Download 12.4 - Factors Affecting Population Growth

UNIT 5: Population Dynamics
12.4: Factors That Affect Population Growth
pg. 610 – 615
Populations have the ability to increase or decrease in size over time. Factors
that can affect the size of a population are called limiting factors, which can
be either biotic or abiotic in nature. Although most of these factors are
interdependent, it is easier to study them individually to understand their
impact on a species population in a given ecosystem.
Limiting Factor – is a factor that limits the growth of a population.
Limiting factors are grouped into either Density-dependent factors, such as
competition or predation, or Density-independent factors, such as weather
and climate.
Density-dependent Factor – is a factor that is influenced by population
density, having a greater impact as the population density increases.
Density-independent Factor – is a factor that influences population
regulation regardless of the population density.
Density-Dependent Factors
The relation between density and Density-dependent factors is if density
should increase so will the density-dependent factors. Density-dependent
factors, such as: competition, predation, disease, and space limitations will
increase in direct proportion as to the increase the density of the population.
Competition – is an interaction in which both competing populations lose
access to some resources.
Interspecific Competition – is competition between members of different
species.
Intraspecific Competition – is competition between members of the same
population for resources.
Predation – is the interaction between predator and prey.
Competition is a density-population factor where the more the individuals
there are the greater the competition for resources for survival. When
resources decrease, the greater the increase in competition between
individuals, but if the resource stay the same but the population 9ncrease
there is still an increase in competition for those resources, such as: food,
water, space, and shelter.
Competition between different species is called Interspecific competition
and when the competition is between individuals of the same species it is
called Intraspecific competition.
Predation is a feeding relationship, where organism eats another organism.
Predation (increase competition) increases when there is more predators
(increase in density of predator) then prey available. Predation (decrease
competition) decreases when there is more prey then predators to eat them.
Disease and over crowding are density-dependent factors that only occur in
high-density populations, overcrowding occurs with an increase in
population in a limited space. Diseases in high density populations are more
easily to spread from one individual to another.
Figure 3: The effects of crowding on growth, size, and survival: a) tadpoles of the frog
grew faster and reached larger adult body size at low densities than at high densities. b)
the annual dune grass was smaller when plants were grown at high density. Density effect
became more accentuated through time as the plants grew larger.
Not Crowded Enough: The Allee Effect
There are some situations where the population is not large enough to
sustain itself. This is known as the Allee Effect, where some plants and
animals which reach a population size too small (density) so they will see a
decrease in their per capita growth rate, the population will continue to
decrease in size, which in turn will lead to an increase in extinction rate of
that species, a decrease in individual fitness.
Allee Effect – is a density-dependent phenomenon that occurs when a
population cannot survive or fails to reproduce enough to offset mortality
once the population density is to low.
Small populations are at a greater risk of extinction due to normal variations
in natality and mortality, resource limitations, habitat availability, predation,
competition, and catastrophic events, such as forest fires and flooding. The
minimum number of individuals required to maintain the population over a
given time period no matter the environmental change or disaster, is known
as the minimum viable population size.
If the species population should drop below its minimum viable population
size, the chance of extinction will increase. The minimum viable population
size is only a prediction which allows ecologists predict targets for species
recovery and re-introduction programs.
Large mammals exist in low-densities and have low reproductive output. It
is difficult for them to recover when decreased population size reduces their
mating choices or opportunities. Changes in habitat quality and quantity can
indirectly affect their survival or reproduction abilities.
Minimum Viable Population Size - is the smallest population size that is
likely to survive both predictable and unpredictable environmental variation;
note that it is only a prediction.
Density-Independent Factors
Factors that may impact a population, no matter the size of the population
are known as density-independent factors. Such factors as weather, climate,
temperature fluctuations, and natural disasters can have a negative affect on
a species ability to survive within their habitat.