SLOTH POPULATION GROWTH RATES AND THE
INFLUENCE OF IMMIGRATION
The analysis herein is, to the best of our knowledge, the
first comprehensive demographic assessment of any vertebrate
species occurring in a shade-grown agro-ecosystem.
Point estimates of k indicated that population growth was
positive for C. hoffmanni when the matrix model was
parameterized with estimates of both true and local adult
survival rates. Indeed, these estimates suggest the local
population is increasing by 3–6% annually and that individuals
initially marked in the agro-ecosystem were
increasing 5–8% annually (i.e. more than replacing themselves).
Nevertheless, lower 95% confidence limits for k
fell slightly below 10, and we could not rule out the
possibility that the population was in decline. However,
we suspect that this is a locally stable population because
C. hoffmanni densities have been high in the study area
since 2004, and we have no evidence indicating that
C. hoffmanni has declined during this period (Vaughan
et al. 2007, this study). Moreover, only a modest amount
of immigration (005–007, depending on AFB) was
needed to produce estimates of k with lower 95%
confidence intervals that did not overlap 10.
In contrast, point estimates of k for B. variegatus indicated
c. 6% annual decline and upper 95% confidence
limits did not overlap 10. Negative population growth
was the case for both estimators of k since true and local
survival were equivalent in the absence of dispersing individuals.
Thus, the local population of B. variegatus did
not appear to be replacing itself and was expected to
decline in the absence of immigration. Adult survival and
reproductive estimates were very similar between species,
and the difference in population growth was largely due
to lower juvenile and subadult survival in B. variegatus.
While we only documented the cause of mortality for a
small number of juveniles and subadults (Table 2), young
B. variegatus may be more susceptible to predation given
this species’ smaller size and more docile behaviour
when handled (M. Z. Peery & J. N. Pauli, personal
observation).