1. Chapter 8
Population Ecology

2. Types of Species in Communities
Native species
species that normally live in a particular community
Nonnative species
also referred to as “invasive” or “alien” species
species that enter a new community either through immigration or introduction
Introduction may be deliberate or accidental
i.e. “killer bees,” Kudzu, zebra mussels, Asian oysters, domesticated animals

3. Indicator Species
species whose decline or migration indicates a significant change (damage) to a particular community
Serve as “early warning sentinels” of environmental degradation
Examples:
Amphibians
Trout
Birds
Aquatic macroinvertebrates

4. Keystone Species
species whose removal from its community may dramatically alter the structure and function of the community
roles:
pollinators
top predators
decomposers

5. Foundation Species
species that alters its habitat in ways that benefit other species
behaviors of such species may influence succession and increase species richness
sometimes identical to the keystone species, while other times serves as a counterbalance
Examples: elephants, kelp, eastern hemlock, mussels

6. Population Distribution
Three general patterns: (see below)
Most populations live in clumps although other patterns occur based on resource distribution.
Figure 8-2

7. Why Clumping? Resource availability varies from place to place.
Living in herds, flocks, or schools provides protection from predators and population declines.
Fish, birds, caribou, antelope, zebra
Predators that live in groups are afforded a better chance of catching prey and getting a meal.
Wolves, hunting dogs
Temporary animal groupings may occur for mating and caring for young.
Dolphin, albatross

8. Changes in Population Size
Populations increase through births and immigration
Populations decrease through deaths and emigration

9. Population Age Structure:
How fast a population grows or declines depends on its age structure.
Prereproductive age: not mature enough to reproduce.
Reproductive age: those capable of reproduction.
Postreproductive age: those too old to reproduce.
Populations with mostly reproductive individuals tend to increase.
Populations with mostly post-reproductive individuals tend to decrease.
Stable populations are equitability distributed among all three categories.

10. Limits on Population Growth: Biotic Potential vs
Limits on Population Growth: Biotic Potential vs. Environmental Resistance
No population can increase its size indefinitely. There are always limits to population growth in nature.
Population change is a balance between:
Biotic potential - the intrinsic rate of increase (r) or the rate at which a population would grow if it had unlimited resources and…
Environmental resistance – all the factors that act to limit the growth of a population.
Together these determine a populations carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat.

11. Exponential and Logistic Population Growth: J-Curves and S-Curves
Exponential or geometric growth – starts slowly but accelerates rapidly as population increases
J-shaped curve plotted on a graph of population vs. time
Logistic growth – exponential growth followed by a steady population decrease until the population size levels off
S-shaped curve
Usually levels off at or near the carrying capacity
Carrying capacity is not fixed

12. Environmental Resistance
Carrying capacity (K)
Population size (N)
Biotic
Potential
Exponential
Growth
Figure 8.3
Natural capital: no population can continue to increase in size indefinitely. Exponential growth (lower part of the curve) occurs when resources are not limited and a population can grow at its intrinsic rate of increase (r) or biotic potential. Such exponential growth is converted to logistic growth, in which the growth rate decreases as the population becomes larger and faces environmental resistance. Over time, the population size stabilizes at or near the carrying capacity (K) of its environment, which results in a sigmoid (S-shaped) population growth curve. Depending on resource availability, the size of a population often fluctuates around its carrying capacity, although a population may temporarily exceed its carrying capacity and suffer a sharp decline or crash in its numbers.
Time (t)
Fig. 8-3, p. 163

13. Population Growth Curves
Exhibit four phases
Lag – phase characterized by low birth rates, when the
population is adjusting to a new environment
Growth – phase which shows a dramatic increase in
population size (B+I > D+E)
Stationary – phase when then population is in dynamic
equilibrium (B+I = D+E)
Death – phase in which the population declines
(B+I < D+E)

14. Can a Population Exceed Its Carrying Capacity?
Members of populations which exceed their resources will die unless they adapt or move to an area with more resources.
Some populations overshoot their carrying capacity.
Reproductive time lag
Causes a dieback or a crash
Some populations may increase their carrying capacity by developing adaptive traits (i.e. natural selection)
Some species maintain their carrying capacity by migrating to other areas.

15. Number of sheep (millions)
Overshoot
Carrying capacity
Number of sheep (millions)
Figure 8.4
Boom and bust: logistic growth of a sheep population on the island of Tasmania between 1800 and After sheep were introduced in 1800, their population grew exponentially thanks to an ample food supply. By 1855, they had overshot the land’s carrying capacity. Their numbers then stabilized and fluctuated around a carrying capacity of about 1.6 million sheep.
Year
Fig. 8-4, p. 164

16. Population overshoots carrying capacity Number of reindeer Population
Crashes
Number of reindeer
Figure 8.6
Exponential growth, overshoot, and population crash of reindeer introduced to the small Bering Sea island of St. Paul. When 26 reindeer (24 of them female) were introduced in 1910, lichens, mosses, and other food sources were plentiful. By 1935, the herd size had soared to 2,000, overshooting the island’s carrying capacity. This led to a population crash, with the herd size plummeting to only 8 reindeer by 1950.
Carrying
capacity
Year
Fig. 8-6, p. 165

17. Population Density and Population Change: Effects of Crowding
Population density: the number of individuals in a population found in a particular area or volume.
A population’s density can affect how rapidly it can grow or decline.
Density dependent factors include biotic factors like disease, competition for resources, predation, and parasitism
Some population control factors are not affected by population density.
Density independent factors include abiotic factors like weather, fire, pollution, and habitat destruction

18. Types of Population Change Curves in Nature
Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically.
Stable: fluctuates slightly above and below carrying capacity.
Irruptive: populations explode and then crash to a more stable level.
short-lived, rapidly reproducing species (i.e. algae, insects)
Cyclic: populations fluctuate according to regular cyclic or boom- and-bust cycles.
close predator-prey interactions
Irregular: erratic changes possibly due to chaos or drastic change.
populations that inhabit unstable or highly variable environments

19. Population size (thousands)
Hare
Lynx
Population size (thousands)
Figure 8.7
Population cycles for the snowshoe hare and Canadian lynx. At one time scientists believed these curves provided circumstantial evidence that these predator and prey populations regulated one another. More recent research suggests that the periodic swings in the hare population are caused by a combination of top-down population control—predation by lynx and other predators—and bottom-up population control. In the latter, changes in the availability of the food supply for hares help determine hare population size, which in turn helps determine the lynx population size. (Data from D. A. MacLulich)
Year
Fig. 8-7, p. 166

20. REPRODUCTIVE PATTERNS
Some species reproduce without having sex (asexual).
Offspring are exact genetic copies (clones).
Others reproduce by having sex (sexual).
Genetic material is mixture of two individuals.
Disadvantages: males do not give birth, increase chance of genetic errors and defects, courtship and mating rituals can be costly (energetically).
Major advantages: promotes genetic diversity, division of labor among the sexes may provide offspring greater protection through critical periods.

21. Reproductive Patterns: Opportunists and Competitors
r-selected species: Large number of smaller offspring with little parental care
K-selected species: Fewer, larger offspring with higher invested parental care
Figure 8-9

22. Reproductive Patterns
r-selected species tend to be opportunists while K-selected species tend to be competitors.
Figure 8-10

23. Survivorship Curves: Short to Long Lives
The way to represent the age structure of a population is with a survivorship curve.
Late loss population live to an old age.
Constant loss population die at all ages.
Most members of early loss populations, die at young ages.
Number of individuals
age

24. WORK CITED
Population Ecology. (1998) Cyber Ed.