Interactions among Populations in Communities
Community Ecology
Interactions among
Populations in Communities
Populations of animals are part of a larger system, known as the community, within which populations of different species interact. The number of species that share a habitat is known as species diversity. These species interact in a variety of ways that can be detrimental (−), beneficial (+), or neutral (0) to each species, depending on the nature of the interaction. For instance, we can consider a predator’s effect on its prey as (−), because the survival of the prey animal is reduced. However, the same interaction benefits the predator (+) because the food obtained from prey increases the predator’s ability to survive and reproduce. Thus, the predatory-prey interaction is + −. Ecologists use this shorthand notation to characterize interspecific interactions because it helps us to view the direction in which the interaction affects each species.
We see other kinds of + − interactions. One of these is parasitism, in which the parasite benefits by using the host as a home and source of nutrition, and the host is harmed. Herbivory, in which an animal eats a plant, is another + − relationship.
Commensalism is an interaction that
benefits one species and neither harms
nor benefits the other (0 +). Most bacteria
that normally inhabit our intestinal
tracts do not affect us (0), but the bacteria benefit (+) by having food and a
place to live. A classic example of commensalism
is the association of pilot
fishes and remoras with sharks (Figure
40-6). These fishes get the “crumbs”
remaining when the host shark makes
its kill, but we now know that some
remoras also feed on ectoparasites of
the sharks. Commensalism therefore
grades into mutualism.
Organisms engaged in mutalism have a friendlier arrangement than commensalistic species, because the fitness of both is enhanced (+ +). Biologists are finding mutualistic relationships far more common in nature than previously believed (Figure 40-7). Some mutualistic relationships are not only beneficial, but necessary for survival of one or both species. An example is the relationship between a termite and protozoa inhabiting its gut. The protozoa can digest wood eaten by the termite because the protozoa produce an enzyme, lacking in the termite, that digests cellulose; the termite lives on waste products of protozoan metabolism. In return, the protozoa gain a habitat and food supply. Such absolute interdependence among species can be a liability if one of the participants is lost. Calvaria trees native to the island of Mauritius have not reproduced successfully for over 300 years, because their seeds germinate only after being eaten and passed through the gut of a dodo bird, now extinct.
Competition between species reduces the fitness of both (− −). Many biologists, including Darwin, considered competition the most common and important interaction in nature. Ecologists have constructed most of their theories of community structure from the premise that competition is the chief organizing factor in species assemblages. Sometimes the effect on one of the species in a competitive relationship is negligible. This condition is called amensalism, or asymmetric competition (0 −). For example, two species of barnacles that commonly occur in rocky intertidal habitats, Chthamalus stellatus and Balanus balanoides, compete for space. A famous experiment by Joseph Connell* demonstrated that B. balanoides excluded C. stellatus from a portion of the habitat, while C. stellatus had no effect on B. balanoides.
We have treated interactions as occurring between pairs of species. However, in natural communities containing populations of many species, a predator may have more than one prey and several animals may compete for the same resource. Thus, ecological communities are quite complex and dynamic, a challenge to ecologists who wish to study this level of natural organization.
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| Figure 40-6 Four remoras, Remora sp., attached to a shark. |
Populations of animals are part of a larger system, known as the community, within which populations of different species interact. The number of species that share a habitat is known as species diversity. These species interact in a variety of ways that can be detrimental (−), beneficial (+), or neutral (0) to each species, depending on the nature of the interaction. For instance, we can consider a predator’s effect on its prey as (−), because the survival of the prey animal is reduced. However, the same interaction benefits the predator (+) because the food obtained from prey increases the predator’s ability to survive and reproduce. Thus, the predatory-prey interaction is + −. Ecologists use this shorthand notation to characterize interspecific interactions because it helps us to view the direction in which the interaction affects each species.
We see other kinds of + − interactions. One of these is parasitism, in which the parasite benefits by using the host as a home and source of nutrition, and the host is harmed. Herbivory, in which an animal eats a plant, is another + − relationship.
 |
| Figure 40-7 Among the many examples of mutualism that abound in nature is the whistling thorn acacia of the African savanna and the ants that make their homes in the acacia’s swollen galls. The acacia provides both protection for the ants’ larvae (lower photograph of opened gall) and honeylike secretions used by the ants as food. In turn, the ants protect the tree from herbivores by swarming out as soon as the tree is touched. Giraffes, however, which love the tender acacia leaves, seem immune to the ants’ fiery stings. |
Organisms engaged in mutalism have a friendlier arrangement than commensalistic species, because the fitness of both is enhanced (+ +). Biologists are finding mutualistic relationships far more common in nature than previously believed (Figure 40-7). Some mutualistic relationships are not only beneficial, but necessary for survival of one or both species. An example is the relationship between a termite and protozoa inhabiting its gut. The protozoa can digest wood eaten by the termite because the protozoa produce an enzyme, lacking in the termite, that digests cellulose; the termite lives on waste products of protozoan metabolism. In return, the protozoa gain a habitat and food supply. Such absolute interdependence among species can be a liability if one of the participants is lost. Calvaria trees native to the island of Mauritius have not reproduced successfully for over 300 years, because their seeds germinate only after being eaten and passed through the gut of a dodo bird, now extinct.
Competition between species reduces the fitness of both (− −). Many biologists, including Darwin, considered competition the most common and important interaction in nature. Ecologists have constructed most of their theories of community structure from the premise that competition is the chief organizing factor in species assemblages. Sometimes the effect on one of the species in a competitive relationship is negligible. This condition is called amensalism, or asymmetric competition (0 −). For example, two species of barnacles that commonly occur in rocky intertidal habitats, Chthamalus stellatus and Balanus balanoides, compete for space. A famous experiment by Joseph Connell* demonstrated that B. balanoides excluded C. stellatus from a portion of the habitat, while C. stellatus had no effect on B. balanoides.
We have treated interactions as occurring between pairs of species. However, in natural communities containing populations of many species, a predator may have more than one prey and several animals may compete for the same resource. Thus, ecological communities are quite complex and dynamic, a challenge to ecologists who wish to study this level of natural organization.
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