Theory of Feeding Strategies - Thomas W. Schoener

Schoener's paper may be the first proper review paper that we've come across. It's dense, long and spattered with math, and one gets the feeling while reading that Schoener has spent a long time pouring over his colleagues' work. The paper itself spans a number of topics related to feeding strategy, and in the interest of brevity I will try to be short in summation.

 General Model of Feeding Strategy
 Schoener starts with the basics of time vs energy, describing two types of feeders. Type I is an organism that searches for food while partaking in other activities such as searching for mates, predator avoidance or displacing invaders, and therefore does not expend energy exclusively on searching. Type II is an organism for which searching for food and other activities are mutually exclusive. Both types must optimize their diet based on the following equation for individual prey items: (e/t)=(Potential Energy - pursuit cost - handling costs)/(Pursuit time + handling time)
If any one prey item's net energy is greater than the feeder's energy requirement (M), then the feeder should focus on that item. If a single prey item does not fulfill the feeder's energy requirement, the feeder should add different items in order of highest to lowest (e/t) to reach its energy requirements. For Type II feeders, any time spent traveling to feeding grounds before any energy is gained must also be taken into account. For them, a balance must be found between searching for food and performing other activities. Either type should optimize their diet in order to increase fitness. We see the gain in fitness by looking at the relation of increased energy to basal metabolic rate. Any diet must satisfy basal metabolic rate, which is related to body weight (W), before the organism can focus on growing or breeding. Large animals tend to be more efficient in storing fat reserves, and so can generally focus less on this basic upkeep. Once BMR is satisfied, optimal diet will benefit organisms by increasing growth rate, and potentially decreasing the time before reproduction, in which case their optimized diet will also benefit their offspring, increase broods per season and individuals per brood. Generally speaking, an organism should try to maximize energy obtained from their diet, however, the benefit from maximizing energy must be balanced with the benefit from performing other tasks such as mating. After all, it doesn't matter how much energy you could potentially give to your offspring if you never actually have any. As such, Schoener points out that feeding time and success in mating are inversely proportional. This is not to say that optimal diet is an unwavering standard for species. Each organism, regardless of feeding type, moves through a progression of feeding periods in which the organism should strive to optimize their net energy based on the requirements put upon them in that feeding period. The optimal diet should be recalculated every period. Optimization can be further viewed based on whether the organism is a time minimizer (generally organisms with a fixed reproductive output) or an energy maximizer.

 Optimal Diet
Schoener here moves on to an examination of the optimal diet. In order to be feeding optimally, organisms must choose an optimal diet, which is based on search time, pursuit time, search and pursuit energy expenditure, pursuit and capture success, potential caloric content and relative abundance of food items. Schoener presents a number of different models for optimal diet proposed by other authors. Hollings examines which prey would be mechanically easiest to predate, MacArthur and Pianka look at the forces of natural selection on eating as a function of pursuit time and search time. Emlen considered when an organism should pass on a prey item if they were aware of another prey item available and Levins & MacArthur examine how much selectivity in diet is most beneficial to the production of offspring. The general consensus being that food scarcity should increase the variety of prey items eaten. Feeders should take larger items closer to their origin before moving further to eat smaller items, and smaller predators should take smaller prey. Furthermore, the more you travel to find food, the more specialized you should be to increase your energy intake; larger organisms should be more specialist for the same reason. Generalism is favored, however, where food items are affected deferentially and unpredictably. Interestingly, Schoener asserts that when all food items are affected equally by feeders, feeders should converge in size.
 Schoener concludes the section with a short discussion of feeding rates, siting Holling's "functional response", which maps three separate intake rates based on food density. The first, linear model based on Lotka-Volterra, being most similar to a filter feeder, is an organism for which eating does not interfere with searching for food. The second is an organism which is limited by its ability to process food. We might think of something like a lion, which could theoretically catch many items when prey density is high, but then must stop capturing prey in order to eat. Finally, the third, making a similar plot to the second, is an organism that is limited by food processing, but also either learns or switches food based on density. This would be something like a honey bee, which must first search for food, but then once food is found remembers its location.

 Optimal Forage Space
Here we explore spacial range for an organism, which is again based on food density, as well as selectivity and metabolism of the feeder. Schoener explains that for any area in which food items are not uniformly distributed, the range size must exceed the size expected based on the above criteria to adjust for food patchiness. Here an organism's connectedness comes into play; the range size for a more connected organism being smaller than that of a less connected organism. Larger organisms should have larger ranges. Carnivores generally have larger ranges than hunting herbivores, which have larger ranges than browsers and grazers, which is based on level of specialization and food density. Efficient pursuers should have larger ranges because it is less energetically expensive for them to pursue prey. Rather than increasing range size, a decrease in food density causes a decrease in the range of items taken from the same area- organisms become more specialist. However, according to "compression hypothesis", the range of patches searched by feeders will decrease when food items are reduced deferentially based on the quality of the patch. Finally Schoener discusses the relation of food resources on territoriality, and the energy an organism should expend to protect a food source. Basically, when invasion is very low, and food resources are dense, there is no reason to protect food. Likewise, when invasion is common and food is scarce, the benefit of protecting a food source is outweighed by the energy expended chasing off invaders. Therefore, range should only be protected when invader frequency is below a certain level, and when food is neither in severe abundance or rarity.

 Optimal Feeding Period
Schoener very briefly discusses when organisms should expand their feeding period, or should utilize more energetically expensive feeding strategies. Generally, when food is scarce, when the feeder has a large energy requirement, or when the feeder has the best opportunity to reproduce, they should expand the feeding period.

 Optimal Foraging Group Size
Finally Schoener explores the role of gregarious behavior in feeding strategy. He presents three potential reasons why organisms may forage in groups and support behind the theories. Each depends on foraging efficiency, risk of predation and the defensible area and its unit cost of defense. The first type of group foraging decreases foraging efficiency, but it is energetically beneficial because it improves another aspect of the organism's life, such as navigation, breeding potential and a decreased risk of predation. The second type has no affect on foraging efficiency, but occurs where food resources are clumped such that multiple individuals can obtain more food together than any could foraging elsewhere alone. And the third type of group foraging increases foraging efficiency by flushing or driving prey, or simply by increasing the size of the food item that can be taken by the feeders collectively. All types of group foraging can be further beneficial by decreasing the overlap in home ranges, thereby decreasing the energy and time spent protecting the area, and decreasing time spent searching a large area. Groups can collectively increase the maximum range of any individual.

Questions:
Schoener makes the assumption in the general model that any organism that does not feed during a period has a reproductive output of 0. Obviously if an organism starves to death, or has no energy to mate, this is true. Are there cases where it is not?

 Is the expenditure of energy in hunting really proportional to BMR? Are there any organisms that function outside the proposed (lambda) of 0.5 to 1?

 Schoener asserts that larger organisms should be more specialized and have larger ranges. Does our knowledge of the past confirm this?

Why would feeders converge in size when food items are uniformly reduced?