Costs of aggregation: shadow competition in a sit-and-wait predator

Abstract: The last decade has seen a surge of interest among biologists in a range of social animal phenomena, including collective behaviour and social networks. In Animal Social Behaviour , authors Ashley Ward and Michael Webster integrate the most up-to-date empirical and theoretical research to provide a new synthesis of the field, which is aimed at fellow researchers and postgraduate students on the topic

Abstract: There is a large body of evidence indicating that predator behavior may strongly influence patterns and processes at the population and community level. Site selection is a major component of fitness in sit-and-wait predators, especially when relocation is rare. Although several review articles dealt with these issues in web-building spiders, this is the first attempt to summarize the effects of biotic and abiotic factors on site selection and relocation in another group of sit-and-wait predators, the pit-building antlions (Neuroptera: Myrmeleontidae). Our synthesis shows that prey abundance may have relatively little effect on pit relocation and that physical properties of the habitat or competition often override its effect. We suggest that owing to a variety of constraints such as physiological constraints or difficulties in assessing site quality, site selection and relocation are not necessarily optimal and thus food intake rate is not maximized. We call for a multi-factorial study on a single species in order to pinpoint the dominant factors and to assess to what extent they influence site selection and relocation. We conclude by proposing new research directions, such as studying whether pit relocation is an adaptive response, when controlling for possible phylogenetic effects.

Abstract: Foraging theory was first developed to predict the behaviour of widely-foraging animals that actively search for prey. Although the behaviour of sit-and-wait predators often follows predictions derived from foraging theory, the similarity between these two distinct groups of predators is not always obvious. In this review, we compare foraging activities of trap-building predators (mainly pit-building antlions and web-building spiders), a specific group of sit-and-wait predators that construct traps as a foraging device, with those of widely-foraging predators. We refer to modifications of the trap characteristics as analogous to changes in foraging intensity. Our review illustrates that the responses of trap-building and widely-foraging predators to different internal and external factors, such as hunger level, conspecific density and predation threat are quite similar, calling for additional studies of foraging theory using trap-building predators. In each chapter of this review, we summarize the response of trap-building predators to a different factor, while contrasting it with the equivalent response characterizing widely-foraging predators. We provide here evidence that the behaviour of trap-building predators is not stereotypic or fixed as was once commonly accepted, rather it can vary greatly, depending on the individual's internal state and its interactions with external environmental factors.

Abstract: Animal groups are highly variable in their spatial structure, and individual fitness is strongly associated with the spatial position of an animal within a group. Predation risk and food gains are often higher at the group peripheries; thus, animals must trade-off predation costs and foraging benefits when choosing a position. Assuming this is the case, we first use simulation models to demonstrate how predation risk and food gains differ for different positions within a group. Second, we use the patterns from the simulation to develop a novel model of the trade-off between the costs and the benefits of occupying different positions and predict the optimal location for an animal in a group. A variety of testable patterns emerge. As expected, increasing levels of satiation and vulnerability to predators and increasing predation risk result in increased preferences for central positions, likely to lead to increased competition or more tightly packed groups. As food availability increases, individuals should first prefer center positions, then edge, and returning to central positions under highest food levels. Increasing group size and/or density lead to more uniform preferences across individuals. Finally, we predict some situations where individuals differing in satiation and vulnerability prefer a range of different locations and other situations where there is an abrupt dichotomy between central and edge positions, dependent on the levels of monopolization of food by peripheral individuals. We discuss the implications of our findings for the structure of groups and the levels of competition within them and make suggestions for empirical tests.

Abstract: Shadow competition occurs in a group of sit-and-wait predators when those closer to a source of mobile prey reduce the feeding success of those further from the prey source. It was examined in territorial juvenile sea trout Salmo trutta in a small stream. The fry formed groups of two to six fish with adjacent territories and a social hierarchy within each group. It was hypothesized that: (i) as group size increased, the mean number of prey eaten per fish within a group decreased and the variability in prey consumption between fish increased; (ii) prey consumption by individual fish decreased with increasing distance from the food source; (iii) group size increased as the mean water velocity immediately upstream from a group, and hence potential drifting food, increased. Five groups of fry were fed on small shrimps released upstream from each group at a rate of one every 15 s over a 10 min period, this procedure being repeated over 5 days to provide five replicates per group. Experiments were performed three times in 1967, 1969 and 1974 to provide information on 45 groups of fry. The first and third hypotheses were supported, but the second was only partially supported. In 1967 and 1969, territory size and shrimp consumption by individual fry decreased with increasing distance from the food source. This also occurred in 1974, except during a critical period for survival when fry density was exceptionally high with large numbers of sea trout lacking territories. This resulted in sea trout fry with the largest territories eating fewer shrimps than those with medium-sized territories because they spent more time defending their territories against sea trout lacking territories. This study is the first to demonstrate shadow competition in a vertebrate species, but has also shown that territorial defence may modify the consequences of shadow competition when densities are high and there is strong competition for the acquisition of a territory.

Abstract: A revised and updated edition of this textbook. As before the study of plant and animal ecology is fully integrated and mathematical and laborotory studies are described where necessary. Examples drawn from real ecological systems ilustrate the complexity of this subject and the involvement of diverse areas: descriptive natural history, physiology, behaviour, field experimentation and mathematical modelling. The authors attempt to encourage the reader to combine all the above facets, so that a balanced view emerges. Emphasis is placed on the clear presentation of mathematical aspects and the material aims to be accessible to the undergraduate with little experience and also stimulating to practising ecologists. This new edition contains two new chapters both of which consider areas of applied ecology which are relevant to contemporary global issues. The first considers the control of abundance; on the one hand the limiting of pest and weed populations, on the other, maintaining the biological diversity of endangered species. The second considers large areas as an extension of the discussion of communities. These "ecosystems" in the newest sense of the word are an important field of study when considering some of the important environmental problems facing mankind.

Abstract: This paper presents an antithesis to the view that gregarious behaviour is evolved through benefits to the population or species. Following Galton (1871) and Williams (1964) gregarious behaviour is considered as a form of cover-seeking in which each animal tries to reduce its chance of being caught by a predator. It is easy to see how pruning of marginal individuals can maintain centripetal instincts in already gregarious species; some evidence that marginal pruning actually occurs is summarized. Besides this, simply defined models are used to show that even in non-gregarious species selection is likely to favour individuals who stay close to others. Although not universal or unipotent, cover-seeking is a widespread and important element in animal aggregation, as the literature shows. Neglect of the idea has probably followed from a general disbelief that evolution can be dysgenic for a species. Nevertheless, selection theory provides no support for such disbelief in the case of species with outbreeding or unsubdivided populations. The model for two dimensions involves a complex problem in geometrical probability which has relevance also in metallurgy and communication science. Some empirical data on this, gathered from random number plots, is presented as of possible heuristic value.

Abstract: Preface Acknowledgements 1. The spider in the ecological play 2. Hungry spiders 3. Competitionist views of spider communities 4. Failure of the competitionist paradigm 5. How spiders avoid competition 6. Impact of spiders on insect populations 7. Anchoring the ecological web 8. Untangling a tangled web 9. Spinning a stronger story References Index.

Abstract: Abundant and ecologically important in many ecosystems as predators of insects (including agricultural pests), spiders have recently gained attention as a result of a number of significant studies. In addition, because of their small size, short lifespan, and the strong influence of genetic control on their behavior, spiders are regarded with keen interest as model organisms in behavioral ecology.

Abstract: Colonial web-building spiders respond to trade-offs between selective forces relative to spatial position within colonies and thus provide support for the selfish herd theory The size distribution of spiders within colonies of Metepeira incrassata, a colonial orb-weaver (Araneae: Araneidae) from tropical Mexico is nonrandom; larger (mature) spiders and females guarding eggsacs are more prevalent in the center, whereas more small (immature) spiders are found on the periphery Experimental field studies with spiders of selected size classes show that larger spiders actively and aggressively seek protected positions in the center of the colony webbing, even though prey availability and capture rates are significantly higher on the periphery Attacks by predatory wasps, other spiders, and hummingbirds are more frequent on the periphery than in the core of the colony Reproductive females on the periphery are at greater risk because they are captured more often than smaller spiders, and if their egg sacs consequently remain unguarded, chances of cocoon parasitism are increased As a result, spiders in the core of the colony have greater reproductive success, producing more egg sacs with greater hatching frequency Colonial spiders thus appear to be making a trade-off between foraging and protection from predation and show a spatial organization predicted by the selfish herd theory The influence of such trade-offs on individual fitness and the structure of colonies is discussed