Plant stem density as a cue in patch choice by foraging juvenile bluegill sunfish

Synopsis Juvenile bluegill sunfish,Lepomis macrochira, are restricted to vegetated habitats by predators. Variation in plant stem density has a significant effect on bluegill foraging success. Given the mosaic nature of this habitat, plant stem density may provide a cue for selecting among patches in which to forage. In this study, juvenile bluegills were offered patches of artificial vegetation differing only in plant stem density as potential foraging sites. Three densities, 100, 250, and 500 stems m^–2 were tested. Fish were presented with a choice between patches (100:250, 250:500, or 100:500). Bluegill foraging rate in, and the number of fish choosing each patch was recorded. Juvenile bluegills showed a preference for those patches which maximized their foraging rate.     

Predator foraging success and habitat complexity: quantitative test of the threshold hypothesis

Summary Numerous studies have demonstrated a negative relationship between increasing habitat complexity and predator foraging success. Results from many of these studies suggest a non-linear relationship, and it has been hypothesised that some threshold level of complexity is required before foraging success is reduced significantly. We examined this hypothesis using largemouth bass (Micropterus salmoides) foraging on juvenile bluegill sunfish (Lepomis macrochirus) in various densities of artificial vegetation. Largemouth foraging success differed significantly among the densities of vegetation tested. Regression analysis revealed a non-linear relationship between increasing plant stem density and predator foraging success. Logistic analysis demonstrated a significant fit of our data to a logistic model, from which was calculated the threshold level of plant stem desity necessary to reduce predator foraging success. Studies with various prey species have shown selection by prey for more complex habitats as a refuge from predation. In this stydy, we also examined the effects of increasing habitat complexity (i.e. plant stem density) on choice of habitat by juvenile bluegills while avoiding predation. Plant stem density significantly effected choice of habitat as a refuge. The relationship between increasing habitat complexity and prey choice of habitat was found to be positive and non-linear. As with predator foraging success, logistic analysis demonstrated a significant fit of our data to a logistic model. Using this model we calculated the threshold level of habitat complexity required before prey select a habitat as a refuge. This density of vegetation proved to be considerably higher than that necessary to significantly reduce predator foraging success, indicating that bluegill select habitats safe from predation.Implications of these results and various factors which may affect the relationships described are discussed.     

Behavioural Response of Juvenile Bluegill Sunfish to Variation in Predation Risk and Food Level

The behavioural response of juvenile bluegill sunfish (Lepomis macrochirus) to predation risk when selecting between patches of artificial vegetation differing in food and stem density was investigated. Bluegill foraging activity was significantly affected by all three factors. Regardless of patch stem density or risk of predation bluegills preferred patches with the highest prey number. During each trial bluegill foraging activity was clearly divided into a between- and within-patch component. In the presence of a predator bluegills reduced their between-patch foraging activity by an equivalent amount regardless of patch stem density or food level, apparently showing a risk-adjusting behavioural response to predation risk. Within patches, however, foraging activity was affected by both food level and patch stem density. When foraging in a patch offering a refuge from predation, the presence of a predator had no effect on bluegill foraging activity within this patch. However, if foraging in a patch with only limited refuge potential, bluegill foraging activity was reduced significantly in the presence of a predator. Further, this reduction was significantly greater if the patch contained a low versus a high food level, indicating a risk-balancing response to predation with respect to within-patch foraging activity. Both these responses differ from the risk-avoidance response to predation demonstrated by juvenile bluegills when selecting among habitats. Therefore, our results demonstrate the flexibility of juvenile bluegill foraging behaviour.     

Phenotypic variation and vulnerability to predation in juvenile bluegill sunfish (<Emphasis Type="Italic">Lepomis macrochirus</Emphasis>)

Bluegill sunfish (Lepomis macrochirus) are known to diversify into two forms specialized for foraging on either limnetic or littoral prey. Because juvenile bluegills seek vegetative cover in the presence of largemouth bass (Micropterus salmoides) predators, natural selection should favor the littoral body design at size ranges most vulnerable to predation. Yet within bluegill populations, both limnetic and littoral forms occur where vegetation and predators are present. While adaptive for foraging in different environments, does habitat-linked phenotypic variation also influence predator evasiveness for juvenile bluegills? We evaluate this question by quantifying susceptibility to predation for two groups of morphologically distinct bluegills; a limnetic form characteristic of bluegills inhabiting open water areas (limnetic bluegill) and a littoral form characteristic of bluegills inhabiting dense vegetation (littoral bluegill). In a series of predation trials, we found that bluegill behaviors differed in open water habitat but not in simulated vegetation. In open water habitat, limnetic bluegills formed more dense shoaling aggregations, maintained a larger distance from the predator, and required longer amounts of time to capture than littoral bluegill. When provided with simulated vegetation, largemouth bass spent longer amounts of time pursuing littoral bluegill and captured significantly fewer littoral bluegills than limnetic fish. Hence, morphological and behavioral variation in bluegills was linked to differential susceptibility to predation in open water and vegetated environments. Combined with previous studies, these findings show that morphological and behavioral adaptations enhance both foraging performance and predator evasiveness in different lake habitats.     

Variation in plant stem density and its effects on foraging success of juvenile bluegill sunfish

Synopsis Juvenile bluegill sunfish, Lepomis macrochirus, are known to use beds of aquatic vegetation as a refuge from predators. This study examines the effects of increasing plant stem density on juvenile bluegill foraging. Three stem densities (100, 250 and 500 stems m⁻²), varying in their refuge potential for bluegills from predators, were tested. Results demonstrate that stem densities chosen as a refuge from predation (i.e. 500 stems m⁻²) significantly reduced bluegill foraging success and increased time required to capture prey. Therefore, juvenile bluegills seeking safety in vegetation may be faced with a trade-off between foraging success and effective refuge from predation when choosing among plant stem densities.     

The Influence of Structural Complexity on Fish–zooplankton Interactions: A Study Using Artificial Submerged Macrophytes

Aquatic macrophytes produce considerable structural variation within the littoral zone and as a result the vegetation provides refuge to prey communities by hindering predator foraging activities. The behavior of planktivorous fish Pseudorasbora parva (Cyprinidae) and their zooplankton prey Daphnia pulex were quantified in a series of laboratory experiments with artificial vegetation at densities of 0, 350, 700, 1400, 2100 and 2800 stemsm^–2. Swimming speeds and foraging rates of the fish were recorded at different prey densities for all stem densities. The foraging efficiency of P. parva decreased significantly with increasing habitat complexity. This decline in feeding efficiency was related to two factors: submerged vegetation impeded swimming behavior and obstructed sight while foraging. This study separated the effects of swimming speed variation and of visual impairment, both due to stems, that led to reduced prey–predator encounters and examined how the reduction of the visual field volume may be predicted using a random encounter model.     

Optimal foraging: food patch depletion by ruddy ducks

Summary I studied the foraging behavior of ruddy ducks (Oxyura jamaicensis) feeding on patchily distributed prey in a large (5-m long, 2-m wide, and up to 2-m deep) aquarium. The substrate consisted of a 4x4 array of wooden trays (1.0-m long, 0.5-m wide, and 0.1-m deep) which contained 6 cm of sand. Any tray could be removed from the aquarium and loaded with a known number of prey. One bird foraged in the aquarium at a time; thus, by removing a food tray after a trial ended and counting the remaining prey, I calculated the number of prey consumed by the bird. I designed several experiments to determine if ruddy ducks abandoned a food patch in a manner consistent with the predictions of a simple, deterministic, patch depletion model. This model is based on the premise that a predator should maximize its rate of net energy intake while foraging. To accomplish this, a predator should only remain in a food patch as long as its rate of energy intake from that patch exceeds the average rate of intake from the environment. In the majority of comparisons, the number of food items consumed by the ruddy ducks in these experiments was consistent with the predictions of the foraging model. When the birds did not forage as predicted by the model, they stayed in the patch longer and consumed more prey than predicted by the model. An examination of the relation between rate of net energy intake and time spent foraging in the food patch indicated that by staying in a patch longer than predicted, the ruddy ducks experienced only a small deviation from maximum rate of net energy intake. These results provided quantitative support for the prediction that ruddy ducks maximize their rate of net energy intake while foraging.     

Consequences of behavioral dynamics for the population dynamics of predator-prey systems with switching

This article explores how different mechanisms governing the rate of change of the predators preference alter the dynamics of predator-prey systems in which the predator exhibits positive frequency-dependent predation. The models assume that individuals of the predator species adaptively adjust a trait that determines their relative capture rates of each of two prey species. The resulting switching behavior does not instantaneously attain the optimum for current prey densities, but instead lags behind it. Several mechanisms producing such lags are discussed and modeled. In all cases examined, our question is whether a realistic behavioral lag can significantly change the dynamics of the system relative to an analogous case in which the predators switching is effectively instantaneous. We also explore whether increasing the rate parameters of dynamic models of behavior results in convergence to the population dynamics of analogous models with instantaneous switching, and whether different behavioral models produce similar population dynamics. The analysis concentrates on systems that undergo endogenously generated predator-prey cycles in the absence of switching behavior. The average densities and the nature of indirect interactions are often sensitive to the rate of behavioral change, and are often qualitatively different for different classes of behavioral models. Dynamics and average densities can be very sensitive to small changes in parameters of either the prey growth or predator switching functions. These differences suggest that an understanding of switching in natural systems will require research into the behavioral mechanisms that govern lags in the response of predator preference to changes in prey density.     

Foraging efficiency of juvenile bluegill, Lepomis macrochirus, among different vegetated habitats

Optimal foraging theory is devoted to understanding how organisms maximize net energy gain. However, both the theory and empirical studies lack critical components, such as effects of environmental variables across habitats. We addressed the hypothesis that energetic returns of juvenile bluegill are affected by environmental variables characteristic of the vegetated habitats. Predicted optimal diet breadths were calculated and compared to prey items eaten by juvenile bluegill to determine if bluegill were foraging to maximize energetic gain. Differences in habitat profitability among vegetated sites were determined by comparing predictions of maximized energetic return rates (cals^-1) with prey contents of bluegill stomachs. Sizes of most prey items eaten by juvenile bluegill throughout the vegetated sites were smaller than the predicted optimal diet breadths. However, inclusion of smaller prey items in the diet did not seem to affect rate of energetic gain. Energetic return rates were maximized at the 1.5 and 2mm prey size classes and declined only slightly with inclusion of smaller prey sizes. Predicted energetic return rates and average mass in bluegill stomachs were related negatively. Average mass in bluegill stomachs also was associated negatively with Elodea canadensis stem densities and percent of light transfer, suggesting that foraging efficiency of bluegill decreased as plant density and percent of light increased. Results of our research indicate that maximization of energetic return rates is dependent upon availability of prey sizes that contribute to optimal foraging. Thus, determination of those habitats that provide the highest availability of benthic invertebrate prey with the least interference by stems is critical. Enhanced foraging capabilities can promote recruitment, faster growth, better body condition and survival.     

Foraging strategy of a non-omniscient predator in a changing environment (I) model with a data window and absolute criterion

1. Almost all the models so far presented assume that predators are omniscient in the sense that they always have complete information about the spatial distribution of prey abundance and its change over time. But this type of model cannot cover the situation where the prey abundance in each patch changes over time due to factors other than predation. The model with a data window and absolute criterion (SAC) here enables us to treat such situations.
2. The strategy of non-omniscient predators can be generally devided into four procedures; collection of information, its memorization, decision of tactics and its execution. SAC involves only two tactics; to stay another time period in the patch the predator is staying presently or to move to another patch chosen at random. The choice of either one of the two tactics is made by comparing the profitability of the current patch estimated by the data window with a pre-determined absolute criterion.
3. Three changing patterns of prey abundance are considered. In the most general pattern good patches have a higher mean profitability than poor patches, but the profitability changes cyclically in each of patches.
4. There are only two possibilities for an optimal strategy; the “patch choice strategy” in which once the predator has taken a good patch, it tries to stay there even when the state becomes poor, and the ‘state choice strategy” in which the predator seeks for only good states in good patches. The condition for which either of the two foraging strategies is superior to the other is specified analytically.

     

Predator interference alters foraging behavior of a generalist predatory arthropod

Interactions between predators foraging in the same patch may strongly influence patch use and functional response. In particular, there is continued interest in how the magnitude of mutual interference shapes predator–prey interactions. Studies commonly focus on either patch use or the functional response without attempting to link these important components of the foraging puzzle. Predictions from both theoretical frameworks suggest that predators should modify foraging efforts in response to changes in feeding rate, but this prediction has received little empirical attention. We study the linkage between patch departure rates and food consumption by the hunting spider, Pardosa milvina, using field enclosures in which prey and predator densities were manipulated. Additionally, the most appropriate functional response model was identified by fitting alternative functional response models to laboratory foraging data. Our results show that although prey availability was the most important determinant of patch departure rates, a greater proportion of predators left enclosures containing elevated predator abundance. Functional response parameter estimation revealed significant levels of interference among predators leading to lower feeding rates even when the area allocated for each predator was kept constant. These results suggest that feeding rates determine patch movement dynamics, where interference induces predators to search for foraging sites that balance the frequency of agonistic interactions with prey encounter rates.     

Stochastic aggregative responses and spatial patterns of parasitism in patchy host-parasitoid interactions

Summary Assuming random search by parasitoids within host-containing patches, and a constant search rate, current host-parasitoid models suggest that positive searching time aggregation by parasitoids in patches of high host density should tend to produce spatially density dependent parasitism at the patch level. However, these models view the aggregative response as a deterministic process, ignoring variability in searching time (T _s) allocation among patches of equal host density, and it is not clear that stochastic analogues of these deterministic models would predict the same result.This question is examined by adding a stochastic aggregative response to the well-known random parasitoid equation, the deterministic equation on which most existing models have been based. Simulations, based on data collected in an earlier laboratory study, indicate that this stochastic model generates very different relationships between parasitoid searching behavior and spatial patterns of parasitism than are predicted using the deterministic approach. The stochastic model suggests that positive aggregative responses, in which patches of high host density receive larger allocations of searching time (on the average) than patches containing lower densities, may fail to produce spatially density dependent parasitism at the patch level if searching time allocation is also more variable at the higher densities. Similarly, a flat response, in which mean searching times do not vary among patches of different host density, may lead to density dependent, density independent, or inversely density dependent parasitism, depending on the variance of the searching time values among patches at different density levels. The different predictions generated by the deterministic and stochastic models can be explained on purely mathematical grounds.When models are written in units of total foraging time (T _TOT), different equations are usually required to describe the spatial features of host-parasitoid and predator-prey interactions. Because the model considered here is written in units of active searching time (T _s) it should, in cases in which the underlying assumptions hold, be capable of describing these different interactions in the framework of a single (unified) equation. This equation may also apply to some plant-herbivore systems and, to indicate its potential generality, might be referred to as a random forager equation.     

Bayesian birds: A simple example of Oaten's stochastic model of optimal foraging

Allan Oaten (1977, Theor. Pop. Biol.12, 263–285) has argued that stochastic models of optimal foraging may produce results qualitatively different from those of the analogous deterministic models. Oaten's model is very general and difficult to understand intuitively. In this paper a simple, tractable model is considered in which the predator searches each patch systematically (without going over the same area twice) until he exhausts the patch or decides the patch is not very good. It is assumed that each patch contains a fixed number of bits, each of which may contain a prey. The number of prey per patch is assumed to have a binomial distribution with n equal to the number of bits and p being a random variable having a beta distribution. After searching each bit the predator decides whether to leave the patch or not according to how many prey it has found. In this paper the best strategy is determined and the long-term rate of feeding is compared with that of the naive animal that searches each patch completely. The advantage of being a Bayesian is determined for a variety of environmental conditions.     

Integrating disparate datasets to model the functional response of a marine predator: A case study of harbour porpoises in the southern North Sea

1. Quantifying consumption and prey choice for marine predator species is key to understanding their interaction with prey species, fisheries, and the ecosystem as a whole. However, parameterizing a functional response for large predators can be challenging because of the difficulty in obtaining the required data on predator diet and on the availability of multiple prey species.
2. This study modeled a multi‐species functional response (MSFR) to describe the relationship between consumption by harbour porpoises (Phocoena phocoena) and the availability of multiple prey species in the southern North Sea. Bayesian methodology was employed to estimate MSFR parameters and to incorporate uncertainties in diet and prey availability estimates. Prey consumption was estimated from stomach content data from stranded harbour porpoises. Prey availability to harbour porpoises was estimated based on the spatial overlap between prey distributions, estimated from fish survey data, and porpoise foraging range in the days prior to stranding predicted from telemetry data.
3. Results indicated a preference for sandeels in the study area. Prey switching behavior (change in preference dependent on prey abundance) was confirmed by the favored type III functional response model. Variation in the size of the foraging range (estimated area where harbour porpoises could have foraged prior to stranding) did not alter the overall pattern of the results or conclusions.
4. Integrating datasets on prey consumption from strandings, predator foraging distribution using telemetry, and prey availability from fish surveys into the modeling approach provides a methodological framework that may be appropriate for fitting MSFRs for other predators.

     

Responses of the trunk routes of a harvester ant to plant density

Messor barbarus is a Mediterranean harvester ant that constructs physically defined trunk routes on the ground to connect nest entrances with foraging areas. Some responses of these trunk routes to plant density (and therefore resource abundance) were analyzed by testing the preferential allocation of different parts (trunk route ends, segments and branching points) in a patchy environment. Maps of grass density in four categories and Messor barbarus trunk routes were compiled for a Mediterranean pasture in Central Spain over four consecutive years. The proportions of the density categories in each year were used to calculate random expected frequencies of the trunk route points and the predominance of higher or lower grass densities. Trunk route ends discriminate and selectively reach patches with a greater abundance of resources in all study years. Branching points are also allocated preferentially in areas with higher vegetation density, but only in years with a predominance of the higher categories of grass density. In these years, the colonies of Messor barbarus have a phalanx strategy at a colonial level, and branching is more profuse. Finally, trunk route segments do not indicate any preference for crossing determined vegetation densities, but rather connect successive branching points or trunk route ends by the shortest route. These results concur with a model of structural strategy change (guerilla — phalanx) (Hutchings 1988) at the level of trunk routes. They are probably constituted by transitory sections with few branches, that expand other more profusely branched sections which are more dedicated to foraging.     

Adaptive foraging by predators as a cause of predator-prey cycles

Summary When foraging has costs, it is generally adaptive for foragers to adjust their foraging effort in response to changes in the population density of their food. If effort decreases in response to increased food density, this can result in a type-2 functional response; intake rate increases in a negatively accelerated manner as prey density increases. Unlike other mechanisms for type-2 responses, adaptive foraging usually involves a timelag, because foraging behaviours do not often change instantaneously with changes in food density or risks. This paper investigates predator-prey models in which there are explicit dynamics for the rate of adaptive change. Models appropriate to both behavioural and evolutionary change are considered. Both types of change can produce cycles under similar circumstances, but under some evolutionary models there is not sufficient genetic variability for evolutionary change to produce cycles. If there is sufficient variability, the remaining conditions required for cycles are surprisingly insensitive to the nature of the adaptive process. A predator population that approaches the optimum foraging strategy very slowly usually produces cycles under similar conditions as does a very rapidly adapting population.     

Evolutionary responses of foraging-related traits in unstable predator–prey systems

The evolutionary responses of predators to prey and of prey to predators are analysed using models for the dynamics of a quantitative trait that determines the capture rate of prey by an average searching predator. Unlike previous investigations, the analysis centres on models and/or parameter values for which the two-species equilibrium is locally unstable. The instability in some models is driven by the predators non-linear functional response to prey; in other models, the cycles are a direct consequence of evolutionary response to selection acting on the trait. When the values of predator and prey traits combine multiplicatively to determine the capture rate, the predators trait shows only a transient response to changes in the preys trait in stable systems. However, when the population densities exhibit sustained oscillations, predators often evolve an increased long-term mean capture rate in response to an increased prey escape ability. Under the multiplicative model, prey in stable systems always evolve increased escape ability in response to an increased predator capture a     

Modeling dietary selectivity by Bornean orangutans: Evidence for integration of multiple criteria in fruit selection

Food patch visitation was compared to the availability of fruit patches of different species during 2 years in a Bornean lowland forest to examine orangutan (Pongo pygmaeus) diet selectivity. Feeding on both the pulp and the seeds of nonfig fruit varied directly with fruit patch availability, demonstrating preference for these foods over fig fruit or other plant parts (bark or leaves). Factors determining fruit selectivity rank were examined through multiple regression analysis. Modeling selectivity for 52 chemically unprotected primate-fruit pulp species revealed strong preferences for species of (i) large crop size (numbers of fruits ripening in an individual patch), (ii) high pulp weight/fruit, and (iii) high pulp mass per swallowed unit of pulp + seed, demonstrating orangutan sensitivity especially to patch size (g of pulp or total energy/patch) and perhaps to fruit handling time. Modeling selectivity for 18 fig species showed that 4 factors significantly influenced fig species rank: crop size, pulp weight/fruit, and 2 chemical variables, percentage digestible carbohydrate and percentage phenolic compounds in the fig fruit pulp. The selectivity rank based on the overall nutrient gain from feeding in the fruit patch (the product of the first 3 variables) is proportionally depressed by the percentage tannin content, demonstrating that orangutans integrate values for these variables in selecting fig patches. The conclusions from these results and from analysis of selectivity for seeds and for other fruit types are that orangutan foraging decisions are strongly influenced by the meal size expected from a feeding visit (i.e., by patch size), that tannins and other toxins deter feeding, and that the energy content, rather than the protein content, of foods is important in diet selection. The foraging strategy of orangutans is interpreted relative to these results and to Bornean fruiting phenology. By integrating spatial, morphometric, and chemical variables in analysis, this study is the first to demonstrate the application of foraging theory to separate out the key variables that determine diet selection in a primate. Multivariate analysis should routinely be applied to such data to distinguish among the many covarying attributes of food items and patches; inferences drawn in previous studies of primate diet selection, which ignore key spatial and morphological variables and rely on univariate correlations, are therefore suspect.     

Dynamics of herbivores and resources on a landscape with interspersed resources and refuges

A tradeoff between energy gain from foraging and safety from predation in refuges is a common situation for many herbivores that are vulnerable to predation while foraging. This tradeoff affects the population dynamics of the plant–herbivore–predator interaction. A new functional response is derived based on the Holling type 2 functional response and the assumption that the herbivore can forage at a rate that maximizes its fitness. The predation rate on the herbivore is assumed to be proportional to the product of the time that the herbivore spends foraging and a risk factor that reflects the habitat complexity; where greater complexity means greater interspersion of high food quality habitat and refuge habitat, which increases the amount of the edge zone between refuge and foraging areas, making foraging safer. The snowshoe hare is chosen as an example to demonstrate the resulting dynamics of an herbivore that has been intensely studied and that undergoes well-known cycling. Two models are studied in which the optimal foraging by hares is assumed, a vegetation–hare–generalist predator model and a vegetation–hare–specialist predator model. In both cases, the results suggest that the cycling of the snowshoe hare population will be greatly moderated by optimal foraging in a habitat consisting of interspersed high quality foraging habitat and refuge habitat. However, there are also large differences in the dynamics produced by the two models as a function of predation pressure.     

Effects of adaptive predatory and anti-predator behaviour in a two-prey—one-predator system

Summary Two prey populations that share a common predator can interact indirectly by causing changes in the predator's foraging behaviour. Previous work suggests that adaptive choice of prey by the predator usually has two related consequences: (i) the predation rate on a particular prey species increases with the relative and/or absolute abundance of that prey; and (ii) increases in either prey population produce a short-term increase in the fitness of the other prey (short-term indirect mutualism between prey). This paper investigates how these two consequences are changed if the prey exhibit adaptive anti-predator behaviour. In this case, the predation rate on a particular prey often decreases as the prey's density increases. The predator then usually exhibits negative switching between prey. However, the presence of adaptive antipredator behaviour does not change the short-term mutualism between prey. In this case, as a prey becomes less common, it achieves a larger growth rate by reducing its anti-predator effort. These results imply that observations of the relationship between prey density and predation rate cannot be used to infer the nature of the behavioural indirect effect between prey that share a predator.