on this threshold, a grower would have ample time to order, receive, and release predatory mites. Predation of spider mites by Phytoseiulus was higher on the tomatoes than the beans. Phytoseiulus populations dipped after introduction on both host plants but recovered more rapidly on beans. The different predation and survival rates on the 2 host plants probably served to keep the 2 population cycles on the 2 hosts out of synchrony, thereby sustaining the population interactions.

Energy flow and community structure in freshwater ecosystems

The goal of this article is twofold: 1) It aims at providing an overview on some major results obtained from energy flow studies in individuals, populations, and communities, and 2) it will also focus on major mechanisms explaining community structures. The basis for any biological community to survive and establish a certain population density is on the one hand energy fixation by primary producers together with adequate nutrient supply and the transfer of energy between trophic levels (‘bottom-up effect’). On the other hand, predator pressures may strongly control prey population densities one or more trophic levels below (‘top-down effect’). Other interpopulation effects include competition, chemical interactions and evolutionary genetic processes, which further interact and result in the specific structuring of any community with respect to species composition and population sizes.

Stochastic lattice gas model for a predator-prey system.

We propose a stochastic lattice gas model to describe the dynamics of two animal species population, one being a predator and the other a prey. This model comprehends the mechanisms of the Lotka-Volterra model. Our analysis was performed by using a dynamical mean-field approximation and computer simulations. Our results show that the system exhibits an oscillatory behavior of the population densities of prey and predators. For the sets of parameters used in our computer simulations, these oscillations occur at a local level. Mean-field results predict synchronized collective oscillations.

Why don't predators have positive effects on prey populations?

Three mechanisms by which increasing predation can increase prey population density are discussed: (1) Additional predation on species which have negative effects on the prey; (2) Predation on consumer species whose relationship with their own prey is characterized by a unimodal prey isocline; (3) Predation on species which adaptively balance predation risk and food intake while foraging. Possible reasons are discussed for the rarity of positive effects in previous predator-manipulation studies; these include the short-term nature of experiments, the large magnitudes of predator density manipulation, and various sources of bias in choice of system and interpretation of results.

A demonstration of asynchronous local cycles in an acarine predator-prey system

Population fluctuations in a continuous predator-prey system consisting of the spider mite,Tetranychus urticae Koch and its predator,Typhlodromus occidentalis Nesbitt, were assessed for almost 2 years (ca. 50 prey generations), on six mini-orchards of young apple trees in a climatically controlled glasshouse. During the first half of the experiment the plants were either ‘connected’ to each other with dowel rods or ‘unconnected’ and separate. In the second half, the plants were all ‘connected’. Population densities of both prey and predator on the unconnected mini-orchards were always higher than on the connected ones. Within the mini-orchards (=local scale), prey and predator populations go through a pattern of large amplitude cycles which continued throughout the experimental period. At the regional scale (mean over all mini-orchards) two types of fluctuations were observed. Large amplitude fluctuations associated with synchrony of the local cycles and small amplitude fluctuations associated with periods when local population cycles are proceeding out of phase. Transition from synchronous to asynchronous cycles took place very fast (within a few weeks), suggesting a mechanism generating asynchrony and possibly also partial refuges, which in turn trigger stabilising mechanisms at a regional scale. Causes of the synchrony are unclear. They may relate to the application of a pesticide (pirimicarb) used to control aphids, but there may just as well be an indirect form of causation, e.g. whereby the factors that promote aphid outbreaks also promote spider-mite population growth and temporary escape from control by the predators. Despite the synchronising factors the predator-prey system persisted, without external inputs, at a fairly small spatial scale, the size of a small glasshouse. Assessing the spatial scale and heterogeneity whereby decoupling of local cycles is sufficient for the cycles to proceed out of phase, remains an important area of experimental research. The experiment reported here provides some first clues and may form a standard for forthcoming experiments to assess the mechanisms promoting asynchrony, persistence and possibly also stability.

Predation and Competition: The Interaction of Two Types of Species Interactions

A perennial endeavor in community ecology is to unravel the relative importance of competition and predation in determining the structure of natural communities. The comparison stems in part from the suggestion that predation affects communities largely through its effects on densities of competing prey species (e.g., Jeffries and Lawton 1984) and that competition is only when predation is weak (e.g., Connell 1975). We suggest that this comparison is misdirected. As we discuss here, predation and competition together can affect prey communities in a multitude of ways that often interact; appreciating the manifold complexities and consequences of this interaction of interactions may prove to be more fruitful than searching for which factor is most important in structuring ecological communities. In this essay, we propose a simple classification of the effects of predation upon the outcome of competition. The basic idea is to distinguish between those effects that are mediated directly or indirectly through changes in prey population densities brought about by predatorinflicted mortality (trophic link indirect effect, sensu Miller and Kerfoot 1987), and those mediated through changes in prey behavior which affect the intensity of competition among the prey species and are brought about in response to changes in the risk of predation (behavioral indirect effect, sensu Miller and Kerfoot 1987). We briefly discuss some of the diverse outcomes to be expected in systems with a mixture of predation and competition, and for illustration, relate theoretical models to a few examples from natural communities. We present the following models as a convenient way for clearly catagorizing the interaction of predation and competition. We present no analysis here, since much of that has already been done elsewhere. One can represent the interaction between a pair of competing species with the following pair of differential equations:

Functional and Numerical Responses of Cheilomenes lunata (Fabricius) (Coleoptera: Coccinellidae) Feeding on the Cowpea Aphid, Aphis craccivora Koch (Homoptera: Aphididae)

The density responsiveness of larvae and adults of Cheilomenes lunata (Fabricius) preying on Aphis craccivora Koch was investigated under laboratory conditions where temperature and relative humidity were uncontrolled and fluctuated between 24-30°C and 43-90% respectively. The feeding rates of C. lunata were positively correlated with the population density of prey. The number of prey consumed daily by each predator stage tested, increased more steeply at lower, than at higher prey densities, exhibiting thus the Type II functional response. Fecundity of C. lunata females also increased to a plateau with increasing prey density.

The impact of predation on the viburnum whitefly, (Aleurotrachelus jelinekii).

In association with a detailed study of the population dynamics of the viburnum whitefly, Aleurotrachelus jelinekii, observations were made on the predators including spiders and five species of insect, notably the mirid Campyloneura virgula and the neuropteran Conwentzia psociformis. Although the species richness of the predator guild was positively correlated with prey population density, at the individual level there was little evidence over twelve years of a density dependent relationship. In these populations natural enemies do not seem to play an important role in the dynamics.

Predation byCheilomenes vicina [Coleoptera: Coccinellidae] on the cowpea aphid,Aphis craccivora [Homoptera: Aphididae]: Effect of prey stage and density

Consumption of larvae and females ofAphis craccivora Koch by 1st and 4th larvae and adults ofCheilomenes vicina (Muls.) was studied under fluctuating temperature (24–30°C). The early aphid instars were consumed in significantly greater numbers than later instars and females. The feeding rates ofC. vicina were significantly positively correlated with the population density of prey. The number of prey consumed daily by each predator stage tested, increased more steeply at lower than at higher prey densities, exhibiting thus the type 2 functional response

Effect of vertebrate pesticides on non-target wildlife populations: Evaluation through modelling

A general modelling methodology to evaluate the potential effects of pesticides on non-target wildlife populations is described, and its use within a resource management framework is demonstrated. Attention is focused on (1) interfacing relatively simple numerical and analytical procedures with available data to specify initial baseline population parameters, (2) representing density-dependent relationships affecting non-target natality and survival rates, and (3) interpreting simulated non-target population responses to different pesticide-use schemes within a management framework. The methodology involves use of a Leslie matrix model with variable age-specific natality and survival rates. Natality and survival rates can vary as a function of non-target population density, the density of competitor, predator, and/or prey populations, and/or abiotic environmental factors. Required model input includes estimates of initial density of the non-target population, age-specific natality rates, the relative magnitudes of age-specific survival rates, the relationship of natality and survival rates to the appropriate population densities and/or abiotic environmental factors, and the magnitude of pesticide-induced mortality. Model output includes estimates of the density of the non-target population over time, the mean lowest density reached, the mean recovery time, and the probability of extinction resulting from a specified scheme of pesticide use. Use of the model within a management framework is demonstrated by evaluating the effects of four hypothetical coyote control schemes, using Compound 1080, on great horned owl populations in the United States.

A predator-prey model for two-species populations with nonlinear interactions and implications for fisheries management.

A predator-prey interaction model is proposed for studying management implications of two fish species populations with nonlinear interactions. Nonlinear interactions seem more plausibl than linear ones in natural pupulations. The present model is built up by adding second order interactive terms to the Lotka-Volterra type linear model proposed by LARKIN. The prey isocline, which is a curve satisfying the equation dN1/dt=0 in the N1 (prey population density)-N2 (predator population density) plane, is classified into the four types based on the shape of the isocline. The predator isocline is also classified into the four types. The present model with the elliptic predator and prey isoclines, can be used to represent all the types of isoclines and can show the general nature of various nonlinear interactions. The responses to exploitation of the present nonlinear predator-prey system are qualitatively different from those of the original linear system. The equilibrium catches may suddenly decline from positive levels to zero even if fishing pressure increases continuously. Such a catastrophic phenomenon is caused by the destabilization of the system. If the single-species MSY (maximum sustainable yield) policies are practiced wihtout taking into account the stability of the system, populations may be threatened with sudden collapses. The MSY policies for populations with nonlinear interactions may be quite dangerous.

Prey Concentrations and Feeding Response in Laboratory-Reared Stage-One Zoeae of King Crab, Snow Crab, and Pink Shrimp

The objective of this study was to determine prey population densities necessary to elicit a successful feeding response in the first zoeae of king crab, Paralithodes camtschatica, snow crab, Chionoecetes bairdi, and pink shrimp, Pandalus borealis, from Kachemak Bay, Cook Inlet, Alaska. In the laboratory, king crab zoeae consumed an average of 0.8, 1.3, 2.6, and 7.6 prey per day in 500-ml beakers in which the number of small copepods equaled 20, 40, 80, and 160 per liter, respectively. At these same prey densities, zoeae of snow crab and pink shrimp ingested an average of 0.5, 1.3, 2.2, 4.8, and 0.7, 1.5, 2.3, 5.3 prey per day, respectively. Prey densities had to equal 80 per liter before zoeae of all three species were able to consistently capture at least one prey item successfully each day. Prey densities of this magnitude are considerably higher than those determined by oblique net tows in the study area, Kachemak Bay, Alaska.

Predator-prey relationships between the rock lobster Jasus lalandii and the mussel Aulacomya ater at Robben Island on the Cape West Coast of Africa

The natural diet and mode of feeding of the rock lobster Jasus lalandii (H. Milne Edwards) was determined in a rock-lobster sanctuary near Cape Town, South Africa. Field observations were tested and confirmed by means of aquarium studies. Rock lobsters feed mainly upon ribbed mussels Aulacomya ater (Molina), which comprise the largest component of the sessile benthic fauna. Mussel remains were found as the major constituent in 97% of the rock-lobster stomachs examined. The density of rock lobsters averaged 8,100 per hectare (0.81 m-2), while mussel biomass averaged more than 5 kg (wet whole weight) m-2 within the same depth range (12 to 30 m). More than 80% of the mussel biomass comprised large individuals between 60 and 90 mm in length. Large rock lobsters (mainly males) were capable of feeding on all sizes of mussels, although many of these were inaccessible to predation. Smaller rock lobsters became progressively more limited in the size range of mussels which they could crack open and consume. Competition between rock lobsters for small mussels appeared to be intense, as mussels of suitable size for feeding were generally in short supply to most of the rock-lobster population. Hence, feeding and growth rates of rock lobsters are likely to be affected by the relative population densities of predator and prey. Growth rates of rock lobsters could be limited by food supplies even in areas where mussel biomass is comparatively large.