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 overconsume 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. clownfishes 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 lifespans 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.