1. Factors that regulate populations
Lecture #3
APES

2. Limits to population Growth
Limiting Factors (resources) – factors that limit
the growth and/or reproduction of an organism
or population.
Examples: predators, limited resources, parasites and
Competitors
Environmental resistance - The combination of abiotic
and biotic factors that may limit population increase
This is what keeps populations in check

3. 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

4. Density Dependent Factors
Density 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

5. 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.

6. 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)

7. 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 are
often times not capable of bringing down an adult individual of their
prey that is in good physical condition.
Who usually gets caught? It is usually the very young, old, sick, injured
or otherwise unlucky animals that are brought down.

8. Parasite-Host balance
Parasitic 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 vectors
have an easy time finding new hosts. This means that infection rates increase, and
die-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 because
there are fewer individuals. This means a decrease in infection and the population
is usually able to recover

9. 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.

10. Conservation Biology
Human 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.

11. 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

12. 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.

13. Genetic Drift

14. 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

15. 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.

16. Ecological Corridor

17. Things to consider when studying and managing metapopulations
Is the habitat a………………….
Sink habitat - Birth rates are less than death rates and the species would disappear if not replenished from a source.
Source habitat - Birth rates are higher than death rates. Surplus individuals can migrate to new locations.
** Linking a sink habitat with a source habitat would do much to improve the viability of the species

18. Metapopulation

19. 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