BIOS 3010: ECOLOGY
Laboratory 12: Stream diversity
Dr. Stephen Malcolm
Heptageniid mayfly nymph
Stonefly adult and nymph (Plecoptera)
Ephemera danica adult
Streams are important and diverse
aquatic habitats that provide links and
flow of resources among both aquatic
and terrestrial ecosystems.
BIOS 3010: Ecology
Streams provide a ready source
of water, which can be used for irrigation
and drinking. In addition, when they are
dammed, streams can provide a
powerful source of energy, which we
sometimes harvest as electricity.
Lab 12: Stream Diversity
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Our interest in streams arises from
their
importance
as
ecological
communities. Streams contain essential
habitats for a wide variety of aquatic
species. Most of us are aware of the
larger vertebrate species that live in and
around streams, such as fish and some
birds, but fewer of us appreciate the
diversity of invertebrates living in stream
habitats.
Stonefly nymph and adult (Plecoptera).
Many of these invertebrates, of course, constitute major prey species of fish
(which are themselves preyed upon by birds such as kingfishers and herons).
Therefore, the health of a stream habitat will be determined in large part by the
abundance and diversity of invertebrates it contains.
Common stream invertebrates include members of many insect orders, including
Diptera (flies), Ephemeroptera (mayflies), Trichoptera (caddisflies), Odonata
(dragonflies), Coleoptera (beetles), Hemiptera (true bugs), Plecoptera (stoneflies) and
even a few Lepidoptera (butterflies and moths). For many species, only the immature
larval stages are aquatic; adults are winged and may disperse far from the stream
habitat. Therefore, many of these species constitute a means by which organic matter
in streams (usually in the form of fallen leaves) is returned to terrestrial habitats.
There are three main goals of today’s laboratory:
(1) First, this is an opportunity for you to become familiar with some organisms
that you might otherwise never meet.
(2) Second, we will use the data we gather to explore several methods of
quantifying community diversity.
(3) Third, if several types of habitats are available within the stream, we will be
able to ask whether some types of stream habitats contribute more than
others to overall stream biodiversity.
BIOS 3010: Ecology
Lab 12: Stream Diversity
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MEASURES OF SPECIES DIVERSITY
“Diversity” is at the same time an intuitively obvious concept but also one that is
difficult to define. Ecologists have been arguing for decades about the best measure of
biological diversity, and will probably debate the subject for decades to come, especially
as humans impact biodiversity with greater intensity over time. Despite lack of
agreement as to the best way to quantify biodiversity, ecologists generally agree that
any measure of diversity should reflect at least two distinct qualities of the community:
(1) the number of species present, and
(2) the evenness of their relative abundances.
The number of species is called species richness, and is abbreviated S. The
evenness of relative abundances is called, strangely enough, evenness, and is usually
abbreviated E. Ecologists generally agree that a community with more species present
is more diverse than one with fewer species, and that a community with more even
relative abundances is more diverse than one with the same number of species but
much less even relative abundances. For instance, there would be general agreement
that Community A is more diverse than Community B, since it has more species and the
same evenness (all species present have equal relative abundances in both
communities). Community B is more diverse than Community C, since Community B
has the same number of species but much greater evenness.
Number of Individuals in Community:
Species 1
Species 2
Species 3
Species 4
Species 5
Species 6
Species 7
Species 8
Species 9
Species 10
Total Number:
BIOS 3010: Ecology
A
10
10
10
10
10
10
10
10
10
10
B
20
20
20
20
20
C
90
6
2
1
1
100
100
100
Lab 12: Stream Diversity
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But how do we generate a single measure that will express the diversity of a
community? There are currently two commonly used diversity indices:
(1) Simpson’s Index, and
(2) Shannon-Wiener Index (not Shannon-Weaver).
Both indices reflect both species richness and evenness.
Simpson’s Index, abbreviated D, is calculated as
D = 1/(Σpi2)
where pi = the proportion of all individuals that belongs to the ith
species
The Shannon-Wiener Index, abbreviated H’, is calculated as
H' = -Σpilnpi
where pi = the proportion of all individuals that belongs to the ith
species
lnpi = the natural log of pi
To convince yourself that both indices reflect species richness and evenness,
calculate D and H’ for the three communities described above.
Both indices use species as the unit of distinction, but we have already discussed
other possibilities. For example, we may judge a community to be more diverse if it
contains more feeding guilds (a group of related or unrelated species that make their
living in roughly the same way), regardless of the number of species present. Can you
think of other limitations of these diversity indices?
In today’s laboratory, we will sample a local stream community to determine:
1.
Whether the absolute abundances of invertebrate species differ among
habitats (e.g. riffles vs. cobble vs. organic sediment vs. logs); and,
2.
Whether the relative abundances of invertebrate species differ among
habitats.
Your data sheet contains written and pictorial descriptions of the most common
aquatic invertebrates. Use the dichotomous key and the pictures together to identify
your samples. We need to be accurate about our identifications.
By now, you should have a good idea of how we will analyze our data. Clearly,
we will compare relative abundances by Chi-square tests. How will we determine
whether absolute abundances differ among stream habitats? Does the choice of
statistical test determine how you should design the experiment?
BIOS 3010: Ecology
Lab 12: Stream Diversity
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