Patch choice under predation hazard

In this paper we study optimal animal movement in heterogeneous environments consisting of several food patches in which animals trade-off energy gain versus predation risk. We derive a myopic optimization rule describing optimal animal movements by fitness maximization assuming an animal state is described by a single quantity (such as weight, size, or energy reserves). This rule predicts a critical state at which an animal should switch from a more dangerous and more profitable patch to a less dangerous and less profitable patch. Qualitatively, there are two types of behavior: either the animal switches from one patch to another and stays in the new patch for some time before it switches again, or the animal switches between two patches instantaneously. The former case happens if animal state growth is positive in all patches, while the latter case happens if animal state growth is negative in one patch. In particular, this happens if one patch is a refuge. We consider in detail two special cases. The first one assumes a linear animal state growth while the second assumes a saturating animal state growth described by the von Bertalanffy curve. For the first model the proportion of time spent in the more profitable and more risky patch increases with profitability of this patch when state growth is positive in both patches. On contrary, if state growth is negative in the less profitable and safer patch, animals spend proportionally less time in the more profitable and more risky patch as its profitability increases. As a function of the predation risk in the more profitable patch the time spent there proportionally decreases. When animal state growth is described by the saturating curve, time spent in the more risky patch is a hump-shaped curve if state growth is positive in both patches. Our results extend the mu/f rule, which predicts that animals should behave in such a way as to minimize mortality risk to resource intake ratio.     

Patch use by Dipodomys deserti (Rodentia: Heteromyidae): profitability,preference, and depletion dynamics

Summary Granivorous desert rodents of the family Heteromyidae forage nonrandomly among microhabitats that vary in substrate, seed densities, and seed species composition. To explore the hypothesis that microhabitat use is sensitive to seed patch profitability, we quantified effects of seed size (1.96 vs. 5.21 mg/seed) and density (0.4–10.6 seeds/cm²) on Dipodomys deserti harvest rates, which is a measure of profitability when expressed as mg of seed taken per min. By manipulating seed density, we created large-seed and small-seed patches of known relative profitability and exposed D. deserti individuals to pairwise choices in the laboratory and field. We used three treatment classes: 1) large-seed patches that were more profitable than small-seed patches (equal seed densities); 2) large-seed and small-seed patches that were equally profitable (small-seed densities somewhat higher): and 3) large-seed patches that were less profitable than small-seed patches (small-seed densities much higher). Harvest rate increased nearly linearly with seed density, and profitability of large-seed patches was greater than small-seed patches of the same density. Cumulative harvest from a patch increased linearly with residence time up to a plateau; this gain curve indicates that animals move systematically within patches and hence avoid resampling already depleted areas. In the laboratory, animals visited small-seed patches first more often and visited them more frequently when they were more profitable than large-seed patches. When large-seed patches were of greater or equal profitability, large-seed patches were preferred by both measures. The expressed preference for large-seed patches, when animals were presented with equally profitable patches, suggests an underlying preference for large seeds. In the field, animals depleted all patches to a constant low profitability, in accord with qualitative predictions of optimal patch use models. These results suggest that patch preferences by D. deserti are affected by the economics of seed harvest.     

Patch Choice Decisions among Ifaluk Fishers

Studies of patch choice decisions among human foragers have failed to explain why foragers do not exclusively exploit the patch with the highest mean profitability. One possible explanation is that profitability rankings are likely to vary daily; however, this instability is not captured when profitabilities are calculated as a sampled average over a longer time span. Here I present data on the patch choice decisions of Ifaluk fishers to evaluate whether men are responding to daily variation in the profitability of their primary fishing patch. Results show that men choose to fish most frequently in the patch with the highest mean profitability. Men fish in alternative patches (alternative from the most profitable patch) when, on that morning or the previous day, return rates in the most profitable patch are lower than the overall mean per capita return rate of alternative patches. Results also indicate that when fishers pursue alternative patches after fishing in the patch with the highest profitability, their mean per capita return rates are generally higher in the alternative patches exploited. However, variance in the profitability of the most profitable patch cannot explain why men exploit two patches, the Nine-mile reef and the dogtoothed tuna patch, which on average have very low profitability. These results and directions for future research are discussed. [Keywords: human behavioral ecology, patch choice decisions, Micronesia]     

Knowing your habitat: linking patch-encounter rate and patch exploitation in parasitoids

According to optimal foraging theory, animals should decidewhether or not to leave a resource patch by comparing the currentprofitability of the patch with the expected profitability ofsearching elsewhere in the habitat. Although there is abundantevidence in the literature that foragers in general are wellable to estimate the value of a single resource patch, theirdecision making has rarely been investigated with respect tohabitat quality. This is especially true for invertebrates.We have conducted experiments to test whether parasitic waspsadjust patch residence time and exploitation in relation tothe abundance of patches within the environment. We used thebraconid Asobara tabida, a parasitoid of Drosophila larvae,as our model species. Our experiments show that these waspsreduce both the residence time and the degree of patch exploitationwhen patches become abundant in their environment, as predictedby optimal foraging models. Based upon a detailed analysis ofwasp foraging behavior, we discuss proximate mechanisms thatmight lead to the observed response. We suggest that parasitoidsuse a mechanism of sensitization and desensitization to chemicalsassociated with hosts and patches, in order to respond adaptivelyto the abundance of patches within their environment.     

Foraging on patchily distributed prey by a cichlid fish (Teleostei,Cichlidae): A test of the ideal free distribution theory

Cichlid fish (Aequidens curviceps) distributed themselves and allocated their foraging time between two drift food patches in close approximation to the patch profitability ratio, as predicted by the ideal free distribution theory. The fish thereby achieved similar average feeding rates in the two patches, in two of three patch profitability ratio experiments. However, one major assumption of the ideal free model was violated, since individual fish differed in their competitive abilities for limited food resources, which resulted in unequal payoffs among individuals within each patch. Individual variation in feeding rates, and thus in competitive ability, was not related to despotism, but perhaps rather to individual differences in perceptual ability and in the ability to learn which patch was currently the more profitable. The strategy used by the fish to assess patch profitability included sampling available patches. However, individual fish switched (sampled) patches with varying frequency. Sampling had an associated cost, since high-frequency switchers had lower feeding rates on average than low-frequency switchers. Differences in foraging strategy among the fish therefore contributed to the observed in-equality in individual payoffs within patches.     

Learning to forage in a regenerating patchy environment: Can it fail to be optimal?

The behaviour of animals foraging along closed traplines of regenerating patches of food has been simulated using a learning rule that determines when an animal should leave the patch at which it is currently feeding to search for another one. The rule causes the animal to stay at the patch as long as it is feeding faster than it remembers doing. The foraging behaviour of one animal, and of two or more animals together, feeding in traplines containing patches of the same and of differing types has been simulated, and in all cases the foraging behaviour generated by the rule allowed the animals to exploit the food very efficiently. The learning model is also responsible for indirect social interactions among animals sharing the same trapline because the feeding of each animal reduces the availability of food for the others. This causes a population of animals to disperse themselves, on average, among patches of food according to the ideal free distribution. The relationship between the learning model and conventional optimal foraging models is examined and it is shown that it is pointless to try to account for learned behaviour in the context of optimal foraging theory.     

Foraging by <Emphasis Type="Italic">Drosophila melanogaster</Emphasis> Larvae in a Patchy Environment

The food resources of Drosophila comprise decaying vegetable matter distributed in patches, yet foraging behavior has not been examined in larvae reared continuously in a patchy environment. Here, the extent and rate of inter-patch movement was studied in larvae of four wild strains of D. melanogaster inhabiting an experimental arena from the egg stage to the third larval instar. The hypotheses were that larvae would forage primarily in the third instar, that larvae would move from low-protein patches at higher rates than from high-protein patches, and that foraging rates would be higher on an agar substrate than on sand. Larvae hatching on a nutrient-poor food patch switched to a nutrient-rich patch during the first instar. The rate of interpatch switching increased with larval age, as did the number of larvae roving on the substrate between food patches. Inter-patch distance affected switching speed---the closer the patches, the faster the switching. High protein patches were preferred over low-protein patches, but there was a bias towards staying on the natal patch. Significant variation among strains in latency to forage, in proportion of larvae that switched patches, and in the rate of roving between patches suggests that there is natural genetic variation for these traits. Larvae switched food patches on a substrate of moist sand as quickly as on an agar substrate.     

Maintaining social cohesion is a more important determinant of patch residence time than maximizing food intake rate in a group-living primate,Japanese macaque (Macaca fuscata)

Animals have been assumed to employ an optimal foraging strategy (e.g., rate-maximizing strategy). In patchy food environments, intake rate within patches is positively correlated with patch quality, and declines as patches are depleted through consumption. This causes patch-leaving and determines patch residence time. In group-foraging situations, patch residence times are also affected by patch sharing. Optimal patch models for groups predict that patch residence times decrease as the number of co-feeding animals increases because of accelerated patch depletion. However, group members often depart patches without patch depletion, and their patch residence time deviates from patch models. It has been pointed out that patch residence time is also influenced by maintaining social proximity with others among group-living animals. In this study, the effects of maintaining social cohesion and that of rate-maximizing strategy on patch residence time were examined in Japanese macaques (Macaca fuscata). I hypothesized that foragers give up patches to remain in the proximity of their troop members. On the other hand, foragers may stay for a relatively long period when they do not have to abandon patches to follow the troop. In this study, intake rate and foraging effort (i.e., movement) did not change during patch residency. Macaques maintained their intake rate with only a little foraging effort. Therefore, the patches were assumed to be undepleted during patch residency. Further, patch residence time was affected by patch-leaving to maintain social proximity, but not by the intake rate. Macaques tended to stay in patches for short periods when they needed to give up patches for social proximity, and remained for long periods when they did not need to leave to keep social proximity. Patch-leaving and patch residence time that prioritize the maintenance of social cohesion may be a behavioral pattern in group-living primates.     

Wild Bornean orangutan (Pongo pygmaeus wurmbii) feeding rates and the Marginal Value Theorem

The Marginal Value Theorem (MVT) is an integral supplement to Optimal Foraging Theory (OFT) as it seeks to explain an animal's decision of when to leave a patch when food is still available. MVT predicts that a forager capable of depleting a patch, in a habitat where food is patchily distributed, will leave the patch when the intake rate within it decreases to the average intake rate for the habitat. MVT relies on the critical assumption that the feeding rate in the patch will decrease over time. We tested this assumption using feeding data from a population of wild Bornean orangutans (Pongo pygmaeus wurmbii) from Gunung Palung National Park. We hypothesized that the feeding rate within orangutan food patches would decrease over time. Data included feeding bouts from continuous focal follows between 2014 and 2016. We recorded the average feeding rate over each tertile of the bout, as well as the first, midpoint, and last feeding rates collected. We did not find evidence of a decrease between first and last feeding rates (Linear Mixed Effects Model, n = 63), between a mid-point and last rate (Linear Mixed Effects Model, n = 63), between the tertiles (Linear Mixed Effects Model, n = 63), nor a decrease in feeding rate overall (Linear Mixed Effects Model, n = 146). These findings, thus, do not support the MVT assumption of decreased patch feeding rates over time in this large generalist frugivore.     

Biological activities as patchiness driving forces in wetlands of northern Belize

Patchiness in wetlands is a common and well documented phenomenon. Oligotrophic wetlands of northern Belize display noticeable vegetation heterogeneity at both large and small scales. In this paper, we document the small scale patches in herbaceous wetlands, describe differences between patches and surrounding wetland habitats and explain patch formation and sustenance. We conducted a survey of patches and confirmed their occurrence by spatial analysis. Patches were distinguished from a surrounding wetland by denser and taller vegetation, higher amount of empty snail shells and elevated soil phosphorus (P). Plants in patches had higher tissue nitrogen (N) and P content and there was also higher total N and P per m² incorporated in plant biomass. In terms of stable isotopes, plants in patches were enriched in ¹⁵N; patch soils were depleted in ¹³C. Observations of focal individuals of Aramus guarauna, limpkin, a wading bird feeding almost exclusively on snails, revealed the origin of the snail shell piles frequently found in patches. An adult limpkin captured on average 18 snails daily, of these 80% were handled in patches and birds often repeatedly used the same patch. Experimental patch creation by adding chicken manure or P to 1 m² plots resulted in higher and denser vegetation with values increasing in order: control, P, manure plots. The effect was significant at both experimental locations six months after the treatment and at one location even 40 months after the treatment. We present a simple mechanistic explanation for nutrient redistribution in wetlands and their eventual accumulation in patches. Both nutrient and isotopic differences result from animal input into patches, e.g. bird droppings or prey remnants. Foraging activity of Aramus guarauna is most likely responsible for patch formation. A positive feedback (repeated use of a suitable patch) is apparently the mechanism sustaining patches in these marsh environments.     

Last Trip Return Rate Influence Patch Choice Decisions of Small-Scale Shrimp Trawlers: Optimal Foraging in São Francisco,Coastal Brazil

Studies using Optimal Foraging Theory to understand human behavior have stated that daily variation in patch profitability could explain mismatches between theoretical predictions and actual behavior. In this paper, we tested whether the return rate of the last fishing trip could predict fishers’ choices to return or choose a different fishing ground for their next trip. We collected data on fishing trips using interviews and direct observation of fishers’ activities at the main landing point in São Francisco, a small-scale shrimp fishing community on Brazil’s southern coast. We found that fishers returned more often to fishing grounds where the return rate of the previous fishing trip was above the average gross return of the environment. Daily variations in patch quality accounted for fishers’ decisions, but other factors may also influence the observed behavior, such as scale of analysis, information exchange, environmental conditions, and economic variables.     

Do cattle egrets gain information from conspecifics when foraging?

Summary We examined whether individual cattle egrets (Bubulcus ibis) base their decisions of where to forage, and how long to stay in a patch, on the behavior of other flock members. Cattle egrets commonly forage in flocks associated with cattle and capture prey at higher rates when they do not share a cow with another egret. Foraging egrets provide cues of the location of prey and their success in capturing prey. Therefore, there is the possibility of information transfer between egrets in a flock. We predicted that egrets should only move to occupied patches when the resident was capturing enough prey that it is profitable for the invader to share the patch or take over the patch. However, egrets did not seem to decide where to forage based on neighbors' rates of energy intake, but rather on the presence or absence of conspecifics in a patch. We also predicted that an egret should remain in a patch until its rate of energy intake dropped to or below the average rate for other egrets within the flock. However, egrets that were foraging more efficiently than the average rate for the flock switched patches sooner than less efficient foragers. Egrets did not appear to increase foraging success by gaining information on patch quality from neighbors.     

On optimal diet in a patchy environment

Optimal foraging theory has dealt with the following questions independently: (1) On what prey types should an individual predator feed (optimal diet)? (2) How long should a predator stay in each patch if prey is patchily distributed (optimal allocation of time to patches) ? This paper explores optimal foraging in patches containing several different kinds of prey. Results obtained by simulation show that deviations from recent predictions are to be expected, particularly for long interpatch travel times and rapid depletion of profitable prey types. In these situations the tactics of feeding as either specialist or as a generalist can be inferior to a tactic which starts as a specialist and then expands the diet after some time in the patch. Furthermore, predators should not necessarily stay longer in a patch if interpatch travel time increases. Some experimental tests of these new predictions are proposed.     

Habitat and Life History Determinants of Antbird Occurrence in Variable-Sized Amazonian Forest Fragments

The highest avian species richness on Earth is found in the Neotropics, with the speciose antbird superfamily (Thamnophilidae, Formicariidae, Grallariidae and Conopophagidae) accounting for 15 percent of South American passerine diversity. Antbird species have divergent life histories and ecological requirements, resulting in considerable interspecific variation in responses to anthropogenic habitat modification. Here, we examine interspecific differences in antbird responses to both habitat fragmentation and perturbation in a region of the so-called ‘Arc of Deforestation’ of southern Brazilian Amazonia in northern Mato Grosso. We surveyed the antbird community of 31 variable-sized forest patches and found that antbird species richness was predominantly affected by patch size and isolation, although forest patch quality was also important. Life history predictors were less important overall in determining patch occupancy and minimum patch area requirements with body mass and geographic range size the most important predictors. Foraging niche was also important; mixed flock followers, bamboo specialists and army-ant followers were all more prone to local extinction in small fragments. Although most Amazonian antbird species are not currently threatened, rates of interfluvial endemism are high and future forest loss may imperil many species currently considered to be of low conservation concern. Lessons learnt in the identification of fragmentation-sensitive genera and guilds may be applicable to other antbird species outside Amazonia, such as those in the Brazilian Atlantic Forest. Ensuring future survival of antbirds across neotropical forest landscapes that retain only a small percentage of their original primary forest cover will rest on protecting remaining large forest patches and maintaining structural and functional connectivity between them.     

Habitat corridors function as both drift fences and movement conduits for dispersing flies

Corridors connect otherwise isolated habitat patches and can direct movement of animals among such patches. In eight experimental landscapes, we tested two hypotheses of how corridors might affect dispersal behavior. The Traditional Corridor hypothesis posits that animals preferentially leave patches via corridors, following them into adjacent patches. The Drift Fence hypothesis posits that animals dispersing through matrix habitat are diverted into patches with corridors because they follow corridors when encountered. House flies (Musca domestica L.), a species that prefers the habitat of our patches and corridors, were released in a central patch (100×100 m) and recaptured in peripheral patches that were or were not connected by a corridor. Flies were captured more frequently in connected than unconnected patches, thereby supporting the Traditional Corridor hypothesis. The Drift Fence hypothesis was also supported, as flies were captured more frequently in unconnected patches with blind (dead end) corridors than in unconnected patches of equal area without blind corridors. A second experiment tested whether these results might be dependent on the type of patch-matrix boundary encountered by dispersing flies and whether edge-following behavior might be the mechanism underlying the observed corridor effect in the first experiment. We recorded dispersal patterns of flies released along forest edges with dense undergrowth in the forest (“closed” edges) and along edges with little forest understory (“open” edges). Flies were less likely to cross and more likely to follow closed edges than open edges, indicating that when patch and corridor edges are pronounced, edge-following behavior of flies may direct them along corridors into connected patches. Because edges in the first experiment were open, these results also suggest that corridor effects for flies in that experiment would have been even stronger if the edges around the source patches and corridors had been more closed. Taken together, our results suggest that corridors can affect dispersal of organisms in unappreciated ways (i.e., as drift fences) and that edge type can alter dispersal behavior.     

Downy woodpecker foraging behavior: foraging by expectation and energy intake rate

Summary I describe an artificial patch system that was used to study the foraging behavior of free-roaming downy woodpeckers (Picoides pubescens) in a woodlot in southeastern Michigan. The artificial patches used were thin logs into which were drilled small holes to hold food items (bits of sunflower seed kernels). Downy woodpeckers would systematically search the holes of a patch for food items and thus by manipulating the food distribution within the patches, the birds could be made to experience differing rates of energy intake while foraging.Simple deterministic theories of optimal foraging in patchy environments indicate that an optimal forager, who experiences a decreasing rate of energy intake while foraging in a patch, should leave a patch when its rate of energy intake falls below the average intake rate for the overall environment. In other words, an optimal forager is continually assessing the quality of a patch and makes decisions as to when to leave a patch via its energy intake rate. When the downy woodpeckers studied could encounter any one of several types of patches each with differing, decreasing rates of energy intake, they followed a patch quality assessment strategy similar to that suggested by theory. Upon encountering a single type of patch for a number of consecutive days, however, the birds appeared to forage according to prior expectations of patch quality and not according to a quality assessment strategy based on energy intake rates. The observed expectations were not related to the number of food items per patch but they appeared to be based on expectations of when or where to leave a patch.     

Antipredator response of pea aphids <Emphasis Type="Italic">Acyrthosiphon pisum</Emphasis> (Hemiptera: Aphididae): effects of predation risks from an alternative patch on a current patch

To escape from predators, herbivorous prey could leave their current patch and relocate to an alternative patch. However, when other predators are present on the new patch, prey are again exposed to predation risk. Thus, patch leaving might be affected by the other predators. We studied patch leaving of pea aphids Acyrthosiphon pisum Harris (Hemiptera: Aphididae) in response to ladybird larvae Harmonia axyridis Pallas (Coleoptera: Coccinellidae) on broad bean Vicia faba L. shoots that were offered as patches for aphids. We tested whether shoot leaving was affected by the presence of predators on alternative shoots under laboratory conditions. Odors from alternative shoots were evaluated as possible cues used by aphids to assess predation risk on the shoots. We exposed aphids to odors from alternative shoots with conspecifics plus either adult or larval ladybirds or larval green lacewings Mallada desjardinsi Navas (Neuroptera: Chrysopidae). Shoot leaving was reduced only when adult ladybirds were present on the alternative shoots compared with controls (i.e., no predators on the alternative shoots). Odors of both adult ladybirds and of conspecifics being attacked by ladybird larvae were required for reduced leaving. Hence, predation risks on current and alternative patches might affect the antipredator responses of aphids.     

Skuas at penguin carcass: patch use and state-dependent leaving decisions in a top-predator

Foraging decisions depend not only on simple maximization of energy intake but also on parallel fitness-relevant activities that change the forager's 'state'. We characterized patch use and patch leaving rules of a top-predatory seabird, the Brown Skua (Catharacta antarctica lonnbergi), which during its reproductive period in the Antarctic establishes feeding territories in penguin colonies. In feeding trials, we observed how skuas foraged at penguin carcass patches and analysed patch leaving decisions by incorporating the estimated state of foraging birds and patch availability.Patches were exploited in a characteristic temporal pattern with exponentially decreasing remaining patch sizes (RPSs) and intake rates. Patch size decreased particularly fast in small compared to large patches and exploitation ended at a mean RPS of 47.6% irrespective of initial size.We failed to identify a measure which those birds equalized upon patch departure from raw data. However, when accounting for the birds' state, we ascertained remaining patch size and intake rates to have the lowest variance at departure whereas food amount and feeding time remained variable. Statistical correction for territory size only and combined with state had lower effects, but remaining patch size remained the measure with lowest coefficient of variation. Thus, we could clearly reject a fixed-time or fixed-amount strategy for territorial skuas and rather suggest a state-dependent strategy that equalizes remaining patch size. Thus our results provide evidence that under natural conditions, territorial skuas adjust their foraging decision on actual energy requirements, i.e. offspring number and age.     

Animal response to nested self-similar patches: a test with woolly bears

To gain insight into how animals respond to resource patchiness at different spatial scales, we envision their responses in environments comprised of nested, self-similar patches. In these environments, all resources reside within the smallest patches, and resource density declines as a constant exponent of patch size. Accordingly, we use simple mathematical formulations to describe a self-similar environment and a null model of how animals should respond to this environment if they do not perceive resource distribution. We then argue that animals that can perceive resource distribution should partition space by reducing the relative time searching between patches as patch size increases. On an experimental landscape, we found that woolly bear caterpillars Grammia geneura could partition space in this manner, but the range of patch sizes over which they did so tended to increase with resource aggregation. Nevertheless, scaling efficiency (i.e. the scaling of search time versus the scaling or resource density) was similar in all distributions when averaged over all patch sizes. These disparate patterns with similar outcomes resulted from differences in caterpillars' abilities to discriminate spatially among patches of different sizes via their movement pathways, and differences in their use of speed to detect resource items. Our work is relevant to the characterization of resource availability from an animal's perspective, and to the linking of optimal foraging theory to the modeling of search behavior.     

Exploitation of food resources by the cockroach Blattella germanica in an urban habitat

The exploitation of food resources by the German cockroach, Blattella germanica (L.) (Dictyoptera: Blattellidae) was investigated experimentally in relation to distance from shelters and depletion of neighbouring food patches. In addition, the dynamics of exploitation of a patch were analysed. Observations were made after dark in a public swimming baths building and each one lasted 3 h. Food patches were placed in rows, at different distances from the shelters. The number of cockroaches in food dishes, in a 20 cm diameter circle round each food dish and in a 60 cm diameter circle round this first circle were recorded.Food items nearest the shelters were exploited first. Exploitation of row 2 and of row 3 food items started later, after row 1 food patches had been depleted. Under these conditions, the moment a food patch was exploited was related to its distance from shelter. Exploitation of food patches occurred in a step-by-step manner, one patch attracting animals when a nearby patch had been depleted, and not following a model of ideal free distribution.Although our experimental food patches were exploited in relation to their distance from shelter, we were able to demonstrate that distance did not influence the dynamics of exploitation of a food item. The mean number of cockroaches on a food patch, whatever its spatial position, increased regularly, reached a maximum at t=–10 min, and then decreased rapidly after all the food had been completely consumed, at t=0 min. The mean number of animals in the 20 cm diameter circle round a food source peaked at t=0 min, then decreased rapidly. This area appeared to be a transit area. The mean number of animals in a 60 cm diameter circle round the food source peaked later, and then decreased slowly. Animals remained in this area longer than in the area closer to the food dish, but their presence there was concomitant with the depletion of the food box.