Essay Notes on Optimal Foraging Theory free essay sample

Foraging Introduction Hunting and escape strategies of predators and prey are probably the result of a coevolutionary arms race (Dawkins 1999). There is an economic approach that the scientific community can use to look at what kinds of prey preds choose to eat. Elner and Hughes (1978) found that when given a choice of different sized mussels, shore crabs Carcinus maenus selected the prey that gives them the highest rate of return. Very small mussels were easy to open but held less nutritional value, and large mussels held much nutritional value, but were too time consuming to break open and so were selected against. The shore crabs were seen to select intermediated sized shells and incorporated suboptimal prey into the diet only in proportion to their relative abundance, where they were chosen against as much as possible. What it is Behaviours such as foraging involve decision making (such as where to search, what to eat), and the subsequent choices have costs as well as benefits. We will write a custom essay sample on Essay Notes on Optimal Foraging Theory or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page The Optimal Foraging Theory dictates that individuals should be designed by natural selection to maximise their fitness. This idea can be used as a basis to formulate optimality models which specify hypotheses concerning the currency for maximum benefit and the constraints on the animal’s performance (Davies et al. , 2012). Behavioural ecology accepts the reality of the constraints and the upper and lower bounds, but the theory seeks to establish how an individual animal organises its own foraging behaviour within these limits. Optimal foraging theory is a fundamental and integral part of behavioural ecology. It aims to establish if an animal’s foraging yields a net gain in energy. This net gain would increase the chance of this animals surviving and of successfully passing on its genes to another generation, an aspect of ecology referred to as fitness. If on the other hand, the animal is not foraging optimally, it will lose weight and condition and therefore the chances of survival and a high reproductive output decrease, i. e. the fitness of the animal decreases. If this happens to the point that the animal dies, then the genetic make up of this animal will not be passed on. How to calculate it For an optimal foraging approach, one must quantify the basic costs of the foraging strategy together with the gains. These are measured in different currencies (kilojoules, kilocalories etc). The first essential step is to calculate the basal metabolic rate (BMR) of the study animal. This can be estimated from very large well-supported databases, that establish a linear relationship between BMR and body mass. Once this has been established, one can estimate the costs of different activities expressed as a multiple of the BMR. Some activities harbour small costs (e. . resting, walking) whereas others are more expensive, such as flight, which is between 3 and 7-8 times the BMR. Following this step, an ethogram can be constructed listing the animal’s daily activities and one can compute the daily energy expenditure of an individual animal or group of animals. From this you can start to examine the foraging behaviour in more detail. Further definitions One must look at the handli ng time, a basic concept of pred/prey relations. This is the time it takes to pursue, subdue, kill and consume the prey. It is exceptionally difficult to measure in the field. It makes sense that the handling time would reduce naturally with experience, and that young, such as recently fledged birds, are more inefficient in prey catching techniques that older more experienced birds. The handling time is a crucial variable. To a certain extent, it is a misnomer as, in many cases, the pred may be unsuccessful and fail to catch the prey, therefore never really â€Å"handling† it. But in attempting to catch this prey, it may have incurred huge costs in pursuing the prey. It’s important to note though, that in the case of herbivores, for the most part the food is immobile and available. A term often used in optimal foraging theory is â€Å"food patch†. Each food patch will have a certain density of prey and these prey animals will have a certain size range. The quality of the patch is determined by prey encounter rate and the â€Å"decision† made by the animal is how long should it stay in the patch and when should it leave. The essence of optimal foraging concerns decisions in relation to what they all â€Å"reward probability† and the key question is how the animal arrives at a decision. Critical discussion In recent years, the simple OFT approach which suggests the maximisation of gain has come under critical scrutiny. It is an important but controversial topic. The most critical view of optimal foraging theory is that it is tautological or not scientific (Pyke 1984). But also, researchers have become interested in the a bility of prey to escape predators. It makes sense to maximise your energy store so that you can utilize it to escape from predators. But the problem is that storing energy is itself a cost. Some animals solve this by hiding or storing prey in their environment e. g. the butcher bird, a shrike, will impale its prey (insects or small vertebrates) on thorns to be consumed at a later date. This caching behaviour is seen in other animals (Smith and Reichman, 1984). Amongst species that do not employ caching.. Numerous studies have shown that takeoff weight in birds is a crucial variable for prey species in order to escape the attack of a pred. If the bird has insufficient weight and is therefore in suboptimal condition, it is vulnerable to being killed, if it doesn’t stave due to its bad condition. Studies of the carcases of killed prey show that they were in poor condition. But equally if too much energy is gained during foraging and is stored, they the prey species may also be in suboptimal condition in terms of escaping from preds. As flight is the predominant escape mode in birds, this is particularly important. Impaired flight abilities due to increased wing loading may increase vulnerability to predation. Following field observations which showed that birds often maintain smaller reserves than expected, Gosler et. al. (1995) stated that there is the implication of a cost to being fat. They demonstrated this by relating the body mass of a prey species the great tit Parus major to various situations; a) to a declining population of predators, b) a predator-free situation, c) when pred populations were recovering and d) when the pred population was fully re-established again. Their research suggested a complex relationship between how net energy is utilised and the presence of a predator species. They found that when the predator species was absent or decreasing, the tits had the highest body weight. Their findings suggested that the target/endpoint of optimal foraging is flexible and depends on the conditions at the time. This mass-dependent predation risk is also an important and researched concept involved in studying trait-mediated effects of predation (Quinn et al. , 2008). In another study examining this trait-mediated effect of predation, Van den Hout et al. (2009) state in order to compensate for increased wing loading, birds are able to independently decrease body mass (BM) or increase pectoral muscle mass (PMM). In their comparison or nearshore and farshore foraging shorebird species, they developed a theory that nearshore foragers should respond to increased predation by increasing their PMM in order to promote speed-based escape, and the farsh ore foragers should decrease BM in order to improve agility for manoeuvring escape. Models Optimization models tend to have two approaches; either the Descriptive model, which is used to predict the choices made by an animal based on its record of past preferences and choices, or the normative model which is based on the choice that the animals ought to make. The principle in both models is that the daily energy expenditure must not be greater than the daily energy intake. Discussion All of the statistical models supporting OFT are supported by the most part where the prey or energy source is stationery and where the cost of obtaining energy are relatively low. The only definite support for OFT comes from laboratory experiments with stationery food/prey. Where animals have to expend high amounts of energy to pursue mobile prey, it has not been possible to prove the case for OFT. The difficulty is two-fold: accurately estimating the costs and benefits and finding a satisfactory statistically based theory that can be used for highly mobile prey.