Population Ecology

Overview Questions What are the major characteristics of populations? How do populations respond to changes in environmental conditions? How do species differ in their reproductive patterns?

Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction? They were over-hunted to the brink of extinction by the early 1900’s and are now making a comeback. 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 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.

How Would You Vote? 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

Little or no parental care and protection of offspring r-Selected Species Cockroach Dandelion Many small offspring Little or no parental care and protection of offspring Early reproductive age Most offspring die before reaching reproductive age Small adults Adapted to unstable climate and environmental conditions High population growth rate (r) Population size fluctuates wildly above and below carrying capacity (K) Generalist niche Low ability to compete Early successional species Figure 8.10 Natural capital: generalized characteristics of r-selected (opportunist) species and K-selected (competitor) species. Many species have characteristics between these two extremes. Fig. 8-10a, p. 168

Fewer, larger offspring High parental care and protection of offspring K-Selected Species Elephant Saguaro Fewer, larger offspring High parental care and protection of offspring Later reproductive age Most offspring survive to reproductive age Larger adults Adapted to stable climate and environmental conditions Lower population growth rate (r) Population size fairly stable and usually close to carrying capacity (K) Specialist niche High ability to compete Late successional species Figure 8.10 Natural capital: generalized characteristics of r-selected (opportunist) species and K-selected (competitor) species. Many species have characteristics between these two extremes. Fig. 8-10b, p. 168

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