Biology Reference
In-Depth Information
Causes of density dependence in two species of small reef fishes were
examined by Holbrook and Schmitt (
2002
). These two fish species find
shelter in branching corals or sea anemones, and are diurnal feeders on
plankton. In both species, density-dependent mortality of juveniles is due
to predation at night, mainly by small resident piscivores. Intraspecific
interference competition between fish trying to shelter in the safest places
increased with increasing population density. The less aggressive fish
became increasingly more exposed to predation.
Forrester et al.(
2002
) used small-scale data on population dynamics, as
discussed above, to simulate large-scale dynamics of reef fish populations.
Many more data are needed to permit a conclusion on whether their
model describes natural conditions well. For large-scale conclusions, it
would be important to know the extent of interconnections between
local populations in a metapopulation. However, as pointed out by Sale
(
2002
), the magnitude of such interconnections is presently unknown on
any spatial scale, and we do not know whether groups of populations
function as metapopulations.
In summary, it is clear that density-dependent processes occur in
certain fish species and under certain conditions. However, we do not
know how widespread these processes are and whether large-scale
dynamics of reef fish populations are regulated by such processes. In
particular, the great variability between regions makes generalizations
difficult. Considering these difficulties, agreement on contentious issues
may be impossible in the foreseeable future.
Kangaroos: fluctuations in rainfall are the primary
determinant of population size, but there is some
''regulation'' by negative feedback
Several species of kangaroo are found in Australia. This discussion is
restricted to the red kangaroo, Macropus rufus. The species is widely
distributed throughout inland Australia, but its greatest density is reached
in the eastern sheep rangelands, for example in northwestern New South
Wales where it can exceed 20/km
2
. The total number for Australia in
1981 was estimated to be 8.4 million (Caughley
1987a
). Because it
competes with sheep (total number in 1981, 133.3 million), it has been
studied extensively, and much of the information is reviewed in Caughley
et al.(
1987
). Many of the study sites are sheep pastures, where kangaroos
and sheep live together, others are in National Parks, which lack sheep.
Male red kangaroos reach a weight of about 83 kg, females 39 kg (Stuart
