General Ecology: Lecture 18
November 16, 2005
I.
Community Structure (Chapter 20, focus also on stream ecology)
A.
The community: defined
1.
Collectively, the populations of organisms that share the same environments
and habitats, and that interact with each other either directly or indirectly.
a.
Similar species are often grouped together within guilds

Example: Filter-feeding aquatic insects such as...
B.
Physical structure
1.
Vertical stratification
a.
Stream example (be able to briefly describe how these are stratified)

Temperature

Oxygen

Flow rates

Organisms

Examples of surface, midwater and bottom-dwelling
organisms
b.
NOTE: Lakes (and the ocean) have even more pronounced
stratification, especially in the summer [ Fig. 30.2]
2.
Horizontal structure: The river continuum. Along the length of the river,
changes occur (be able to briefly describe differences in these factors along
the river continuum)
a.
Bottom substrate
b.
Input of leafy matter as a primary food source
c.
Size of the organic particulates
d.
Primary production by in-stream autotrophs
e.
Temperature
f.
Shade
g.
NOTE: All these differences in turn affect the distribution of stream
organisms [Fig. 30.21]
C.
Biological Structure
1.
Species dominance
a.
May be dominant in number and/or impact on the community.

What is dominant in our stream sample.

Lampreys and fish populations (freshwater).

Keystone predators (later in lecture)
2.
Species diversity
a.
Number of different species (or families)

Good for a first approximation.
b.
The Shannon-Weiner index: based in probability theory, provides a
more fine-tuned measure

Takes into account two factors

total number of species (or families, etc…)

II.
evenness, or relative abundance, of the groups

For a given number of families, higher diversity
index is obtained if there are approximately equal
numbers of individuals for each group rather than
dominance by a single group
3.
Species abundance [Fig. 20.7]
a.
Can calculate a “rank abundance” to see whether a community is
dominated by just a few species (even though there may be many) or
whether there is more evenness

Compare to Shannon-Weiner index.

Note that we are doing a “crude” version of this as part
of the IBI calculation (% dominance)
Influences of population interactions on community structure/food webs and
community structure
A.
Overview
B.
Competition and community structure
1.
Examples:
a.
Zebra mussels vs. local bivalves in rivers and lakes

Change caused by zebra mussels?
b.
Himalayan blackberry

Change caused by Himalayan blackberries?
c.
Why do introduced species do so well?
2.
“Bottom-up” hypothesis of diversity
a.
Defined: The abundance and diversity of primary producers (the
“bottom”) affects the diversity at higher levels of the food web.
b.
Stream-related hypothesis: Diversity in different parts of a river is, at
least in part, a consequence of the amount and diversity of primary
producers.

Part 1: High diversity in the mid-reaches occurs because all
major sources of primary production are generally found in this
part of a stream. This gives rise to a diversity of primary and
secondary consumers because there are more feeding niches.

Examples of these feeding niches

Part 2: High diversity in the mid-reaches occurs because the
higher levels of primary production, in total, reduces
competition among species
C.
Predation and community structure
1.
Top-down hypothesis of diversity:
a.
Defined: levels of diversity are controlled by predators
b.
Classic example: Pisaster ochraceus, the ochre star (native
inhabitant), is a keystone predator in the community (Bob Payne)

How does Pisaster’s presence maintain/increase
diversity? (HINT: Two major mechanisms)
c.
Example 2: Introduced predators may decrease diversity. This
example is pertinent to our studies of stream ecology.
Invading Trout Reduce Forest Spiders by Altering the Stream Food
Web That Supplies Their Prey (excerpt from Ecological Society of
America. Study published in the journal “Ecology”, Nov. 8, 2004.)
A team of researchers from the U.S. and Japan have shown that exotic
species can have strong effects that degrade not only the ecosystems they
invade, but also spread to adjacent ecosystems as well. Colden Baxter, Kurt
Fausch and Phillip Chapman from Colorado State University collaborated
with Masashi Murakami of Hokkaido University to conduct a large
experiment in a watershed of northern Japan. The study showed that nonnative rainbow trout usurped forest insects that fell into the stream, depriving
native trout of more than 80% of their diet. This forced the native fish to
feed primarily on insect larvae that live on the stream bottom, which
decreased adult insects emerging from the stream to the forest. In turn, this
caused a 65% reduction in forest spiders that, like other forest-dwellers such
as birds and bats, prey on insects emerging from the stream. This research
demonstrates that species invasions can decouple critical connections
between ecosystems like streams and forests and have strong effects that
propagate across their boundaries to generate ecological surprises
D.
E.
Parasitism
1.
Avian malaria in Hawaii (brought by introduced mosquitoes)
2.
Tumors on fish locally known to be caused by parasites (trematode parasites
in Newburg Pool fish).
Mutualism
1.
Coral reef communities.
Study questions
1.
What is a community?
2.
How are communities physically structured? List/describe the two types of physical
structuring discussed in the lecture and text, and use examples from streams/rivers to
illustrate your answer.
3.
Be able to describe several changes that occur along a river in both physical structure and
primary productivity from the headwaters to the mouth (i.e. the river continuum).
4.
What are the major measurements used to describe the biological structure of a
community?
5.
What is a dominant species? Can a species be dominant even if it is not abundant?
Explain.
6.
What aspects of species diversity does the Shannon-Weiner diversity index take into
account?
7.
What information can “rank abundance” give you that the Shannon-Weiner diversity
index does not?
8.
Provide an example where competition has clearly shaped biological structure. Name the
competitor(s) involved and how the community was affected.
9.
Why do introduced species often fare so well in competitive interactions with native
species?
10.
Use the example of the river continuum to explain the bottom-up hypothesis of
community diversity. Also, explain how the bottom-up hypothesis is related to
competition.
11.
Use the example of Pisaster ochraceus in the intertidal to explain the top-down hypothesis
if community diversity.
12.
Provide an example where parasitism has clearly impacted a community. Name the
parasite involved and how the community was affected.
13.
Is there any evidence, locally, of parasites impacting stream communities? If so, describe.
14.
Discuss whether the bottom-up and top-down hypotheses are mutually exclusive. In other
words, do you think it would be possible to have both bottom-up and top-down effects
occurring at the same time? Think specifically about the stream community and how this
might (or might not) work. [NOTE: This question was not explicitly covered in class.
Requires your own creative thinking!]
15.
Explain how it was possible for an aquatic predator, the non-native trout, to cause a
reduction of spiders. Be sure follow the food web links all the way from the trout to the
spider.
16.
In what other ways are the stream and terrestrial ecosystem linked? Think about the
physical environment as well as the organisms. Also, try to think of organisms we did not
include in our studies.