Competition

Categories: Ecology; ecosystems; evolution

Competition is a major driving force in evolution, the process by which living organisms change over time, with better-adapted species surviving and less well-adapted species becoming extinct. Evolution begins with mutation, changes in the nucleotide sequence of a gene or genes, resulting in the production of slightly altered genes which encode slightly different proteins. These altered proteins are the expressed traits of an organism and may give the organism an advantage over its competitors. The organism outcompetes its rivals in the environment, and hence the environment favors the better-adapted, fitter organism, a process called natural selection. A mutation may help an organism in one environment but may hurt it in a different environment (for example, an albino squirrel may flourish in snowy regions but may not do as well in warm regions). Mutations are random events whose occurrence can be increased by chemicals called mutagens or by ionizing radiation such as ultraviolet light, X rays, and gamma radiation.

Species Interactions

Natural selection influences the distribution and abundance of organisms from place to place. The possible selection factors include physical factors (temperature and light, for example), chemical factors such as water and salt, and species interactions. According to ecologist Charles Krebs, species interactions include four principal types: mutualism, which is the living together of two species that benefit each other (for example, fungi and algae living together as lichens); commensalism, which is the living together of two species that results in a distinct benefit (or number of benefits) to one species while the other remains unhurt (for example, plants called epiphytes that grow on other plants); predation, which is the hunting, killing, and eating of one species by another (examples include insects eating plants or snails eating algae); and competition, which is defined as an active struggle for survival among all the species in a given environment.

Competition is related to the acquisition of various resources: food, space, and pollinators. Food is an obvious target of competition. All organisms must have energy in order to conduct the cellular chemical reactions (such as respiration) that keep them alive. Photoautotrophic organisms (plants, algae, cyanobacteria) obtain energy by converting sunlight, carbon dioxide, and water into organic molecules, a process called photosynthesis. Photoautotrophs, also called primary producers, compete for light and water. For example, oak and hickory trees in eastern North America grow taller than most pines, thereby shading smaller species and eventually dominating a forest. A few other autotrophs, such as chemoautotrophic bacteria, obtain their energy from inorganic chemical reactions rather than from sunlight.

All other organisms—animals, zooplankton, and fungi—are heterotrophs, also called consumers; they must consume other organisms to obtain energy. Heterotrophs include herbivores, carnivores, omnivores, and saprotrophs. Herbivores (plant eaters such as rabbits and cattle) derive their energy from eating plants. Carnivores (meat eaters such as cats and dogs) eat other heterotrophs in order to get their energy needs met. Omnivores (such as humans) eat plants and animals. Saprotrophs (such as fungi and bacteria) decompose dead organisms to meet their energy requirements. Life on earth functions by intricately complex food chains in which organisms consume other organisms in order to obtain energy. Ultimately, almost all energy in organisms comes from the sun.

Types of Competition

Intraspecific competition occurs among individual members of the same population, for example, when sprouts grow from seeds scattered closely together on the ground. Some seedlings will be able to grow faster than others and will inhibit the growth of less vigorous seedlings by overshadowing or overcrowding them.

Interspecific competition involves two or more different species trying to use the same resources. All green plants depend on photosynthesis to derive the energy and carbon they need. Different areas or communities favor different growth characteristics. For plants with high light requirements, a taller-growing plant (or one with more or broader leaves) will have a competitive advantage if its leaves receive more direct sunlight than competitors. If, on the other hand, the species cannot tolerate too much sun, a shorter-growing species that can benefit from sheltering shadows of larger plants nearby will have the competitive advantage over other shade-loving plants.

Competition in nature leads to certain species dominating an environment and evolving to adapt to it, while the outcompeted species move elsewhere or become extinct. Humans are able to utilize the results of competition among species for their own benefit. This principle is useful in agriculture. Certain plant species (such as sunflowers and peach trees) release chemicals into the soil that inhibit the growth of other plants that might otherwise compete with them. Plant competition can be used to fight the growth of weeds and is useful in understanding which plants are compatible.

Competition is also useful in medical microbiological research. Certain fungal species produce and secrete molecules called antibiotics, which kill bacteria. Antibiotics give these fungi an edge in their competition against bacteria. Laboratory production of these antibiotics can treat bacterial diseases in humans.

Bibliography

Barbour, Michael G., ed. Terrestrial Plant Ecology . 3d ed. Menlo Park, Calif.: Benjamin/Cummings, 1999. Covers the entire breadth of modern plant ecology, blending classic topics with the results of new research, using as little jargon as possible.

Hartl, Daniel L. Principles of Population Genetics . 3d ed. Sunderland, Mass.: Sinauer Associates, 1997. Hartl’s work, aimed at graduate-level biologists, is a detailed mathematical approach to species interactions and evolution. Approaches the study of populations from a genetic and statistical viewpoint and provides numerous examples and references.

Keddy, Paul A. Competition. 2d ed. New York: Kluwer, 2000. A somewhat controversial yet wide-ranging study of competition in all the natural kingdoms, using many nontechnical examples easily understood by the layperson. Argues that the challenge in ecology is to determine when and where each kind of competition is important in natural systems, focusing on variants of competition such as intensity, asymmetry, and hierarchies.

Krebs, Charles J. Ecology: The Experimental Analysis of Distribution and Abundance. 5th ed. San Francisco: Benjamin Cummings, 2001. This advanced textbook is a wonderfully comprehensive survey of ecology. Numerous experiments are cited, with a thorough but understandable discussion of mathematical modeling in ecology. Includes many references.

Raven, Peter H., and George B. Johnson. Biology . 5th ed. Boston: WCB/McGraw-Hill, 1999. Beautifully illustrated, clearly written, well-diagramed introductory biology textbook for undergraduates. Chapter 23, “Population Dynamics,” is a clear presentation of species interactions, such as competition, that contains numerous examples.