Population Ecology. 54 Population Ecology. 54.1 How Do Ecologists Study Populations? 54.2 How Do Ecological Conditions Affect Life Histories? 54.3 What Factors Influence Population Densities? 54.4 How Do Spatially Variable Environments Influence Population Dynamics?
all the individuals of a species in a given area.
Population structure describes the age distribution of individuals, and how those individuals are spread over the environment.
The number of individuals per unit area or volume is the population density.
Density has strong influence over how individuals react with one another and with populations of other species.
Change the pop density
# and density of individuals
rates of demographic events
locations of individuals
physiological & environmental data
Figure 54.2 Hydrogen Isotopes Tell Where Migratory American Redstarts Molted Their FeathersWhat are molecular marker examples? Useful?
H isotopes -- where American redstarts molt during migrations
Isotopes in feathers reflect the latitude at which the feathers grew
strong latitudinal gradient of these isotopes in precipitation
Mobile animals: capture, mark, recapture method
Proportion of marked individuals in the new sample is used to estimate population size:
ONLY if marked individuals randomly mix with the unmarked ones, and both are equally likely to be captured
Some animals learn to avoid traps, or learn that traps provide food and become “trap-happy.”
Table 54.1 Life Table of the 1978 Cohort of the Cactus Finch on Isla Daphne (Part 1)
life table:track a group of individuals born @ same time (cohort)
Survivorship: # still alive at later dates
fecundity: # offspring produced in a time interval
Table 54.1 Life Table of the 1978 Cohort of the Cactus Finch on Isla Daphne (Part 2)
For cactus finch, mortality rate was high during the first year, then dropped.
Mortality rate fluctuated year to year because the birds are dependent on seed production, which fluctuates with rainfall.
Figure 54.3 Survivorship Curves: survivorship in relation to age---3 MAIN PATTERNS!
Figure 54.4 Age Distributions (Age Structure Diagrams) Change over Time
average family size increased from 2.5 to 3.8 children
describes how it allocates time and energy among the various activities throughout its life
can vary dramatically
single offspring per reproductive episode....or lots!
Some reproduce only once and then die (salmon, agave)
Why are wildlife
Black rockfish females continue to grow throughout their lives
large females produce many more eggs than small ones
Eggs from older females contain oil droplets that are food for developing fish, so offspring have abetter chance for survival.
Intensive fishing off Oregon from 1996–1999 reduced average age of females from 9.5 to 6.5 years.
Age reduction decreased number of eggs produced and average growth rates of offspring.
Maintaining populations of this species may require no-fishing zones where females can grow to large sizes.
Influence of predation
If predator fish are excluded (by waterfalls) fish have much lower mortality rates
When reared in the lab, guppies from the high predation site matured earlier, produced more eggs, and produced more offspring per brood.
ΔN/Δt = rate of change of pop over time
r = net reproductive rate
Over short time periods, close to rmax
northern elephant seals were hunted to near extinction:
populations grew exponentially on some islands after hunting was stopped
The environmental carrying capacity (K) # of any particular species that can be supported in an environment
Environmental limits birth rates decrease, death rates increase
availability of resources
Diseases and parasites
S-shaped curve = logistic growth
Growth stops when N = K
pop density increases food supplies may be depleted
Predators may be attracted to high densities of prey, increasing death rate.
Diseases can spread more easily.
Figure 54.9 Regulation of an Island Population of Song Sparrows (Part 1)
Figure 54.9 Regulation of an Island Population of Song Sparrows (Part 2)
Figure 54.9 Regulation of an Island Population of Song Sparrows (Part 3)
12 yr study
Population size fluctuated significantly.
Death rates high during cold, snowy winters, regardless of population density.
# breeding males limited by territorial behavior
more breeding females fewer offspring each one fledged
more birds alive in autumn less chance juveniles would survive the winter
More stable population # in species with long-lived individuals and low reproductive rates.
Insect pops fluctuate more than birds and mammals.
Environmental factors can change carrying capacity for species.
Figure 54.10 Individuals Born during Years of Good Reproduction May Dominate Populations (1)
Figure 54.10 Individuals Born during Years of Good Reproduction May Dominate Populations (2)
likely to fluctuate
boreal forests: many birds and mammals eat conifer seeds.
trees reproduce synchronously and episodically
Mortality rates can be high in years with poor seed production.
Size of individuals
Length of time a species has lived in an area
small body size higher pop densities
require less energy to survive vs larger
mammal species worldwide
No natural pathogens and predators very high population densities
Zebra mussels were introduced to the Great Lakes in 1985.
They spread rapidly and reached densities much higher than in their native Europe.
Giant sequoias are restricted to a few groves in the southern Sierra Nevada but Douglas firs are widespread and abundant
Numbers of births and growth of individuals tend to be highest when population is below carrying capacity.
If humans wish to maximize the number of individuals harvested from a population, we should try to maintain it below carrying capacity.
Hunting seasons are established with this goal in mind.
In fast reproducing populations,
harvest rates can be high.
Growth rates of individuals are often density-dependent, so harvesting pre-reproductive individuals allows others to grow faster.
Some fish populations can be harvested on a sustained basis because
a few females can produce enough eggs to maintain the population!!!
Many fish have been overharvested and population sizes reduced.
Cod and haddock on Georges bank were so heavily exploited that fishing had to be stopped to allow populations to recover.
Whaling has also resulted in declining populations.
Most whale populations have failed to recover.
Whales are large animals with slow reproductive rates. Many adults are needed to produce a small number of offspring.
The International Whaling Commission was established to guide recovery of whale populations.
Member countries voted to ban all commercial whaling, but some members now lobby to restore harvest of non-endangered species.
Lack of a market for whale meat may in the end cause the demise of commercial whaling.
Humans wish to decrease the size of populations of many pest species.
Reducing population numbers below carrying capacity stimulates higher birth rates and growth of the population.
A more effective approach is to remove the resources for the population, (e.g., making garbage unavailable for rats).
Humans introduce other species to control pests, such as the cactus moth to control Opuntia cacti in Australia.
Sometimes the introduced predator or parasite fails to control the pest; or worse, begins to attack other species.
Toads were introduced into Australia to control cane beetles in sugar cane fields.
The toads couldn’t reach the beetles high on the sugar cane plants, but have been an ecological disaster for other species.
They are poisonous, reproduce quickly, and outcompete native amphibians.
The size of the human population now contributes to most environmental problems.
Human social organization and specialization has allowed us to increase the carrying capacity for humans.
Earth’s current carrying capacity for humans
biosphere’s ability to absorb our by-products, especially CO2 from fossil fuels;
our willingness to cause extinction of other species to accommodate our increasing use of Earth’s resources.