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Extended Lecture Outline |
Chapter 27: The Structure Of Biological Communities |
A. GENERAL FEATURES OF COMMUNITIES
27.1 A community is not a superorganism.
a. Frederick Clements, a pioneer of modern ecology, proposed that a community could be viewed as a kind of "superorganism" with self-correcting internal control mechanisms.
b. The superorganism idea contradicts two other concepts in biology:
1. that organisms are opportunists that take advantage of any evolutionary opportunity,
2. that genomes evolve in a way that tends to perpetuate them, at the expense of any other genome.
c. The superorganism idea also violates the principle of parsimony, as there is no need to postulate mechanisms that favor the community as a whole when current postulates of natural selection in individual species can account for observed or theorized evolutionary changes.
d. If communities were tightly integrated associations, one would expect to find specific combinations of species that fit together well.
1. There are plant associations that have been found, including patches of communities in Eastern North America.
e. However, each species, including those in the aforementioned plant communities, is distributed independently of others, and their distributions can be explained by gradual changes in the environment of each area.
1. Robert Whittaker's studies on plant distribution in the Great Smoky Mountains (Figure 27.1) support this idea.
2. As the environment gradually changes, so does community structure, and there are not sharp discontinuities between one plant association and another.
27.2 Species abundance varies within a community and from the tropics to the poles.
a. The diversity of organisms in most communities can be measured by counting the number of species.
b. The result (Figure 27.2) shows that a few individuals of rare species and large numbers of common species are found.
c. This distribution is called the broken stick model (Figure 27.2)
d. Wallace Arthur, a British ecologist, observed that, going from the polar regions to the tropics, species richness increases regularly (Figure 27.3).
e. Two main hypotheses have been proposed to account for this observation:
1. Historical opportunities for speciation are greater in the tropics, as isolated populations are more likely to survive,
2. Climate stability and energy availability in the tropics favors greater speciation and also supports greater numbers of established species
27.3 Community structure may be dominated by keystone species.
a. Robert Paine suggested that many communities contain a keystone species, whose role is so central that removing it would drastically upset the community.
b. Paine studied the sea star Pisaster ochraceous (Figure 27.4) in a littoral zone on the Pacific coast.
c. When Pisaster individuals were removed from the area, the community shifted to one dominated by the blue mussel, as evidence that the presence of Pisaster kept the populations of all prey species low enough to prevent competition for space.
d. Kangaroo rats in the Chihuahuan desert, and elephants in Africa are also thought to be keystone species, due to their effects on the plant communities in which they live.
B. NICHES AND RELATIONSHIPS IN THE COMMUNITY
27.4 Relationships within a community are complicated but may be analyzed into many binary relationships.
a. Recall, a community is structured fundamentally by its trophic relationships among all the members of its food web.
b. The relationships among thousands of species may defy description.
c. The interactions between two species, known as binary relationships, are thus used to describe the effects of one species on another in a community.
d. Table 27.1 shows six types of relationships formed by either positive, negative, or neutral interactions.
e. Neutralism means the lack of any direct interaction between two species, with no effect on the community structure.
f. Amensalism is a relationship detrimental to one species and neutral to the other, but may be found not to exist in the long run.
g. The four remaining relationships are predation/parasitism, commensalism, mutualism, and competition.
27.5 The concept of an ecological niche is problematic.
a. A discussion of interspecific competition must deal with the concept of an ecological niche.
b. Niche literally means nest, and implies the space occupied by an organism.
c. Because habitat also describes the space occupied by an organism, a niche, as distinguished from a habitat, has been defined as the organism's occupation.
d. Various definitions offered for the term niche are covered in Concepts 27.1.
e. G. Evelyn Hutchinson pictured a niche as an n-dimensional space where each dimension is a variable affecting the life of the species (Figure 27.5), and where three variables at a time can be used to picture their affects on the organism.
f. A niche depends on the interactions of the species with the community's other species.
g. The fundamental niche of a species is the volume it could occupy in the absence of any competitors.
h. The realized niche of a species is the one it is forced to occupy in the presence of its competitors.
i. A.G. Tansley illustrated this difference in a 1927 study of two British bedstraw plants, where each outcompeted the other in its preferred soil, but grew equally well in either acidic or basic soil in the absence of competition.
27.6 Two species cannot occupy the same niche—or can they?
a. It has long been common wisdom in biology that interspecific competition must be short-lived and limited, otherwise, in competing for the same resources in the same habitat, either:
1. one species would outcompete the other, or
2. one would evolve the ability to occupy a different niche and remove itself from competition.
b. G. F. Hause showed that species of Paramecium either won out over each other in competition for the same growth medium, or ended up occupying different areas of a culture tube, as a result of competition in the same space.
c. Garrett Hardin formulated the competitive exclusion principle: Completely competing species cannot coexist.
d. Thus, potentially competing species can only coexist through niche differentiation, by either using different resources in the same space, or dividing the space into two areas that do no overlap.
e. Robert MacArthur reported this type of differentiation in the feeding patterns of five species of warblers living in the same tree in New England (Figure 27.6).
f. Potentially competitive carnivorous beetles divide the space on leaves, where they feed on herbivorous insects (Figure 27.7).
g. Species can also divide a space temporally, as do swifts, feeding on the wing during the day, and nighthawks, feeding on the wing at night.
h. Ecologists are virtually certain that two species cannot coexist in total competition, but describing the actual amount of interspecific competition has been difficult.
i. Resource or scramble competition occurs when organisms use common resources that are limited.
j. Interference or contest competition occurs when organisms seeking a resource harm one another in the process.
k. The degree of competition between species also varies with the trophic level, and the availability of unoccupied niches for herbivorous insects has indicated a lack of competition at the herbivory level in some cases.
l. Other studies, of African squirrels in an evergreen rain forest, for example, have shown strong niche differentiation among herbivores.
m. Moving away from competition is an important factor in microevolution and speciation, and niche differentiation must always occur at some point in speciation.
27.7 Niche differentiation is commonly determined by subtle chemical factors.
a. Members of each species occur only in certain patches within an ecosystem, but a central question about communities is what really determines where each species will occur.
b. Specific chemical factors were shown to keep four species of fruit flies separate from one another as each fed on yeasts growing on cactus in the Sonoran desert (Figure 27.8).
C. CHEMICAL INTERACTIONS IN THE COMMUNITY
27.8 The members of a community are in perpetual "arms races" with one another.
a. The members of a community must continually adapt to one another.
b. Adaptations may be morphological, behavioral, or biochemical.
c. Allelochemic interactions involve a kind of chemical warfare called allelopathy.
d. Though a species may enhance its position through such adaptations, this is only temporary, as other species evolve other mechanisms to improve their positions.
27.9 Many organisms use allomones in chemical warfare against other species.
a. Several species make allomones, substances whose sole function is to repel other species, especially predators.
b. Many examples of allomones and their deterrent functions are known:
1. Digitalis, a steroid glycoside derived from foxglove, causes heart attacks when consumed in nonregulated quantities (it is used in regulated quantities to control heart problems in humans and their pets).
2. Buttercups contain protoanemonin, and larkspurs contain delphinine, both of which poison the nervous system.
3. Plants of the genus Hypericum produce hypericin, which causes skin irritations and extreme sensitivity to light.
4. Conifers produce phytoecdysones, which mimic insect ecdysones, and prevent insect larvae from developing into adults.
5. Meloid beetles have cantharidin in their blood, a disagreeable substance to most predators.
6. Some grasshoppers produce cardiac glycosides and histamine, which causes pain and inflammation.
7. Ants produce formic acid.
8. The millipede Apheloria corrugata produces and stores mandelonitrile separately from an enzyme that, when mixed with it, converts it into benzalehyde and deadly hydrogen cyanide (Figure 27.9).
27.10 Some organisms create intolerable conditions for others.
a. Plants can aggressively take over space, and usually do so with chemical adaptations.
b. Where light is the limiting factor in growth, some plants have acquired the ability to grow in the shade, or to grow taller and shade out their competition.
c. Other plants, such as the black walnut, Juglans nigra, release juglone from its leaves, which washes into the soil and inhibits the growth of many other plants around it.
d. When environmental conditions in an area are stable, species tend to hold one another in check, but in a disturbed environment, where a secondary succession is beginning, for example, an aggressive plant can take over.
e. In secondary successions in the midwest, the common sunflower, Helianthus annuus, moves in and produces allomones that inhibit the growth of other species.
f. Certain grasses can tolerate these substances, however, and invade the region to become dominant members of the second stage of succession.
g. Fungi and certain bacteria make antibiotics, metabolic inhibitors that arrest the growth of other microorganisms in their environments.
27.11 Many species react to kairomones produced by other species.
a. A kairomone is a substance that gives a selective advantage to a species that receives it.
b. Figure 27.10 shows some fungi that can trap nematode worms by growing in loops that expand when stimulated by a worm that slips into a loop; the fungi are thought to be responding to kairomones produced by the worms, as they produce no loops in the absence of the worms.
c. A predator may have an odor that, when detected by a prey, allows the prey to defend itself; this odor would be characterized as a kairomone.
d. For example, whelks can distinguish herbivorous from carnivorous starfish by their chemical features.
D. PREDATION, SYMBIOSIS, AND CAMOUFLAGE
27.12 Predation is an essential activity in every community.
a. Communities are based on trophic, or predatory, relationships.
b. At each trophic level, the animals that eat are the predators, and those that are eaten are the prey.
c. In any natural, established relationship, the predator is unlikely to jeopardize its prey populations in the long run.
d. Prey can use a refuge in a density-dependent way (Figure 27.11).
1. When a prey population is small, individuals can hide from predators to some extent.
2. In a large prey population, individuals are forced out into exposed places.
e. Predators respond to changes in prey populations in at least three ways:
1. In numerical response, as the prey population increases, the predator population also increases.
2. In functional response, as one prey species increases, the predator concentrates more on that species; many predators form a search image, a set of stimuli characteristic of one type of prey, which they use in hunting.
3. In aggregative response, when one prey population increases, predators are likely to move into the area and concentrate on it.
f. In some situations, a predator does not hurt its prey species, and even keeps it healthier that it would be otherwise, as shown by Paul Errington in studies of Iowa muskrats, and by L. David Mech, in studies of Isle Royale wolves.
g. A predator's beneficial effect on its prey can be described in terms of the recruitment of a population (Figure 27.12), the difference between the birth rate and death rate at any time.
h. Recruitment is severely reduced when a population approaches its carrying capacity, but recruitment rises if the population is held to a lower level by predation.
i. In other cases, predators severely limit prey populations in restricted areas, as shown by Oliver Pearson in studies of a grassland, weed, and mammal ecosystem in California (Figure 27.13).
j. A third situation shows that predators of very small animals may be even more effective than larger carnivores in controlling prey populations; this was the case for the 1880s California citrus crop, as it was attacked by an insect that was later controlled by a predator, the ladybird beetle.
k. The effects of predators on their prey may be functions of patterns of predation behavior.
27.13 Predator and prey populations may change in cycles.
a. It is expected that the sizes of prey and predator populations are closely correlated, and the mutual relationship between them has been expressed by a set of equations (Figure 27.14).
b. Figure 27.15 shows the highly correlated fluctuations in experimental populations of a bean weevil and its parasite.
27.14 Many organisms engage in symbiotic relationships.
a. Symbiosis is the term for a whole series of intimate cooperative relationships between species.
b. Mutualism is the term used when these relationships are mutually beneficial.
c. A symbiotic relationship can be categorized according to the strength of the interaction and the degree of benefit to one species or the other (Figure 27.16).
d. Phoresis includes loose interactions in which a larger animal, the host, is not hurt by carrying around a smaller one, the phoront, which obtains mobility and a place to live.
e. Commensalism occurs when a smaller organism shares the host's food and also its living space.
1. The protozoan Ellobiophyra (Figure 27.17) lives in the gills of a clam, sharing the stream of food that comes in, but not taking enough to damage the host.
2. Some fish live commensally among sea anemones, with a chemical that prevents their sting, and sharing food and shelter among the tentacles.
f. Parasitism occurs when a smaller parasite lives on a larger host and depends metabolically on it, damaging the host in the process.
g. The association between the pearlfish and some echinoderms borders on parasitism (Figure 27.18) and is known as endocommensalism.
h. Ectoparasites such as ticks, which live on the outside of the body, and endoparasites such as worms, which live on the inside, have been common among some human populations.
i. Most animals harbor a wide assortment of parasites (Figure 27.19).
j. Brood parasitism in birds is an unusual phenomenon involving certain birds laying their eggs in the nests made by other species, which then raise their offspring.
k. The European cuckoo is dividing into breeding lines known as gentes, each specialized for parasitizing a different host (Figure 27.20).
l. Mutualism is a relationship from which both parties benefit.
1. Hermit crabs (Figure 27.21) often carry anemones on their backs; the anemones protect the crab and have improved feeding due to the mobility of the crab.
2. Wood-eating termites harbor wood-digesting protozoans in their intestines; both species benefit from the glucose released from the wood ingested by the termites (Figure 27.22).
3. Lichens are a specific mutualistic association between a fungus and an alga.
m. A cleaning symbiosis is a relationship in which one species gets some of its food by cleaning another.
1. Tickbirds clean the African rhinoceros.
2. Egyptian plovers are allowed to walk on African crocodiles (Figure 27.23) and pick leeches from their gums.
3. The barracuda allows the smaller hogfish to swim into its open mouth to clean it (Figure 27.24).
27.15 Many animals are camouflaged by their forms and colors.
a. Cryptic coloration, as illustrated by the peppered moths (Figure 2.20) camouflages the organism.
b. Many insects have evolved to look like parts of plants (Figure 27.25).
c. Birds often have plumage that blends with their nest materials (Figure 27.26).
d. Disruptive coloration is illustrated by a zebra's stripes, or a leopard's spots (Figure 27.27), which stand out against a plain background, but disguise the animals against a broken background.
e. Search images are patterns acquired by predators to focus on in repeated hunting, and may account for the brilliantly colored Morpho butterflies (Figure 27.28), which fold their wings when they land, and thus disappear out of the search image produced by their predator.
f. Yellow-rumped warblers are also obvious and may produce a search image when flying, but not when they land and cover their yellow patch (Figure 27.29).
27.16 Some animals are protected by warning coloration.
a. In some species, bright colors have been selected for their role in communication within the species, as for attracting the opposite sex.
b. In other species, aposematic coloration, or warning coloration, tells predators to leave an organism alone; many distasteful insects make use of this coloration.
c. Müllerian mimicry is a phenomenon in which a number of species evolve a common color pattern that warns predators of their threat; the black and yellow stripes on wasps and bees (Figure 27.30) are an example of this.
27.17 Mimics may survive by imitating warning coloration.
a. Batesian mimicry occurs when a harmless or palatable species called a mimic survives by evolving to resemble a dangerous model.
b. The distasteful monarch butterfly is a model for the unrelated viceroy butterfly (Figure 27.31).
c. Batesian mimicry poses an interesting ecological problem that can be analyzed with the game theory illustrated in Section 49.2, where the mimic and the model are engaged in a contest that is a disadvantage to the model if a predator happens to eat a mimic first and follows by eating several models before altering its predation behavior.
d. Models are therefore under selective pressure to change so they don't resemble mimics.
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