Data from: Life history evolution in guppies VIII: the demographics of density regulation in guppies (Poecilia reticulata)

In prior research, we found the way guppy life histories evolve in
response to living in environments with a high or low risk of predation is
consistent with life history theory that assumes no density-dependence. We
later found that guppies from high predation environments experience
higher mortality rates than those from low predation environments, but the
increased risk was evenly distributed across all age/size classes. Life
history theory that assumes density-independent population growth predicts
that life histories will not evolve under such circumstances, yet we have
shown with field introduction experiments that they do evolve. However,
theory that incorporates density regulation predicts this pattern of
mortality can result in the patterns of life history evolution we had
observed. Here we report on density manipulation experiments performed in
populations of guppies from low predation environments to ask whether
natural populations normally experience density regulation and, if so, to
characterize the short term demographic changes that underlie density
regulation. Our experiments reveal that these populations are density
regulated. Decreased density resulted in higher juvenile growth, decreased
juvenile mortality rates and increased reproductive investment by adult
females. Increased density causes reduced offspring size, increased fat
storage by adult females and increased adult mortality.