Principles of terrestrial ecosystem ecology.pdf

260 11. Trophic Dynamics
terrestrial herbivores selectively feed on young
tissues with a high concentration of digestible
energy and low concentrations of cellulose and
lignin. These young tissues have high concentrations
of nitrogen and phosphorus. Because
of selective herbivory on nutrient-rich tissues,
a larger proportion of plant-derived nutrients
cycle through plant-based trophic systems than
is the case for carbon.
Terrestrial herbivores not only select
nutrient-rich tissues but cycle nutrients more
rapidly than do plants. Plants resorb about half
the nitrogen and phosphorus from leaves
during senescence, so plant litter generally has
only half the nitrogen and phosphorus concentrations
as does the live tissue eaten by
herbivores (see Chapter 8). For this reason,
herbivory is at least twice as important an
avenue for nitrogen and phosphorus cycling in
terrestrial ecosystems as it is for biomass and
energy. The turnover time for nutrients in terrestrial
herbivores is often shorter than in the
plants on which they feed. Many terrestrial
animals, particularly carnivores and homeotherms,
eat more nutrients than they require for
growth, due to the large energetic costs of
movement, and in the case of homeotherms, for
temperature regulation.Animals excrete excess
nutrients in inorganic form or as urea, which is
quickly hydrolyzed in soils (see Chapter 9). In
Frequency (% of observations)
25
20
15
10
5
0
Terrestrial
Freshwater
summary, terrestrial herbivores speed nutrient
cycling in at least two ways: (1) by removing
plant tissues that are more nutrient-rich than
would otherwise return to the soil in litterfall
and (2) by returning nutrients to the soil in
forms that can be directly used by plants (Fig.
11.6).
The ratio of elements required by plants and
herbivores determines the nature of element
limitation in organisms and the patterns of
nutrient cycling in ecosystems. Both aquatic
and terrestrial plants require nitrogen and
phosphorus in a ratio of about 15:1 (the
Redfield ratio; see Chapter 8) (Fig. 11.11). The
N:P ratio in herbivores is generally less than
in plants, particularly in lakes (Elser et al. 2000).
Lake herbivores must therefore concentrate
phosphorus more strongly than nitrogen to
meet their nutritional demands and tend to
excrete the excess nitrogen (Elser and Urabe
1999). In this way, herbivory speeds the recycling
of nitrogen, relative to phosphorus,
making nitrogen more available to phytoplankton
and reinforcing the phosphorus limitation
that characterizes many lakes. Differences in
N:P ratios among grazers in lakes illustrate the
importance of this effect. Daphnia is a rapidly
growing cladoceran grazer that has a higher
phosphorus concentration (lower N:P ratio)
than the more slowly growing copepods. Under
Plants Invertebrate herbivores
40
30
20
10
Terrestrial
Freshwater
0
2.3 5 10 15 20 25 2.3 5 10 15 20 25
N:P ratio N:P ratio
Figure 11.11. Frequency distribution of N : P ratios
in terrestrial and freshwater ecosystems in (A) plants
and (B) invertebrate herbivores. Ratios are mass of
nitrogen relative to mass of phosphorus. The N : P
ratio is lower in herbivores than in the plants on
A B
which they feed, particularly in fresh-water ecosystems.
Herbivores therefore preferentially retain
phosphorus and excrete nitrogen to the environment.
(Redrawn with permission from Nature; Elser
et al. 2000.)