MOUSE – SPRING 2011
That there is an incredible variety of life on the planet is a trite statement, at this
point in the course. The question is why? And there are other questions: Why is it that we have
more diversity at the equator than at the poles? Why do we have more diversity in tropical
rainforest than in the deserts? And how much has it changed over evolutionary time, and why?
Ecological vs Evolutionary Measurements
– There are two approaches to the biodiversity
issue, one ecological, the other evolutionary. They are related but not the same. An ecologist is
concerned with what is out there now, and how it changes in contemporary time, as a
consequence of the ecological forces that are operative. An evolutionary biologist is concerned
with what has happened over the last 3.5 billion years, and why (in some larger sense) it has.
– An ecologist goes out to a pond, a lake, a forest inventory plot (a hectar or so),
and simply counts all the different species within a certain group, say insects or fungi, or
trees or orchids, or whatever. There are problems of finding everything, and the taxonomic
challenge is terrifying – that’s a lot of taxa for one person to know, but it’s basically a
counting exercise. One can compare the biodiversity of different places and situations.
– A student of evolution has to be more indirect. Both the time and spatial
coverage are much larger, so one ends up collecting records of the taxa that have been found
from musea, herbaria, and published reports in several different literatures. The catalogue is
always incomplete and imperfect, and there are several different taxonomic styles involved.
– Let’s start with the ecological approach, because that is driven by its
own set of interests. In a given locality, the species present are some subset of those available
from the immediate region, providing the potential colonizers. That species pool is determined
by the range of ecological opportunity in the region, as well as the history of that biota.
– The factors that can drive a species to local extinction are: (a) severe
climatic events, (b) rapid changes in the habitat, (c) an overdose of predation, parasitism or
disease, (d) competition, (e) loss of food or other resources, on which the organism depends,
or (f) demographic randomness (in small populations).
Wallace (1878) took the view that climatic evenness would promote species persistence, but
later data don’t support that view very closely. In fact, a modicum of local disturbance is
beneficial to biodiversity, because it allows the habitat to support species at different stages
of ecological succession. Either total disruption or total stasis lead to reduced biodiversity.