Environmental Engineering Reference
In-Depth Information
13.2 (a) Major energetic characteristics of ectothermic and
endothermic vertebrates. (b) Major energetic characteristics of
large terrestrial mammals and Homo sapiens.
thermic rates of over 30% and over 10%, respectively.
Most ectotherms also pour much of their production
into copious offspring. This strategy limits parental sur-
vival and precludes repeated reproduction, but it makes
all such species both very efficient colonizers and persis-
tent pests. In contrast, endotherms reproduce more
slowly but repeatedly.
Endotherms are unsurpassed masters of both rapid and
long-distance locomotion. Their metabolic scopes are
mostly around 10 for mammals (
>
30 for some species)
and up to 15 for birds. The maximum aerobic power of
endotherms is commonly 1 OM higher than in ecto-
therms. Larger animals have lower specific locomotion
costs, but both massive mammals and tiny birds under-
take long seasonal migrations powered by lipid stores. In
terms of minimal transportation cost, swimming is the
most efficient way of locomotion, running the most
demanding. Flying falls in between, but energetic consid-
erations limit the size of flyers to less than 15 kg. Most of
the time spent in motion is in search of food, but forag-
ing, even in the absence of predators, is not simply a mat-
ter of minimized energy expenditures. The objective is
selection of specific nutrients rather than maximization
of net energy returns.
Herbivores are not generally energy-limited; their
numbers are kept in check by predators whose existence
is clearly food-limited. In the most favorable environ-
ments herbivores consume up to 60% of NPP, but typical
grazing rates are below 10% and less than 1% for verte-
brates. Energy transfer efficiency between plants and pri-
mary consumers is 1%-15%, whereas carnivores consume
5%-25% of available prey. Inevitably, large losses during
energy transfers between successive trophic levels limit
the biomasses of heterotrophs to small fractions of stand-
ing phytomass. An inverse relation between body size
and density means that there may be a few MJ/m
2
of
decomposer biomass (bacteria and fungi) but not even 1
kJ/m
2
of birds and insectivorous and carnivorous mam-
mals. Large carnivores must be rare; even in the richest
ecosystems their mass does not surpass 1%-2% of all her-
bivorous zoomass. The global total of heterotrophic bio-
mass is highly uncertain, mainly because of the dominant
but invisible or hidden bacterial and fungal mass. Wild
vertebrates add up to just over 0.1% of continental
