BIOLOGY Volume 1 - A Guide to General Biology - 2004

10. ORGANISMS AND THE ENVIRONMENT

10.3. Ecosystems and Energy Flow

10.3.5. Efficiency of Energy Transfer: Productivity

The Study of the productivity of various ecosystem components essentially involves tracing the diverse Pathways of Energy transfer within the ecosystem. Energy enters the biotic community via producers, and the rate at which it is stored by them in the form of edible organic matter is termed primary productivity. This parameter is crucial because it determines the total amount of energy available to all organisms in the community, i.e., the total biomass of the ecosystem.

10.7. Besides plants, which other groups of organisms contribute to the primary productivity of an ecosystem?

As noted in Section 10.3.1, The amount of solar radiation reaching the Earth's surface depends on geographical latitude and landscape features, such as altitude and slope aspect. The fraction of this radiation assimilated by plants is determined by its quality and The Structure of the plant canopy. In Britain, the average amount of energy reaching plants is approximately 1 · 10⁶ kJ · m^-2 · yr^-1. Of this amount, 95–99% is immediately lost through reflection, heating, and evaporation. The remaining radiant energy (1–5%) is absorbed by chlorophyll and used to synthesize organic molecules. The total chemical energy accumulated by plants is called gross primary production (GPP). Approximately 20–25% of GPP is expended by the plant itself in Respiration and Photorespiration, while the remainder contributes to an increase in its biomass, known as net primary production (NPP). It is this energy that is potentially available to the next trophic level.

By consuming other organisms, heterotrophs transfer matter and energy from one trophic level to another. However, not all consumed food goes into increasing consumer biomass. Part of the energy is lost through respiration; other losses include metabolic end products (excretes); furthermore, food is not entirely digested, and this indigestible fraction is lost as feces (excrement).

Class="center">Consumed food = Growth + Respiration + Excrement + Excretes

However, taking the ecosystem as a whole, no waste is generated: what is excreted or discarded by some serves as food for others—decomposers and detritivores.

Some Components of the equation above can be readily measured in domestic animals or wild animals in the laboratory. Growth is measured as an increase in biomass, or better yet, as an increase in the ENERGY VALUE OF the Organism per unit of time. The amount of feed consumed over the same period, as well as the feces and urine produced, can easily be weighed. Knowing these values, one can calculate through simple arithmetic how much energy was expended on respiration.

What remains for heterotrophs after all inevitable losses—fueling their growth, tissue repair, and reproduction—is termed secondary production (regardless of the trophic level).

Figure 10.10 clearly demonstrates that energy is lost at each link of the food chain, and its length obviously depends on the magnitude of these losses. The First stage—The conversion of solar radiation into primary production—is the least efficient. Subsequently, as Energy is transferred from one trophic level to another, energy losses are much smaller. The average transfer efficiency from autotrophs to herbivores is close to 10%, and from one animal to another, around 20%. In general, herbivores assimilate food less efficiently than carnivores because plants contain high amounts of Cellulose and other substances that are typically indigestible and are egested in the feces rather than converted into secondary production.

Fig. 10.10. Energy Flow in a grazing food chain. All values are expressed in kJ · m^-2 · yr^-1. R – energy losses via respiration; E – energy losses transferred from the grazing food chain to the detrital food chain via excretes (urine, etc.) and excrement. C – consumption by organisms of a higher trophic level.

Energy expended on respiration is no longer available to other organisms. However, the fraction corresponding to excretes and feces is not lost to the ecosystem: it is utilized by decomposers and detritivores. Similarly, any dead organism, fallen leaves, twigs, and branches form the foundation for new detrital food chains. Detrital chains are often very complex and much less studied than the traditionally examined grazing chains. Nevertheless, in terms of energy transfer, they are frequently of greater importance.

The proportion of NPP flowing directly into the detrital food chain depends on the type of ecosystem. In a forest, the bulk of primary production enters the detrital food chain rather than the grazing one: litter, forest floor, and soil humus are the primary sites of activity for forest heterotrophs, even though these organisms are usually inconspicuous. Conversely, in the upper layers of the ocean or in intensively managed pastures, more than half of the NPP may enter the grazing food chain. In the most intensive agricultural systems, the potential role of detritus-based food production is largely ignored.

The figures cited in this section are calculated on an annual basis. If an ecosystem is in equilibrium—i.e., not undergoing directional change through succession (Section 10.6.1)—its total biomass remains unchanged after such a period. All energy converted into primary production passes to subsequent trophic levels and is gradually dissipated along the way, leaving no net accumulation of NPP. However, such a steady state in ecosystems is quite rare. For example, a young forest grows over the course of the year, sequestering a portion of solar energy within the tree biomass. The annual period is a convenient baseline for evaluating the energetic aspects of an ecosystem because a year encompasses all seasonal changes occurring within it. For instance, primary productivity typically peaks in spring and summer when autotrophic growth accelerates, after which secondary productivity begins to predominate.

One of the reasons for the close scrutiny of ecosystem energetics is that this field of ecology is intimately linked to human food and fuel production. It allows for the analysis of agricultural system efficiency and the proposition of Ways to improve them. Because energy is lost at each trophic level, it is evident that for omnivores like humans, vegetarianism is a more energetically efficient way of exploiting ecosystems (Table 10.2). However, when analyzing dietary efficiency, other factors must also be considered. For example, animal protein typically provides more Essential Amino Acids than plant protein, although certain legumes, particularly soybeans, come close in this regard. Furthermore, animal Tissues are easier to digest because their Cells are not enclosed by rigid walls like those of plants, which must first be broken down. Finally, in many ecosystems, animals concentrate matter gathered over vast areas of land unsuitable for agriculture. Examples include livestock grazing on low-quality pastures (sheep in Britain, reindeer in Scandinavia, elands in East Africa) or fisheries.

Table 10.2. Efficiency of agricultural food chains in Great Britain