Environmental Engineering Reference
In-Depth Information
produces fewer but larger neonates of greater competi-
tive ability.
Borrowing the terminology of growth equations, Mac-
Arthur and Wilson (1967) labeled the first strategy r se-
lection (r being the intrinsic rate of increase) and the
other one K selection (K being the upper asymptote of
population size). Clearly, r selectionists are great oppor-
tunists, pouring a much larger part of their metabolism
into reproduction, and this makes them into obnoxious
pests and efficient colonizers. But this prodigious chan-
neling of energy into offspring severely limits the survival
of parents and the chances for repeated reproduction.
Endoparasites are notable exceptions to this rule; tape-
worms reproduce copiously and survive for up to 15
years. In contrast, adaptation to limited resources makes
the larger K selectionists long-term occupants in more
stable settings. Most heterotrophs do not operate at
these extremes; their reproductive strategies tend toward
the r or the K end of the continuum.
Biochemical commonalities of the production of
gametes and the growth of embryos and neonates put
the maximum theoretical net efficiency of organizing
food-derived monomers into zoomass polymers at about
96%, an impressive figure by any standard. This is only a
theoretical maximum. Actual rates (considering digestive,
molecular turnaround, molecular transport, and mechan-
ical inefficiencies) would be just over 70%. Actual effi-
ciencies can be measured in proliferating unicellular
organisms; net values are 50%-65% for bacteria; 40%-
50% for protozoan and yeast. Estimates for invertebrates
show gross growth conversion efficiencies of 30%-65%
for mollusca, 35%-55% for crustaceans, and 25%-60%
for insects (Calow 1977).
Given the prominence of eggs in avian life and the fre-
quent complexities of reproductive behavior among birds,
ornithologists were among the earliest students of the
energetics of reproduction and growth. Ricklefs (1974)
put the energy requirements of testicular growth at no
more than 0.4%-2% of BMR during the period of rapid
gonadal gain. Female gonadal growth claims between
l.5%-6% of BMR, a small expenditure compared to the
cost of eggs, a function of their energy density and their
rate of formation. Egg energy density is related to the de-
velopment of hatchlings. Precocial species, above all, wa-
terfowl, have heavier yolks (30%-40% of egg weight) and
shells, and their eggs have up to 7.6 kJ/g; in contrast,
small altricial birds (born blind and featherless) have
eggs containing as little as 4.1 kJ/g.
But the extreme case of high energy investment in an
egg is New Zealand's kiwi, whose large eggs (at 400 g-
435 g, roughly one-fifth of the female's weight) contain
10 kJ/g (yolk is 61% of total weight) and provide nutri-
tion for one of the longest incubation periods (70-74
days), resulting in a fully feathered chick with adult-type
plumage (Calder 1978). The energy cost of eggs among
precocial species is commonly in excess of 110%, even
130%, of their BMR, whereas for passerines and raptors
the markup may be as low as 30%-35%. Incubation of a
chicken egg containing 360 kJ produces a chick (160 kJ)
and leaves behind nearly 110 kJ of unused biomass,
yielding a gross conversion rate of 60%. Incubating a
clutch of eggs equal to body weight is less demanding
for large birds, usually requiring markups of between 20%
and 80% of BMR, whereas in the smallest species the en-
ergy investment is equivalent to 100%-300% of BMR.
The energy cost of mammalian gonadal growth is neg-
ligible compared to the investment in the gravid uterus,
embryo, and enlarged mammary glands. Birth weights
and gestation periods increase as power functions of ma-
ternal M; the larger species produce neonates of rela-
