Environmental Engineering Reference
In-Depth Information
control larger amounts of trophic energy than smaller
ones, and no herbivorous mammal could thus outstrip
another energetically only because of its larger size. But
Damuth's (1993) regressions of 39 mammalian dietary
groups revealed slopes ranging from
รพ
0.42 (for temper-
ate South American frugivores-herbivores) to
1.30 (for
North American and Asian carnivores). A reexamination
based on ecological density data for nearly 1,000 terres-
trial mammal populations showed that density scales as
M
0
:
75
only for animals with body masses between 0.1
and 100 kg (Silva and Downing 1995). The overall rela-
tion is distinctly nonlinear, which means that previous
log-linear models overestimated the densities of small
species and underestimated those of
was very close to assumed universal BMR scaling of
M
0
:
75
, the ranges were seen to be simply a function
of metabolic needs (McNab 1963). Reality is more com-
plex. Examination of species belonging to 124 mamma-
lian and avian genera showed the ranges of carnivorous
passeriformes scaling as M
1
:
75
, herbivorous galliformes
as M
1
:
39
, omnivorous rodents as M
0
:
97
, and herbivorous
rodents as M
0
:
81
(Mace, Harvey, and Clutton-Brock
1983). Range overlap is a major reason why space used
by animals increases at steeper rate than would otherwise
be expected. Small animals can maintain small, defensi-
ble, and hence overwhelmingly exclusive home ranges,
but spatial constraints on effective defense mean that
exclusivity of home range use declines with rising M
and that large mammals may lose to neighbors over 90%
of resources available within a home range (Jetz et al.
2004).
Size is much less predictably related to trophic levels.
Elton (1927) was the first ecologist to recognize the
declining numbers of heterotrophs in higher food web
levels while pointing out their often increasing size. A
decade later Hutchinson redefined the principle of the
Eltonian pyramid of numbers in terms of productivity
and opened the way for Lindeman's (1942) pioneering
research on energy transfers in Wisconsin's lake Men-
dota in terms of progressive efficiencies (assimilation at
level n/assimilation at level n
1). Lindeman's results
showed primary producers assimilating 0.4% of the in-
coming energy, while the primary consumers incorpo-
rated 8.7%, secondary consumers 5.5%, and tertiary
consumers 13% of all energy that reached them from the
previous trophic level. These results led to the formula-
tion of the often-invoked 10% law for typical interlevel
energy transfers, and Lindeman thought that the
progressively higher efficiencies at higher trophic levels
large mammals
(fig. 4.10).
More important, densities were well described by
M
0
:
75
for only about half of all studied populations (for
the smallest and largest mammals, allometric exponents
were significantly different from zero). The energy use
of mammalian populations is not independent of M;it
appears to increase linearly with M in small mammals,
varies little for those up to 100 kg, and rises rapidly for
the largest animals. The absence of an energy equivalence
rule was confirmed by Schmid, Tokeshi, and Schmid-
Araya (2000) in their analysis of population density and
body size of invertebrates in two stream communities.
But Carbone and Gittleman (2002) found that 10 t of
prey biomass supports about 90 kg of carnivores regard-
less of body mass and that the ratio of carnivore number
to prey biomass scales almost perfectly to the reciprocal
of carnivore mass, M
1
:
048
.
The higher total energy needs of larger heterotrophs
dictate that space used by animals must increase with M.
An early analysis of home ranges for small mammals
yielded the allometric ratio of M
0
:
63
, and because this
