Previously, we considered populations of a single species. But species are not found in isolation.
Interactions among species, such as competitions and consumption can:
o Affect the distribution and abundance of the interacting species.
o Be agents of natural selection and thus affect the evolution of the interacting species.
The assemblage of species found in a biological community changes over time and is primarily a
function of climate and chance historical events.
A biological community consists of interacting species, usually living within a defined area.
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 of the
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.
Three broad categories of interactions between species are –/ –,
+/ –, or +/+ relationships.
A fourth category is a +/0 relationship called commensalism, in which the members of one
species benefit while having no effect on the members of the other species.
This section focuses on three key themes:
o Species interactions may affect the distribution and abundance of a particular species.
o 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.
o The outcome of interactions among species is dynamic and conditional.
Competition is a –/– interaction that occurs when individuals use the same limiting resources.
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 population growth.
Interspecific competition occurs between members of different species.
There are several mechanisms of interspecific competition:
o Consumptive competition occurs when two species consume the same resources.
o Preemptive competition exists when one species makes space unavailable to other
o Overgrowth competition happens when one species grows above another.
o Chemical competition or allelopathy takes place when one species produces toxins that
negatively affect another species.
o Territorial competition arises when a mobile species protects its feeding or breeding
territory against other species.
o Encounter competition occurs where two species interfere directly for access to specific
Using the Niche Concept to Analyze Competition
A niche can be thought of as the range of resources that a species is able to use or the range of
conditions it can tolerate. Community Ecology
A niche can have multiple dimensions – food, shelter, temperature
Interspecific competition occurs when the niches of two species overlap.
o A) One species eats seeds of a certain size range
o B) Partial niche overlap: competition for seeds of intermediate size.
If the human population continues to grow, which of the following outcomes are likely
o A.Stabilizing at the carrying capacity
o B.Reaching a peak and then crashing
o C.Expanding crop capacity through lunar farms
o D.Continuing to grow at a high rate
o E.A or B are possible Community Ecology
In looking at Figure 53.1, what conclusions can be drawn?
o A) Both species compete for all sizes of seeds other than those of intermediate size.
o B) Both species compete for seeds of intermediate size.
o C) Both species eat all sizes of seeds.
o D) Both species compete for all sizes of seeds.
What Happens When One Species Is a Better Competitior?
The competitive exclusion principle states that it is not possible for species within the same
niche to coexist.
Asymmetric competition occurs when one species suffers a much greater fitness decline than
the other. (=a poorer competitor)
In symmetric competition, each species experiences a roughly equal decrease in fitness. (=equal
If asymmetric competition occurs and the two species have completely overlapping niches, the
stronger competitor is likely to drive the weaker competitor to extinction.
The Niche concept is important here:
o There is an important distinction between a species’ fundamental niche—the resources
it uses or conditions it tolerates in the absence of competitors—and its realized niche—
the resources it uses or conditions it tolerates when competition occurs.
o If asymmetric competition occurs and the niches of the two species do not overlap
completely, the weaker competitor will only live in its realized niche, not its
Niche Overlap Leads to Competitive Exclusion or Restricted Habitat Use
Competition exclusion occurs when competition is asymmetric and niches overlap completely.
When competition is asymmetric and niches do not overlap completely, weaker competitors use
Experimental Studies of Competition
Experimental evidence supports competitive exclusion of Chthamalus barnacles from the lower
intertidal zone by Balanus barnacles.
o Chthamalus in hot dry splash zone
o Balanus in cool wetter intertidal zone 13
Mechanisms of Coexistence:
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
Semibalanus –good at competing, bad at tolerating dry, hot splash zone.
Chthamalus– poor at competing, good at surviving in dry hot conditions.
Because competition is a –/– interaction, there is strong natural selection on both species to
The predicted eventual outcome of evolution through natural selection is that either or both
species have a genetic 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.
Competition Can Lead to Niche Differentation Community Ecology
Natural selection favours
individuals that do not
Consumption is a +/– interaction that occurs when one organism eats another.
There are three major types of consumption:
o Herbivory is the consumption of plant tissues by herbivores.
o Parasitism is the consumption of small amounts of tissues from another organism, or
host, by a parasite.
o Predation is the killing and consumption of most or all of another individual (the prey)
by a predator.
Like predators, they are lethal.
Like parasites, they are smaller than the host.
Wasps lay eggs on caterpillars; larvae hatch out and consume caterpillars from the inside.
How Do Prey Defend Themselves?
Prey have adaptations that reduce their likelihood of becoming victims. These adaptations are
responses to natural selection exerted by predators.
Constitutive or standing defenses are defenses that are always present. Constitutive defenses
include camouflage, schooling, weaponry, and mimicry. Also, aposematic colouration
Mimicry is the close resemblance of one species to another.
o Müllerian mimicry is the resemblance of two harmful prey species.
o Batesian mimicry is the resemblance of an innocuous prey species to a dangerous prey
species. Community Ecology
The Constitutive Defense of Mimicry
o Mimicry can protect both dangerous and harmless species.
Müllerian mimics Batesian mimics
Paper wasp Bumblebee Honeybee Hornet moth Wasp beetle Hoverfly
Inducible defenses are defenses produced only when prey are threatened.
o For example, a study of the thickness of mussel shells demonstrated that thicker shells
are an inducible defense produced by the presence of crabs.
Are Animal Predators Efficient Enough to Reduce Prey Populations?
Species interactions have a strong impact on the evolution of predator and prey populations.
Experiments have supported the hypothesis that predators play a role in the density-dependent
growth of prey populations.
Why Don’t Herbivores Eat Everything-Why is the World Green?
Biologists who compiled the results of more than 100 studies raised the question of why
herbivores don't eat more of the available plant food.
Three hypotheses have been proposed to answer this question:
o The top-down control hypothesis suggests that predation or disease limits herbivores.
o The poor-nutrition hypothesis proposes that plants are a poor food source in terms of
the nutrients they provide for herbivores, especially nitrogen. This in turn limits
o The plant-defense hypothesis suggests that plants defend themselves effectively
enough to limit herbivores.
The data to date suggest that top-down control, nutrient-poor plants, and plant defenses are all
important in limiting herbivores.
Adaptation and Arms Races
When consumers and prey interact over time, a coevolutionary arms race begins. Consumers
evolve traits that increase their efficiency; in response, prey evolve traits that make them
unpalatable or elusive, which leads to selection on consumers for traits that counter the prey