Section 20.2 – Interactions in Ecological Communities
An ________________________________ is an association of interacting populations that inhabit a
defined area. In any community, individuals of many communities may compete for limited resources.
The interactions between individuals, within the same population or from different populations, are a
driving force between population dynamics – the changes that occur in populations over time.
A. Intraspecific Competition
a. The competition for limited resources among members of the same species is called
_______________________________________.
i. Intraspecific competition is a density-dependent factor that limits the growth of
a population
ii. It plays an important role in natural selection as some individuals may have a
competitive advantage that makes them more likely to survive long enough to
reproduce, thereby passing on this competitive trait to their offspring.
b. In some species intraspecific competition exists between parents and offspring so in
order to avoid ________________________ some plant species disperse seeds or
depend on animals to eat their fruit and transfer the seeds to other locations. Some
insects undergo complete metamorphosis so offspring and adults have different needs,
so competition is reduced.
B. Interspecific Competition
a. Competition between two or more populations for limited resources, such as nutrients,
light, or living space, is referred to as ______________________________________.
b. Interspecific competition results in no two species sharing the exact same ecological
niche. Competition is less fierce when populations have even slightly different niches.
c. Often, non-native species, when introduced, compete with native species and result in
changing of the structure of the ecological community.
C. Producer-Consumer Interactions
a. Predators are organisms that kill and consume other organisms, known as ________.
b. The producer-consumer relationship puts selective pressure on both partners.
Producers that are more difficult to catch or less desirable to consume are more likely to
survive. In addition, the scarcity of a producer species is a factor that can limit a
consumer species’ population.
c. Predator-prey interactions are one factor in the _______________________________
observed in populations of some northern species.
Large prey populations  more food, energy for predators  increased
predator population  decrease prey population  more competition for
predators  decrease in predator population  increase in prey population 
repeat cycle
d. Predator-prey interactions also influence natural selection for adaptive traits. Predators
that have traits that make them more successful at hunting are likely to survive to pass
those traits onto their offspring. Prey that have traits that are more successful at
resisting predation are likely to survive and pass those traits on to their offspring.
e. Producers and consumers typically ________________________ as a result of their
interactions. Some examples of traits that producers and prey have developed (or been
created with) include:
i. Bitter-tasting chemicals, protective thorns, sharp quills, cryptic coloration or
camouflage (___________________________________), mimicry to look like a
well-defended model (known as Batesian mimicry), or mimicry of two or more
species that both have undesirable traits so predators avoid both (known as
Müllerian mimicry) all are possible defense mechanisms that some species
employ to protect against consumers or predators.
D. Symbiotic Relationships
a. The direct or close relationship between individuals of different species that live
together is called _________________________. Often these relationships involve an
organism that lives or feeds in or on another organism which is called the host. Three
forms of symbiosis are:
i. Mutualism
1. When both partners in a symbiotic relationship benefit or depend on it
in order to survive, their relationship is called ____________________.
a. Example: Lichen is a combination of an alga and a fungus. Their
mutualistic relationship allows them to grow on exposed, bare
rock, where neither one would survive on its own. The algae
photosynthesizes, which provides food for both organisms and
the fungus produces an acid that dissolves rock, releasing
minerals that are needed by the algae.
ii. Commensalism
1. ____________________________ is a symbiotic relationship in which
one partner benefits and the other partner neither benefits nor is
harmed.
a. Example: A shark does not appear to be affected from its
relationship with suckerfish. Suckerfish use their sucker-like
dorsal fins to hold fast to the shark’s body in order to receive
protection and bits of food from the shark.
iii. Parasitism
1. _____________________________ is a form of symbiosis in which one
partner the parasite, benefits at the expense of its host.
a. Example: Mistletoes are plants that live on trees and shrubs and
do not have their own developed leaves so they obtain food by
growing roots directly into the bark and phloem of conifer trees.
The interaction weakens the trees and predisposes them to
disease.
2. Parasites are among the most successful species in the world affecting
almost all species of wild and domesticated plants and animals. They
play an important role in limiting the growth of host populations,
potentially helping the host population as a whole by reducing the
density of the population and eliminating the weaker hosts.
E. Succession: Community Change over Time
a. ___________________________ is the sequence of invasion and replacement of
species in an ecosystem over time. Succession is driven by abiotic factors, such as
climate, and biotic factors, such as interspecific competition for changing available
resources.
b. ___________________________________ begins when no soil is present, such as on
bare rocks left behind a retreating glacier.
i. The first species to colonize an area and initiate succession form the pioneer
community. The first species tend to be small and opportunistic, able to grow in
harsh conditions.
ii. As this first species dies, soil starts to form so that nutrient content, moisture
content, and pH change to allow larger species to grow in the area.
iii. Due to interspecific competition and the changing habitat, some populations
are better able to survive and replace those that are unable to survive.
iv. The latecomers in the process of succession form a
___________________________.
c. An ______________________________________ is an event that changes the structure
of a community – sometimes destroying all actively growing organisms.
d. ___________________________________ is the recolonization of an area after an
ecological disturbance such as a forest fire or flood.
i. Soil nutrients are usually not destroyed in an ecological disturbance and often
seeds and roots of vascular plants remain buried in the soil and are able to grow
following a disturbance.
ii. Secondary succession also includes changes in the composition and number of
species over time.
e. Ecological disturbances are important for many plants and animals as they provide new
opportunities for species to grow. Understanding the role of ecological disturbances in
structuring communities is currently an important area of investigation in ecology.