Biology Reference
In-Depth Information
biomass, which in turn is determined by rainfall. Rainfall affects pasture
with little time-lag. However, kangaroos also affect pasture biomass, at
least under certain conditions. They have little or no effect when their
density is low, or when a drought sets in, but kangaroo grazing tends to
lower the peaks and troughs of pasture biomass, and in the long term they
lower pasture biomass to about 60 % of the mean level on ungrazed lands,
from about 500 kg/ha to about 300 kg/ha. In other words, according to
Caughley, the feedback loop (more pasture leads to more kangaroos,
which in turn reduces pasture biomass leading to a reduction in kangaroo
density, etc.), makes the system centripetal. Although this loop is often
weak, in the long run it leads to a weak interaction, i.e., it has a weak
regulatory effect.
In conclusion, the predominant factor affecting population size of
kangaroos is rainfall. Repeated and unpredictable droughts lead to popu-
lation collapses, resulting in nonequilibrium conditions; but regulation of
population sizes due to density dependence (negative feedback) also plays
some role.
