An animal may be considered a nutrient pool. Things present within each animal, other than genetic materials, have been consumed and retained. Depending on age and growth rates, the mass can be substantial -- all relative and depending on definitions, from amoebae to re-introduced elk. Depending on density and behavior, the collected nutrients are distributed. They are mobile pools and may enter or leave an area through migration. Predation and harvesting animals may remove nutrients from an area. Given the evident vast root systems of plants and the great energy expended by them to collect minute amounts of dispersed nutrients, dead animals or animal feces and urine represent nutrient pools. These pools are utilized first by scavengers, then detritivores or decomposers (including fungi, bacteria, and others), then for plants and, once again, the herbivores or plant consumers.
Scavenging is common and rapid; few dead animals are seen; conspicuous deer antlers, when found, show tooth marks of gnawing animals. Nutrients are accumulated in the standing forest biomass (about net primary production of 18 kg/ha/year in the average deciduous temperate mesic forest (Perry:1994:301)), incorporated into animal cells made mobile (but now immobilized to plants), sequestered in stable humic compounds, or released in the soil (mineralized) where it may be taken by mycorrhizae fungi of plant roots or leached to sub soils or runoff and made unavailable to the forest community. (In some cases, mammals purposely take soil (mineral or "salt licks".)
Plants have low nutrient mass (compared to carbon) and herbivores, plant eaters - from insects to deer - must gather and process at high energy costs large amounts of plant material to gain the elements for life (nutrients) and the large amounts required (often annually) for reproduction.
| Nutrient concentrations of tree leaves (from Perry 1994:451) |
|||
| Oak | Ash | Maple | |
| Nitrogen | 2.59 | 1.79 | 1.33 |
| Phosphorus | 0.05 | 0.18 | 0.14 |
| Calcium | 1.15 | 2.12 | 1.87 |
| Potassium | 1.04 | 1.50 | 1.38 |
The nearby table suggests the low amounts of nutrients available (e.g., to a browsing deer) and the work required to accumulate the mass of the animal's body. As Perry generalized, "...herbivores can compensate somewhat for low nutrient concentrations in their diet by eating a large quantity." The energy and time costs that are associated with high levels of carbon consumption have low payoff in nutrients or survival gains.
Secondary chemicals (e.g., tannic acid) may thwart these efforts to gain nutrients.
The reproductive adult herbivore represents vast embodied energy (Odum 1983) required to reach that state. At its death (whether mouse of deer) about 90% is lost as moisture but the critically short nutrients (due to availability, not mass per unit area) are released within a small area potentially for uptake. The receiving structures must be present and active at the time. The annual requirements (critically specific in time and place) for N (proteins), P (ATP for energy bonds), and Ca (bone, muscle and nerve function) are enormous.
The percent of calcium (dry weight) in temperate forest plants is 1.57 (varying from 0.73 to 3.0), that of phosphorus, about 0.26, it too varying (0.10 to 0.60)(Perry 1994:347).
Phosphorus, even when abundant, is tightly bound in clays and may not be readily available to plants (Perry 1994:424). The carbon : phosphorus ratio in temperate forests is 630 : 1 (Perry 1994:415), requiring processing vast plant-mass to achieve the essential animal nutrients. Plant leaves are on average about 70% water, 27% organic, and 3 % minerals.
Carnivores have the same needs as herbivores. Some spend little energy and lay in wait, attacking prey when it comes near. Others spend great energy (and take high risks) in seeking, capturing, and processing animals to meet their energy and nutritional needs.
Omnivores have similar needs that may be met somewhat opportunistically.
The scarified seeds in the stomach or crop content of a dead granivore have an especially favorable place for germination and early plant growth.
References
Odum. H.T. 1983. Systems ecology: an introduction, John Wiley and Sons, New York, NY. 644pp.
Perry, D.A. 1994. Forest ecosystems. Johns Hopkins Univ. Press, Baltimore, MD 649pp.
Kimmins, J.P. 1997. Forest ecology, Macmillan Pub. Co., New York, NY 531pp.
Submitted by Robert H. Giles, Jr.
This Web site is maintained by R. H.
Giles, Jr.
Last revision June 5, 2001