Efficiency Yield and Stability

In many circumstances, ecological efficiency can be increased by more intensive predation, but only at the cost of reducing the availability of energy for population maintenance and probably lowering the total yield from that population. For example, if the fishing rate is excessive, a prey population may have a very high ecological efficiency, but also a dangerously low population size.

Whole organisms or parts of organisms in the focus population may be eaten. Feeding on another organism does not automatically infer the death of that organism. Parts of living plants may be eaten without killing the plants as in the feeding of bovines on grasses. Certainly, most macroscopic plants are partially eaten by herbivores as can be confirmed by examining almost any leaf. Almost all leaves have their outline broken by missing portions and their surfaces marked by tunnels, galls, or holes.

Some animals can be eaten in such a way as to not kill them. For example, parrotfish browse on the branches of coral without killing the entire coral. Stone crab claws are commercially harvested by breaking them off the living animals that are then released to presumably regenerate new claws. Clams are often fed upon by fishes in such a way as to leave the animals alive, removing only vulnerable parts of their anatomy - particularly the siphons.

Obviously, there are many procedures by which a focus population may avoid predators or at least minimize the harm from higher trophic levels. Mechanisms range from the flight of small birds before a hawk, the secretion of foul odors by many insects, and digging further into sediments by clams in the wake of crab attacks. Correspondingly, there are an endless variety of ways in which predators attack their prey. All parasites can be considered predators that have evolved ways of feeding that do not immediately kill their prey.

Most of the solar energy impinging on green plants dissipates as heat. The rest drives the photosynthetic process, converting light energy to chemical energy in organic molecules. The photosynthetic organisms usually use most of the sugars and carbohydrates that they produce for growth, reproduction, and repair.

Usually there is not a perfect local balance between photosynthetic energy fixation and energy losses to respiration. Residual material, particularly in oligotrophic situations, is a relatively small fraction of the organic material produced by the photosynthesizers in that ecosystem. This residual material may be washed away and used by some distant organisms, or buried, joining sediment or adding to fossil fuel or to the brown color of soil. The existence and condition of organic material in sediments depends broadly on the availability of water and oxygen for decomposer bacteria and molds.

The relative quantities of consumed energy that are dissipated as heat, used in growth and reproduction, stored as fats or oils, or passed on to higher trophic levels, vary among organisms, and vary with different times and circumstances. However, heat and buried or dissolved carbon compounds are the universal ends of all energy that enters an ecosystem.

Because some potential energy is converted to heat at each trophic level, the total energy per unit time that flows through the system must decrease with trophic level. If the maintenance cost per gram of live standing crop tissue is approximately independent of trophic level, a pyramid of standing crop abundance is generated, the classical 'Eltonian pyramid'.

When growth and metabolic rates (per calorie-day maintained) are very high in the lower trophic levels, and relatively low in organisms at higher trophic levels, the trophic pyramid can be inverted - with small standing crops of plants and herbivores turning over rapidly at the bottom while maintaining large populations at higher trophic levels. Examples include some aquatic systems that are based on phytoplankton.

Those organisms that are not food for any other organisms in the system have by definition ecological efficiency equal to zero. While the ecological efficiency of a top predator population is usually considered to be zero, but parasites on or in these predators can be taken as an even higher trophic.

In fact, there is probably no animal or plant population that actually has ecological efficiency of zero. Occasionally, even elephants, wolves, and tigers, if they are very young or very old, fall to predators or carry parasites, and even the driest and most toxic of vegetation usually shows obvious signs of having been partially consumed by some herbivore or mold.

In addition to feeding predators, energy is used in processes of growth and reproduction. Plants and animals both do work in moving internal fluids and intracellular parts. Animals are more mobile than plants, and a correspondingly larger portion of their ingested energy is expended as movement against external forces and friction.

In a growing population, the rate of energy consumption is greater than the rate at which energy is discharged from the population. At its upper limit, ecological efficiency is greatest when a population is rapidly growing and such a short time interval is considered that no deaths or losses of potential energy from individuals have occurred. At the lower limit, ecological efficiency of the focus population goes to zero as a starving population approaches extinction or if there are no predators.

In short, ecological efficiency will vary with conditions. Ecological efficiency will approach a steady state only if abundance, age distribution, and size distribution of the organisms in the focal population are not changing and neither are the predation rate and the distribution of age and size of the prey.

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