Effect on community structure
- Key People:
- Robin Wall Kimmerer
Predators can greatly affect the structure of communities. For example, seed predators commonly scour the ground for each year’s seed crop, eating most of the seeds produced by many different species each year. To defend against these predators, certain plants are thought to resort to mast seeding, which is the production of many seeds by the plant every two or more years in regional synchrony with other plants of the same species. Mast seeding is an effective defense because the seed predators become satiated before all the seeds have been consumed. The consequence of mast seeding for the organization of communities is that, instead of a few new seedlings establishing themselves every year, major pulses occur over time during which new plants become established and old plants die. Many conifers in boreal forests exhibit mast seeding as do other species such as bamboos. Some bamboo species grow for 100 years or more before producing seeds. Then all at once the bamboo plants over a large geographic region will set seed and die in the same year.
Competition
Competition is a powerful form of interaction in the organization of communities, but it differs from other forms of antagonistic and mutualistic relationships in that no species benefits from the interaction. In competitive interactions, species evolve either to avoid each other, to tolerate the presence of the other, or to aggressively exclude the other.
Types of competition
Species compete for almost every conceivable kind of resource, and the same two species may compete for different resources in different environments. Hole-nesting birds compete for tree holes, plant species compete for pollinators and seed dispersers, and male birds compete for preferred sites to defend as territories for attracting females. Species may compete for many resources simultaneously, but often one resource, called the limiting resource because it limits the population growth of each species, is the focus of competition. Moreover, the ways in which species compete vary with the resources. In some cases, species compete by capturing resources faster than their competitors (exploitation competition). Some plant species, for example, are able to extract water and nutrients from the soil faster than surrounding species. In other cases, the two species physically interfere with one another (interference competition) by aggressively attempting to exclude one another from particular habitats.
The effects of competition
Over evolutionary time, the effects of competition on species can vary. In some environments, the effects may be highly asymmetrical, and, at the extreme called amensalism, the survival or growth of one species may be inhibited and the other(s) not affected. The weaker competitor will either go extinct locally, diverge from the other species in its use of resources, or evolve an increased competitive ability. All three outcomes have been observed in natural and experimental populations studied by ecologists.
Species diverge from one another through competition, with the result that they fill different niches within the community. The great differences in bill size and shape that some of Darwin’s finches in the Galapagos have evolved have resulted from competition. This process, called character displacement, results as natural selection favours those individuals in each species that compete least with individuals of the other species. Experimental studies of coexisting seed-feeding rodents in the deserts of North America have shown that these species have evolved differences in size and other characteristics to minimize competition.
By evolving in response to one another, many competitors may be able to coexist regionally over the long term but not locally. Within any local area, one species may generally be driven to extinction by the other. Which species wins locally will depend on the physical environment, the genetic makeup of each of the competing species, and their interactions with other species in the community. Even subtle changes in the environment can affect which species wins. Experiments with species of flies (Drosophila) have shown that, when all other factors are held constant, small variations in temperature or in the percentage of ethanol in the larval environment can determine which species outcompetes the other. Hence, the continued coexistence of some competing species may depend critically on multiple populations of both or all species being distributed over a number of environments throughout a region (see population ecology: Metapopulations).
Commensalism and other types of interaction
In commensal interactions, one species benefits and the other is unaffected. The commensal organism may depend on its host for food, shelter, support, transport, or a combination of these.
One example of commensalism involves a small crab that lives inside an oyster’s shell. The crab enters the shell as a larva and receives shelter while it grows. Once fully grown, however, it is unable to exit through the narrow opening of the two valves, and so it remains within the shell, snatching particles of food from the oyster but not harming its unwitting benefactor. Another form of commensalism occurs between small plants called epiphytes and the large tree branches on which they grow. Epiphytes depend on their hosts for structural support but do not derive nourishment from them or harm them in any way.
Many other kinds of interaction, however, range from antagonism to commensalism to mutualism, depending on the ecological circumstances. For example, plant-feeding insects may have large detrimental effects on plant survival or reproduction if they attack small or nonvigorous plants but may have little or no effect on large or vigorous plants of the same species. Some human diseases may cause only temporary discomfort or be life-threatening, depending on the age and physical condition of the person.
No interaction between species fits neatly into the categories of antagonism, commensalism, or mutualism. The interaction depends on the genetic makeup of both species and the age, size, and physical condition of the individuals. Interactions may even depend on the composition of the community in which the interaction takes place. For example, the moth Greya politella pollinates the flowers of a small herb called the prairie star (Lithophragma parviflorum). The female moth pollinates while she lays eggs (oviposits) in the corolla of the flower. As she pushes her abdomen down into a flower, pollen adheres to her. She flies on to the next flower to lay more eggs, where some of the pollen rubs off onto the stigma of the flower, causing pollination to occur. Although this unusual pollination mechanism is very effective in some local populations, in other communities different pollinators such as bee flies and bees are so common that their visits to the flowers swamp the pollination efforts of the moths. As a result, pollination by the moths makes up a very tiny proportion of all the pollinator visits that occur within that community and probably has little effect on plant reproduction or natural selection. This moth, therefore, is a commensal in some populations and a mutualist in others, depending on the local assemblage of pollinator species.
The coevolutionary process
As pairs or groups of species interact, they evolve in response to each other. These reciprocal evolutionary changes in interacting species are called coevolutionary processes, one of the primary methods by which biological communities are organized. Through coevolution local populations of interacting species become adapted to one another, sometimes even evolving into new species.
The study of coevolution
To understand how coevolution shapes interactions within communities, researchers must distinguish between traits that have coevolved and those that were already present in ancestors before the interspecific interaction began. For example, hummingbirds use their wings and bills to reach the nectar within flowers. A hummingbird with a long bill may have evolved its bill as a result of coevolution with a particular species of flower; however, its wings are not the result of coevolution. Wings were already present in birds before hummingbirds evolved. Therefore, both the evolutionary ecology and the history (phylogeny) of the interacting species must be studied. The phylogeny indicates when each species arose within a lineage and when each new trait made its first appearance. The ecological studies can then show how each of those traits has been shaped by and used under different ecological conditions.
The study of the coevolution between moths of the family Prodoxidae and their host plants illustrates the interplay of phylogeny and ecology. Prodoxid moths include some species that have become major pollinators of plants. These pollinators include yucca moths (of the genera Tegeticula and Parategeticula) and Greya moths (see above Commensalism and other types of interaction). Greya moths inadvertently, or passively, pollinate the flowers they visit, but their close relatives the yucca moths purposely, or actively, perform this function. Female yucca moths collect and carry pollen on specialized appendages attached to their mouthparts. They visit yucca flowers to lay their eggs in the floral ovary, and their offspring feed on the developing seeds. While visiting each flower, a female moth takes some of the pollen she is carrying and places it directly on the stigma of the flower. Her offspring are therefore guaranteed developing seeds on which to feed. The yuccas have evolved to depend solely on these moths for pollination. Unlike many other plant species, they do not produce nectar or any other reward for pollinators and so do not waste energy to attract pollinators. They lose some of their seeds to the yucca moth larvae, but this is the cost of coevolution with this highly efficient pollinator.
Phylogenetic studies have shown that the loss of nectar production in yuccas and the evolution of active pollination in yucca moths are novel traits that have arisen through coevolution, as the relatives of yuccas produce nectar, and the relatives of yucca moths, the Greya moths, do not actively pollinate their host plants. Some other aspects of the interaction make use of traits that did not coevolve between yuccas and yucca moths. Instead, the traits were present in ancestors. Laying eggs in flowers and local specialization to one plant species are two traits that are common to all the close relatives of yucca moths, regardless of the plants on which they feed.
Therefore, by combining ecological and phylogenetic studies, researchers can piece together the history of coevolution between these species. The coevolved mutualism between yuccas and yucca moths began when their ancestors inadvertently became more successful at survival and reproduction as a result of their interactions. Yuccas that did not waste energy on nectar production to attract other pollinators achieved an advantage over those plants that did; yucca moths that ensured the availability of developing seeds for their offspring by actively pollinating the flowers in which they laid their eggs also gained an advantage over populations that did not do so. The process undoubtedly involved many other twists and turns along the way, but the combination of evolutionary ecological and phylogenetic studies allows at least part of the coevolutionary process to be reconstructed.
All coevolved interactions are similar to those between yuccas and yucca moths in that natural selection operates on traits that are already present within species, molding them in new ways by favouring new mutations that fine-tune the relationship. Tinkering rather than engineering is how the biologist François Jacob described the process of evolution, and his analogy certainly extends to the coevolutionary process. Coevolved interactions are not designed from scratch for maximum efficiency. Instead, evolution fiddles with existing structures and behaviours and adapts them to perform new functions—in effect, jury-rigging them. Consequently, the organization of biological communities reflects this makeshift nature of adaptation and coevolution.
The coevolutionary “arms race” versus reduced antagonism
Nothing is absolutely predictable about the direction of coevolution. How an interaction coevolves depends not only on the current genetic makeup of the species involved but also on new mutations that arise, the population characteristics of each species, and the community context in which the interaction takes places. Under some ecological conditions, an antagonistic interaction between two species can coevolve to enhance the antagonism; the species “build up” methods of defense and attack, much like an evolutionary arms race. Under other ecological conditions, however, the antagonism may be lessened, resulting in reduced antagonism.
