Unit Eight. The Living Environment
The previous section described the “winner take all” results of competition between two species whose niches overlap. Other relationships in nature are less competitive and more cooperative.
The plants, animals, protists, fungi, and prokaryotes that live together in communities have changed and adjusted to one another continually over millions of years. For example, many features of flowering plants have evolved in relation to the dispersal of the plant’s gametes by animals (figure 35.21). These animals, in turn, have evolved a number of special traits that enable them to obtain food or other resources efficiently from the plants they visit, often from their flowers. In addition, the seeds of many flowering plants have features that make them more likely to be dispersed to new areas of favorable habitat.
Figure 35.21. Pollination by bat.
Many flowering plants have coevolved with other species to facilitate pollen transfer. Insects are widely known as pollinators, but they're not the only ones. Notice the cargo of pollen on the bat's snout.
Such interactions, which involve the long-term, mutual evolutionary adjustment of the characteristics of the members of biological communities, are examples of coevolution. Coevolution is the adaptation of two or more species to each other. In this section, we consider the many ways species interact, some of which involve coevolution.
Symbiosis Is Widespread
In symbiotic relationships, two or more kinds of organisms live together in often elaborate and more or less permanent relationships. All symbiotic relationships carry the potential for coevolution between the organisms involved, and in many instances the results of this coevolution are fascinating. Examples of symbiosis include lichens, which are associations of certain fungi with green algae or cyanobacteria (see chapter 18). Other important examples are mycorrhizae, the associations between fungi and the roots of most kinds of plants. The fungi expedite the plant’s absorption of certain nutrients, and the plants in turn provide the fungi with carbohydrates. Similarly, root nodules that occur in legumes and certain other kinds of plants contain bacteria that fix atmospheric nitrogen and make it available to their host plants.
The major kinds of symbiotic relationships include: (1) mutualism, in which both participating species benefit; (2) parasitism, in which one species benefits but the other is harmed; and (3) commensalism, in which one species benefits while the other neither benefits nor is harmed. Parasitism can also be viewed as a form of predation (discussed later), although the organism that is preyed upon does not necessarily die.
Mutualism is a symbiotic relationship among organisms in which both species benefit. Examples of mutualism are of fundamental importance in determining the structure of biological communities. Some of the most spectacular examples of mutualism occur among flowering plants and their animal visitors, including insects, birds, and bats. As we discussed in chapter 32, during the course of their evolution, the characteristics of flowers have evolved in large part in relation to the characteristics of the animals that visit them for food and, in doing so, spread their pollen from individual to individual. At the same time, characteristics of the animals have changed, increasing their specialization for obtaining food or other substances from particular kinds of flowers.
Another example of mutualism involves ants and aphids. Aphids, also called greenflies, are small insects that suck fluids with their piercing mouthparts from the phloem of living plants. They extract a certain amount of the sucrose and other nutrients from this fluid, but they excrete much of it in an altered form through their anus. Certain ants have taken advantage of this—in effect, domesticating the aphids (figure 35.22). The ants carry the aphids to new plants, where they come into contact with new sources of food, and then consume as food the “honeydew” that the aphids excrete.
Figure 35.22. Mutualism: ants and aphids.
These ants are tending to aphids (small green organisms), feeding on the "honeydew" that the aphids excrete continuously, moving the aphids from place to place, and protecting them from potential predators.
Parasitism is a symbiotic relationship that may be regarded as a special form of predator/prey relationship, discussed later. In this symbiotic relationship the predator, or parasite, is much smaller than the prey, or host, and remains closely associated with it. Parasitism is harmful to the host organism and beneficial to the parasite, but unlike a predator/prey relationship, a parasite often does not kill its host. The concept of parasitism seems obvious, but individual instances are often surprisingly difficult to distinguish from predation and from other kinds of symbiosis.
External Parasites. Parasites that feed on the exterior surface of an organism are external parasites, or ectoparasites. Lice, which live their entire lives on the bodies of vertebrates— mainly birds and mammals—are normally considered parasites. Mosquitoes are not considered parasites, even though they draw food from birds and mammals in a similar manner to lice, because their interaction with their host is so brief.
Internal Parasites. Vertebrates are parasitized internally by endoparasites, members of many different phyla of animals and protists. Invertebrates also have many kinds of parasites that live within their bodies. Bacteria and viruses are not usually considered parasites, even though they fit our definition precisely.
Internal parasitism is generally marked by much more extreme specialization than external parasitism, as shown by the many protist and invertebrate parasites that infect humans. The structure of an internal parasite is often simplified, and unnecessary armaments and structures are lost as it evolves.
Commensalism is a symbiotic relationship that benefits one species and neither hurts nor helps the other. In nature, individuals of one species are often physically attached to members of another. For example, epiphytes are plants that grow on the branches of other plants. In general, the host plant is unharmed, and the epiphyte that grows on it benefits. Similarly, various marine animals, such as barnacles, grow on other, often actively moving, sea animals like whales and thus are carried passively from place to place without harming their hosts. These “passengers” presumably gain more protection from predation than they would if they were fixed in one place, and they also reach new sources of food. The increased water circulation that such animals receive as their host moves around may be of great importance, particularly if the passengers are filter-feeders.
Examples of Commensalism. The best-known examples of commensalism involve the relationships between certain small tropical fishes and sea anemones, which are marine animals that have stinging tentacles (see chapter 19). Certain species of tropical fish have evolved the ability to live among the tentacles of sea anemones, even though these tentacles would quickly paralyze other fishes that touched them. The anemone fishes feed on the detritus left from the meals of the host anemone, remaining uninjured under remarkable circumstances.
On land, an analogous relationship exists between birds called oxpeckers and grazing animals such as cattle or rhinoceroses. The oxpecker birds spend most of their time clinging to the animals, picking off parasites and other insects, carrying out their entire life cycles in close association with the host animals.
When Is Commensalism Commensalism? In each of these instances, it is difficult to be certain whether the second partner receives a benefit or not; there is no clear-cut boundary between commensalism and mutualism. For instance, it may be advantageous to the sea anemone to have particles of food removed from its tentacles; it may then be better able to catch other prey. Similarly, the grazing animals may benefit from the relationship with the oxpeckers or cattle egrets if the bugs that are picked off of them are harmful, such as ticks or fleas. If so, then the relationship is mutualism. However, if the birds also pick at scabs, causing bleeding and possibly infections, the relationship may be parasitic. In true commensalism, only one of the partners benefits and the other neither benefits nor is harmed. If the grazing animals are not harmed by either the ticks that are eaten off of their bodies or by the oxpeckers that feed off of them, then it is an example of commensalism.
Key Learning Outcome 35.10. Coevolution is a term that describes the long-term evolutionary adjustments of species to one another. In symbiosis, two or more species live together. Mutualism involves cooperation between species, to the mutual benefit of both. In parasitism, one organism serves as a host to another organism, usually to the host's disadvantage. Commensalism is the benign use of one organism by another.