5. Adaptive Radiation
83
and van Alphen 1998; Seehausen et al. 1997). I believe that a similar process of
sexual selection in brightly lit, clear water may also help account for the astounding
diversity and visual splendor of coral reef fish. In both rift lakes and coral reefs, the
factors favoring exceptional water clarity - and thus, in part, high species diversity
- are likely to be the same: adjacent deep water basins (allowing nutrients to purged
by gravity from the water column, in the form of sinking plankton, feces, and dead
plants and animals); lack of seasonality (reducing seasonal turnover of the water
column and consequent re-injection of nutrients and particulates into the photic
zone); and high temperatures (favoring rapid rates of growth - and, ultimately, of
nutrient scavenging - by phytoplankton and zooplankton). It should be recognized,
however, that high water clarity may not only favor sexual selection based on visual
cues, but increase the selective pressures caused by visually orienting predators as
well. Those predation pressures are almost surely important causes for the evolution of mouth brooding, strong territoriality, and use of refuge holes in rocky reefs
in cichlids - all of which should increase speciation rates by limiting dispersal -
and for the evolution of strong territoriality, striking coloration, and the use of
refuge holes in coral reefs in coral reef fish.
Interactions with natural enemies might explain some of the remarkable species diversity of many groups of tropical woody plants. Tropical forests not only
include the most diverse plant communities on earth - with up to 470 tree and liana
species per hectare - they also display well-marked patterns of plant species richness along several ecological gradients (see review by Givnish 1999). Of these,
perhaps the most striking is the rise in woody plant diversity with increasing rainfall and decreasing seasonality in the Neotropics (Gentry 1982, 1988; Wright 1992;
Clinebell et al. 1995) and Hawaii (Aplet et al. 1998). Interactions with natural
enemies may be an important factor helping create this gradient and foster high
speciation rates in woody plants native to wet tropical forests (Givnish 1999). Most
plant enemies arc small, soft-bodied, desiccation-intolerant insects, nematodes, and
pathogenic fungi, and are frequently host-specific within the tropics. As argued by
Janzen (1970) and Connell (1971) - and decisively supported by the data of Wills
et al. (1997) -- attacks by host-specific natural enemies can create patterns of density-dependent mortality that can generate and maintain high levels of tropical tree
diversity. Givnish (1999) made the Janzen-Connell hypothesis context-specific:
wet, humid, aseasonal conditions should put the fewest physical limits on the populations of desiccation-intolerant plant enemies and thereby generate and maintain
high levels of tree diversity, and foster high rates of speciation and differentiation
by pathogens in tropical woody groups. In addition to the factors discussed by Givnish
(1999), high temperatures may elevate thc feeding rates of herbivores and thereby
increase the potential for density-dependent mortality in their hosts. Indeed, Wilf
and Labandeira (1999) recently used the fossil record to show that insect damage to
leaves was greater in Wyoming during the Paleocene than during the cooler Eocene.
A higher tempo of predation at elevated temperatures may thus be an additional
factor promoting high diversity in tropical forests and coral reefs.
High tree diversity on rainy, humid, aseasonal sites in the tropics may also be
promoted by the high density of understory stems on such sites (Givnish 1999).
83
and van Alphen 1998; Seehausen et al. 1997). I believe that a similar process of
sexual selection in brightly lit, clear water may also help account for the astounding
diversity and visual splendor of coral reef fish. In both rift lakes and coral reefs, the
factors favoring exceptional water clarity - and thus, in part, high species diversity
- are likely to be the same: adjacent deep water basins (allowing nutrients to purged
by gravity from the water column, in the form of sinking plankton, feces, and dead
plants and animals); lack of seasonality (reducing seasonal turnover of the water
column and consequent re-injection of nutrients and particulates into the photic
zone); and high temperatures (favoring rapid rates of growth - and, ultimately, of
nutrient scavenging - by phytoplankton and zooplankton). It should be recognized,
however, that high water clarity may not only favor sexual selection based on visual
cues, but increase the selective pressures caused by visually orienting predators as
well. Those predation pressures are almost surely important causes for the evolution of mouth brooding, strong territoriality, and use of refuge holes in rocky reefs
in cichlids - all of which should increase speciation rates by limiting dispersal -
and for the evolution of strong territoriality, striking coloration, and the use of
refuge holes in coral reefs in coral reef fish.
Interactions with natural enemies might explain some of the remarkable species diversity of many groups of tropical woody plants. Tropical forests not only
include the most diverse plant communities on earth - with up to 470 tree and liana
species per hectare - they also display well-marked patterns of plant species richness along several ecological gradients (see review by Givnish 1999). Of these,
perhaps the most striking is the rise in woody plant diversity with increasing rainfall and decreasing seasonality in the Neotropics (Gentry 1982, 1988; Wright 1992;
Clinebell et al. 1995) and Hawaii (Aplet et al. 1998). Interactions with natural
enemies may be an important factor helping create this gradient and foster high
speciation rates in woody plants native to wet tropical forests (Givnish 1999). Most
plant enemies arc small, soft-bodied, desiccation-intolerant insects, nematodes, and
pathogenic fungi, and are frequently host-specific within the tropics. As argued by
Janzen (1970) and Connell (1971) - and decisively supported by the data of Wills
et al. (1997) -- attacks by host-specific natural enemies can create patterns of density-dependent mortality that can generate and maintain high levels of tropical tree
diversity. Givnish (1999) made the Janzen-Connell hypothesis context-specific:
wet, humid, aseasonal conditions should put the fewest physical limits on the populations of desiccation-intolerant plant enemies and thereby generate and maintain
high levels of tree diversity, and foster high rates of speciation and differentiation
by pathogens in tropical woody groups. In addition to the factors discussed by Givnish
(1999), high temperatures may elevate thc feeding rates of herbivores and thereby
increase the potential for density-dependent mortality in their hosts. Indeed, Wilf
and Labandeira (1999) recently used the fossil record to show that insect damage to
leaves was greater in Wyoming during the Paleocene than during the cooler Eocene.
A higher tempo of predation at elevated temperatures may thus be an additional
factor promoting high diversity in tropical forests and coral reefs.
High tree diversity on rainy, humid, aseasonal sites in the tropics may also be
promoted by the high density of understory stems on such sites (Givnish 1999).