Environmental Engineering Reference
In-Depth Information
sizes and rapid turnovers mean that oceanic phytomass
equals less than 1/1000 of continental stores. Best esti-
mates of global terrestrial NPP are 50-60 Gt C, with
forests contributing nearly half, grasslands almost one-
third. Marine NPP (45-50 Gt C) is almost as large as
the continental total. NPP efficiencies are highest in wet-
lands (
>
2%) and rarely surpass 1.5% in forests. The ter-
restrial mean is about 0.3%; the oceanic average is 1 OM
lower because of the nutrient-poor euphotic zone. Net
ecosystem productivity is highest in grasslands and crop-
lands, lowest in forests. The energetic imperatives of
forest growth favor evergreens in arid and boreal envi-
ronments (where the cost of annual replacement of leaves
would be prohibitive), limit the number of surviving ma-
ture trees per unit area (self-thinning process), and deter-
mine the efficiency of nutrient use.
Heterotrophs found it profitable to feed on the abun-
dant polymers synthesized by autotrophs or to eat other
heterotrophs. Of their two principal metabolic choices,
aerobic respiration has a decisive energetic advantage (1
OM higher energy gain per unit of matter) over anaero-
bic fermentation; all higher organisms use it. Complex
plant carbohydrates are the most abundant source of
feed energy for heterotrophs; proteins (critical building
blocks of animal tissues) are used as energy sources only
when the supply of carbohydrates and lipids is limited.
As with autotrophs, ATP is the energy carrier of the intri-
cate heterotrophic metabolism, whose regularities on the
organismic level are best expressed by many allometric
equations (exponential relations related to body mass).
Most heterotrophic BMRs scale as body mass raised to
powers of 0.66 or 0.75, and disputes continue regarding
the value and universality of the exponents and the rea-
sons for their regularity.
The requirements of reproduction, growth, and loco-
motion raise the total energy need substantially above
BMR, but thermoregulation is generally the highest
constant endothermic energy burden. Ectothermic
thermoregulation is overwhelmingly behavioral (choice
of suitable environments), although some insects can be-
come temporarily endothermic, and tuna and sharks have
specialized endothermic muscles. Ectotherms cannot be
active in thermally extreme environments, nor can they
be as competitive in optimal temperatures as endotherms
(fig. 13.2). Carrying a uniform thermal environment
confers enormous survival and competitive advantages
on endotherms, and they have radiated to virtually every
terrestrial niche. But this evolutionary strategy has a high
energetic cost. In order to be above the ambient tem-
perature most of the time (to facilitate cooling through
cutaneous evaporation and sweating), the body tempera-
tures of endotherms are relatively high 36
C-42
C),
close to the thermal decay threshold of proteins. Main-
tenance of these temperatures requires much higher
feeding rates than in ectotherms and limits the share of
energy that can be diverted to reproduction and growth.
Endothermy also limits the size of the smallest animals
because the rising specific metabolic rates with falling
mass require higher frequency of feeding in smaller crea-
tures. Larger animals with sufficient fat stores go through
extreme cold with only minimum lowering of body tem-
peratures or remain active thanks to their superior insula-
tion, but they face greater difficulties in coping with
extreme heat. Endothermic feed assimilation efficiencies
are generally higher than in ectotherms (70%-90%), but
endotherms pay an energetic price in terms of much
lower growth and ecological growth efficiencies. These
rates are invariably much below 5% compared with ecto-
