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Extended Lecture Outline |
Chapter 26: Population Structure And Dynamics |
26.1 Competition in communities occurs both within and between species.
a. Ecology is about resources, such as nutrients and space, and how they are distributed and divided among the organisms in communities.
b. Populations and communities are both shaped by limited resources and competition.
c. One of Darwin's critical observations was that organisms are in competition for limited resources and that most do not survive.
d. Competition can be both intraspecific, between members of the same species, and interspecific, between members of different species.
e. This chapter covers intraspecific competition; Chapter 27 covers interspecific competition.
26.2 All populations are dispersed in specific habitats over a certain geographic range.
a. A population is a group of individuals of the same species in a given area.
b. The boundaries of the area occupied by the population may shift, as individuals enter and leave, or as environmental conditions change.
c. Every species occupies a certain geographic range, where its members are likely to be found (e.g. gray squirrels are found in eastern North America).
d. Within a range, individuals select only a limited habitat (e.g. a marsh, or a forest) and the habitats for members of different species may be quite different within the same range.
e. A population is dispersed, or distributed over its range in accordance with its resources.
f. Individuals may be dispersed uniformly, randomly, or in clusters (Figure 26.1).
g. The environment has a patchiness (Figure 26.2) to it because of the uneven distribution of resources or because of differences in soil, temperature, or topography.
h. The size of the patches is described as the graininess of the environment, which is defined by the organism's ability to move.
i. Fine-grained environments are made of small patches that are easily traversed.
j. Course-grained environments are made of large patches that are confining.
k. A decaying log (Figure 26.3) is fine-grained for a bird, and coarse-grained for a slug.
l. Uniform dispersion often results from antagonism between individuals, as in the case of plants that produce substances that inhibit the growth of their own seedlings (Figure 26.4).
m. Plants tend to grow in clusters when soil conditions are patchy.
n. Animals may exist in clusters due to social interactions like breeding, or hunting, that define territories (Section 26.10).
o. Dispersion can change with time, as with migrating birds (Figure 26.5).
p. Population density, the number of individuals per unit or volume, is a measure of structure and change in the population.
q. Absolute density over the entire range is different from ecological density, measured within a limited habitat.
r. Mark-recapture is a commonly used technique for measuring density.
s. Sidebar 26.1 addresses cases of enormous population size.
26.3 Populations tend to grow exponentially.
a. All natural populations tend to grow in size, though their growth may not be obvious or constant, due to changing environmental conditions.
b. Exponential growth can produce large numbers in a short time (Figure 26.6; Sidebar 26.2).
c. Figure 26.7 shows the growth rate of ring-necked pheasants on Protection Island in the 1930s.
d. In a growing population of size N, measured over intervals of time, Æt, the size difference between any two generations, ÆN, is proportional to N; that is, Æ N/N = rÆt.
e. In this equation, r is the intrinsic growth rate of the population, the increase in the population during any time interval; the growth equation is commonly written to emphasize that the growth rate is proportional to the size of the population; that is, ÆN/Æt = rN.
f. This growth rate is equal to the difference between natality, or birth rate, b, and mortality, or death rate, d; r = b — d.
g. The net reproduction rate is a measure of growth based on the ratio of numbers from one year to the next: R = Nx + 1/Nx, where the subscript is an index that counts generations.
h. As long as R is greater than 1, the population is growing exponentially.
i. If R remains constant, after n generations, Nn = RnN.
26.4 Populations of many organisms tend to remain quite stable in size.
a. Gilbert White, one of the first naturalist-ecologists, studied swifts in the village of Selbourne in southern England in 1778.
1. White observed that only eight pairs of swifts lived in the village year after year.
2. In 1983, the swift population was examined again. Although there were considerable changes in the village, only twelve pairs of swifts were found to be living there.
3. This case is often cited to show the stability of bird populations over many generations.
4. Other censuses of birds and mammals have shown that populations tend to remain stable if the environment is not drastically disturbed.
b. Populations can change dramatically in the short term, even over a single generation.
1. Figure 26.8 illustrates a typical situation involving insects.
2. The chief factor that holds population size down is predation.
c. The populations of most birds and mammals tend to remain constant, but other kinds of organisms may experience sharp increases and decreases in numbers.
26.5 All populations are limited to maximum size.
a. The carrying capacity of a natural population (denoted by K) is the point where the population has reached maximum size and stops growing.
1. Some populations actually exceed their carrying capacity, but these populations usually "crash" as a result.
2. The logistical
growth equation is as follows: 
3. A graph of the logistical growth equation (Figure 26.9), is described as sigmoid or S-shaped, and it levels off at K.
b. V. C. Wynne-Edwards devised an idea to explain the limitation of population size.
1. He surmised that group selection favors populations that develop mechanisms to keep their sizes within the limits of their resources.
2. He defined group selection as competition between groups; individual selection results from competition between individuals.
3. Most population biologists give this hypothesis little credence do to the fact that it doesn't consider "cheaters" that might be defined as mutants.
26.6 Populations may be limited by resources or by predation.
a. Populations can be limited in three ways:
1. populations can be limited by random catastrophes such as weather,
2. populations can be limited by limited resources,
3. populations can be limited by predators, where predation is defined most broadly to include herbivory (herbivores eating phototrophs).
b. One classic idea in ecology is that populations are primarily limited "from below" by the resources available to them.
1. Many populations reach a limit because their members are competing for limited resources.
2. Competition takes the form of either exploitation or interference.
3. An example of exploitation of resources would be when each member of a population exploits the supply of food maximally and independently. The result is that some members of the population get less food than they need.
4. In animal species where males compete actively for females, two males engaged in combat are obviously interfering with each other.
5. Intraspecific competition for resources ultimately affects the ability of individuals to survive and to reproduce.
c. Some ecologists have proposed that many populations are limited primarily "from above" by predators feeding on them.
1. In 1960, Nelson Hairston and his associates argued that herbivores are not food-limited, since there is plenty of food available; they are not catastrophe-limited, so they must be limited by predation.
2. This group also argued that carnivores are food-limited, because there are no predators above them to limit their populations.
3. They added that decomposers are food-limited because, by definition, they are the organisms that consume organic debris.
d. Neither the "from above" or "from below" arguments on population limitation have been proven. It is important to remember, when studying populations, that they may be limited from both directions.
26.7 Some factors that limit population size depend on population density.
a. The factors that limit population growth are divided into those that are independent of population density and those that become more effective as population density rises.
1. Density-independent factors affect large and small concentrations of organisms equally.
2. Density-dependent factors affect a crowded population much more than a sparse one.
b. As a population grows, various factors that reflect its density could start to change the behavior or physiology of its members, so that growth eventually stops (Figure 26.10).
26.8 Density-dependent factors include nutrient limitation and physiological pressures associated with high density.
a. Population biologists have documented density-dependent limitations by at least two kinds of factors: by limited nutrients and by the accumulation of wastes and related stress factors associated with high density.
b. Nutrients can limit population growth.
1. Food is the most obvious resource that may reduce survival and fecundity.
2. As crowded individuals find it harder to get adequate nourishment, they may die before they can mature and reproduce, or they may produce fewer, less viable offspring (Figure 26.11).
c. Waste accumulation and other stress factors can limit population growth.
1. Dense populations impose stresses on their members.
2. Classical experiments by R. N. Chapman and by Thomas Park on flour beetles (Tribolium confusum; Figure 26.12) show how these stresses can limit population density.
3. Park showed that male flour beetles release the pheromone ethylquinone when another beetle interrupts them during mating.
4. Park also showed that egg-laying in flour beetles is inhibited by the presence of ethylquinone. Thus, ethylquinone acts as a population control device.
d. In vertebrates with well-developed nervous and endocrine systems, the stresses of dense populations may affect reproduction through the stress syndrome (Figure 26.13).
1. The stress syndrome, first described by Hans Selye, holds that often the factors that produce stress (injury, fear, anxiety, overcrowding) often activate the adrenal glands to produce hormones.
2. One hormonal effect is to inhibit the infammatory reaction that fights infections, making animals under stress more susceptible to disease.
3. Stress inhibits the growth of reproductive organs.
26.9 Members of a population tend to adopt strategies that maximize their use of resources and minimize competition.
a. Every organism has a limited amount of energy, which it can allocate in various ways for growth, maintenance, and reproduction.
1. The Principle of Allocation says that an organism may allocate its energy in various ways, but by allocating more energy to one activity, it reduces the energy available for others.
2. An individual organism does not necessarily actually make allocation decisions, but rather follows the strategy of growth and reproduction inherent in its genome.
b. Intraspecific competition, competition among members of the same species, certainly limits population sizes, but it is also this pressure on a population that creates natural selection for factors and mechanisms that maximize the fitness of individuals.
c. When animals are free to move and seek resources, populations achieve an ideal free distribution (Figure 26.14).
1. An ideal free distribution is the distribution of organisms who are free to optimize their access to resources, and their densities in the areas will be proportional to the relative richness of the areas (Figure 26.15).
26.10 Territories help allocate resources and control populations.
a. An important alternative strategy for dividing resources is territoriality.
1. Territoriality means claiming an area for one's own and defending it actively, rather than simply roaming freely through a habitat.
2. When animals defend a territory, they retain control over a set of resources, and territoriality is one way of limiting the number of individuals who try to share those resources.
3. Generally, individuals without a territory do not breed. This leads to relatively stable numbers of individuals inhabiting each territory year after year (Figure 26.16).
b. Because an animal must expend energy to defend an area's resources, territoriality is only advantageous when the animal obtains more energy than it expends (Figure 26.17).
26.11 Survivorship curves show different patterns of reproduction.
a. Population size depends on a balance between the rates of natality and mortality.
b. Considering an entire species, the critical factor is the number of offspring that survive to reproductive age and the number of reproductively successful offspring they produce.
c. Each population allocates some of its resources to reproduction, and tends to survive when just enough offspring survive to replace itself in the next generation.
d. Different kinds of organisms have evolved different reproductive strategies, as shown by their survivorship curves (Figure 26.18).
e. Members of a population that are born at the same time are termed a cohort.
f. Curves for survivorship of cohorts of various species form a spectrum.
g. Type I survivorship characterizes birds and mammals.
h. Type II survivorship characterizes many species with steady mortality rates.
i. Type III survivorship characterizes many plants and invertebrates that produce large numbers of offspring.
26.12 Different kinds of selection produce two extremes within a spectrum of lifestyles.
a. An opportunistic species is one such as an insect that reproduces rapidly and exploits the resources of a suitable area.
b. An equilibrium species is one such as a woodland mammal that lives in a stable environment for many generations at an equilibrium size, and competes with other animals for limited resources.
c. These two species experience either a high growth rate, r, in the case of the insect, or a steady-state population, K, in the case of the mammal.
d. These two species are therefore subject to two types of selection, either r-selection, or K-selection, respectively.
e. Table 26.1 summarizes features that are generally correlated with these two types of selection.
26.13 The human population is growing far beyond its ecological limits.
a. Figure 26.19 shows how the human population has grown over time.
b. The rapid growth that has ensued since modern agriculture and medicine have improved has been termed a population explosion.
c. Figure 26.20 shows the age structure of a population, which shows the number of people in each age bracket.
d. A population can be divided into prereproductive, reproductive, or postreproductive individuals.
e. The age structure of a stable population has a uniform width with regard to these categories.
f. As the human population grows, it puts enormous pressure on the world's ecosystems, and humans cannot be expected to survive if it continues at its recent rate.
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