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An Introduction to Ecology
(Selected concepts from Ch 18, 19, and 20)
Biology 160, Murkowski
We’ll use a few excerpts from these chapters to help orient us to an exciting area
of specialty within the field of biology, ecology! As you read, remember that the words in bold
are vocabulary terms I would expect you to understand in a quiz or exam. If they’re not clear
as you read, check your textbook, ask a colleague, or drop me a note!
Introduction: Once you’re familiar with the diversity of life,
you’re well-equipped to start asking questions about how
organisms interact with each other and the environment.
This is ecology! There are several levels at which you can
study ecology. I’ll highlight a few in the sections that
Levels of Ecology: Ecologists work at many different levels.
Some ecologists, for example, are concerned with
questions of organismal ecology. These folks want to know
how a particular species or population adapts to their
abiotic environment. This is a big area of study in these
days of rapid climate change! The ecologists tracking the
so-called “killer bees” are, for example, very interested in
how abiotic factors like temperature affect the spread of
these aggressive creatures!
Population ecology is the next level up. These ecologists study
the factors (both biotic and abiotic) that regulate populations
over time. The growth of populations can usually be
characterized by one of two types of growth curves,
exponential and logistic, as shown at left. (Ignore the
equations, unless you like that sort of thing!!) Exponential
growth is typical in an ideal environment with unlimited
resources. As you can imagine, this type of growth is pretty
rare in healthy ecosystems. Instead, populations usually start
to run out of resources, slowing their growth rate. This results in
a logistic growth model where the population gradually
approaches its carrying capacity (K). The carrying capacity is
the number of individuals in a population that the environment can sustainably support. When
we look around the natural world, we see that some populations over-shoot their carrying
capacity, causing sharp die-offs that bring population numbers back down. Others experience
a more gradual decrease in their growth rate, gently leveling off at their carrying capacity.
These reductions in a population’s growth rate can be caused by both density-dependent, and
density-independent factors. The density-dependent factors, as their name suggests, depend
on the number of individuals in the population. One of the most common forms of a densitydependent limit on growth is intraspecific competition. (Remember that the prefix “intra” tells
us within!) This is competition from other members within your species for the same limited
resource. Resources like food supply, nesting sites, predation, and even toxic waste products
can all limit a population’s growth in a density-dependent fashion. In contrast, densityindependent factors limit a population’s growth regardless of the size of that population.
Weather is often a factor as either heat or cold can reduce a population’s growth regardless of
its size (as can floods, fires, and other natural disasters).
This observation begs the question, “where are
humans at in terms of our carrying capacity?”
There are lots of ways to begin to ask this
question. One common approach to answering
this question is to begin to quantify the amount of
resources used by each of us, and compare it to
the amount of resources available in a given
area. As the image at right shows, much of the
world, including those of us in the USA are above
the “break-even” red line, meaning we use more
resources than our environment can sustainably
This is, of course, an average. There are lots of
factors that contribute to your overall resource use. It can be an eye-opening experience to
calculate your own “ecological footprint” and see how you stack up based on your everyday
choices! We’ll try an example of this on our in-class activity this week!
A Few Other Final Thoughts: Our ecology lab introduced several other important ecological
concepts, including trophic levels, energy pyramids, and a variety of ecological roles species
can play in their community. I encourage you to read through the handout a second time and
make sure these roles are clear:
Keystone Species: This is a term we borrow from architecture. It
describes the uppermost stone in an arch. As you can probably
imagine from the image at left, if you remove this stone, the entire arch
collapses. In ecology, we use this idea to describe not the most
abundant species, but one whose presence is required to stabilize the
entire species. Hint: As you think back on the Yellowstone video,
consider how many different species were impacted by the
reintroduction of the wolves.
Foundation Species: Ecologists use this term to describe species that
have a disproportionate role in supporting other members of the
community. You can also often think of these species as literally
providing a foundation (the habitat) for a wide variety of other species
in the community. If you think about our little friend on the left, for
example, his presence creates freshwater ponds, marshes, and
wetlands that provide habitat for an enormous assortment of other
plants and animals.