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Key Concepts • Interactions among species, such as competition, consumption, and mutualism have two main outcomes: (1) They affect the distribution and abundance of the interacting species, and (2) they are agents of natural selection and thus affect the evolution of the interacting species. The nature of interactions between species frequently changes over time.
Key Concepts • The assemblage of species found in a biological community changes over time and is primarily a function of climate and chance historical events. • Species richness is higher in large islands near continents than in small, isolated islands, due to differences in immigration and extinction. Species richness is also higher in the tropics and lower toward the poles, but the mechanism responsible for this pattern is still controversial.
Introduction • A biological community consists of interacting species, usually living within a defined area. • Biologists want to know how communities work, and how to manage them in a way that will preserve species and create an environment that people want to live in.
Species Interactions • Because the species in a community interact almost constantly, the fate of a particular population may be tightly linked to the other species that share its habitat. • Biologists analyze interactions among species by considering the effects on the fitness—the ability to survive and produce offspring—of the individuals involved.
Species Interactions • A relationship between two species that provides a fitness benefit to members of one of the species is a + interaction. Such a relationship that hurts members of one of the species is a – interaction. A relationship that has no effect on the members of either species is a 0 interaction.
Species Interactions • There are four general types of interactions among species in a community: • Competition occurs when individuals use the same resources—resulting in lower fitness for both (/). • Consumption occurs when one organism eats or absorbs nutrients from another, increasing the consumer’s fitness but decreasing the victim’s fitness (+/). • Mutualism occurs when two species interact in a way that confers fitness benefits to both (+/+). • Commensalism occurs when one species benefits but the other species is unaffected (+/0).
Three Themes • As you analyze each type of species interaction, watch for three key themes: • Species interactions may affect the distribution and abundance of a particular species. • Species act as agents of natural selection when they interact. In biology, a coevolutionary arms race occurs between predators and prey, between parasites and hosts, and between other types of interacting species. • The outcome of interactions among species is dynamic and conditional.
Competition • Competition is a –/– interaction that lowers the fitness of the individuals involved. When competitors use resources, those resources are not available to help individuals survive better and produce more offspring. • Intraspecific competition occurs between members of the same species. • Because intraspecific competition for resources intensifies as a population’s density increases, it is a major cause of density-dependent growth. • Interspecific competition occurs when members of different species use the same limiting resources.
Using the Niche Concept to Analyze Competition • Early work on interspecific competition focused on the concept of the niche—the range of resources that the species is able to use or the range of conditions it can tolerate. • Interspecific competition occurs when the niches of two species overlap.
When One Species Is a Better Competitor • The competitive exclusion principle, formulated by G. F. Gause, states that it is not possible for species within the same niche to coexist. • The hypothesis was inspired by a series of experiments Gause did with similar species of the unicellular pond-dweller Paramecium. • Grown in separate cultures, both species exhibited logistic growth. • When the two species grew in the same culture together, only one species exhibited logistic growth; the other species was driven to extinction.
When One Species Is a Better Competitor • Asymmetric competition occurs when one species suffers a much greater fitness decline than the other. • In symmetric competition, each species experiences a roughly equal decrease in fitness. • If asymmetric competition occurs and the two species have completely overlapping niches, the stronger competitor is likely to drive the weaker competitor to extinction.
When One Species Is a Better Competitor • Gause’s experiments illuminated an important distinction: • A species’ fundamental niche is the resources it uses or conditions it tolerates in the absence of competitors. • A species’ realized niche is the resources it uses or conditions it tolerates when competition occurs. • If asymmetric competition occurs and the niches of the two species do not overlap completely, the weaker competitor will move from its fundamental niche to a realized niche, ceding some resources to the stronger competitor.
Experimental Studies of Competition • Joseph Connell performed a series of experiments to test the competitive exclusion principle. • Connell used a common experimental strategy in competition studies—removing one of the competitors and observing the response by the remaining species. • Experimental evidence supports competitive exclusion of Chthamalus barnacles from the lower intertidal zone by Balanus barnacles.
Fitness Trade-Offs in Competition • The ability to compete for a particular resource is only one aspect of an organism’s niche. • If individuals are extremely good at competing for a particular resource, they are probably less good at enduring drought conditions, warding off disease, or preventing predation―there is a fitness trade-off.
Mechanisms of Coexistence: Niche Differentiation • Because competition is a –/– interaction, there is strong natural selection on both species to avoid it. • The predicted eventual outcome is an evolutionary change in traits that reduces the amount of niche overlap and the amount of competition. • This change in resource use is called niche differentiation or resource partitioning. • The change in species’ traits is called character displacement.
Mechanisms of Coexistence: Niche Differentiation • Peter and Rosemary Grant recently documented character displacement in Galápagos finches. • After a severe drought in 1977, selection favored larger beak size in the medium ground finch, Geospiza fortis. • Only those individuals with larger beaks were able to crack open the fruits of their major food source, Tribulus cistoides.
Mechanisms of Coexistence: Niche Differentiation • A second severe drought occurred in 2003; by this time the large ground finch, Geospiza magnirostris, had become established on the island. • The Grants’ measurements revealed that this time, only the smallest-beaked G. fortis individuals survived. • Data on feeding behavior indicated that G. magnirostris were outcompeting G. fortis for Tribulus cistoides; only G. fortis that could eat extremely small seeds efficiently could survive.
Competition and Conservation • One of the goals of conservation biology is to keep biological communities intact. • One of the major threats to communities is invasive species. • Recent experiments have shown that communities that contain a large number of different species are more resistant to invasion than communities with a smaller number of species. • In other words, competition can help communities resist invasion.
Consumption • Consumption is a +/– interaction that occurs when one organism eats another. • There are three major types of consumption: • Herbivory is the consumption of plant tissues by herbivores. • Parasitism is the consumption of small amounts of tissues from another organism, or host, by a parasite. • Predation is the killing and consumption of most or all of another individual (the prey) by a predator.
Constitutive Defenses • Natural selection strongly favors traits that allow individuals to avoid being eaten. • Constitutive or standing defenses are defenses that are always present and include: • Avoidance (hiding, with or without camouflage, or running, flying or swimming away). • Poison (many plants lace their tissues with compounds that are toxic to consumers). • Schooling and flocking behaviors that confuse predators. • Fighting back, with the use of weaponry or toxins.
Constitutive Defenses • Some of the best-studied constitutive defenses involve mimicry—the close resemblance of one species to another. • There are two forms of mimicry: • Müllerian mimicry is the resemblance of two harmful prey species. • Batesian mimicry is the resemblance of an innocuous prey species to a dangerous prey species.
Inducible Defenses • Although constitutive defenses can be extremely effective, they are expensive in terms of the energy and resources that must be devoted to producing and maintaining them. • Many prey species have inducible defenses—defensive traits produced only in response to the presence of a predator. • Inducible defenses are efficient energetically, but they are slow—it takes time to produce them. • For example, mussels have thicker shells and attach more strongly to a substrate only in the presence of crabs.
Inducible Defenses • The data on the mussels and crabs are correlational in nature, however, so they are open to interpretation and criticism by critics of the induced defense hypothesis. • Biologists tested the hypothesis more rigorously by conducting experiments on mussel thickness in and out of the presence of crabs. • Results support the hypothesis that mussels do in fact increase their investment in defense in the presence of crabs.
