LECTURES FOR ZOO 1010—CHAPTER 2
Animal Ecology
BASED ON HICKMAN ET AL. ANIMAL DIVERSITY
3rd Edition
Chapter Prologue—Every Species Has Its Niche: Earth’s biomass is organized into a
hierarchy of interacting units: an individual organism, a population, a community,
and finally an ecosystem, the most complex of all natural systems. The place where
an animal lives is its habitat, what it does in that habitat is its niche. A niche is
subject to evolution and, once established, no other species in the community can
evolve to exploit exactly the same resources. This phenomenon is explained by the
competitive exclusion principle, i.e., that no two species will occupy the same niche.
Different species are able, therefore, to form an ecological community in which each
has a different role in their shared environment.
Ernst Haeckel introduced the term ecology in the mid-19th century, which is defined
as the “relation of the animal to its organic as well as inorganic environment.”
Ecology is not longer restricted animals and today it is a highly synthetic science that
incorporates everything we know about behavior, physiology, genetics, and evolution
of animals to study interactions between populations of animals and their
environments. The major goal of ecological studies is to understand how these
diverse interactions determine geographical distribution and abundance of animal
populations. Such knowledge is crucial for ensuring continued survival of many
populations when their natural environments are altered by human activity.
The Hierarchy of Ecology:
Ecology is studied as a hierarchy of biological systems in interactions with their
environments. This hierarchy extends from the individual organism, to the
population, community, ecosystem, and biosphere. At the community level,
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species interact in a variety of ways, such as predation, parasitism, and
competition. At the ecosystem level, communities of organisms interact with the
abiotic or nonliving environment through two principal processes, energy flow
and materials cycling.
An animals environment is composed of all conditions that directly affect it chances
for survival and reproduction. Environmental factors directly used by an animal
are called resources, and are either expendable or nonexpendable. The physical
space where an animal lives is its habitat. Animals of any species have certain
environmental limits of temperature, moisture, and food within which they can
grow, reproduce, and survive. A suitable environment must simultaneously meet
all the requirements for life. These environmental limits collectively define the
animal’s niche. A distinction is made between an animal’s fundamental niche,
which defines its potential role, and its realized niche, the subset of potentially
suitable environments that an animal actually experiences.
Animal populations consist of demes of interbreeding members sharing a common
gene pool. Each population or deme has a characteristic age structure, sex ratio,
and growth rate.
Each new generation of biparental metazoan animals begins with a cohort of
individuals born at the same time.
Animal species have different characteristic patterns of survivorship from birth until
death of the last member of a cohort. There are three principal types of
survivorship.
Populations of animal contining multiple cohorts exhibit age structure.
Population growth is the difference between rates of birth and death. All populations
have an inherent ability to grow exponentially. This ability is called the intrinsic
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rate of increase or r. Population growth may be regulated intrinsically by the
carrying capacity of the environment, extrinsically by competition between
species for a limiting resource, or by predators or parasites. Density independent
abiotic factors can limits, but not truly regulate, population growth.
Communities consist of populations that interact with one another in any of several
ways, including competition, predation, parasitism, commensalisms, and
mutualism. These relationships are results of coevolution among populations
within communities. Guilds of species avoid competitive exclusion by character
displacement, the partitioning of limited resources by morphological
specialization. Keystone predators are those that control community structure
and reduce competition among prey, which increases species diversity. Parasites
and their hosts evolve a benign relationship that ensures their coexistence.
Ecosystems consist of communities and their abiotic environments. Animals occupy
the trophic levels of herbivorous and carnivorous consumers within ecosystems.
All organisms have an energy budget consisting of gross and net productivity,
and respiration. For animals, respiration usually is at least 90% of this budget.
Thus, transfer of energy from one trophic level to another is limited to about 10%,
which in turn limits the number of trophic levels in an ecosystem. Ecological
pyramids of energy depict how productivity decreases in successively higher
trophic levels of food webs.
Ecosystem productivity is a result of energy flow and material cycles within
ecosystems. All energy is lost as heat, but nutrients and other materials including
pesticides are recycled. No ecosystem, including the global biosphere, is closed
because they all depend upon imports and exports of energy and materials from
outside.