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Organisms in populations interact with one another in complex ways, where individuals compete for resources such as food, shelter, and mates. These interactions are costly since individuals invest energy to acquire the resource, therefore there are a variety of strategies that organisms adopt to gain an advantage over their competitors. This is observed clearly with polymorphic mating systems, in which one sex, predominantly males, display multiple mating strategies. For example, dominant male sea lions defend harems of females up on the beach, whereas non-dominant males try to increase their chances of mating by staying in or near the water, where they mate with females that have left the beach temporarily 1. Other types of strategies may determine how often an individual will fight for a resource, or how willing they are to cooperate with others.
In order to understand how different behavioral strategies evolve, ecologists turn to the game theory, which is a mathematical modeling approach that investigates the outcomes of multi-individual interactions, in which the payoff for any one individual depends its own strategy as well as the strategies of the others 2. In this approach, the relative cost of the interaction and the benefits obtained from the resource determines the net gain or, in some cases, loss incurred by the organism. Different strategies can be assigned costs and benefits based on those faced by an organism. For example, fighting for control of a resource may provide a large benefit, but it also comes with costs that reduce an organism’s net gain. On the other hand, a strategy to avoid conflict will reap fewer benefits but incurs no costs.
The organisms with the best interaction strategy maximize their net gain, which in turn will contribute to their fitness. Therefore, over the course of evolutionary time, one strategy may arise that outcompetes all others in a population. This is called an evolutionary stable strategy (ESS) 2. Populations evolve to adopt this strategy once it arises through mutation or is introduced through migration. Therefore, these strategies are mostly genetic or adopted at a young age, and changes in the strategies used by a population over time are determined by the action of natural selection. This concept is often illustrated using the Hawk-Dove game, which compares the success of two strategies for obtaining resources 3. In this example, “hawks” are aggressive, and always fight for resources. “Doves,” on the other hand, are passive, and never fight for resources. In an interaction between two doves, resources are split evenly. When a hawk and a dove interact, the hawk always wins and obtains all of the resources. However, when two hawks interact, they split resources evenly and suffer the cost of their conflict as well 3. Assessing these interactions over consecutive interactions allows modeling of how competing strategies in an evolving population fare against each other, thus emergence of ESS in an experimental setting.
As seen with the doves in the Hawk-Dove example, organisms not only compete against each other, but also display cooperative behaviors. The risk of being a population of 100% doves is the arrival of a cheater, or an individual that does not cooperate 4. Cheaters can invade and outcompete the residents, therefore many cooperating populations have strategies in place to prevent invasion, including the ability to switch strategy as necessary or to identify and cheaters, and in some cases, to communicate this information to others in their group to decrease the chances of cheater success 4.
The existence of altruism, or a reduction in an organism’s immediate fitness in order to benefit another’s, in wild populations has been held up as a counterpoint to the theory of natural selection, however game theory shows how altruism can evolve under certain conditions. Assuming organisms can identify one another or reasonably expect to interact again in the future, acts of apparent altruism may actually be beneficial over time, as an organism can count on the favor being returned. This is seen when flocks of birds or herds of mammals feed collectively – one individual may sound an alarm call when they spot a predator, making them more vulnerable to attack 5. The net benefit, however, of others frequently doing the same makes this act adaptive. Similarly, a vampire bat may regurgitate its food to feed hungry individuals. When it is unable to find food in future, it may benefit from the same behavior of another vampire bat 6.
In addition to the interactions within a single species, the evolution of social interactions can take place between species as well. Besides predator-prey interactions, different species can compete for the same resources and develop strategies to gain advantage over others. However, individuals of a species may also interact cooperatively with members of another species. Interspecies interaction that require cooperation include mutualisms, or situations in which two organisms provide mutual benefits to one another. Many plants form mutualisms with nitrogen-fixing bacteria in the soil, whereby the plants provide complex sugars to bacteria in exchange for nitrogen 7. Should bacteria fail to provide nitrogen, then plants can reduce the amount of sugars available.
Since cooperative interactions depend on either being able to identify or retaliate against cheaters, it is possible for invaders to take advantage of existing mutualisms in new environments. Invasive species have the potential to be closely related enough to local species that they are able to form mutualisms with other local species, but distantly related enough that existing recognition or defense mechanisms are ineffective. In Hawaii, an invasive dinoflagellate that forms a mutualism with coral takes more resources than local dinoflagellates. This has a negative impact on coral instead of the expected beneficial one 8. Therefore, studying intra- and interspecies interactions not only allows understanding the development of behavioral strategies in evolving populations but is also essential to assess the behavioral phenotypes of invasive organisms to develop effective strategies against them.