Environmental Engineering Reference
In-Depth Information
more complicated than was previously thought and was certainly not automatic. May has
studied the mathematics of diversity-stability theory and sees no reason to suppose that
stability increases with diversity. Others quote examples of species-poor communities
which are stable and species-rich communities which are unstable. Many species-poor
communities can be very resilient, with the plants recovering quickly from an unusual
drought, for example. Such are the species-poor heather moorlands of upland Britain. In
contrast nearby species-rich limestone grassland communities can show very low
resilience.
Other ecologists emphasize the value of links which have evolved over time (
co-
evolutionary links
); human-modified systems (agro-ecosystems) have no co-evolutionary
links between the interacting species. Farms are really ecosystems with a haphazard
collection of species selected by the farmer. It should also be borne in mind that, among
natural communities, stable complex systems have survived whilst unstable complex
systems have disappeared (Plate 23.3). Other workers have discovered that in some
ecosystems the more connected the ecosystem (the higher the connectance), the more
amplified small variations become as they propagate through the well connected system.
This of course is counter to the hypothesis of MacArthur.
The picture regarding diversity and stability is complicated and unclear. As we have
seen (pp. 463-71), there are various definitions of both diversity and stability. In the past
different relations between different indices have been investigated, and so perhaps it is
not surprising that there are different answers. What is clear is that ecosystem stability
and community stability are dependent on environmental stability. A stable environment
will promote community stability, and species-richness is then likely to increase over
time.
The main conclusions seem to be as follows. First, the more species that are present in
a community:
