Transcript Factors that regulate populations

Factors that regulate populations
Lecture #3
APES 2009-2010
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
•
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 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
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 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.
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
Plant-Herbivore Balance
This can only occur if there is a predator-prey or hostparasite 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 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.
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 metapopulations
Is 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