E R Pianka, University of Texas, Austin, TX, USA © 2008 Elsevier B.V. All rights reserved.

Further Reading

Environment refers to all of the physical and biotic factors directly influencing a given organism or organismic unit as well as anything affected by it. As such, the environments of most organisms are exceedingly complex and multidimensional. Autecology is the study of the interactions between an individual, a population, or a species and its total environment. In contrast, the study of the ecology of entire communities is known as synecology. The Encyclopaedia Britannica says that autecology is also called 'species ecology'. Sometimes autecology has also been equated with physiological ecology.

Autecological studies take place at the level of individuals, populations, or an entire species, and they must deal with a myriad of physical environmental variables such as light, humidity, temperature, pH, concentrations of gases such as carbon dioxide and oxygen, availability of water and various other nutrients, among others. The goal is to understand the requirements, life history, and behavior of a particular target organism or species. Thermoregulation and water balance are often considered. Of course, to be complete, an autecology must also consider the biotic environment, which can be quite complex as it includes habitat, microhabitats, interactions with other species, as well as factors such as nesting sites, foraging tactics, diet, both intraspecific and interspecific competition, parasites, predators, and mutualists, such as pollinators and/or seed dispersers.

Autecologies differ greatly among various taxa. Many bacteria and fungi are decomposers, plants are primary producers (autotrophs), whereas animals are consumers (heterotrophs). Some species live in soil or mud, others live on surfaces, and still others in three-dimensional space. For microbes and protists, surface tension and pH can be critical, but larger organisms are better buffered and hence protected against such environmental factors. Smaller organisms have shorter generation times and faster rates of increase than larger species, but because larger species live longer, they must encounter a wider array of environmental conditions than do smaller, shorter-lived, species. Microbes evolve at much faster rates than larger organisms. Aquatic organisms have fundamentally different problems to solve than terrestrial ones. Marine fish are approximately isotonic with sea-water, whereas fish that live in freshwater must excrete large amounts of water in order to maintain high plasma ion concentration levels. Phytoplankton have exceedingly short life spans and very high rates of increase, as compared to the zooplankton that feed on them. Annual plants germinate, grow, and reproduce in less than a year, whereas perennials such as tree species may live for many hundreds of years. Early in secondary succession, fast-growing sun-loving trees predominate, whereas climax species are slow growing, shade tolerant, and can set seed underneath canopies of their own species. For these reasons, shade tolerance and growth rates are inversely correlated among tree species. Similarly, insects live in a time-space scale quite different from those experienced by most vertebrates. Because many insects are aquatic as larvae but terrestrial as adults, ecological niches of such insect larvae differ profoundly from those of adults. For example, lepidopteran caterpillars are herbivorous, whereas most adults feed on nectar and cannot replenish nitrogen reserves; as a result, adult butterflies must budget larval amino acids among a finite number of offspring -once larval reserves are used up, adults can no longer reproduce. Colonizing and dispersal abilities also vary widely among organisms. Plants and some invertebrates such as snails are sessile or very sedentary, whereas creatures such as birds and whales may move thousands of kilometers during their lifetimes. Hosts are 'islands' that must be colonized by parasites; as a result, most parasites have evolved high fecundity and good dispersal ability. Some parasites exploit intermediate hosts such as mosquitoes to find their way to their final hosts. Other parasites manipulate their hosts and change host behavior in ways that facilitate transmission to new hosts; for example, the virus that causes rabies passes itself on via saliva when a rabid animal bites another uninfected animal. Whereas predators kill their prey outright and consume them on the spot, parasites must exploit their hosts over a period of time. Parasites may evolve to become benign to their hosts because a virulent strain that kills its host also dies itself. However, some parasitic organisms such as influenza viruses are in a race against their host's immune systems - they must therefore be virulent enough to propagate themselves and spread to new hosts before their current host annihilates them.

Botanists embraced the autecological approach early, and the term was used up until about 1990, but it has fallen into disuse lately as ecologists have come to realize that one cannot understand the ecology of any organism without knowing the entire complex network of interacting organisms within which a given target organism is embedded.

Ecologists now appreciate the importance of indirect effects mediated by way of other organisms, sometimes several trophic levels away from an organism under consideration. Such 'trophic cascades' can be considered from two perspectives. For example, so-called 'bottom-up effects' can occur when climatic variables alter populations of primary producers, which changes productivity and reverberates upwards by changing population sizes of their herbivores to affect food supply for carnivores at higher trophic levels. In contrast, 'top-down effects' may take place when factors influencing predators in turn impact their prey populations (herbivores), which then cascade downwards to affect plant populations. With three trophic levels, the carnivore and plants are indirect mutualists because the carnivore, by eating the herbivore, reduces predation pressure on the plant - by producing food for the herbivore, plants raise herbivore population density, thereby indirectly benefiting the carnivore by increasing its food supply. The old adage ''the enemy of my enemy is my friend'' applies. Such an interaction is called a 'food chain mutualism' (panel 'c' below). An example was provided by Power et al:. fish-eating bass prey upon herbivorous minnows in pools of an Oklahoma creek. When bass were removed (pools were fenced to keep these predators out) and minnow densities raised, the standing crop of algae diminished. With the readdition of bass, minnows retreated to shallow water and algal densities increased significantly over the next two weeks. Indirect effects are the products of direct effects, so that a negative direct effect times another negative direct effect becomes a positive indirect effect. If a fourth trophic level is added (panel 'd' below), top-down versus bottom-up effects are no longer symmetric (bottom-up effects remain beneficial, but top-down effects become detrimental with an odd number of negative links between trophic levels). In addition to such vertical indirect effects between trophic levels, they can also occur horizontally within a given trophic level. In such a situation, a competitor sandwiched between two other competitors can result in a 'competitive mutualism', because the two competitors on either end both benefit each other via their joint competitive inhibition of the shared competitor between them (panel 'e' below). These, and some other indirect interactions, are depicted in Figure 1.

Direct effects are shown with solid arrows, indirect effects with dashed arrows. Pointed headed arrows point to the party benefiting, whereas circle-headed arrows

designate detrimental effects. In all except panels 'c' and 'd', C's represent consumer species, and P's represent prey or plant producers. In panels 'c' and 'd', C's are carnivores, H is an herbivore, and P is a producer. Indirect effects take longer to occur than direct effects and are usually weaker, especially if path lengths are long. Nevertheless, because there are many more indirect effects than direct effects, the former can be important. A given target species may have both direct and indirect interactions with another species and these could be opposite in sign effectively canceling out each other.

In a long-term study of the interactions between seed-eating ants and rodents, Munger and Brown and Brown et al. found exploitation competition (panel 'a' above) in the short term, but over a longer time period of decades, they suggested that seed-eating ants and rodents were indirect mutualists with ants eating small seeds and rodents eating large seeds. Because small seeded plants compete with plants with large seeds, this system corresponds to the one shown in panel 'f' of Figure 1. An indirect mutualism or facilitation occurs between Cx and C2 arising from the products of two negative direct links times one direct positive link with total indirect path lengths of three. Many other indirect interactions with still longer path lengths are also possible.

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