Summary Physical barriers divide
the population of giant tortoises (
Geochelone gigantea Schweigger)
on Aldabra into several sub-populations of different density, which
nevertheless are similar genetically. We measured individual growth rates
in each sub-population. Mortality was estimated using data from Bourn and
Coe (1979). Reproduction and recruitment were studied using data from
previous work (Swingland and Coe 1979) and our own estimates of clutch
size, egg weight, and laying frequency from 1975 to 1981.
Individual growth rates were strongly dependent only on
individual size and sub-population density and not on age or sex. Within a
sub-population, the relationship between specific growth rate and size
(linear measure) was best fitted by a Gompertz model, except for very
young tortoises which grew faster in volume, though not in weight, than
expected. Animals at high densities grow slowly to a small size whereas
those at low densities grow fast to a large size. At very high density
many juveniles remain at a small size without growing or
maturing.
Mortality of larger (> ca. 5 years old) animals
was independent of density, but did depend on size in the highest-density
sub-population, as predicted by the Gompertz growth model.
Reproduction and recruitment were negatively
density-dependent over the whole density range (5 to 35 animals
ha-1) studied. Clutch size and laying frequency were strongly
influenced by sub-population density, but egg weight was not. Laying
frequency varied within sub-populations according to rainfall (presumably
via annual food supply).
All except one sub-population are seen as stages in the
development of the same interactive system. Competition between
individuals is nearly, but not purely, of scramble type. The remaining
sub-population is either a distinct interactive system in which food
supply for very young animals is important, or it is a non-interactive
system controlled by the effect of natural enemies on very young animals.
This suggests that the equilibrium density and/or dynamics of giant
tortoise populations are highly sensitive to mortality factors affecting
very young animals.
In low density sub-populations the animals are large, have
many young, low relative reproductive effort, and a short generation time.
In high density sub-populations they are small, have few young, high
relative reproductive effort, and a long generation time. This variation
is largely phenotypic. It is anomalous with respect to r-K life
history theory but is a logical consequence of indeterminate growth
combined with size-determined risk and benefit functions and may have
contributed to the giant tortoises' success as island
colonisers.
and
J. Hamilton1