Environmental Engineering Reference
In-Depth Information
TIME
The Time Hypothesis is one of the oldest suggestions, and is based on the length of time
which has been available in
Figure 23.3
Changes in diversity with latitude: (a) breeding
birds in the wetlands of Finland; (b) sawfly species in
Eurasia.
tropical forests for speciation mechanisms to evolve. It is argued that the absence of
glaciation in tropical regions during the Quaternary era means that they contain a larger
legacy of species from pre-glacial times; the implication is that the number of species in
mid and high latitudes will catch up in time, hence their lower diversities are temporary.
This is a doubtful hypothesis, for several reasons. The geological fossil record shows that
the diversity gradient existed before the Quaternary, and seems to have been evident
throughout most geological time. Also, as research in the tropical forests continues to
increase our understanding of them, it becomes clear that they did suffer major climatic
changes in the Quaternary; in fact it is difficult to find habitats on the Earth which did not
suffer disruption and disturbance during that time.
One aspect of the Time Hypothesis which is ecologically significant is the variation in
diversity during the course of an ecological succession. The early stages of successions
have low diversity, whether we are considering sand dunes, salt marshes, bare rock
surfaces or wet lakeside habitats. These are rigorous environments where only a few
opportunist species can survive. As succession progresses through time, and growing
conditions ameliorate, the successional communities become richer and richer in plant,
animal and insect species as more and more newcomers arrive by processes of
replacement and immigration. Low diversity at the pioneer stage of the succession is
replaced by higher diversity in the middle stages. In this sense, time is clearly an
important local factor for such local situations. In the final stages of the succession,
however, in the climax community, diversity usually declines; a small number of
dominant species become established and shade out the larger number of
