Individual ecology can improve our overall understanding of ecological systems, as well as providing detailed predictions for individual sites. This page summarises some of the general insights into individual behaviour and population ecology that have arisen through our research.
Predator and prey size determine feeding rate
An early limitation in the development of individual-based models for wading birds was a reliable and quick method for predicting the rate at which these animals consumed food. Measuring feeding rate can be time consuming, and so if this were required for each new site, individual-based models could have taken a number of years to apply, limiting their usefulness for addressing pressing environmental issues. The breakthrough was to discover that two main variables controlled the rate at which the birds consumed prey biomass: the body mass of the bird; and the mass of the prey being consumed. Larger birds consume prey biomass at a higher rate, and birds costuming larger prey also have a higher rate of consuming prey biomass. Since bird and prey mass can be measured quickly, this has allowed individual-based models to be developed within a much shorter timescale, sometimes with a couple of months, once the abundance and size distribution of the prey have been measured.
Goss-Custard, J.D., West, A.D., Yates, M.G., Caldow, R.W.G., Stillman, R.A., Bardsley, L., Castilla, J., Castro, M., Dierschke, V., Durell, S.E.A. le V. dit, Eichhorn, G., Ens, B.J., Exo, K.-M., Fernando, P.U.U., Ferns, P.N., Hockey, P.A.R., Gill, J.A., Johnstone, I., Kalejta-Summers, B., Masero, J.A., Moreira, F., Nagarajan, R. V., Owens, I.P.F., Pacheco, C., Perez-Hurtado, A., Rogers, D., Scheiffarth, G., Sitters, H., Sutherland, W.J., Triplet, P., Worrall, D.H., Zharikov, Y., Zwarts, L. & Pettifor, R.A. (2006) Intake rates and the functional response in shorebirds (Charadriiformes) eating macro-invertebrates. Biological Reviews, 81, 1-29.
General rules determine the strength of food competition
Another early limitation in the rapid development of individual-based models for wading birds was lack of understanding of the strength of interference competition between the birds. Interference occurs when animals either fight over prey, avoid more dominant individuals, or when prey become harder to capture at high competitor densities (e.g. they retreat into burrows). The breakthrough occurred when a small-scale individual-based model considering local competitive interactions was used to predict how the strength of interference depended on processes including the time to consume prey, the distance over which birds fight for food and avoid competitors, the speed of movement, and the distance and time for which prey avoid the birds. These processes are relatively straightforward to measure or estimate, allowing the strength of interference competition to be quickly predicted and incorporated into individual-based models. These principles were developed on wading birds, but can potentially be applied to a much wider range of forager systems.
Stillman, R. A., Poole, A. E., Goss-Custard, J. D., Caldow, R. W. G, Yates, M. G. & Triplet, P. (2002) Predicting the strength of interference more quickly using behaviour-based models. Journal of Animal Ecology, 71, 532-541.
Food requirements of animal populations
Shellfish are frequently exploited by humans but are also an important food resource for birds such as the oystercatcher. The setting of appropriate shellfish quotas requires the amount of food required by the shellfish to be exploited. The amount harvested can then set set according to the difference between the total shellfish stock and the amount required by the birds. Although simple to explain estimating the amount required by the birds has proved complicated, and when underestimated can lead to cases in which high mortality has been caused by insufficient food after shellfishing has occurred. The amount required in the environment is not simply the amount consumed by the birds, as not all of the food can be found, some birds are exclude from some of the food by competition and other factors reduce shellfish abundance. We have used individual-based models to determine that in order to maintain survival of oystercatcher, up to six times the amount consumed must be left in the environment. Although our predictions have been restricted to waders, the principles on which these are based can be applied much more widely.
Goss-Custard, J. D., Stillman, R. A., West, A. D., Caldow, R. W. G., Triplet, P., Durell, S. E. A. Le V. dit & McGrorty, S. (2004) When enough is not enough: shorebirds and shellfishing. Proceedings of the Royal Society, London, Series B, 271, 233-237.