Population Ecology

Characteristics of Populations Geographic distribution Also called range The area inhabited by a population Density Growth rate

Population Density Population Density: Spatial Distribution: The number of organisms per unit area Spatial Distribution: Dispersion: The pattern of spacing a population within an area 3 main types of dispersion Clumped Uniform Random The primary cause of dispersion is resource availability 3

POPULATION DYNAMICS AND CARRYING CAPACITY Most populations live in clumps although other patterns occur based on resource distribution. TPS: Why do you think populations “clump”?

Changes in Population Size: Entrances and Exits Populations increase through births and immigration Populations decrease through deaths and emigration

Population Limiting Factors Population Ecology: Population Characteristics Population Limiting Factors Population growth rate How fast a given population grows Factors that influence this are: Natality (____ rate) Mortality (_____ rate) Emigration (the number of individuals moving _________ a population) Immigration (the number of individuals _________ a population) TPS: if Natality = Mortality, what change occurs in the population? birth death away from moving to 6

Population Limiting Factors Population Ecology: Density-independent factors Density-independent factors Factors that limit population size, regardless of population density. These are usually abiotic factors They include natural phenomena, such as weather events Drought, flooding, extreme heat or cold, tornadoes, hurricanes, fires, etc. Reflection: what limiting factors are common in your ecosystem? 7

Population Limiting Factors Population Ecology: Density-dependent factors Density-dependent factors Any factor in the environment that depends on the number of members in a population per unit area Usually biotic factors These include Predation Disease Parasites Competition Infer: what biotic limiting factor is probably represented in this graphic? 8

What population do you think this is? Population Ecology: Population Growth Rate

So, what do you think is going to happen to the human population? We will probably reach our carrying capacity. Our growth rate will start to look like most organisms, which is the Logistic Growth Model Carrying Capacity (k) What letter does this curve kind of look like?

TPS: What are the implications for the human population? Limits on Population Growth: Biotic Potential vs. Environmental Resistance No population can increase its size indefinitely. Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off. TPS: What are the implications for the human population?

Limits to exponential growth Population Density (the number of individuals per unit of land area or water volume) increases as well Competition follows as nutrients and resources are used up Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat

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)

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. Reflection: What resources might the reindeer population be lacking that would cause this type of data?

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. Look at the trends in data during 1875 and 1925. What are possible reasons why these trends are different?

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.

Updates Online The latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at www.thomsonedu.com to access InfoTrac articles. InfoTrac: One Hatchling at a Time. Brownsville Herald (Brownsville, TX), July 2, 2006. InfoTrac: Where the Cattle Herds Roam, Ideally in Harmony With Their Neighbors. Jim Robbins. The New York Times, July 11, 2006 pF3(L). InfoTrac: A nudge for nature. Milwaukee Journal Sentinel, July 10, 2006. Earth Island Institute Environmental Defense: Creating a Conservation Community in Oregon’s Williamette Valley Marine Bio: Habitat Conservation