BIOS 3010: ECOLOGY

Laboratory 12: Stream diversity

Dr. Stephen Malcolm

Heptageniid mayfly nymph

Ephemera danica adult

Streams are important and diverse aquatic habitats that provide links and flow of resources among both aquatic and terrestrial ecosystems.

Stonefly adult and nymph (Plecoptera)

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.

Stonefly nymph and adult (Plecoptera).

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.

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.

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:

A B C

Species 1 10 20 90

Species 2 10 20 6

Species 3 10 20 2

Species 4 10 20 1

Species 5 10 20 1

Species 6 10

Species 7 10

Species 8 10

Species 9 10

Species 10 10

Total Number: 100 100 100

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/(Sp_i²) where p_i = the proportion of all individuals that belongs to the i^th species

The Shannon-Wiener Index, abbreviated H’, is calculated as

H' = -Spilnpi where p_i = the proportion of all individuals that belongs to the i^th species

lnp_i = the natural log of p_i

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?