Adaptation is the acquisition of traits that allow a species to survive in its environment. Genetic traits are passed from generation to generation, and individuals with traits that give an advantage in survival are more likely to reproduce, and their offspring inherit the beneficial traits. The basic idea of evolution is that species change over generations because individuals compete for scarce resources. The process of better-selected individuals passing their traits to the next generation is called natural selection. Changes in traits occur because of random mutations in the DNA of individuals. During the course of a species’ existence, some mutations are thought to have given those individuals an advantage under the selection pressures in their environment at that time. The result is a species population that differs from those of preceding generations. Species are limited in where they can live. A habitat is the place or set of environmental conditions in which a particular organism lives. The term ecological niche is a more functional description of both the role played by a species in a biological community and the set of environmental factors that determine its distribution. Some species are generalists, others are specialists. Endemic species occur only in one area (or one type of environment). The principle of competitive exclusion states that no two species can occupy the same ecological niche for long. Resource partitioning is the process in which species evolve to use resources differently, or to use different resources. The development of a new species is called speciation. Speciation that occurs when populations are geographically separated is known as allopatric speciation. Speciation that occurs within one geographic area is known as sympatric speciation.
Competition among members of the same species is called intraspecific competition, whereas competition between members of different species is called interspecific competition. A predator is any organism that feeds directly on another living organism, whether or not this kills the prey. Predator-prey relationships exert selection pressures that favor evolutionary adaptation, producing physical and behavioral changes in a process known as coevolution. Symbiotic relationships can be nonantagonistic, even beneficial. Mutualism results in a mutual benefit to one more species. Commensalism is a type of symbiosis in which one member clearly benefits and the other apparently is neither benefited or harmed. Parasitism, a form of predation, may also be considered symbiosis because of the dependency of the parasite on its host. A keystone species plays a critical role in a biological community that is out of proportion to its abundance.
A community’s primary productivity is the rate of biomass production, an indication of the rate of solar energy conversion to chemical energy. Only 0.1 to 0.2 percent of the absorbed energy is used by chloroplasts to synthesize carbohydrates. Abundance is an expression of the total number of organisms in a biological community, while diversity is a measure of the number of different species, ecological niches, or genetic variation present. The abundance of a particular species is often inversely related to the total diversity of the community. As a general rule, diversity decreases but abundance within species increases as we go from the equator toward the poles. Ecological structure is the spatial distribution or organization of individuals and populations within a community. Complexity in ecological terms refers to the number of species at each trophic level and the number of trophic levels in a community. We can identify three kinds of stability or resiliency in ecosystems: constancy (lack of fluctuations in composition or functions), inertia (resistance to perturbations), and renewal (ability to repair damage after disturbance). The relationships that occur at the boundary between one habitat and its neighbors are called edge effects. Ecotone refers to either a boundary between adjacent communities or a gradual transition zone between biomes.
Ecological succession is the transition from one community to another. In primary succession, land that is bare of soil is colonized by living organisms where none lived before. When an existing community is disturbed, a new one develops from the biological legacy of the old in a process called secondary succession. In primary succession on land, the first colonists are hardy pioneer species, often microbes, mosses, and lichens that can withstand a harsh environment with few resources. A disturbance is any force that disrupts the established patterns of species diversity and abundance, community structure, or community properties. Ecologists generally find that disturbance benefits many species, much as predation does, because it sets back supreme competitors and allows less-competitive species to persist. Disturbance-adapted species tolerate or even require occasional disturbances. Introduced species can cause profound community change.
—March 2021
Lance Jones 