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Virtually no species has exclusive access to any given set of resources. All species must compete with other species for at least some resources. Resources are simply the components of the environment—such as food, water, light, and space—that are required by all species. The full set of these resources, in part, determines the niche of a species, defined as the physical and biological conditions required for growth, reproduction, and survival. Thus a species’ niche defines where it can and cannot live. Although a species might be physiologically able to live under a wide range of conditions, competitors may restrict its use of those resources. Thus every species has a fundamental niche, defined by its physiological capabilities, and a realized niche, defined by its interactions with other species.

As you saw in Key Concept 54.1, intraspecific competition for resources is the main reason why populations do not grow indefinitely, and instead eventually reach some limit defined by their carrying capacity. Interspecific competition—competition among different species—affects individuals in much the same way. At some point an essential resource, or set of resources, may be in such short supply that competition becomes a factor in a population’s growth. Such a resource that is in short supply is referred to as a limiting resource. At one extreme, if a species can prevent another species from using essential resources, the inferior competitor may become locally extinct, a result called competitive exclusion. In reality, though, most species show some sort of competitive coexistence, or the ability to coexist with one another despite sharing limiting resources. Competitive coexistence is maintained through a variety of ecological processes.

Assuming competitive coexistence occurs, competition can manifest itself either directly or indirectly. Interference competition occurs when one species directly interferes with or excludes another species’ access to a limiting resource. Interference competition can take many forms, from physical exclusion to chemical warfare among the competitors. A graphic example involves two ant species: the desert ant Conomyrma bicolor and the honeypot ant Myrmecocystus mexicanus. These species occupy the same habitat type—arid areas containing little vegetation—and they feed on similar foods—the sugary excretions of aphids and other sap-feeding insects. When C. bicolor workers find the entrance of a honeypot ant nest, they pick up small stones in their mandibles, carry them to the rim of the nest opening, and drop them down the hole—up to 200 stones in a 5-minute interval. This activity stops the honeypot ants from foraging and temporarily limits competition for food with the desert ant.

Exploitation competition occurs when a limiting resource is available to all competitors but the outcome of the interaction depends on the relative efficiency with which each species uses the resource. Exploitation competition affects the availability of a resource for another species but not in an exclusionary way, as we see with interference competition. Exploitation competition may lead to coexistence, provided that the species relying on the same resource have ways to divide up, or partition, that resource. For example, in the American Southwest at least three species of bees consume the nectar of the shin dagger agave (Agave schottii). The three bee species differ in where and when they collect shin dagger nectar. Honey bees tend to forage in places with the greatest numbers of shin dagger flowers, bumblebees in places with intermediate numbers of flowers, and carpenter bees where flowers are few and far between. Honey bees also tend to be most active when nectar output is greatest. With their larger nests and greater numbers of offspring to support, honey bees require greater foraging efficiency and greater energy intake. Foraging sites that are not worth their time are left to the other bees.

The bee–shin dagger agave example is one in which closely related bee species compete for agave nectar. But competition is not limited to closely related species; it commonly occurs among distantly related species as well. Bats and hummingbirds also visit shin dagger agave for its nectar. Even humans harvest agave (albeit the leaves rather than the nectar) to be used as an alternative for sugar.

Another important aspect of competition is that it is often asymmetrical (unequal): one species can be more negatively affected by the interaction than the other. Clearly this is the case under conditions of competitive exclusion, when one species drives the other to local extinction. But even when species coexist under competition, competitive success forms a continuum, as you saw with red and gray squirrels in the United Kingdom (see Figure 55.2). The effects of competition can vary for many reasons. Morphological, physiological, or behavioral differences can affect a species’ ability to obtain limiting resources. In addition, the magnitude of competitive interactions can vary depending on environmental conditions and how they affect the competitive ability of individual species. We’ll discuss how the outcome of competitive interactions can be modified by environmental factors below, but first let’s consider the ways in which species can divide limiting resources to reduce their competitive effects on one another.