5.5: Principle of Allocation and Evolutionary Trade-offs

Evolutionary Tradeoffs

When a population adapts to one set of environmental conditions its fitness (ability to survive and reproduce) decreases in other environments. For example a polar bear tolerates the extreme cold and weather of the arctic, but would die of overheating in the tropics. This tradeoff is caused, at least in part, by limited access to resources and energy.

Definition: Principle of Allocation

The Principle of Allocation states that each individual organism has a finite (limited) quantity of resources that it can use for all necessary life processes, including growth, reproduction, acquiring nutrients and resources, escaping predators or pathogens, and other processes. Resources such as nutrients or energy that an individual invests in one process cannot also be invested in other processes, meaning that not all life functions can be simultaneously maximized.

The principle of allocation leads to trade-offs, which are the relationships between the benefits of resource investment decisions in one context compared to the costs of those decisions in another context. You are already familiar with the principle of allocation through the concept of time. Each individual has 24 hours each day to invest in all the tasks that need to be accomplished. The more time a student invests in studying, for example, the less time is available for other tasks, such as sleeping. The trade-off can also be thought on in terms of costs and benefits. Investing more time in social activities may increase the number of friends you have (benefit), but will also decrease the amount of time available to study and likely result in lower grades (cost).

This concept is a cornerstone in both evolutionary biology and ecology. Species balance the principle of allocation and its associated trade-offs differently, resulting in a wide variety of strategies and traits that organisms use to accomplish life tasks. This variety of strategies and traits contributes to the diversity of species that exist on Earth.

Watch the video for examples of evolutionary tradoffs.

This lecture is part of a Coursera course called Evolution Today. It was given by Prof. Dr. Menno Schilthuizen and Dr. Marijn van der Zee from The Institute Biology Leiden and Dr. Rutger from Naturalis.

Plants face particularly complex trade-offs when it comes to growth since investing in above-ground growth (leaves, stems, etc) reduces access to below-ground resources and vice versa. The availability of resources above ground (sunlight, CO2) and below ground (water, other nutrients) consequently influences patterns of plant resource investment in growth. An important consideration in these trade-offs is which nutrient is most limiting for plant growth in a given environment.

Optimal Foraging Theory

Animals have a variety of acquisition strategies available to them and face choices about what food sources to pursue. Each food source and each consumption strategy have a balance of nutritional benefits and energetic costs. At first glance, one might expect that evolution would favor foraging behaviors that are as energetically efficient as possible. This sort of reasoning has led to what is known as the optimal foraging theory, which predicts that animals will select food items that maximize their net energy intake per unit of foraging time.

Optimal foraging theory quantifies the benefits (in terms of energy content) of various prey choices and the costs (in terms of energy inputs for capturing, consuming, and digesting food) in order to predict the most energetically beneficial foraging choice.The net energy (in calories) gained by feeding on each kind of food available to a foraging animal is simply the energy content of the food minus the energy costs of pursuing and handling it.

For example, consider a population of shore crabs (Carcinas maenas) that primarily feed on blue mussels (Mytilus edulis) (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Carcinas maenas crabs (left) and their primary prey, blue mussels (Mytilus edulis). (Left: CC-BY-SA 3.0; Ar Rouz via Wikimedia, Right: CC-BY-SA 2.0; Benutzer Darkone via Wikimedia)

Many foragers do preferentially use food items that maximize the energy return per unit time. Shore crabs, for example, tend to feed primarily on intermediate-sized mussels, which provide the greatest energy return. In order to consume the meat of the mussel, the crab must crack open its shell. Large mussels are more difficult to crack open, but contain more meat. Smaller crabs take more time and expend more energy in breaking open mussels than larger crabs. Consequently, what size of mussel provides the optimal energy return on the crab’s energy investment depends on the size of the crab.

Scientists quantified the amount of time it took for crabs of various sizes to crack open mussels of different size as well as the nutritional benefit of mussels of different size and used these data to estimate the value of different size of mussel prey for different sizes of predator crabs. Scientists then tracked prey choice by crabs when presented with a variety of prey sizes and discovered that actual prey choice closely matched the patterns of prey value estimated by optimal foraging theory. Many other animals also behave to maximize energy acquisition.

The key question, however, is whether increased energy resources acquired by optimal foraging leads to increased reproductive success. In many cases, it does. In a diverse group of animals that includes ground squirrels (Figure \(\PageIndex{2}\)), zebra finches, and orb-weaving spiders, the number of offspring raised successfully increases when parents have access to more food energy.

Figure \(\PageIndex{2}\): Optimal foraging in this golden mantled ground squirrel pays off with increased reproductive success.. (Copyright; author via source)

In other cases, however, the costs of foraging seem to outweigh the benefits. An animal in danger of being eaten itself is often better off to minimize the amount of time it spends foraging. Many animals alter their foraging behavior when predators are present, reflecting this trade-off between food and risk.

Optimal foraging theory focuses entirely on energy costs of feeding and energy return from prey tissue consumed. Other environmental factors may drive organisms to make choices different from what optimal foraging theory predicts. If capturing high quality prey items exposes the organism to danger, then animals may choose less energetically valuable prey in safer areas of the habitat. Additionally, some plants contain rare nutrients that herbivores require in their diet and so grazers may seek out these plants even if they are not energetically beneficial.

Attribution

This page is a modified derivative of:

• The Principle of Allocation by Laci M. erhart-Barley, licence CC BY-NC-SA 3.0
• Resource Acquisition in Plants by Laci M. erhart-Barley, licence CC BY-NC-SA 3.0
• 4.3: Resource Acquisition in Animals by Laci M. erhart-Barley, licence CC BY-NC-SA