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Chapter 21
On the Tracks of Wolves and Moose
Young Gray Wolf-Romeo
• Every summer and a few weeks in winter,
scientist investigate the Isle Royale’s pack of
wolves and herd of moose
• Studies for almost 50 years, it is the longestrunning predator prey study in the world.
– Scientists would like to understand the dynamic
fluctuations of the wolves and moose.
– They are trying to learn about ecology
• Ecology is the study of the interactions
between organisms, and between organisms
and their nonliving environment (land, water,
– The Isle Royale is a great place to study ecology
– Uninhabited by humans, protected national park,
undistributed by hunting, logging, settlement
– No other predators or prey on the island
– Island just the right size to study ecology
• Ecologists can study
organisms at a
number of levels:
the individual, the
population, the
community, and the
• At the individual level, ecologists can study
how an individual organisms fares in its
• At the population level, ecologists study a
group of individuals of the same species living
and interacting in the same region.
• At the community level, ecologists study
interacting populations of different species.
• At the ecosystem level, ecologists study all the
living organisms in an area and the nonliving
components of the environment with which
they interact.
• Ecology is a multidisciplinary science. It draws
not only on many areas of biology but also on
many other branches of science, including
geography and meteorology as well as
– Living organism in an area and the nonliving
components of the environment with which they
The study
• Vucetich is a population ecologist
• Interested in the number of wolves and
• Much of the counting is done by air.
– Wolves are easy to find
– Moose are solitary animals and dark colored
• Count a few moose and extrapolate for the entire island
Distribution patterns
• Moose are relatively solitary creatures
– Different types of distribution patterns
Random Distribution for moose
– Random dispersion of individual roaming moose
on the island.
– The moose are hard to count for research studies.
– One type of distribution pattern
• The distribution pattern, or the way that
organisms are distributed in geographic space,
depends on resources and interactions with other
members of the population.
– For moose, being solitary may help protect them from
predation – harder to see in forest
– A truly random distribution is rare in nature
– Resources may be unevenly distributed, etc.,
– Distribution pattern are different for wolves and
moose- wolves clump
Distribution patterns
• Distribution patterns generally reflect behavioral or
ecological adaptations. Three different types:
Distribution patterns
• A random
distribution may
individuals to
maximize their
access to
Distribution patterns
• A clumped
distribution may
result when
resources are
distributed across
the landscape, or
when social
behavior dictates
Distribution patterns
• Uniform
usually results
from territorial
• Few Moose swam to Isle Royale in 1900
• Moose population exploded on the Isle Royale
due to abundant food supply and no natural
• By 1920, more than a 1000
• Rapid increase in growth rate
Population growth
• The growth rate of a population is the difference
between the birth rate and the death rate of a given
Population growth
• Ecologists describe two general types of population
growth: exponential growth and logistic growth.
Population growth
• Exponential growth is the unrestricted growth
of a population increasing at a constant
growth rate.
• When a population is growing exponentially, it
increases by a certain fixed percentage every
generation. Increase 20% each year, for
Population growth
• Unrestricted growth is rarely, if ever, found
unchecked in nature.
• As populations increase in numbers, various
environmental factors such as food availability
and access to habitat limit an organism’s
ability to reproduce.
Population growth
• When populationlimiting factors slow
the growth rate, the
result is logistic
growth―a pattern
of growth that starts
rapidly and then
• Disease, food
Population growth
• Eventually, after a period of rapid growth, the
size of the population may level off and stop
• At this point, the population has reached the
environment’s carrying capacity ―the
maximum number of individuals that an
environment can support given its space and
• Moose arrived in 1900
• By 1929 they had eaten most of the
• Moose exceeded the island’s carrying capacity
in 1935.
• The population dropped due to starvation
– What happened next to help the moose?
– When did the wolves arrive?
Population growth
• Carrying capacity places an upper limit on the
size of any population;
• No natural population can grow exponentially
forever without eventually reaching a point at
which resource scarcity and other factors limit
population growth.
Population growth
• Ecologists use a
variety of data to
monitor the health of
– Moose dropping
– Moose bones
– Wolf scat
– Urine –soaked snow
Population growth
• Population density is the number of organisms
per given area. Wolves kill moose
– As the population density of a species increases,
individuals of that species may face food shortages.
• This is an example of a density-dependent
factor―a factor whose influence on population
size and growth depends on the number and
crowding of individuals in the population.
– Wolf predation and plant abundance have greater
effect when moose population is large
Population growth
Biotic refers to the living components of an
environment. Not all density-dependent
factors are biotic.
– Food, predators, diseases
• Nonliving, or abiotic, factors like weather and
habitat can also influence population size in a
density-dependent manner.
Population growth
• Density-independent factors take a toll on the
population no matter how large or small the
population is.
• Most, but not all, density-independent factors
are abiotic.