Factors that regulate populations

Lecture #3APES 2009-2010

Limits to population GrowthLimiting Factors (resources) – factors that limit the growth and/or reproduction of an organism or population. Examples: predators, limited resources, parasites andCompetitors

Environmental resistance - The combination of abiotic and biotic factors that may limit population increase

– This is what keeps populations in check

Factors that Regulate Population Growth

• Intrinsic factors - operate within or between individual organisms in the same species

• Extrinsic factors - imposed from outside the population• Biotic factors - Caused by living organisms. Tend to be

density dependent.• Abiotic factors - Caused by non-living environmental

components. Tend to be density independent, and do not really regulate population although they may be important in increasing or decreasing numbers. Example: Rainfall, storms

Density Dependent FactorsDensity Dependant Limiting Factors - these limiting factors depend on the density of

the population. The population size is reduced by decreasing natality or increasing mortality.

As the population density decreases for a species, environmental resistance decreases.

Example: Predator-Prey oscillations

Density Dependent Factors Continued

• Intraspecific Interactions - competition for resources by individuals within a population– As population density approaches the carrying

capacity, one or more resources becomes limiting.• Control of access to resources by territoriality;

owners of territory defend it and its resources against rivals.

• Stress-related diseases occur in some species when conditions become overcrowded.

Limiting factors continued

Density Independent Limiting Factors: environmental factors that affect population size regardless of the population’s density Many density independent limiting factors are abiotic factors.

Examples: long periods of hot or cold weather, natural catastrophes ( forest fires, floods, volcanic eruptions)

Predator-Prey balance

Predator – prey balances - Regulation of a population by a predator • This is the best-known mechanism of population balance. • Example: wolves and deer

Prey species are not wiped out completely because predators areoften times not capable of bringing down an adult individual of theirprey that is in good physical condition. Who usually gets caught? It is usually the very young, old, sick, injuredor otherwise unlucky animals that are brought down.

Parasite-Host balanceParasitic organisms • Much more abundant and ecologically important than predators in population

control • All species of plants and animals (even microbes) can be infected with parasites As the population density of a parasite host increases, parasites and their vectorshave an easy time finding new hosts. This means that infection rates increase, anddie-off results Vector ~ the organism that carries a parasite from one host to another. Example ~ Mosquitoes (disease carrying insects) carry West Nile virus

As the population density of the hosts decrease, transfer of infection is less becausethere are fewer individuals. This means a decrease in infection and the population is usually able to recover

Plant-Herbivore Balance This can only occur if there is a predator-prey or host-

parasite balance

• If there is no predator to kill off a prey species that is a herbivore, the herbivores will overgraze the land.

Example ~ Elk or Deer in an area where wolves have been

eradicated from.

Conservation BiologyHuman activities have caused much extinction

due to alteration of habitats, pollution, hunting and other forms of exploitation.

A Critical question in conservation biology is the minimum population size of a species required for long term viability.

What we can learn from islands

Special case of islands – Island biogeography - small islands far from

a mainland have fewer terrestrial species than larger, closer islands

–How this relates to conservation biology: Fragmented habitats are going to support few terrestrial species

Conservation Genetics

• In a large population, genetic diversity tends to be preserved. A loss/gain of a few individuals has little effect on the total gene pool.

• However, in small populations small events can have large effects on the gene pool.

• Genetic Drift– Change in gene frequency due to a random event

• Founder Effect– Few individuals start a new population.

Genetic Drift

Population Viability Analysis• Minimum Viable Population (aka ‘critical number’) is the

minimum population size required for long-term survival of a species.

– The number of grizzly bears in North America dropped from 100,000 in 1800 to 1,200 now. The animal’s range is just 1% of what is once was and the population is fragmented into 6 separate groups.

– Biologists need to know how small the bear groups can be and still be viable in order to save the grizzly.

• If a population is depleted below its critical number needed to provide support, the surviving members become more vulnerable, breeding fails, and extinction is almost inevitable

Metapopulations

• Metapopulation - a collection of populations that have regular or intermittent gene flow between geographically separate units

• Wildlife biologists feel that creating corridors will increase metapopulation size and improve the long term viability of species suffering from habitat fragmentation.

• Corridor – a land bridge that frees migration of flora and fauna in both directions.

Ecological Corridor

Things to consider when studying and managing metapopulationsIs the habitat a…………………. – Source habitat - Birth rates are higher than death

rates. Surplus individuals can migrate to new locations.

– Sink habitat - Birth rates are less than death rates and the species would disappear if not replenished from a source.

** Linking a sink habitat with a source habitat would do much to improve the viability of the species

Metapopulation

These definitions are assigned by the U.S. Fish and Wildlife service

Threatened - Species in which populations that

are declining rapidly because of human impacts.

Endangered - Species in which the population is

near what scientists believe to be its critical number