Rain forest canopy cover, resource availability, and life history evolution in guppies

Life history traits in guppies (Poecilia reticulata) vary geographically al ong a predator assemblage gradient, and held experiments have indicated tha t the association may be causal; guppies introduced from high predation sit es to low predation sites have evolved the phenotype associated with low pr edation in as few as seven generations. It has long been recognized, howeve r that low predation sites tend to have greater forest canopy cover than hi gh predation sites. Stream differences in canopy cover could translate into stream differences in resource availability, another theoretically potent agent of selection on life history traits. Moreover, new computer simulatio ns indicate that the high predation phenotype would outcompete the low pred ation phenotype under both mortality regimes. Thus, predation alone may not be sufficient to explain the observed life history patterns, Here we show that food availability for guppies decreases as forest canopy cover increases, among six low predation streams in the Northern Range of T rinidad. Streams with less canopy cover received more photosynthetically ac tive light and contained a larger standing crop of algae (the primary food of guppies), as measured by algal pigment:, (chlorophylls and carotenoids) on both natural cobble and artificial tile substrates, but did not contain a greater biomass of guppies (per square meter of streambed). Consequently, algae availability for guppies (in micrograms of algal pigments per millig ram of guppy) increased with decreasing canopy cover. The biomass of guppie s and algae both decreased after a series of floods, with no net effect on algae availability. Field mark-recapture studies revealed that female and j uvenile guppies grew faster. and that the asymptotic size of mature males w as larger, in streams with less canopy cover. Canopy cover explained 84% of the variation among streams in algae availability which, in turn, explaine d 93% of the variation in guppy growth rates. Laboratory "common garden" ex periments indicated that the stream differences in growth and adult male si ze in the field were largely environmental (nongenetic). These results stro ngly suggest that stream differences in canopy cover result in consistent s tream differences in food availability, independent of predation. Our preliminary data indicate that some life history traits (offspring size and litter size) vary genetically along the canopy cover gradient, among l ow predation streams, in the same direction a's along the predation gradien t. Another recent study shows that food availability is higher at high pred ation sites than at low predation sires, partly as an indirect effect of pr edators reducing guppy densities. Further research is required to disentang le the direct effects of predation from those of resource availability in t he evolution of life histories.