96
L. B. SLOBODKIN
a particularly interesting problem since as top trophic level carnivores,
their ecological efficiency as we have been using the term is zero by
definition. Nevertheless, they produce a significant amount of slime.
This slime is in one sense part of the flatworm population since it
serves to entangle food organisms and t o facilitate the locomotion of
the worms. It almost certainly serves as food for various bacteria, and,
considering the catholic taste of amphipods and isopods, may serve
t o feed them also. We might, therefore, consider the slime as yield from
the flatworm. If we did, we would also, of necessity, have to consider
non-predatory mortality as constituting yield for the same reason.
With mortality estimated at 48 kcal/m2/year, mucus production at
89 and energy income at 131, we would have a fantastically high
efficiency for flatworms.
This forces attention on a logical problem associated with our
original definitions of yield and efficiency. In the Daphnia experiments,
if the experimenter did not take any yield, the ecological efficiency was
considered t o be zero. There was, nevertheless, potential energy leaving
the populations as non-predatory mortality, faecal material and cast
skins. Given an appropriate bacterial flora, we might then have considered the energy relations of the system algae - Daphnia - bacteria. Slobodkin (1959) calculated that 5% of the consumed algae is
accounted for by mortality in the absence of predation in Daphnia.
Predation of any kind competes with other sources of mortality and
at maximum predation intensity it seems almost possible to completely eliminate non-predatory mortality.
The logical problem is whether or not t o include energy-rich exudates, wastes and deposits as yield. Arbitrarily our discussion has been
confined t o actively acquired yield, that is, yield which would not
have been acquired by the predator without some activity ofi the
predator’s part, specifically not including activities of scavengers,
bacteria and others which might consume potential energy after it
has passively departed from the prey population. This has been a
matter of convenience. Quantitatively, inclusion of passively-lost
potential energy as yield from each population will raise the ecological
efficiency estimates t o varying degrees. The maximumincrease in the case
of Daphnia will be somewhat greater than 5% at low active predation
levels and somewhat less at high predation levels. The 5% estimate of
Slobodkin (1959) did not include cast skins. High predation lowers the
rate of non-predatory mortality. Determination of yield by Teal’s technique of substracting respiration and standing crop change from energy
income provides a maximum estimate which will include both actively
acquired yield to predators and the yield t o scavengers and bacteria.
Steady state conditions were not demonstrated by Teal but the
L. B. SLOBODKIN
a particularly interesting problem since as top trophic level carnivores,
their ecological efficiency as we have been using the term is zero by
definition. Nevertheless, they produce a significant amount of slime.
This slime is in one sense part of the flatworm population since it
serves to entangle food organisms and t o facilitate the locomotion of
the worms. It almost certainly serves as food for various bacteria, and,
considering the catholic taste of amphipods and isopods, may serve
t o feed them also. We might, therefore, consider the slime as yield from
the flatworm. If we did, we would also, of necessity, have to consider
non-predatory mortality as constituting yield for the same reason.
With mortality estimated at 48 kcal/m2/year, mucus production at
89 and energy income at 131, we would have a fantastically high
efficiency for flatworms.
This forces attention on a logical problem associated with our
original definitions of yield and efficiency. In the Daphnia experiments,
if the experimenter did not take any yield, the ecological efficiency was
considered t o be zero. There was, nevertheless, potential energy leaving
the populations as non-predatory mortality, faecal material and cast
skins. Given an appropriate bacterial flora, we might then have considered the energy relations of the system algae - Daphnia - bacteria. Slobodkin (1959) calculated that 5% of the consumed algae is
accounted for by mortality in the absence of predation in Daphnia.
Predation of any kind competes with other sources of mortality and
at maximum predation intensity it seems almost possible to completely eliminate non-predatory mortality.
The logical problem is whether or not t o include energy-rich exudates, wastes and deposits as yield. Arbitrarily our discussion has been
confined t o actively acquired yield, that is, yield which would not
have been acquired by the predator without some activity ofi the
predator’s part, specifically not including activities of scavengers,
bacteria and others which might consume potential energy after it
has passively departed from the prey population. This has been a
matter of convenience. Quantitatively, inclusion of passively-lost
potential energy as yield from each population will raise the ecological
efficiency estimates t o varying degrees. The maximumincrease in the case
of Daphnia will be somewhat greater than 5% at low active predation
levels and somewhat less at high predation levels. The 5% estimate of
Slobodkin (1959) did not include cast skins. High predation lowers the
rate of non-predatory mortality. Determination of yield by Teal’s technique of substracting respiration and standing crop change from energy
income provides a maximum estimate which will include both actively
acquired yield to predators and the yield t o scavengers and bacteria.
Steady state conditions were not demonstrated by Teal but the