Individual-based ecology
Brief introduction
Individual-based ecology predicts how animal populations are affected by environmental change from an understanding of the responses to change of the individuals that comprise these populations.
This approach is founded on a thorough understanding of the behaviour and ecology of animals, and their individual responses to environmental change. Individual-based models are then used to integrate individual behaviour and produce population level predictions.
This site describes how individual-based ecology has been used to predict the responses of coastal birds to environmental change, and highlights the potential range of systems to which the approach could be applied in the future.
What are individual-based models?
Individual-based models predict the mortality rate and body condition of animals.
They are based on the principle that individuals within a population vary (e.g. in their ability to find or compete for food), but that each individual behaves in ways that maximise its own chances of surviving and reproducing (e.g. they feed where food is abundant or competitors are rare).
Real animals also behave in this way, and so model animals should respond to environmental change in the same way that real ones would.
Models are run over the course of a season (e.g. winter), and predict the proportion of animals which survive in good condition and the proportion which die (e.g. before spring).
To be applied to a particular site the models require information about the number of animals present, the quantity, quality and distribution of the food supply and the time for which feeding areas are available.
Example of an individual-based model
This animation shows an individual-based model of shorebirds on the Exe estuary, England. It shows the exposure and covering of intertidal patches through the tidal cycle. The food supply available differs between the different coloured patches. Individual birds of different species (shown in different colours) move to their preferred feeding areas when these are exposed by the tide, or move to the roost if no feeding patches are available. The model runs for one winter and predicts the proportion of birds which survive and the body condition of the survivors.
The model environment can be changed to simulate any form of environmental change. Change may be caused by human disturbance, shellfishing, agriculture, habitat loss or climate change. For example, the effects of habitat loss can be incorporated by reducing the area of one or more patches. Human disturbance can be incorporated by excluding birds from areas when people are present and increasing their energy requirements when they take flight. Climate change can affect either the area of habitat available or the energy requirements of birds.
When the model environment is changed, model birds will adapt their behaviour (e.g. change their feeding location or prey, or feed for longer) to minimise the effect of this change on their survival chances.
Underpinning research
Individual-based models are based on fundamental research into the behaviour and ecology of birds, and the population dynamics of their prey populations.
Individual Variation
A key element of these models is that individuals within a population vary in their abilities of finding or competing for food. Much basic research, particularly on colour-ringed oystercatchers, has measured the amount of variation within animal populations, and predicted the consequences of this variation for population ecology.
Foraging behaviour
The rate at which an animal feeds depends on the density of available food and the density of its competitors. Research has shown that a few simple measurements (e.g. prey and bird mass, time taken to find and eat food, movement speed) can be used to predict feeding rate in different environments, and how this is affected by food and competitor density.
Prey populations
Individual-based models predict the effect of food distribution, abundance and quality on bird populations. Research has been conducted to understand and predict the dynamics and distribution of these prey populations.
Human disturbance
Disturbance from humans can cause birds to expend energy while taking flight, and reduces the time and space available for feeding. But does disturbance from humans cause more birds to die? Birds may simply take flight because they can move to other, equally good feeding areas. The response of birds to disturbance has been measured, and used to predict the population consequences of disturbance.
Applied research
Individual-based models have been applied to a wide range of environmental issues, including the effects of shellfishing and aquaculture, human disturbance, habitat loss and the assessment of site quality. This page summarises the range of sites and issues for which models have been developed.
Click on a site on the map for details of that case study
Future applications
We have mainly developed models for wintering shorebirds and wildfowl in coastal, European habitats. However, the issues that these models address occur world-wide. These models are potentially applicable to any problem relating to habitat changes, climate change, pollution, human disturbance, and to any other species providing suitable data can be collected. This page highlights some potential applications of individual-based ecology.
Wintering farmland birds
Many farmland birds, such as sparrows, finches and buntings, are declining throughout much of their ranges. Declines are usually attributed to intensification of agriculture. In Britain, several species are thought to be declining due to increased winter starvation caused by decreases in the abundance of crop and weed seeds. Individual-based models could be used to predict how changes in seed abundance are linked to changes in winter starvation rate.
Wintering garden birds
Many bird species (e.g. thrushes, finches, sparrows and tits) consume artificial food, such as peanuts, bread and seed, put out in gardens. This food can make an important contribution to the over-winter survival of some species, which are suffering declines in their natural food in farmland. Although, these are not natural habitats, all the principles on which our individual-based models are built also apply. Individual-based models could be used to determine the importance of artifical food for increasing the survival and body condition of garden birds.
Farmland breeding shorebirds
Populations of shorebirds breeding in farmland habitats, such as snipe, lapwing and redshank, have declined greatly in recent decades. These declines have often been attributed to changes in agricultural practices. For example, improved water drainage and lowered watertables reduces the availability of invertebrate prey, such as worms, for birds. Individual-based models could be used to predict how breeding success is related to habitat features, such as site wetness and vegetation height.
Further reading
Overviews of Individual-based models
Goss-Custard, J. D., Stillman, R. A., West, A. D., McGrorty, S., Durell, S. E. A. le V. dit & Caldow, R. W. C. (2000) Role of behavioural models in predicting the impact of harvesting on populations. In: Behaviour and Conservation (eds. M. Gosling and W. J. Sutherland). Cambridge University Press, Cambridge. pp 65-82.
Norris, K. J. & Stillman, R. A. (2002) Predicting the impact of environmental change. In: Conserving bird biodiversity (eds. K. J. Norris and D. Pain). Cambridge University Press, Cambridge. pp 180-201.
Stillman, R. A. (2003) Predicting wader mortality and body condition from optimal foraging behaviour. Wader Study Group Bulletin, 100, 192-196.
Application of models to specific problems
Stillman, R. A, Goss-Custard, J. D., West, A. D., Durell, S. E. A. le V. dit, Caldow, R. W. G., McGrorty, S. & Clarke, R. T. (2000) Predicting mortality in novel environments: tests and sensitivity of a behaviour-based model. Journal of Applied Ecology, 37, 564-588.
Stillman, R. A, Goss-Custard, J. D., West, A. D., McGrorty, S., Caldow, R. W. G., Durell, S. E. A. le V. dit, Norris, K. J., Johnstone, I. G., Ens, B. J., van der Meer, J. & Triplet, P. (2001) Predicting oystercatcher mortality and population size under different regimes of shellfishery management. Journal of Applied Ecology, 38, 857-868.
Stillman, R. A, West, A. D., Goss-Custard, J. D., Caldow, R. W. G., McGrorty, S., Durell, S. E. A. le V. dit, Yates, M., G., Atkinson, P. W., Clark, N. A., Bell, M. C., Dare, P. J. & Mander, M. (2003) An individual behaviour-based model can predict shorebird mortality using routinely collected shellfishery data. Journal of Applied Ecology, 40, 1090-1101.
West, A. D., Goss-Custard, J. D., McGrorty, S., Stillman, R. A, Durell, S. E. A. le V. dit, Stewart, B., Walker, P., Palmer, D. W., & Coates, P. (2003) The Burry shellfishery and oystercatchers: using a behaviour-based model to advise on shellfishery management policy. Marine Ecology Progress Series, 248, 279-292.
Caldow, R. W. G., Beadman, H. A., McGrorty, S., Stillman, R. A., Goss-Custard, J. D., Durell, S. E. A. le V. dit, West, A. D., Kaiser, M. J., Mould, K. & Wilson, A. (2004) A behavior-based modeling approach to reducing shorebird-shellfish conflicts. Ecological Applications, 14, 1411-1427.
Durell, S. E. A. le V. dit, Stillman, R. A., Triplet, P., Aulert, C., Biot, D. O. dit, Bouchet, A., Duhamel, S., Mayot, S. & Goss-Custard, J. D. (2005) Modelling the efficacy of proposed mitigation areas for shorebirds: a case study on the Seine estuary, France. Biological Conservation, 123, 67-77.
Stillman, R. A, West, A. D., Goss-Custard, J. D., McGrorty, S., Frost, N. J., Morrisey, D. J., Kenny, A. J. & Drewitt, A. L. (2005) Predicting site quality for shorebird communities: a case study on the Humber estuary, UK. Marine Ecology Progress Series, 305, 203-217.
Durell, S. E. A. le V. dit, Stillman, R. A, Caldow, R. W. G., McGrorty, S., West, A. D. & Humphreys, J. (2006) Modelling the effect of environmental change on shorebirds: a case study on Poole Harbour, UK. Biological Conservation, 131, 459-473.
Goss-Custard, J. D., Burton, N. H. K., Clark, N. A., Ferns, P. N., McGrorty, S., Reading, C. J., Rehfisch, M. M., Stillman, R. A., Townend, I., West, A. D. & Worral, D. H. (2006) Test of a behaviour-based individual-based model: increased winter mortality in a shorebird following habitat loss. Ecological Applications, 16, 2215-2222.
Durell, S. E. A. le V. dit, Stillman, R. A, McGrorty, S., West, A. D., Goss-Custard, J. D. & Price, D. (2007) Predicting the effect of local and global environmental change on shorebirds: a case study on the Exe estuary, UK. Wader Study Group Bulletin, 112, 24-36.
West, A. D., Yates, M. G., McGrorty, S. & Stillman, R. A (2007) Predicting site quality for shorebird communities: a case study on the Wash embayment, UK. Ecological Modelling, 202, 527-539.
General insights
Goss-Custard, J. D., West, A. D., Stillman, R. A, Durell, S. E. A. le V. dit, Caldow, R. W. G., McGrorty, S. & Nagarajan, R. (2001) Density-dependent starvation without significant food depletion. Journal of Applied Ecology, 70, 955-965.
Goss-Custard, J. D., Stillman, R. A, West, A. D., Caldow, R. W. G. & McGrorty, S. (2002) Carrying capacity in overwintering migratory birds. Biological Conservation, 105, 27-41.
Goss-Custard, J. D., Stillman, R. A, Caldow, R. W. G., West, A. D. & Guillemain, M. (2003) Carrying capacity in overwintering birds: when are spatial models needed? Journal of Applied Ecology, 40, 176-187.
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.
West, A. D., Goss-Custard, J. D., Durell, S. E. A. le V. dit & Stillman, R. A (2005) Maintaining estuary quality for shorebirds: towards simple guidelines. Biological Conservation, 123, 211-224.
Caldow, R. W. G, Stillman, R. A, Durell, S. E. A. le V. dit, West, A. D., McGrorty, S., Goss-Custard, J. D., Wood, P. J. & Humphreys, J. (2007) Benefits to shorebirds from invasion of a non-native shellfish. Proceedings of the Royal Society, London, Series B, 274, 1449-1455.
Foraging behaviour
Stillman, R. A, Caldow, R. W. G., Goss-Custard, J. D. & Alexander, M. J. (2000) Individual variation in intake rate: the relative importance of foraging efficiency and dominance. Journal of Animal Ecology, 69, 484-493.
Stillman, R. A, Goss-Custard, J. D. & Alexander, M. J. (2000) Predator search pattern and the strength of interference through prey depression. Behavioral Ecology, 11, 597-605.
Yates, M. G., Stillman, R. A & Goss-Custard, J. D. (2000) Contrasting interference functions and foraging dispersion in two species of shorebird (Charadrii). Journal of Animal Ecology, 69, 314-322.
Stillman, R. A, Bautista, L. M., Alonso, J. C. & Alonso, J. (2002) Modelling state-dependent interference in common cranes. Journal of Animal Ecology, 71, 874-882.
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.
Poole, A. E., Stillman, R. A & Norris, K. J. (2006) A video-based method for measuring small-scale animal movement. Animal Behaviour, 72, 1205-1212.
Stillman, R. A & Simmons, V. L. (2006) Predicting the functional response of a farmland bird. Functional Ecology, 20, 723-730.
Gillings, S., Atkinson, P. W., Bardsley, S. L., Clark, N. A., Love, S. E., Robinson, R. A., Stillman, R. A. & Weber, R. G. (2007) Shorebird predation of Horseshoe Crab eggs in Delaware Bay: species contrasts and availability constraints. Journal of Animal Ecology, 76, 503-514.
Poole, A. E., Stillman, R. A., Watson, H. K. & Norris, K. J. (2007) Searching efficiency and the functional response of a pause-travel forager. Functional Ecology, 21, 784-792.
Human disturbance
West, A. D., Goss-Custard, J. D., Stillman, R. A, Caldow, R. W. G., Durell, S. E. A. le V. dit & McGrorty, S. (2002) Predicting the impacts of disturbance on wintering wading birds using a behaviour-based individuals model. Biological Conservation, 106, 319-328.
Stillman, R. A & Goss-Custard, J. D. (2002) Seasonal changes in the response of oystercatchers Haematopus ostralegus to human disturbance. Journal of Avian Biology, 33, 358-365.
Stillman, R. A, West, A. D., Caldow, R. W. G. & Durell, S. E. A. le V. dit (2007) Predicting the effect of disturbance on coastal birds. Ibis, 149 (Suppl. 1), 73-81.
For further information please contact
Richard Stillman
Telephone: +44 (0)1202 966782
PhD students
David Baker – Farmland bird behaviour and ecology
Antonio Uzal Fernandez – Sika deer ecology
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