Mayfly distribution assessment

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

For details of a use of biotic indexes see the page on problems caused by fertilisers.
Ecosystems
Streams

FRESHWATER ECOLOGY

One of the major problems that ecologists are faced with is how many samples to take. One way of solving this is to plot what is known as a taxon accretion curve. The number of samples taken (x-axis) is plotted against the number of new species found (y-axis). In freshwater ecology the number of samples required to obtain 80% of the species from a site is generally considered to be acceptable. The shape of the taxon accretion curves also gives us valuable information about the differences between sites. A steep taxon accretion curve may indicate a less heterogeneous site than one with a shallow slope.

Species Diversity

Another way of comparing two sites is by comparing their species diversity. This can be done by simply comparing the number of species in the sites. However one site, while having a large number of species, may in fact be dominated by large numbers of one species. A site with a more even spread of species might be considered to have a greater species diversity. A simple species diversity index is the Berger-Parker Dominance Index.

Bn = Nmax/n

Nmax = number of individuals of the most common taxa. n = total no. of individuals   You also need to know the Simpson index.

Similarity Indexes

These are used to compare sites on the basis of how many species they have in common. A simple one is the Sorensen's Index.

S = 2 c/ (A+B)

C= no. of species in common. A and B are the two sites. These can be used in a more complex analysis of a number of sites called Average Linkage Cluster Analysis.

 

Biotic Indexes

These are used to compare the water quality of different sites based on the principle that some species are characteristic of clean water. The BMWP scoring system is the one currently used by the Environment Agency. These techniques to can be used to compare different freshwater sites. However good identification skills are important in order to get any of these techniques to work.