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Optimal Foraging and Ideal Free Distribution:
Examining the free distribution and foraging behaviour of ducks at Jellie Park.
The aim of this experiment is to demonstrate the distribution pattern predicted by Ideal Free Distribution (IFD) by examining whether the ducks at Jellie Park duck pond do indeed follow the distribution predicted.
IFD predicts that the equilibrium proportion of foragers between two patches will equal the proportion of total resources available at each patch. (Tyler and Gilliam, 1995) Algebraically this is expressed as:
N1/N2=R1/R2
Where N1 and N2 are the numbers of foragers at patches 1 and 2 respectively and R1 and R2 represent the amount of resources to be found at the respective patches. (Kennedy and Gray, 1993)
There are several core assumptions that IFD is based on, these are that foragers 1) have perfect knowledge of resource availability, 2) have negligible travel costs in terms of using energy, 3) are equal competitors and that 4) higher competitor density reduces individual resource gains. (Kennedy and Gray, 1993) Because these assumptions are obviously not valid in practice, IFD makes imprecise predictions about forager distribution. IFD has however, been shown to be an accurate model of the trends shown by such distributions, (Kennedy and Gray, 1993) and work by Tyler and Gilliam (1995) to include energy cost in the model, has helped in making it more precise.
Methods
For the experiment, the lab class was separated into two feeding stations, each station representing a resource patch about 10 metres apart. Each station had a feeder, a timer and a team of counters. A 6-metre radius (estimated by the counters) from the feeder defined the patch area. The experiment was run in a series of five trials, each lasting 5 minutes. In each trial, a predetermined amount of bread was thrown simultaneously into the patch every 30 seconds and the number of ducks at each patch counted simultaneously every minute. In each trial, the total amount of bread thrown was six pieces. In the first trial the resource ratio was 1:5 (station 1 : station 2). In the subsequent trials, the ratio was altered to 1:2, 1:1, 2:1 and 5:1 respectively. This substantial range of ratios is important because it is at extreme ratios where deviation from IFD is most apparent. (Kennedy and Gray, 1993)
The counts from the first 2 minutes of each trial were discarded to allow for movement between patches and so that duck numbers could settle at equilibrium. The counts for the last 3 minutes were then averaged across the class to ensure accuracy and then averaged again to give a figure for the equilibrium number for each trial.
There were several species of duck present at each station, Mallards, Scaup and Grey ducks. The large male Mallards tended to dominate the stations, patrolling the area where the bread pieces were landing. At first, there was little competition between the ducks at station 2 as station 2 began with the majority of resources. They soon became more aggressive as less food became available, going so far as to chase off other ducks. The largest ducks were the first to leave the patch as the other patch began receiving more resources, leaving the remaining ducks to form a hierarchy with the larger ducks holding the central area of the patch and the smaller ducks darting in from the periphery when food became available. When tabulated, the movement of ducks did not follow the IFD prediction although it did follow the general trend.
This is to be expected as in practice the assumptions that IFD is based on are invalid. The premise that the ducks have perfect knowledge of resource distribution is obviously untrue. Without perfect knowledge, the ducks will tend to overutilise the patch that started with the higher resources, staying longer than they should for optimal foraging, and missing out on the higher resource availability at the other patch. Tyler and Gilliam (1995) add to this with a costs model that factors energy expenditure and threat of predation into the likelihood of an individual switching patch.
It is also untrue that all competitors are equal. The largest ducks dominated the station when the ratio was favourable, then switched as soon as the other became more favourable. A review of various IFD studies by Kennedy and Gray (1993) found that, ??better competitors were over represented in the high resource sites and poorer competitors were over represented in the poorer sites.?
It is clear that the IFD?s value is not in predicting the exact distribution of animals. Its real value lies in its ability to provide us with a framework to guide our thinking on the behaviour of foraging animals. (Lab Manual, 2005)
References ?
Kennedy, M and Gray R.D (1993). Can ecological theory predict the distribution of foraging animals: A critical analysis of experiments on the Ideal Free Distribution. Oikos 68: 158 - 166
Tyler, J and Gilliam J.F. (1995). Ideal free distributions of stream fish: a model and test with minnows, Rhinicthys Atratulus. Ecology 76: 580 ? 592
Biol 112 Lab Manual (2005). University of Canterbury. Pg. 55 - 61
Clam, A (2005). The ideal distribution of ducks is one per 5 square metres: A study. pg 115 - 127
can you believe it took me all night? the frikken sun has come up! well actually with all the procrastination i guess it's not that surprising. holy shish kekab! I've got a lecture in an hour and a half. better catch some zzzz's. Although, I could always do that later....
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