Chapter 8Population Ecology

They were over-hunted to the brink of extinction by the early 1900’s and are now making a comeback.

Core Case Study: Southern Sea Otters: Are They Back

from the Brink of Extinction?

Figure 8-1

Core Case Study: Southern Sea Otters: Are They Back

from the Brink of Extinction?

Sea otters are an important keystone species for sea urchins and other kelp-eating organisms.

Figure 8-1

POPULATION DYNAMICS AND CARRYING CAPACITY

Most populations live in clumps although other patterns occur based on resource distribution.

Figure 8-2

Changes in Population Size: Entrances and Exits

Populations increase through births and immigration

Populations decrease through deaths and emigration

Age Structure: Young Populations Can Grow Fast

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.

Limits on Population Growth: Biotic Potential vs. Environmental

Resistance

No population can increase its size indefinitely. The intrinsic rate of increase (r) is the rate at

which a population would grow if it had unlimited resources.

Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat.

Exponential and Logistic Population Growth: J-Curves and S-Curves

Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off.

Figure 8-4

Exponential and Logistic Population Growth: J-Curves and S-Curves

As a population levels off, it often fluctuates slightly above and below the carrying capacity.

Figure 8-4

Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size

Members of populations which exceed their resources will die unless they adapt or move to an area with more resources.

Figure 8-6

Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size

Over time species may increase their carrying capacity by developing adaptations.

Some species maintain their carrying capacity by migrating to other areas.

So far, technological, social, and other cultural changes have extended the earth’s carrying capacity for humans.

What do you think?

Can we continue to expand the earth's carrying capacity for humans? a. No. Unless humans voluntarily control their

population and conserve resources, nature will do it for us.

b. Yes. New technologies and strategies will allow us to further delay exceeding the earth's carrying capacity.

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.• e.g. biotic factors like disease

Some population control factors are not affected by population density.• e.g. abiotic factors like weather

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. Cyclic: populations fluctuate and regular cyclic or

boom-and-bust cycles. Irregular: erratic changes possibly due to chaos or

drastic change.

Types of Population Change Curves in Nature

Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources.

Figure 8-7

Case Study: Exploding White-Tailed Deer Populations in the United States

Since the 1930s the white-tailed deer population has exploded in the United States. Nearly extinct prior to their protection in 1920’s.

Today 25-30 million white-tailed deer in U.S. pose human interaction problems. Deer-vehicle collisions (1.5 million per year). Transmit disease (Lyme disease in deer ticks).

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.

Major advantages: genetic diversity, offspring protection.

Sexual Reproduction: Courtship

Courtship rituals consume time and energy, can transmit disease, and can inflict injury on males of some species as they compete for sexual partners.

Figure 8-8

Reproductive Patterns:Opportunists and Competitors

Large number of smaller offspring with little parental care (r-selected species).

Fewer, larger offspring with higher invested parental care (K-selected species).

Figure 8-9

Reproductive Patterns

r-selected species tend to be opportunists while K-selected species tend to be competitors.

Figure 8-10

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 population, die at

young ages.

Survivorship Curves: Short to Long Lives

The populations of different species vary in how long individual members typically live.

Figure 8-11