free-floating clumped masses, or attached to the substratum, have temperature-
corrected hatch times that are on average3.3 times longer than those for
unprotected free planktonic eggs at equivalent egg masses. Furthermore, not
only can differences in the egg-hatch time be observed, but data on the two- to
four-cell cycle times in marine invertebrate embryos also demonstrate similar
separation on the basis of egg protection strategy (Strathmann, Staver &
Hoffman, 2002 ). Differences in egg-hatch times not only occur between taxa on
the basis of egg-protection strategy, but also within single taxa such as the
copepods. As the sac-carried eggs of copepods are around an order of magnitude
less vulnerable than are broadcast eggs (seeprevious section), the patterns
found in this analysis qualitatively agree with expectations from an evolution-
ary perspective: the period of time animals spend in some life-history stages
appears to be negatively related to their vulnerability. There is also clear evi-
dence of trade-offs between different life-history attributes. The shorter hatch
times in broadcasting copepods are not short enough to offset the higher
mortality in the egg stage in comparison to sac spawners (i.e. for a given number
of eggs laid there will be, on average, more hatchlings from sac spawners than
broadcasters). Broadcasters appear to make up for this reduced survival over the
egg period in other ways: although the two groups have similar amounts of
energy to invest in eggs (Hirst & Bunker, 2003 ), broadcasters achieve a higher
rate of fecundity than sac spawners by producing relatively smaller eggs
(Bunker & Hirst, 2004 ). It is interesting that although sac-spawning copepods
are not uncommon in the marine environment, the broadcasters are generally
dominant, whereas in freshwater almost all pelagic copepods are sac spawners.
It is suggested here that a broadcast strategy would simply be too costly on egg
survival in freshwaters, where the water column is relatively shallow and
benthic predation intense, so that any eggs which settled out would have poor
chances of survival.
Conclusion
This chapter has shown how life-history analysis can predict the evolution of
body size in different environmental conditions, and can help explain the
scaling of important vital rates with body mass and temperature. Like the
book as a whole, this chapter illustrates the importance of body size throughout
ecology, including microevolution, population dynamics, and the structure and
functioning of communities. Functional relationships between organism life
histories and higher levels of ecological organization, including ecosystem
functioning, are becoming increasingly appreciated, and act both up and
down levels of organization. The importance of community structure on life
history (including adult and offspring size), is highlighted, for instance, in
recent models of optimal life history that invoke selective pressures that depend
on the community size spectrum (Thygesenet al., 2005), and on indirect
LIFE HISTORIES AND BODY SIZE 49