Ecology (p. 720)
35.1
What Is Ecology? (p. 720; Fig. 35.1, 35.2, 35.3)
A. Ecology is the study of how organisms that live in a place interact with each other and their
physical habitat. Ecology also encompasses the study of the distribution and abundance of
organisms.
B. Levels of Ecological Organization
1. Populations are members of the same organism that live together and potentially breed
with one another.
2. Species consist of all the populations of a particular organism.
3. Communities are made up of populations of different species that utilize different
resources within the habitat that they share.
4. Ecosystems include the communities together with their nonliving habitats; ecosystems
are affected by the flow of energy and cycling of nutrients.
5. Biomes are major terrestrial assemblages of plants, animals, and microorganisms that
occur over wide geographic areas as predicted by climate.
6. All the world’s biomes together with the marine and freshwater assemblages constitute
the biosphere.
C. The Environmental Challenge
1. Temperature
2. Water
3. Sunlight
4. Soil
Populations (p. 722)
35.2
Population Range (p. 722; Figs. 35.4, 35.5, 35.6)
A. Groups of interacting organisms of the same species within a given area are called a
population.
B. Population Ranges
1. Not all organisms occur in all habitats throughout the world, not even humans.
C. Range Expansions and Contractions
1. Ranges of populations are not static; they change over time.
35.3
Population Distribution (p. 724; Figs. 35.7, 35.8)
A. Randomly Spaced
B. Uniformly Spaced
C. Clumped Spacing
D. The Human Effect
E. Dispersal Mechanisms
35.4
Population Growth (p. 726; Figs. 35.9, 35.10, 35.11)
A. Characteristics of populations include population size (number of individuals), population
density (number per unit area), population dispersion, and population growth.
B. The Exponential Growth Model
1. Populations have the capacity to grow.
2. The biotic potential r is the rate at which a population can increase when there are no
limits to its growth.
3. In mathematical terms:
population growth rate = riN
where ri = the intrinsic rate of natural increase (the innate capacity for growth)
and N = population size.
4. The actual rate of increase r includes births, deaths, immigrants, and emigrants:
r = (births -deaths) + (immigrants - emigrants)
5. The innate capacity for increase for a population is exponential.
6. When a small population is added to a new area of very favorable habitat, growth rate
will be nearly exponential.
B. Carrying Capacity
35.5
35.6
35.7
1. Exponential growth does not last long; at some point, the living space or food or shelter
becomes limited, and the population growth rate slows down.
2. A population stabilizes at a certain size called the carrying capacity, which is the number
of individuals the habitat is able to support indefinitely.
C. The Logistic Growth Model
1. The growth curve of a population that is approaching its carrying capacity can be
represented by the logistic growth equation:
Population growth rate = rN [(K – N)/K]
where K = the carrying capacity, r = realized rate of population increase, and N =
population size.
2. This type of growth can best be represented by a sigmoid growth curve showing that
growth is rapid at first when the population is small, then tapers off as population size
approaches carrying capacity.
The Influence of Population Density (p. 728; Figs. 35.12, 35.13)
A. Density-Independent Effects
1. Density-independent effects regulate population size regardless of the density of
individuals.
2. Density-independent factors are primarily those associated with the external
environment, such as weather or natural disasters.
B. Density-Dependent Effects
1. The effect that these factors have on population growth depends on population size.
2. These effects grow stronger as the population size increases.
C. Maximizing Population Productivity
1. In ecosystems that are exploited by humans, the aim is to maximize productivity by
harvesting the population early in the rising portion of its growth curve where the point
of maximal sustained yield lies.
Life History Adaptations (p. 729; Fig. 35.14; Table 35.1)
A. Some life-history adaptations favor near-exponential growth.
1. Populations that are r-selected reproduce many offspring early, care for them little, have
a high mortality rate, and are small in size.
B. Other life-history strategies favor more competitive logistic growth.
1. Populations that are K-selected are good competitors, have few young, and exhibit
parental care.
C. Most populations exhibit a combination of these two extremes.
Population Demography (p. 730; Fig. 35.15)
A. Demography is the statistical study of populations.
B. Age Structure
1. A group of individuals of the same age is a cohort.
2. Every cohort has its own fecundity and mortality.
3. The relative number of individual’s in each cohort defines a population’s age structure.
C. Sex Ratio
1. The proportion of males and females in a population is the sex ratio.
D. Mortality and Survivorship Curves
1. One way to represent age distribution is to construct a survivorship curve.
2. This indicates survivorship, the percentage of an original population that survives to a
given age.
3. Three types of general survivorship curves exist.
4. Type I, characteristic of humans, shows a low infant mortality, followed by a long period
of high survivorship, and then a high mortality of the oldest individuals.
5. Type II, as found in the hydra, is characterized by an equal chance of dying at any age.
6. In type III, organisms like oysters suffer a very high juvenile mortality, followed by a
stable survivorship to old age.
How Competition Shapes Communities (p. 731)
35.8
Communities (p.731; Fig. 35.16)
A. All the populations of a given area make up a community.
35.9
B. We recognize a community largely because of the presence of its dominant species, but many
other kinds of organisms are also characteristic of each community.
C. A community exists in a place because the ranges of its species overlap there.
The Niche and Competition (p. 732; Figs. 35.17-35.20)
A. Each organism plays a role within its ecosystem.
B. Its role is called a niche—its way of interacting with others in its community and with its
ecosystem.
C. The Realized Niche
1. The entire range of factors an organism could potentially exploit is its fundamental niche.
2. However, fundamental niches are rarely seen in nature because other organisms are
present that use similar resources.
3. Competition over resources occurs, and an organism's niche is narrower than its true
potential.
4. We say this is the realized niche.
D. Competitive Exclusion
1. In the 1930’s, ecologist Gause studied competition in three species of Paramecium and
formulated his principle of competitive exclusion.
E. Niche Overlap
1. Experiments have shown that when there is niche overlap, two species can survive, but
their niche breadth is narrowed.
2. Competitive Exclusion
a. When two species compete intensely, sometimes one can be excluded from the
habitat, a process called competitive exclusion.
b. This can be seen in mature ecosystems where organisms rarely occupy their
fundamental niche and, instead, share resources with their neighbors.
F. Resource Partitioning
1. Competitive exclusion is rare in nature because organisms evolve slight differences in
food choice and habitat preference when competition is strong.
2. Resource partitioning is seen when species divide up resources to create their realized
niches.
3. Resource partitioning is often seen in sympatric species versus allopatric species.
4. Sympatric species also tend to evolve differences in morphology and behavior to reduce
competition, a process called character displacement.
Species Interactions (p. 736)
35.10
Coevolution and Symbiosis (p.736; Figs. 35.21,35.22)
A. Coevolution
1. Since organisms continually interact within their ecosystem, they can coevolve with one
another; coevolution is the adaptation of two or more species to each other.
B. Symbiosis Is Widespread
1. In symbiosis, two species live in intimate contact with each other.
2. All symbiotic relationships carry the potential for coevolution between the organisms
involved.
3. Examples of symbiosis include lichens, mycorrhizae, and leafcutter ant–fungi
associations.
4. Kinds of Symbiosis
a. Types of symbiosis include commensalism, mutualism, and parasitism.
C. Mutualism
1. Mutualism is a symbiotic relationship in which both species benefit.
2. Lichens are made up of an alga and a fungus, and both derive benefit from the
association.
a. The fungus is fed by the alga, and the fungus protects the alga from drying.
3. Ants and Acacias
a. The mutualistic association between ants and certain tropical species of Acacia
plants is a striking example of mutualism.
b.
35.11
The ants derive food and shelter from living in thorns of the plants, and the plants
outcompete surrounding plants because the ants remove nearby shading leaves and
contribute organic matter with nitrogen to the plant.
D. Parasitism
1. Parasitism is a symbiotic association in which the parasite benefits by deriving food and
shelter from the host, and the host is harmed somewhat by the parasite.
2. External Parasites
a. External parasites, or ectoparasites, are common, such as lice or mosquitoes.
b. Parasitoids are insects that lay eggs on living hosts and are common among wasps.
3. Internal Parasites
a. Vertebrates are parasitized internally by endoparasites involving many different
phyla of animals.
b. Internal parasitism is marked by extreme specialization.
4. Brood Parasitism
a. Brood parasitism occurs when birds lay their eggs in the nests of other species.
E. Commensalism
1. In commensalism, one species benefits from the association, but the other neither
benefits nor is harmed.
2. Examples of Commensalism
a. Several examples can be found in marine organisms.
b. Barnacles riding around on whales benefit by receiving food and protection, and the
whale is not harmed.
c. Tropical fish receive protection from sea anemones and eat their leftovers, and the
sea anemones are not harmed.
3. When Is Commensalism Commensalism?
a. There are no clear boundaries between commensalism and mutualism, and it is
sometimes difficult to determine whether one or both species benefit.
Predator-Prey Interactions (p. 734; Figs. 35.23, 35.24)
A. Predation is the consuming of one organism by another.
B. Predator-prey interactions also show examples of coevolution.
C. Predator-Prey Cycles
1. Predators will often deplete populations of prey unless prey have a refuge in which they
can hide.
2. Even if prey populations become very small, they can usually recover once predator
pressure lets up.
3. In nature, predator and prey population sizes can cycle.
4. Prey population sizes are limited by predator pressure: when predators are abundant, prey
populations decline; when predator populations decline, prey populations recover.
5. Predator population sizes increase in response to more available prey.
6. Predator populations are thus limited by their food supply.
D. Cycles in Hare Populations: A Case Study
1. Population cycles, such as that described above, have been observed in hares and
lemmings.
2. In the classic example of the snowshoe hare “10-year cycle,” two factors appear to be
generating the cycle: food plants and predators.
E. Predation Reduces Competition
1. Predators greatly reduce competitive exclusion, and this increases diversity, by reducing
the numbers of individuals of competing species.
2. It is a mistake to try to eliminate the top predator from a community of organisms.
3. Predation helps to prevent prey populations from being driven to extinction.
35.12
Mimicry (p. 739; Fig 35.25)
A. Some animals evolve coloration that mimics that of poisonous species; the unprotected mimic
gains an advantage by looking like the distasteful one.
B. Batesian Mimicry
1. An example of Batesian mimicry is the viceroy butterfly mimicking the poisonous
monarch butterfly.
C. Müllerian Mimicry
1. In Müllerian mimicry, several unrelated but protected animal species come to look like
each other.
2. This is seen in a variety of stinging wasps that all employ the yellow and black warning
coloration.
3. Any predator seeing that color pattern leaves the wasp alone.
4. For any type of mimicry to be effective, the mimic must also act like its model and live in
the same type of habitat if predators are to be deceived.
D. Self-Mimicry
1. Self-mimicry involves adaptations where one animal body part comes to resemble
another body part.
2. This has evolved in both predator and prey.
3. Prey, such as many moths and butterflies, have “eye spots” that function in startling the
predator, allowing time for escape.
4. Predators can use mimicry to simulate bait and lure prey closer for a successful attack.
Community Stability (p. 741)
35.13
Ecological Succession (p. 741; Fig. 35.26)
A. Succession is the orderly replacement of one community with another.
B. Secondary Succession
1. Secondary succession occurs in areas where an existing community has been disturbed.
C. Primary Succession
1. In contrast, primary succession occurs on bare, lifeless substrates, such as those left
behind when a glacier retreats or when a volcanic island emerges.
2. When a bare rock or new habitat opens up, the first organisms reaching it are the
pioneers.
3. They can grow and reproduce rapidly, and they have broad niches so they can survive
easily over a wide range of conditions.
4. They form and improve the soil, and new arrivals move into the area.
5. The pioneers are poor competitors, so the new arrivals quickly displace them in the
habitat.
6. Over time, still other, larger species of plants move in, until a climax community is
reached.
7. The climax community is stable and similar to the communities characteristic of the area.
D. Why Succession Happens
1. Succession happens because species alter the habitat and the resources available in it
through tolerance, facilitation, and inhibition.