Chapter 14: Populations Section 14-2: Why Populations Stop Growing

Population Limiting Factors Population density = the number of organisms in a given area Density-dependent limiting factors are those that operate more strongly on large, dense populations than on small, less- crowded ones Species whose populations are controlled by these factors tend to be more stable

Population Limiting Factors There are four density-dependent limiting factors: – Competition – Predation – Parasitism – Crowding

Competition Individuals compete with each other for food, water, space, sunlight, etc. Only some individuals will get what they need to survive and reproduce, some will only be able to live and not reproduce, and others may starve to death or die from lack of shelter The more individuals in an area, the sooner they will use up available resources Can affect carrying capacity

Predation Most species exist as food for other species, and in nature predators and their prey usually coexist for long periods of time Prey develop defenses against their predators, and predators develop counter defenses Even with this accommodations, there is not always a balance between predators and prey

Predation

Parasitism A parasite is an organism that takes nourishment from its host Parasites weaken their hosts, cause disease, and in some cases death – usually not right away Because parasites are host-specific, they are more effective when their hosts are present in large numbers Crowding of individuals helps parasites travel from one host to another

Crowding Most organisms have a built in behavioral need for a certain amount of space If organisms become too over-crowded, they may fight among themselves, leading to high levels of stress, disrupting homeostasis of hormone levels Can lead to a weakened immune system, or can lead to changes in behavior that cause individuals to neglect, kill, or even eat their own offspring

Population Limiting Factors Density-independent limiting factors do not depend on population density or size Examples are weather and human activity