Transcript Population Dynamics Population dynamics • The pattern of any process, or the interrelationship of phenomena, which affects growth or change within a population. •

Population Dynamics
Population dynamics
• The pattern of any process, or the
interrelationship of phenomena, which affects
growth or change within a population.
• The study of the factors that affect the
growth, stability, and decline of populations, as
well as the interactions of those factors.
• A term that describes the ways in which a given
population's numbers grow and shrink over time,
as controlled by birth, death, and emigration or
immigration.
A wildlife population is a group of
individuals of the same species that
have some basis of commonality.
Populations can be linked to a feature in
the landscape, to other populations, or a
time period for example.
All populations have the potential
for exponential growth
All populations have the potential for
explosive growth i.e. the growth of a
population without any constraints;
therefore, the population will grow at an
ever-increasing rate.
naturalsciences.sdsu.edu/classes/lab2.7
/glossary.html
But typically, a population in an
environment stops growing exponentially
long before it reaches the environmental
carrying capacity.
What reasons can you think of ?
Population growth is limited by abiotic and
biotic interactions.
Limiting factors
Are those elements that prevent a population
from attaining its biotic potential (maximum
growth rate)
Limiting factors are categorized into densitydependent and density- independent factors
Density dependant factors
Predation - predators may be attracted
to areas with higher densities of their
prey.
Competition for resources - depletion
of food supply = poorer nutrition =
increased death rates = decreased birth
rates.
Parasitism
Infectious disease - diseases can
spread more easily in dense populations
than in sparse populations.
Density independent factors
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hurricanes
floods
fire
pollution
habitat destruction
Age Structure
Lifespan
Sex Ratio
Natality & Mortality
Interspecific Dynamics
Intraspecific Dynamics
Territoriality & Home Range
Dispersal
Carrying Capacity
Age Structure
The proportional amounts of young and old
age classes reveal much about a
population.
There should be some kind of balance
among the classes and the "proper" balance
will vary by species and season.
Generally, the age structure can be
depicted by a triangle, with the numerous
young on the bottom and very few older
individuals at the top.
"Age" might be measured in years, weeks,
or days, depending upon the species
considered.
At the end of the food-rich season, the
youngest age classes are usually swollen.
The winter will kill many individuals, but
usually the young and very old experience
the highest mortality rates.
Humans sometimes have a strong impact on
the age structure of a population.
White-tailed deer have few animals
beyond 4.5 or 5.5 years largely because
of hunting pressure, although an
individual is capable of living a decade or
more.
A heavily fished lake may reduce the
number of sizable (older) adults to the
point where breeding might be reduced.
Lifespan
Obviously, different species have
different life spans.
Most insects complete their life cycles
during the warm season.
Some have multiple generations during that
time.
Other species live for years and
individuals must have adaptations and
adequate habitat to survive regular periods
of food-shortages and inclement weather.
Species toward the end of food chains
are usually much longer-lived that those
in the beginning. Long-lived species
have strategies that favour the survival
of fewer individuals. Shorter-lived
species generally utilize the opposite
strategy (r & K strategists)
The combination of lifespan and age
structure reveal much about the general
health of a population, either a wildlife
population or a stand of trees.
Sex Ratio
Each species has an "ideal" sex ratio.
Usually this is somewhere around 50:50, but not
necessarily.
Honeybees, for example, have almost no males.
A particular sex ratio will help maximize
"fecundity", or the ability of a species to
produce new individuals.
Males of some species will mate with as many
females as possible.
Other species, such as swans and geese, tend
to be more monogamous.
Natality and Mortality
Natality is the inherent ability of a
population to increase in numbers.
Mortality deals with the level of death
within a population.
These terms are usually expressed as
rates that reflect pressures to increase
and decrease population size.
The size of a population is impacted by
many factors, which vary over time.
At a particular point in time, natality
factors or mortality factors may
dominate, causing a population to
increase or decrease.
Some factors are fairly predictable,
such as the average clutch/litter size or
the onset of winter.
Other factors, such as extreme
weather events or disease epidemics,
can have great impacts but are not
predictable.
Interspecific Dynamics
These are relationships among or between
species.
The predator-prey relationship is a wellknown example of an interspecfic
dynamics.
Interspecific dynamics can be antagonistic
or beneficial. Lichens are two species (an
algae and a fungus) working in concert to
the benefit of both.
This is called a "mutualistic" relationship.
A "commensal" relationship is where one
species requires another, but the host
is relatively unaffected.
Another kind of interspecific
relationship would be parasitic.
Mosquitoes draw blood essential to the
completion of their life cycle, at the
expense of another species.
Species that require something from
another species are termed "obligate".
When the relationship is beneficial but
not required, it is termed facultative".
Intraspecific Dynamics
There are relationships among
individuals of a population.
Competition for food, shelter, and other
requirements are common examples.
Mating and establishing territories are
other examples.
A species might be colonial in nature or
live primarily as individuals.
There are many life strategies.
Territoriality and Home Range
An individual or population of a species may
actively mark and/or defend a particular area.
The male robin that challenges anything
resembling another male robin is expressing
"territoriality".
A "home range" is the amount of space an
animal needs acquire the resources to meet its
needs.
A predator such as a wolf may have a home
range of many square miles, while an earthworm
has almost none.
The amount of area for either a territory
or home range is not necessarily
constant. It often varies with the season.
After the breeding season, male robins
resume a gregarious nature.
Ruffed grouse will "expel" their young
before the onset of winter because winter
home ranges are larger than summer home
ranges.
The young animals must seek their own new
habitat.
Dispersal
Dispersal is movement of individuals or
their offspring into or out of an area.
Dispersal allows individuals to colonize
new areas of crop fields.
Dispersal, along with birth and death
rates, regulates population size, and
plays an important role in evolution
through mixing of genes between
populations.
Dispersal is accomplished through
immigration (movement into a
population), emigration (movement out
of a population) or migration (frequent
movement into or out of a population
area).
Winters and dry seasons result in less
available food and water.
Animals have a wide range of strategies to
accommodate these seasonal fluctuations.
Migration is one such strategy.
Autumn bird migration is the most
familiar. Many species of birds fly south
more because of food shortages, rather
than cold temperatures.
Carrying Capacity
The physical and biological resources of
an area, varying with the season, will
support only so many individuals.
This maximum amount called the
"carrying capacity".
When most species approach their
particular carrying capacity, mortality
factors overtake natality factors and
the population growth declines.
For some species, this ecological
balancing-act is fairly regulated without
great fluctuations.
With other species, there is a normal
"boom and bust" cycle.
Ruffed grouse and snowshoe hare
populations are good examples.
There are a few species that can
maintain high population densities long
enough to actually damage their habitat
and substantially reduce the carrying
capacity.
Deer and moose are classic examples of
species than can damage their habitat.
Humans may very well fall into this
category, as well.
Life tables & survivorship
curves
A survivorship curve is a graph showing
the number or proportion of individuals
surviving at each age for a given species or
group (e.g. males/females).
Survivorship curves can be constructed for
a given cohort (a group of individuals of
roughly the same age) based on a life
table.
Type I: Convex
The line of the Type I survivorship curve begins
high on the graph and continues across all young
and middle-age groups. It indicates a low death
rate of young and middle-aged individuals. A
large proportion of individuals of each age
survive to the next age; thus, a large proportion
live to old age. The curve drops steeply as
individuals reach old age, because mortality
becomes high at old age. This type of curve is
associated with humans and other large
mammals, that produce few offspring during
their lifetimes. The young are dependent on
parental care for an extended period of time.
Type II: Linear
Type II survivorship curves exhibit a constant,
downward slope. The same proportion of the
population survives at all ages. Type II is the
intermediate between Type I and Type III
curves, indicating a constant mortality rate and
birth rate. Type II is associated with many
small mammals, birds, various invertebrates,
some reptiles and annual plants follow this
pattern of survival. Young are dependent upon
parental care, but for a shorter period of time.
Type III: Concave
Type III is the opposite of Type I. The curve
begins with an immediate, steep, downward slope,
and levels off while individuals are still young. In
these populations, mortality remains high among the
very young. The shallow slope continues through old
age. This type of survivorship curve indicates that
if individuals survive their youth, they will likely
survive to old age. Type III curves are associated
with fish and other organisms that produce
thousands of young, but provide little to no parental
care.
• Type I – most mortality among older
individuals
• Type II – constant rate of mortality
• Type III – highest mortality in young