2 results
23 - State-dependent foraging rules for social animals in selfish herds
- from Part III - Action selection in social contexts
- Edited by Anil K. Seth, University of Sussex, Tony J. Prescott, University of Sheffield, Joanna J. Bryson, University of Bath
-
- Book:
- Modelling Natural Action Selection
- Published online:
- 05 November 2011
- Print publication:
- 10 November 2011, pp 523-537
-
- Chapter
- Export citation
-
Summary
Summary
Many animals gain benefits from living in groups, such as a dilution in predation risk when they are closely aggregated (referred to as the ‘selfish herd’). Game theory has been used to predict many properties of groups (such as the expected group size), but little is known about the proximate mechanisms by which animals achieve these predicted properties. We explore a possible proximate mechanism using a spatially explicit, individual-based model, where individuals can choose to rest or forage on the basis of a rule of thumb that is dependent upon both their energetic reserves and the presence and actions of neighbours. The resulting behaviour and energetic reserves of individuals, and the resulting group sizes, are shown to be affected both by the ability of the forager to detect conspecifics and areas of the environment suitable for foraging, and by the distribution of energy in the environment. The model also demonstrates that if animals are able to choose (based upon their energetic reserves) between selecting the best foraging sites available, or moving towards their neighbours for safety, then this also has significant effects upon individuals and group sizes. The implications of the proposed rule of thumb are discussed.
Introduction
When animals form groups, it is often assumed that each individual faces various costs and benefits of group membership (Giraldeau and Caraco, 2000; Krause and Ruxton, 2002; Pulliam and Caraco, 1984). For example, within a foraging group, benefits could come through an increased likelihood of finding food or detecting predators, while costs could come through increased competition for resources, or increased visibility to predators. Much theoretical work has been conducted examining how the trade-off between these costs and benefits can determine the stable size of a group (Clark and Mangel, 1984; Ekman and Rosander, 1987; Giraldeau and Caraco, 2000; Higashi and Yamamura 1993; Sibly, 1983), and how these predictions match with empirical observations (Krause and Ruxton, 2002). However, although these studies have considered which group sizes should be stable from a functional perspective, little work has been conducted examining the proximate mechanisms resulting in the formation of these groups: recent models (e.g., Flierl et al., 1999; Juanico et al., 2003) have considered the actions of individuals following extremely simple rules of thumb. However, as noted by Krause and Ruxton (2002), little consideration has been given to making these rules realistic. State-dependent models of behaviour (Clark and Mangel, 2000; Houston and McNamara, 1999) offer us a means of predicting realistic rules, by considering which behaviours at a particular moment in time an animal with a given state set (such as its energy reserves, or the environment it currently occupies) should conduct in order to maximise some measure of its fitness. Therefore, unlike previous spatially-explicit models considering group formation behaviour, the model presented in this chapter bases its rules upon the results of state-dependent models (Rands and Johnstone, 2006; Rands et al., 2003, 2008).
Intake rates and the functional response in shorebirds (Charadriiformes) eating macro-invertebrates
- John D. Goss-Custard, Andrew D. West, Michael G. Yates, Richard W. G. Caldow, Richard A. Stillman, Louise Bardsley, Juan Castilla, Macarena Castro, Volker Dierschke, Sarah. E. A. Le. V. dit Durell, Goetz Eichhorn, Bruno J. Ens, Klaus-Michael Exo, P. U. Udayangani-Fernando, Peter N. Ferns, Philip A. R. Hockey, Jennifer A. Gill, Ian Johnstone, Bozena Kalejta-Summers, Jose A. Masero, Francisco Moreira, Rajarathina Velu Nagarajan, Ian P. F. Owens, Cristian Pacheco, Alejandro Perez-Hurtado, Danny Rogers, Gregor Scheiffarth, Humphrey Sitters, William J. Sutherland, Patrick Triplet, Dave H. Worrall1, Yuri Zharikov, Leo Zwarts, Richard A. Pettifor
-
- Journal:
- Biological Reviews / Volume 81 / Issue 4 / November 2006
- Published online by Cambridge University Press:
- 24 July 2006, pp. 501-529
- Print publication:
- November 2006
-
- Article
- Export citation
-
As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free-living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual-based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro-invertebrates. Intake rate is measured as the ash-free dry mass (AFDM) per second of active foraging; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type II (‘disc equation’) formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching.
A review of 30 functional responses showed that intake rate in free-living shorebirds varied independently of prey density over a wide range, with the asymptote being reached at very low prey densities (<150/m−2). Accordingly, most of the many studies of shorebird intake rate have probably been conducted at or near the asymptote of the functional response, suggesting that equations that predict intake rate should also predict the asymptote.
A multivariate analysis of 468 ‘spot’ estimates of intake rates from 26 shorebirds identified ten variables, representing prey and shorebird characteristics, that accounted for 81% of the variance in logarithm-transformed intake rate. But four-variables accounted for almost as much (77.3%), these being bird size, prey size, whether the bird was an oystercatcher Haematopus ostralegus eating mussels Mytilus edulis, or breeding. The four variable equation under-predicted, on average, the observed 30 estimates of the asymptote by 11.6%, but this discrepancy was reduced to 0.2% when two suspect estimates from one early study in the 1960s were removed. The equation therefore predicted the observed asymptote very successfully in 93% of cases.
We conclude that the asymptote can be reliably predicted from just four easily measured variables. Indeed, if the birds are not breeding and are not oystercatchers eating mussels, reliable predictions can be obtained using just two variables, bird and prey sizes. A multivariate analysis of 23 estimates of the half-asymptote constant suggested they were smaller when prey were small but greater when the birds were large, especially in oystercatchers. The resulting equation could be used to predict the half-asymptote constant, but its predictive power has yet to be tested.
As well as predicting the asymptote of the functional response, the equations will enable research workers engaged in many areas of shorebird ecology and behaviour to estimate intake rate without the need for conventional time-consuming field studies, including species for which it has not yet proved possible to measure intake rate in the field.