THINKING LAB
Shrinking Polar Bears and
Expanding Snow Geese
Background
An incredible variety of living things inhabits the biosphere.
This biotic diversity reflects the abiotic diversity of Earth.
Through the process of evolution, populations have evolved
adaptations that enhance their survival and reproductive
ability in diverse habitats. The result is the diversity of life.
During the last 100 years, scientists have reported
significant changes in the world’s ecosystems. While some
of these changes seem to have been beneficial for some
species, they seem to have had negative effects on others.
For example, dramatic changes have been observed in
populations of polar bears (Ursus maritimus) and lesser
snow geese (Chen caerulescens) that breed in Canada’s
arctic. Field surveys of polar bears have revealed that
populations are declining, fewer cubs are born each year,
and individual bears are smaller and weigh less than what
has been considered typical for members of this species. In
contrast, snow goose populations have tripled since 1968.
Their numbers are so high that the feeding activity of these
geese is causing substantial damage to the habitats where
they and other species breed and overwinter.
Why do you think these changes are happening? In
completing this assignment, you will draw on your
knowledge of the nature of ecology and evolution. You
might choose to work on one of these two species, and
then compare your findings with those of students who
worked on the other species.
You Try It
1.Describe the ecological niches of the polar bear and the
snow goose. Include diet, preferred habitat, time of
peak activity during a day, yearly activities such as
migration, and other relevant information.
2.Would you consider the polar bear to be a generalist
or a specialist? What about the snow goose? Explain
your answer.
3.To which trophic levels do the polar bear and snow
goose belong?
4.Describe the feeding, sensory, and locomotory
adaptations that improve the ability of each species to
survive and reproduce in its habitat and niche. What
features allow each species to cope with environmental
stresses (for example, temperature or moisture
extremes) in its habitat?
5.Draw a map illustrating the approximate range of each
species.
6.Find out as much as you can about the evolutionary
history of these species. What other species are they
most closely related to? When might they have
appeared as a species?
7.How might changes that currently appear to be
occurring in Earth’s climate affect the future evolution
of each of these species?
8.Estimate what chance each of these species has for
surviving for the next 100 years. Does either face
possible extinction? Why or why not?Chapter 13 Ecological Principles • MHR 441make the energy-rich molecules they need to fuel
their life processes. Instead, they must obtain these
molecules by consuming other organisms, either
autotrophs or other heterotrophs. Therefore, they
are referred to as consumers.
Herbivores that eat autotrophs are termed
primary consumers, since they are the first eaters
in the chain. On land, insects, snails, grazing
mammals, and birds and mammals that eat seeds
and fruit are the most common herbivores. In
aquatic ecosystems, this role is often taken by
heterotrophic protists, various types of small
invertebrate animals, and some species of fish.
Carnivores that eat mainly herbivores are
secondary consumers. Spiders, frogs, and insect-
eating birds are examples of secondary consumers.
In most ecosystems, these secondary consumers
are themselves eaten by other carnivores, which
are known as tertiary consumers(the third set
of eaters). There may also be higher levels of
consumers above these.
The members of another consumer group, often
referred to as decomposers, obtain their energy-rich
molecules by eating leftover or waste material
derived from all the trophic levels, including the
feces of living organisms, dead bodies, or body
parts (for example, fallen leaves). Decomposers are
very important to every ecosystem. Their role is to
break large molecules (that were once part of living
organisms) down into smaller ones and return them
to the abiotic environment. Thus, decomposers
return vitally important nutrient elements such as
carbon and nitrogen to the soil and air. These
materials can then be used again by autotrophs
to make new energy-rich organic molecules.
Decomposers are an ecosystem’s recyclers, ensuring
that the biosphere’s limited supply of required
nutrients is not lost.