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continue particularly because regions of highest diversity in the tropics are
strongly affected (e.g., deforestation of the Amazon rainforests, destruction
of coral reefs as the result of human activities leading to coral bleaching,
etc.) resulting in conditions that resemble those in earlier eras of mass
extinctions discussed earlier in this chapter. Also, these conditions were
typical of nonsaturation and evolutionary nonequilibrium; it took many
millions of years until species numbers recovered and were finally exceeding
numbers before the extinctions.
A better-founded view on how widespread extinctions really are could
be reached by obtaining quantitative data (even if only approximate) for
the various parameters in the diversification equation discussed above. In
particular, it is important to note that most parasite groups are much more
diverse in tropical than in cold environments, and some (although probably
not all) parasite groups are also relatively more diverse at low latitudes
(i.e., there are more parasite species per host species at low than at high
latitudes), as demonstrated by Rohde and Heap ( 1998 ) for monogenean
gill parasites of marine teleost fish. Whereas there are about as many
monogenean species as marine teleost species on the gills at high latitudes,
in the tropics there are twice as many. Therefore, mass extinctions in
tropical ''hot spots'' will have a magnified impact on parasite faunas.
In conclusion, we can state that periods of stasis, i.e., apparent equili-
brium, existed in evolutionary history, at least for marine benthos which
is the best known. There were also repeated severe reductions in diversity
(mass extinctions). However, overall, these periods were superimposed
on a ''walk'' to ever increasing diversity which has continued to the
Recent. This walk is now threatened by human impacts that have caused
mass extinctions over thousands of years at accelerating speed. Mass
extinctions are likely to accelerate even further if present trends continue
or become stronger (e.g., due to global warming), resulting in severe
evolutionary nonequilibrium. Estimates of species lost or threatened are
underestimates, because parasites, which constitute a very large proportion
of the total fauna, have been largely ignored.
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