Environmental Engineering Reference
In-Depth Information
5
Zoomass Harvests
Terrestrial trophic pyramids are invariably broadly based, with phytomass (primary
producers) being commonly 20 times more abundant than the mass of herbivores
(primary consumers), and the zoomass in the highest trophic level (this may be the
third level in the simplest ecosystems, and only a few terrestrial communities go
beyond the i fth level) may be equal to just 0.001% of the phytomass. These realities
limit the mass of heterotrophs that could be proi tably hunted (or collected): many
small herbivores (insects, rodents) are not worth the effort, while the largest carni-
vores are too scarce and too dangerous to hunt. The situation is reversed in the
ocean as phytoplankton's short life span limits the total amount of standing auto-
trophic biomass and results in inverted trophic pyramids, with the aggregate het-
erotrophic biomass being commonly twice, and up to four times, as large as the
biomass of primary producers and with some i sh and marine mammals being fairly
massive.
There are no universally valid rules as far as energy transfers between successive
trophic levels are concerned. Lindeman's (1942) pioneering work on conversion
efi ciencies in Wisconsin's Lake Mendota found that the primary consumers incor-
porated nearly 9%, secondary consumers a bit over 5%, and tertiary consumers
about 13% of all energy that was available at the previous trophic level—and this
led to the often cited conclusion that typical energy transfers between successive
trophic levels (ecological efi ciencies) average about 10%. Subsequent research dis-
proved this simplistic conclusion, as studies in aquatic ecosystems showed transfer
efi ciencies as high as 40% and as low as 2%, but the10% rule is still frequently
invoked.
Endothermy exacts its high metabolic cost, as only 1%-3% of ingested phyto-
mass is converted to new zoomass by birds and mammals; in contrast, many ecto-
therms invest roughly an order of magnitude more of their energy intake (10%-25%)
