The relative Mean Species Abundance of originally occurring species (MSA) is an indicator describing biodiversity changes with reference to the original state of ecosystems. It is defined as the average abundances of originally occurring species relative to their abundance in the original, pristine or mature state as the basis. As such it is an indicator of biodiversity intactness, and also describes the process of homogenisation.
When people intervene in ecosystems, some species decrease in abundance and distribution. At the same time a few other, opportunistic, species increase in abundance, replacing the original ones. If the intervention or disturbance by humans increase, many more species will decrease in abundance with extinction as the final step for some of them. Other species will increase and some new species replacing the original ones. If these new species are the same species as everywhere else, homogenisation of ecosystems occurs. MSA describes this process by tracking the abundances of the original species.
MSA is quantified by using datasets derived from peer reviewed publications comparing disturbed sitautions with the original ones. The observed abundances of species in the disturbed situations are divided by the abundances found in the original system described in the same publication. These relative values are capped at 1, as to avoid compensation by increasing species beyond their 'original' abundance over decreasing species.
After human intervention, remarkably local ‘species richness’ may initially increase due to new, opportunistic, species. Because this limited group is becoming more and more dominant, ecosystems lose their characteristic species and become more and more alike: homogenisation. As a result, the number of species at a location may stay (nearly) the same, but these will be more and more species found anywhere.
The homogenisation process can be illustrated with the ‘fishing down the food web’ figure of Prof. Daniel Pauly (University of British Columbia). Overfishing results in a strong decrease of large, long-lived species. In the course of time, only small, short-lived species remain.
Irrespective we deal with forest, marine or agriculture ecosystems, the underlying homogenisation process is analogous.