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The Optimal Foraging Theory: Food selection in beavers based on tree species, distance, and size

The paper examines the relationship between optimal foraging theory and central foraging by using a beaver as a model. It also analyzes beaver food choice by noting species from woody vegetation, distance from water, status (not chewed and chewed), and the circumference of trees neighboring a beaver pond in South Carolina. Beavers avoid some species of trees that are closer to the water. No preference regarding tree circumference was noticed. These data show that beaver food choice agrees with optimal foraging theory.

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Introduction

The aim of this experiment is to explore the theory of central place foraging and the theory of optimal foraging using a beaver as the main model animal. Foraging is a term used to refer to a mammalian behavior linked with food searching. Optical foraging theory undertakes that animals eat in a way that maximizes their net energy intake rate per unit time (Schoener 904). An animal may either minimize time spent nurturing (time minimizer) or maximize its everyday energy intake (energy maximizer) to meet the minimum requirements. All herbivores normally behave as energy maximizers (Belovsky 209), and achieve the maximizing behavior by choosing a high quality food and having low-handling time (Schoener 903).

Central place theory describes all animals that gather food and keep them in a fixed place in a central place that is their home range (Jenkins 741).The other factors linked with optimal foraging theory are also applicable to central place theory. Central place theory expects that the retrieval cost linearly increase with distance of reserve from the central place (Belovsky 210; Jenkins 740). When it comes to choosing far from the central place, central place feeders become very selective because they have to spend energy and time carrying it back to the storage place (Schoener 901).

The main aim of this experiment was to determine the beaver food selection based on the trees species, distance, and size. Since all beavers are central place feeders (Jenkins 740) and energy maximizers (Belovsky 208; Jenkins 742), they make the perfect test animals for optimal foraging theory. The beavers eat many types of herbaceous plants together with bark, eaves, and twigs of many species of woody plants growing near water source (Jenkins 741).Examining trees that are either chewed or not chewed can gain accurate assessment of food choices among tree species (Jenkins 740). The aim of this experiment was to study about optimal foraging theory. The aim was to know whether beavers put into action optimal foraging theory when choosing food.

 The hypothesis was that the beavers in this research will prefer trees closest to the water a having a small circumference. Since energy yield of the tree species can significantly vary, it can be also hypothesized that the beavers will exhibit preference for some species of trees over the others regardless of distance from central area or the size of the circumference. The central place theory and the optimal foraging theory helped to understand that like other herbivores, beavers will maximize the net rate of energy intake per unit time. For beavers to maximize energy, they will choose trees that require the smallest retrieval cost are closer to the central place (water). Since the beavers are trying to maximize energy, it can be further hypothesize that they will likely choose some species of a tree over the others on the fact of nutritional value.

Methods

This research was carried out at a research pond owned by South Carolina State University. The research area was about 28 meters in width and 100 meters in length and was situated along the edges of the pond. Beyond this size, no beaver activity was observed. The species, the distance from the water, status (chewed or not-chewed), and the circumference for each tree in the research area were noted. The worked was equally divided among four groups learners working in quadrants owing to the large number of trees sampled. Each group had a contribution to the total data collected.

In order to analyze the data collected with respect to the beaver preference of a certain tree species, a chi-squared test was done. The test we conducted was to determine (1) if chewed trees were considerably smaller or larger than trees not chewed, and (2) if avoided trees were considerably far from the water than the chosen tree. The mean tree circumference and mean tree distance from water were also noted.

Results

Generally, the beavers showed some preference for certain species of trees, and this preference was based on the distance from central place. The measurements recorded at the study location show that beavers avoid muscle wood and oaks and a substantial food preference (P<.05, d.f. =9, x2=447.26). No particular preference or avoidance was noted in the other tree species. Mean distance of about 8.42 meters away from the not-chewed trees was substantially larger than a mean distance of 6.13 meters for the chewed trees (P<.05, d.f.=268, t=3.49). Tree species that the beavers avoided were not notably far from the water than the selected trees (P>.05, d.f. =268, t=.4277). For the selected trees, no major difference in the circumference was observed between the tree that were chewed (mean of 12.80 cm) and not chewed (mean of 12.80 cm) (P>.05, d.f.=268, t=1.52).

Discussion

Even though beavers are generally described as herbivores, the findings in this study relating to the species selection shows that beavers are very selective in their choice of food. This finding is with the hypothesis that beavers are very likely to exhibit preference to certain species of trees. Even though beaver choice for some species of trees can be related to nutritional value, more information is required to determine why the beavers choose some trees over others. Some studies argue that beavers normally avoid trees with chemical defenses, which make the tree inedible to the beavers (Schoener 902). These studies further argue that trees having soft wood and this can possibly explain the noted avoidance of oak and muscle wood in our study.

This result showed that chewed trees species that were closer to water source account for energy and time spent hauling and gathering. This is in harmony with optimal foraging theory and totally agrees with the hypothesis that the beavers will choose trees that are closer to the water. The net energy yield of a tree decreases as the distance from water increases because food farther away is very likely to increase retrieval and search time. The finding is the same as Belovsky’s finding of inverse relationship between percentage of plants cut and the distance from water.

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Lack of any observable difference in the mean circumference between not chewed and chewed trees do not agree with the hypothesis that beavers normally prefer small trees to large ones. The hypothesis was based on the idea that the branches from large trees will need a lot of energy to haul and cut than the ones from small trees. The finding agrees with the study done by Schoenerthat suggest that value of all trees should reduce with distance from water and the beavers should benefit from selecting larger branches from larger trees at any distance. This will explain why there was no meaningful difference in the circumference between not-chewed and chewed trees.

This experiment gave the chance of observing how a given mammal chooses food that maximizes the energy gain in accordance with optimal foraging theory. Even though beavers obey optimal foraging theory, no optimal diet likelihoods can be made without additional information on the energy costs of cutting some tree species and the relative nutritional value of the tree species. Other information about predacious risk and its role in food assortment is also needed. Finally, since many students were taking samples in the field, some errors might have occurred, which might have affected the precision and accuracy of the measurements. It can be suggested that other students repeat this study to verify the findings.

Conclusion

The main purpose of this experiment was to learn about optimal foraging theory by measuring the tree selection in beavers. It is known now that optimal foraging theory enables to foretell food-seeking behavior of beavers using variations in the tree species and the distance from their central place. It emerged that food selection and foraging behaviors is not normally straightforward. For example, beavers choose larger branches at any distance from water although cutting the larger branches would increase the energy requirements. This is a fine line between energy expenditure and energy intake in beavers that any given theory has not predicted.

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