24.1. What Is a Biological Community (p. 406)
A. A community is a group of many different populations that interact with one another in
the same environment.
1. Communities vary in size and sometimes have boundaries that are difficult to
determine.
2. A fallen log supports a community but a passing bird can eat one of its members.
3. A forest may appear distinct but gradually fades into surrounding areas.
B. Community Composition and Diversity
1. Species composition is a list of the species within a community; it does not
reveal relative abundance.
2. Species diversity consists of two factors: richness and evenness.
a. Species richness is number of species; forest with 20 tree species has
more richness than a forest with 12.
b. Species evenness is the number of individuals within each population; a
forest with 76 yellow poplars and one American elm differs from
a forest with 40 of both species.
3. Number of species in a community increases as we move from the poles to the
equator.
4. The individualistic model by H. L. Gleason states each population is there
because of its adaptations.
a. A species range is based on its tolerance for abiotic factors including
light, water, salinity, etc.
b. A plot of tolerance to conditions usually gives a bell-shaped curve. (Fig.
24.2a)
5. Frederick Clements proposed the interactive model of community structure.
a. A community was simply a higher level of organization arising from cell
to tissue to organism, and so forth to community.
b. Just as organ cells are adapted to each other, a community had species
adapted to each other.
c. Communities are classified; the same species were found in the same
community, living in equilibrium.
6. Modern ecology supports the individualistic model.
a. F. H. Talbot and co-workers built artificial reefs and set them in a uniform
tropical lagoon.
1) Of 42 species that colonized the reefs, there was only a 32%
similarity reef-to-reef.
2) From month to month, 20-40% of species changed.
3) Reef species composition appears to depend on chance migrations.
b. Certain animals occur near their food source.
c. Community structure depends on abiotic and biotic factors.
C. Island Biogeography (Fig. 24.3)
1. Robert MacArthur and E. O. Wilson developed the theory of island biogeography.
2. Compared to a far island, a nearby island is likely to have more species because
immigration is easier.
3. A large island is more likely to have more species because a large island has more
resources.
4. "Islands" include patches of forest surrounded by cropland, etc.
5. This concept pertains to conservation; larger reserves are needed to preserve more
species.
6. An environment with variable patches presents more diverse habitats.
7. Stratification is an increase in vertical living spaces; a tree canopy provides a high-
rise habitat.
8. An equilibrium point is reached when the rate of species immigration matches the rate
of species extinction.
24.2. Communities Organized (p. 409)
A. Interaction Between Populations is Complex
1. Interactions include competition for resources, predator-prey interaction, parasite-
host interaction, etc.
2. Competition for limited resources has a negative effect on population size of both
species.
3. Predation and parasitism increase the predator population at the expense of the
prey and host populations.
4. In parasitism, one species is benefited, the other is harmed.
5. In commensalism, one species is benefited, the other is neither benefited nor harmed.
6. In mutualism, both species are benefited.
B. Habitat and Ecological Niche (Fig. 24.4)
1. A habitat is where an organism lives and reproduces in the environment.
2. An ecological niche is a role an organism plays in its community, including its habitat
and its interactions with other organisms.
a. The fundamental niche is the niche it occupies when independent of
interactions.
b. The realized niche is the niche it occupies when the species it interacts with
are present.
C. Competition Between Populations
1. Interspecific competition occurs when different species utilize a resource that is in
limited supply.
2. If the resource is not in limited supply, there is no competition.
3. Lotka and Volterra (1920s) developed a formula: competition would favor one species,
eliminate the other.
4. Gause grew two species of paramecia in one test tube; only one survived when they
were grown together.
5. Competitive exclusion principle: no two species can occupy the same niche at the
same time. (Fig. 24.5)
6. Over time, either one population replaces the other or the two species evolve to
occupy different niches.
7. If it appears two species are occupying the same niche, there must be slight
differences; Gause found two species of paramecium could coexist if one fed on
bacteria at the bottom of the tube and the other fed at top.
8. Niche partitioning occurs when species have shifted niches so they no longer
directly compete. (Fig. 24.7)
a. Gause's paramecium now fed on distinct top and bottom populations of
bacteria.
b. Three species of Darwin's finches on an island have three sizes of beaks for
small, medium, and large seeds.
c. When three species live on separate islands, beak sizes are intermediate; this
is character displacement. (Fig. 24.6)
d. Five species of warblers in same tree niches actually spent time in different
tree zones to avoid competition.
e. Swallows, swifts, and martins fly in mixed flocks eating aerial insects, but have
different nesting sites, etc.
f. Above examples are deduced from already completed partitioning; Joseph
Connell studied competition occurring in barnacles that consistently shift to
match shoreline tidal zones; by removing large Balanus barnacles from the
lower zone, the smaller barnacles easily moved in. (Fig. 24.8)
D. Predator-Prey Interactions
1. Predation occurs when one organism (predator) feeds on another (prey).
2. In a broad sense, it includes not only single predator-prey kills, but also filter feeding
whales that strain krill, parasitic ticks that suck blood, and herbivorous deer that eat
leaves.
3. Predator-Prey Population Dynamics
a. Some predators reduce the densities of their prey.
1) When Gause reared the protozoans Paramecium caudatum and
Didinium nasutum together in culture, Didinium ate all the
Paramecium and then died of starvation.
2) When cactus was introduced to Australia, it spread wildly without
competition on the desert; when a natural predator moth was
introduced, the cactus populations plummeted dramatically.
b. In nature, predator-prey relationships can result in persistent populations of
both predator and prey populations, though both may fluctuate over time.
1) Often a graph of predator-prey population densities shows
regular peaks and valleys with the predator population lagging slightly
behind the prey; two reasons are possible.
2) The biotic potential of the predator may be great enough to
over consume the prey, and the prey population declines and so does
the predator.
3) Or the biotic potential of the prey is unable to keep pace and the prey
population overshoots the carrying capacity and suffers a crash.
4. Classic Case of the Snowshoe Hare and Canadian Lynx (Fig. 24.10)
a. Careful records of pelts of both animals for over a century demonstrated
regular fluctuations.
b. To test whether the lynx or hare food supply was causing the cycling, three
experiments were done.
1) A hare population was given a constant supply of food and predators
were excluded; the cycling ceased.
2) Hare populations were given constant food supply but predators were
not excluded; cycling continued.
3) Predators were excluded but no food was added; the cycling
continued.
c. The interpretation of these results is that both a hare-food cycle and a
predator-hare cycle combine to produce the overall effect.
d. Grouse population also cycle, perhaps because the lynx switches to grouse
when the hare populations decline; thus predators and prey do not normally
exist as simple two-species systems.
E. Prey Defenses and Other Interactions
1. Prey have evolved a variety of antipredator defenses.
2. Plant adaptations for discouraging predation include sharp spines, tough leathery
leaves, poisonous chemicals in their tissues, and chemicals that act as hormone
analogues to interfere with insect larval development.
3. Animals have defenses that include
a. camouflage for concealment; this also requires behavior (stillness) (Fig.
24.11),
b. fright of the predator (Fig. 24.20b),
c. warning coloration (Fig. 24.13c), and
d. vigilance and association with other prey for better warning.
F. Mimicry
1. Mimicry occurs if one species (the mimic) resembles another species (the model)
possessing an antipredator defense.
2. Batesian mimicry, named for Henry Bates, is a form of mimicry in which one
species that lacks defense mimics another that has successful defenses (e.g., the
species shown in Fig. 24.13c, d and e resemble a wasp).
3. Müllerian mimicry, named for Fritz Müller, is where several different species with the
protective defenses mimic one another (e.g., stinging insects all share same black
and yellow color bands). (Fig. 24.14a and b)
G. Symbiotic Relationships
1. Symbiosis: a close relationship between members of two populations.
2. Parasitism
a. Parasitism is similar to predation in that the parasite derives nourishment from
the host.
b. All viruses are always parasitic and parasites occur in all kingdoms of life.
c. Endoparasites are small and live inside the host.
d. Ectoparasites are larger and remain attached to the body of hosts by
specialized organs or appendages.
e. Many parasites have several hosts, some of which may serve to transport
(vector) the parasite among hosts.
f. Association between parasite and host is coevolved; parasites are specific
and require certain species as hosts.
g. Malaria
1) The malaria protozoan has a sexual portion of its life cycle within
vector mosquitoes.
2) The asexual stages occur in the human body (liver and circulatory
system).
3) The human immune system detects surface proteins on pathogens;
the malaria parasite has numerous genes and changes its protein
surface often.
h. Lance Flukes
1) Ants are the vector; ants infected with the fluke cling to blades of
grass.
2) Infested ants are therefore eaten by grazing sheep and thus
transmitted to the host.
i. Snail Worms
1) Worms of the genus Leucochloridium parasitize snails of the genus
Succinea.
2) As the worms mature, they invade the snail's eyestalk and resemble
edible caterpillars.
3) Birds are attracted and eat the snails.
4) The parasites release their eggs and can only complete development
inside the urinary tracts of birds.
3. Commensalism
a. Commensalism is a symbiotic relationship where one benefits and the other is
neither harmed nor benefited.
b. It is difficult to determine true commensalism because it is difficult to ensure
host is not harmed.
c. Possible examples include the following:
d. barnacles that attach themselves to the backs of whales and the shells of
horseshoe crabs;
e. remora fish that attach themselves to the bellies of sharks;
f. epiphytic plants grow in the branches of trees to receive light but take no
nourishment from the tree; and
g. clown fishes that live within the tentacles of sea anemones for protection. (Fig.
24.15)
h. Some relationships are also so loose that it is difficult to know if they are true
commensalism.
1) Cattle egrets feed near cattle because the egrets flush insects as
they graze.
2) Baboons and antelopes forage together for added protection.
4. Mutualism
a. Mutualism is a symbiotic relationship between two species where both benefit.
b. Mutualism can be found among organisms in all kingdoms of life.
c. Examples include the following:
1) Bacteria in the human intestinal tract are provided with food but
provide us with vitamins.
2) Termites can only feed on wood because their gut contains protozoa
that can digest cellulose.
3) Mycorrhizae are symbiotic associations between roots of fungal
hyphae and plants.
4) Flowers and insect pollinators may represent a shift from insects
eating pollen to eating nectar.
5) Lichens are made of algae (produce food) and fungi (preserve
water), although the algae can survive alone.
d. Classic Example of the Ant and the Acacia Tree (Fig. 24.16)
1) In tropical America, the bullhorn acacia provides a home for ants in
its hollow thorns.
2) The acacia also provides ants food from nectaries, and protein
nodules called Beltian bodies.
3) In return, the ant protects the plant from herbivores and other plants
that might shade it.
4) When the ants on an experimental tree were killed with insecticide,
the tree also died.
e. Tree-Ant-Caterpillar Complex
1) Trees in the genus Croton have nectaries that feed ants.
2) The ants have a mutualistic relationship with Thisbe caterpillars that
feed on Croton saplings.
3) Thisbe caterpillars also offer nourishment to ants, keeping them
nearby.
4) The caterpillar releases the same chemical that causes ants to
defend an ant colony.
5) The result is that caterpillars are protected while feeding on the trees.
f. Cleaning Symbiosis (Fig. 24.17)
1) Crustacea, fish, and birds act as cleaners to a variety of vertebrate
clients.
2) Large fish in coral reefs line up at cleaning stations and wait their
turn to be cleaned by small fish.
3) The possibility of feeding on host tissues as well as on ectoparasites
complicates this case of mutualism.
H. Interactions and Coevolution
1. Coevolution occurs when two species adapt in response to selective pressure
imposed by the other.
2. Symbiotic organisms (parasites, commensals, and mutualists) are especially prone to
coevolution.
3. Flowers and insect pollinators are coevolved; their parts are specialized to get
pollination accomplished.
4. A faster cheetah selects for gazelles with better escape mechanisms; this produces
an "arms race."
5. Cuckoos are coevolved to successfully parasitize other birds' nests. (Fig. 24.15)
a. They must lay an egg that mimics the host bird's egg.
b. Cuckoos must lay eggs within seconds while the host bird is absent briefly in
the afternoon.
c. They must leave host eggs in nest to prevent host birds from deserting; the
early-hatching cuckoo chick will hatch first and expel other eggs from the
nest.
24.3. Community Structure Changes Over Time (p. 420)
A. Communities change over both short and long intervals of time due to continental drift,
glaciation, etc.
B. Ecological Succession
1. Ecological succession is the successive set of stages that a community undergoes
over time, following a disturbance.
2. Primary succession begins in a barren habitat lacking soil, such as barren rock, or
following volcanic eruption.
3. Secondary succession begins with colonization of habitat that has a soil but has
been disturbed, such as in an abandoned cronfield. (Fig. 24.18)
a. First year, remains of corn plants.
b. Second year, wild grasses (pioneer species) grow.
c. By the fifth year, sedges join the mature grasses.
d. During the tenth year, there is a mixture of shrubs and trees.
4. F. E. Clements proposed in 1916 the climax-pattern model of succession---that
succession leads to a climax community characteristic for an area.
a. A climax community has a community composition that depends on climate.
1) Dry climates eventually produce deserts.
2) Wet climates proceed to forests.
3) Intermediate moisture will result in grasslands, shrubs, etc.
4) Soils will influence the developing community.
b. Each stage facilitates the occurrence of the next stage (facilitation model).
1) Shrubs cannot grow on dunes until dune grass has developed soil.
2) Grass-shrub-forest occurs sequentially. (Fig. 24.19)
5. The inhibition model challenges Clements' view of succession.
a. Colonists hold onto their space and inhibit growth of the plants until the
colonists die.
b. Death releases resources that allow different, longer-lived species to invade.
6. The tolerance model provides another view of succession.
a. Sheer chance may determine which seeds arrive first; successional stages
may reflect maturation time.
b. Trees merely take more time to develop; however, facilitation and inhibition of
growth may be taking place.
C. Equilibrium and Communities
1. Stability of communities is seen in two ways: resistance to change, and recovery
once a disturbance occurred.
2. A deciduous forest changes after it regrows its leaves after an insect infestation.
3. A chaparral community is resilient to fire and quickly returns to its normal state.
D. The Intermediate Disturbance Hypothesis (Fig 24.20)
1. Fire, wind, severe weather, and water erosion are abiotic and external factors that
cause disturbances.
2. If disturbances affect one type of patch and not another, the effect of patchiness is to
provide overall stability.
3. If widespread disturbances occur frequently, diversity is limited and a community will
be dominated by rapid growth, short lifespan (r-strategists) colonizers.
4. When disturbances are less widespread and infrequent, species with slow growth
rates and long life spans will (K-strategists) dominate.
5. Intermediate disturbance hypothesis states a moderate level of disturbance yields
highest community diversity.
6. Therefore, too much disturbance, or not enough, may threaten diversity of tropical
rainforests and coral reefs.
7. Archeological remains show the Maya cultivated huge areas from 300 to 900 AD; the
civilization collapsed, and 1,200 years later the community composition is still different
from a local tropical rainforest.
F. Predation, Competition, and Biodiversity
1. Predation by a particular species can reduce competition and increase diversity.
a. Robert Payne removed the starfish Pisaster from test areas along the coast
of North America. (Fig. 24.21)
b. In the control area, there was no change in the numbers of species.
c. In the removal area, the mussel Mytilus increased in number and excluded
other invertebrates and algae from attachment sites.
2. Such predators that regulate competition and maintain diversity are called keystone
predators.
3. Predation has changed Barro Colorado Island.
a. This island was formed in Panama from damming a river in the 1910s.
b. Island biogeography predicts fewer species can survive on islands; jaguar,
puma and ocelot are now gone.
c. Therefore, the medium-sized coatimundi increased in numbers; it is a
predator of bird eggs.
d. Thus, the numbers of bird species is less on the island than is expected for its
size.
4. Introduction of exotic species is devastating if they are not held in check by predators
and competitors.
5. Elephants feed on shrubs and trees and keep woodland habitats in grassland stage,
benefiting other grazers.