Populations Why they stop growing

Population density The number of organisms in a given area Population growth is limited by several factors: One set of which depends on population density Another set does not depend on population density

Density Dependent Limiting Factors Population limiting factors that operate more strongly on larger, denser populations than on small less crowed populations These are: competition, predation, parasitism and crowding

Competition The more individuals the sooner they use up the available resources The fewer the individuals the longer resources last One of the most important factors in determining the carrying capacity of an environment Can lower birth rates (fewer survivors); increase death rates

Predation Most species serve as food for other species Predators and prey usually coexist for long periods of time Prey evolved mechanisms (defenses) to avoid predators Predators evolved mechanisms (counter-defenses) to better capture prey Predator prey populations always change over time (see graph page 139)

Parasitism Organism that take nourishment from a host Most effective when hosts are present in large numbers Parasites are host specific High density means access to more hosts Successful parasites do not kill their hosts

Crowding Organisms have built in behavioral need for space (hunting, nesting) Stress from crowding can interfere with hormone and immune systems Changes in behavior can lead to less offspring being successful (no mating, eating offspring)

Density Independent Limiting Factors These factors limit populations regardless of how large or small the population is Mostly abiotic (does not involve living things) Weather is most important (hurricanes, cyclones, rain, drought) Fires Physical disruption of the environment (earthquakes, volcanoes, avalanche, rock slides, mudslides, floods) Human activity (plowing, clear cutting, slash and burn, use of toxins on crops)

Review Population density Density dependent limiting factors Density independent limiting factors