Ecology can be defined as the study of the relationships of organisms with their living and nonliving environment. Most ecologists are interested in questions involving the natural environment. Increasingly, however, ecologists are concerned about degradation associated with the ecological effects of humans and their activities. Ultimately, ecological knowledge will prove to be fundamental to the design of systems of resource use and management that will be capable of sustaining humans over the longer term, while also sustaining other species and natural ecosystems.
The subject matter of ecology is the relationships of organisms with their biological and nonliving environment. These are complex, reciprocal interactions; organisms are influenced by their environment, but they also cause environmental change, and are components of the environment of other organisms.
Ecology can also be considered to be the study of the factors that influence the distribution and abundance of organisms. Ecology originally developed from natural history, which deals with the richness and environmental relationships of life, but in a non-quantitative manner.
Although mostly a biological subject, ecology also draws upon other sciences, including chemistry, physics, geology, mathematics, computer science, and others. Often, ecologists must also deal with socioeconomic issues, because of the rapidly increasing importance of human impacts on the environment. Because it draws upon knowledge and information from so many disciplines, ecology is a highly interdisciplinary field.
The biological focus of ecology is apparent from the fact that most ecologists spend much of their time engaged in studies of organisms. Examples of common themes of ecological research include: (1) the physical and physiological adaptations of organisms to their environment, (2) patterns of the distribution of organisms in space, and how these are influenced by environmental factors, and (3) changes in the abundance of organisms over time, and the environmental influences on these dynamics.
The universe can be organized along a spectrum of levels according to spatial scale. Ordered from the extremely small to the extremely large, these levels of integration are: subatomic particles ... atoms ... molecules ... molecular mixtures ... tissues ... organs ... individual organisms ... populations of individuals ... communities of populations ... ecological landscapes ... the biosphere ... the solar system ... the galaxy ... the universe. Within this larger scheme, the usual realm of ecology involves the levels ranging from (and including) individual organisms through to the biosphere. These elements of the ecological hierarchy are described below in more detail.
In the ecological and evolutionary contexts, an individual is a particular, distinct organism, with a unique complement of genetic information encoded in DNA. (Note that although some species reproduce by nonsexual means, they are not exceptions to the genetic uniqueness of evolutionary individuals.) The physical and physiological attributes of individuals are a function of (1) their genetically defined capabilities, known as the genotype, and (2) environmental influences, which affect the actual expression of the genetic capabilities, known as the phenotype. Individuals are the units that are "selected" for (or against) during evolution.
A population is an aggregation of individuals of the same species that are actively interbreeding, or exchanging genetic information. Evolution refers to changes over time in the aggregate genetic information of a population. Evolution can occur as a result of random "drift," as directional selection in favor of advantageous phenotypes, or as selection against less well-adapted genotypes.
An ecological community is an aggregation of populations thar are interacting physically, chemically, and behavioral in the same place. Strictly speaking, a community consists of all plant, animal, and microbial populations occurring together on a site. Often, however, ecologists study functional "communities" of similar organisms, for example, bird or plant communities.
This level of ecological organization refers to an aggregation of communities on a larger area of terrain. Sometimes, ecological units are classified on the basis of their structural similarity, even though their actual species may differ among widely displaced locations. A biome is such a unit, examples of which include alpine and arctic tundra, boreal forest, deciduous forest, prairie, desert, and tropical rainforest.
The biosphere is the integration of all life on Earth, and is spatially defined by the occurrence of living organisms. The biosphere is the only place in the universe known to naturally support life.
Less than 1 percent of the solar energy reaching Earth's surface is absorbed by green plants or algae and used in photosynthesis. However, this fixed solar energy is the energetic basis of the structure and function of ecosystems. The total fixation of energy by plants is known as gross primary production (GPP). Some of that fixed energy is used by plants to support their own metabolic demands, or respiration (R). The quantity of energy that is left over (that is, GPP - R) is known as net primary production (NPP). If NPP has a positive value, then plant biomass accumulates over time, and is available to support the energy requirements of herbivorous animals, which are themselves available as food to support to carnivores. Any plant or animal biomass that is not directly consumed eventually dies, and is consumed by decomposers (or detritivores), the most important of which are microorganisms such as bacteria and fungi. The complex of ecological relationships among all of the plants, animals, and decomposers is known as a food web.
Compared with the potential biological "demand," the environment has a limited ability to "supply" the requirements of life. As a result, the rates of critical ecological processes, such as productivity, are constrained by so-called limiting factors, which are present in the least supply relative to the biological demand. A limiting environmental factor can be physical or chemical in nature, and the factors act singly, but sequentially. For example, if a typical unproductive lake is fertilized with nitrate, there would be no ecological response. However, if that same lake was fertilized with phosphate, there would be a great increase in the productivity of single-celled algae. If the lake was then fertilized with nitrate, there would be a further increase of productivity, because the ecological requirement for phosphate, the primary limiting factor, had previously been satiated.
This example illustrates the strong influence that the environment has on rates of processes such as productivity, and on overall ecological development. The most complex, productive, and highly developed ecosystems occur in relatively benign environments, where climate and the supplies of nutrients and water are least limiting to organisms and their processes. Tropical forests and coral reefs are the best examples of well-developed, natural ecosystems of this sort. In contrast, environmentally stressed ecosystems are severely constrained by one or more of these factors. For example, deserts are limited by the availability of water, and tundra by a cold climate.
In a theoretically benign environment, with an unlimited availability of the requirements of life, organisms can maximize the growth of their individual biomass and of their populations. Conditions of unlimited resources might occur (at least temporarily), perhaps, in situations that are sunny and well supplied with water and nutrients. Population growth in an unlimited environment is exponential, meaning that the number of individuals doubles during a fixed time interval. For example, if a species was biologically capable of doubling the size of its population in one week under unlimited environmental conditions, then after one week of growth an initial population of N individuals would grow to 2N, after two weeks 4N, after three weeks 8N, after four weeks 16N, and after eight weeks it would be 256N. A financial analogy will help to put this tremendous rate of population increase into perspective: an initial investment of $100 growing at that rate would be worth $25,600 after only 8 weeks.
Clearly, this is an enormous rate of growth, and it would never be sustainable under real-world ecological (or economic) conditions. Before long, environmental conditions would become limiting, and organisms would begin to interfere with each other through an ecological process known as competition. In general, the more similar the ecological requirements of individuals or species, the more intense is the competition that they experience. Therefore, competition among similar-sized individuals of the same species can be very intense, while individuals of different sized species (such as trees and moss) will compete hardly at all.
Competition is an important ecological process, because it limits the growth rates of individuals and populations, and influences the kinds of species that can occur together in ecological communities. These ecological traits are also profoundly influenced by other interactions among organisms, such as herbivory, predation, and disease.
The larger objective of ecology is to understand the nature of environmental influences on individual organisms, their populations, and communities, on ecoscapes and ultimately at the level of the biosphere. If ecologists can achieve an understanding of these relationships, they will be well placed to contribute to the development of systems by which humans could sustainably use ecological resources, such as forests, agricultural soil, and hunted animals such as deer and fish. This is an extremely important goal because humans are, after all, completely reliant on ecologically goods and services as their only source of sustenance.
Science. Gale Research, 1996. Reproduced in Student
Resource Center College Edition. Farmington Hills,
Mich.: Gale Group. September, 1999.