because copepods are neither going extinct nor taking over the world. The
assumption of constant fecundity with age in copepods was made because this
group has determinate growth: with no adult moults or great increase in adult
body size. Equation (3.2) was then used to derive mortality-rate estimates for sac
spawning species (i.e. those species that carry their eggs from laying up to the
point of hatch), with the same value for mortality rates across eggs and post-
hatch stages. Approximately equal mortality across all stages including eggs has
been demonstrated for sac spawners in the field (Ohman & Wood, 1996 ; Liang &
Uye, 1997 ). Hirst and Kiørboe ( 2002 ) also altered this approach for broadcasting
species (whose eggs do not remain attached to the female but rather are free) to
allow for the highly vulnerable eggs of this group (see their study for details).
When compared with the limited and highly variable measures of field mor-
tality rates, the model predictions matched well with the average rates and
patterns, and with adult longevity with respect to temperature and body size
(Hirst & Kiørboe, 2002 ).
The body-mass scaling of copepod mortality was compared with a general
relationship for other marine organisms: pelagic invertebrates; the eggs, juven-
iles and adults of fish; and mammals (data from McGurk, 1986 ). Copepods have
much flatter slopes than the general relationship (Fig.3.5), suggesting that the
smaller bodied copepods are avoiding mortality that other marine animals of a
similar size are not. Copepods have especially well-developed predator sensory
systems (the first antennae) and strong escape responses, which may in part
account for their ability to escape mortality. Verity and Smetacek ( 1996 ) have
suggested that the remarkably homogeneous body form observed across the
range of free-living epipelagic copepods has resulted from strong predation
pressures.
Thus, life-history analysis can be applied to identify global patterns of vital
rates in populations of diverse species belonging to a large taxonomic group
(copepods). Copepods are the dominant metazoan grazers of marine phyto-
plankton, provide an important trophic link to fish, are key exporters of parti-
culate organic carbon from the upper mixed layer of the ocean (see Kiørboe,
1998 ), and play a pivotal role in regenerating nutrients in the vast oligotrophic
areas of ocean (Banse, 1995 ). Therefore, this application of life-history analysis is
likely to be important for understanding oceanic ecosystem function and global
biogeochemistry.
Improving predictions of global patterns of life history
Gilloolyet al.(2002) have produced a model of development times for animals,
including egg-hatch times and egg-to-adult times, with the primary determi-
nants being body mass and temperature. They corrected development times for
mass (as development time/mass0.25) and plotted these against temperature (as
Tc/(1þ(Tc/273)), whereTcis the temperature in 8 C: this term represented a
LIFE HISTORIES AND BODY SIZE 47