Biology 31, Las Positas College, Fall 2003
Population & Community Ecology
Chapters 40 & 42, Starr, 5th ed, p 684-701 & 718-735

I-Population Ecology

A-Structure & Growth
B- Human Population Growth

II-Community Ecology

A-Overview
B-Interspecific Interactions
C-Disturbances
D-Human Impact

I-Population Ecology
Ecology – study of interactions among organisms & between organisms & the environment.
Population Ecology – focuses on the factors that influence a population’s size, growth rate, density, and features of population structure
Population – group of individuals of the same species living in a given area at a given time.

A-Structure & Growth

1. Population Density
Density = the number of organisms or individuals in a population/unit area. Influences survival.

Population Growth

Exponential Growth Model

J-shaped curve – Number of organisms added = multiply growth rate (birth rate – death rate) X number of organisms in area.
Ideally a population can grow exponentially but it stops as shortages of growth factors eventually limit population = biotic potential.

Wastes accumulate and kill population, run out of resources and population collapses, predators wipe out huge population, etc. Logistic Growth Model

Carrying capacity = the number of individuals within a population that can be supported within a particular environment for an indefinite period.

Population wavers around carrying capacity.
As population grows and its density increases, competition among organisms for resources such as food, shelter, light and mating sites increases and toxic waste products accumulate. Factors that result from the growth of a population regulate its subsequent growth.

Regulation of Population Growth

1. Density-independent limiting factors operate regardless of population size.
2. Density-dependent limiting factors are in effect when a population size increases in a given area.

a. competition for resources
b. waste products accumulate
c. interactions within community may take over
d. sometimes -> population cycles

B- Human Population Growth

Better agriculture, advanced technology, medical care, -> more survivors and decreased mortality -> more people.
Will we reach carrying capacity of the Earth?
Population keeps increasing.
Some resources already declining
People strive for higher standard of living than just having enough food to survive.
If more people or people live better lives, Earth will need to increase capacity or population must decrease.
If want to avoid "bust" of "boom-and-bust" cycle, must voluntarily decrease birth rate.

Age Structure & Population Growth

Expanding-more prereproductively competent potential than numbers than will die
Stable-about same number will give birth as will die, every year
Shrinking-more will die than will give birth

Younger people more productive than older ones. Need younger people to do work and help older people.
If young people reach old age, will need to be helped by more younger people.
How long can this continue? Carrying capacity of world.

II-Community Ecology

A-Introduction
Population = individuals of the same species.
Community = grouping of populations of different species living together in a particular area at a particular time.

Influenced by habitat which includes physical and chemical conditions, such as topography, soil, weather -> availability of food and other resources -> adaptive traits of species -> interactions among species. Independent factors = arrival and disappearance of species and physical disturbances in habitat.

Habitat = actual area in which organism resides.

Niche = role of organism within an ecosystem. Refers to the organism's use of the biotic and abiotic resources in its environment. = space, food, temperature, appropriate conditions for mating, requirements for moisture.

Community Depends Upon

Diversity – the variety of different species = species richness & the relative abundance of the different species

Prevalent form of vegetation – types & structural feature of plants, -> kinds of animals in community

Stability – community’s ability to resist change & return to its original species composition after being disturbed. Depends on community & on disturbance.

Trophic structure – feeding relationships among species making up the community.

B-Interspecific Interactions

1. Competition - Organisms of different species that live near one another strive to obtain the same limited resources. Can be through behaviors or merely by being there.

Competitive Exclusion – some species are so much more adapted to environment that they entirely eliminate other species.

Ecological Niche – competition often results in niche subdivision, i.e. competitors use slightly different resource.

Resource Partitioning – two species use same resources, but different segment of it.

2. Predation - An organisms of one species kills and eats an organism of another. Includes animals eating animals, animals eating plants and plants eating animals.

Controls numbers
Provides evolutionary pressure
Plants and herbivore are in constant evolution to try to protect themselves and get enough nutrients to survive.

Other factors, such as competition, weather, etc. also influence predator, prey numbers.
Predation can -> species diversity in community. Prevents well-adapted species from becoming dominant and killing everything.

3. Symbiotic Relationships

a. Parasitism - An organism of one species (the parasite) lives in or on another (the host). The parasite benefits from this relationship, whereas the host is usually harmed.

b. mutualism – two species live together in close association, both benefiting from the relationship

C-Disturbances – natural & unnatural that change a biological community.

Succession - the dynamic process of change during which a sequence of communities replaces one another in an orderly and predictable way.

1. Primary succession - takes place in areas not previously supporting organisms. Lakes formed from the retreat of glaciers, or volcanic islands.

Plants and animals form climax community (a community in which the mix of plants and animals becomes stable; the last stage of succession).

2. Secondary succession - in areas that have been disturbed and that were originally occupied by organisms. . Also may take place after natural disasters such as forest fire or a volcanic eruption producing ash.

Cyclic nondirectional changes – repetitive changes in a habitat

Restoration Ecology

D-Human Impact

Land use – farming, logging, grazing etc. -> changed mix of organisms
Introduced species – may be more adapted to environment and kill natives -> change in mix of organisms