WHAT IS THE LAW OF THE ECOLOGICAL TITHE OR 10%? - BIOLOGY - 2025

The law of tithing ecological, ecological law or of 10% raises how energy travels in its derivation by different trophic levels. It is also often argued that this Law is simply a direct consequence of the second Law of Thermodynamics.

Ecological energy is a part of ecology that is concerned with quantifying the relationships that we have outlined above. It is considered that Raymond Lindemann (specifically in his seminal work of 1942), was the one who established the foundations of this area of ​​study.

Figure 1. Trophic Network. Source: By Thompsma, from Wikimedia Commons

His work focused on the concepts of food chain and web, and on the quantification of the efficiency in the transfer of energy between the different trophic levels.

Lindemann starts from the incident solar radiation or energy that a community receives, through the capture carried out by plants through photosynthesis and continues to monitor this capture and its subsequent use by herbivores (primary consumers), then by carnivores (secondary consumers) and finally by decomposers.

What is the ecological tithe law?

After Lindemann's pioneering work, trophic transfer efficiencies were assumed to be around 10%; in fact, some ecologists referred to a law of 10%. However, since then, multiple confusion has been generated regarding this issue.

Certainly there is no law of nature that results in precisely one tenth of the energy that enters one trophic level being transferred to the next.

For example, a compilation of trophic studies (in marine and freshwater environments) revealed that transfer efficiencies by trophic level ranged between approximately 2 and 24%, although the mean was 10.13%.

As a general rule, applicable to both aquatic and terrestrial systems, it can be said that the secondary productivity by herbivores is usually located approximately, an order of magnitude below the primary productivity on which it is based.

This is frequently a consistent relationship that is maintained in all foraging systems and that tends to become pyramidal-type structures, in which the base is provided by the plants and on this base a smaller one is established, of the primary consumers, on which another (even smaller) secondary consumers is based.

Organization levels

All living things require matter and energy; matter for the construction of their bodies and energy to carry out their vital functions. This requirement is not limited to an individual organism, but extends to higher levels of biological organization that these individuals can conform.

These levels of organization are:

• A biological population: organisms of the same species that live in the same specific area.
• A biological community: set of organisms of different species or populations that live in a certain area and interact through food or trophic relationships).
• An ecosystem: the most complex level of biological organization, made up of a community related to its abiotic environment - water, sunlight, climate and other factors - with which it interacts.

Trophic levels

In an ecosystem the community and the environment establish flows of energy and matter.

The organisms of an ecosystem are grouped according to a "role" or "function" that they fulfill within the food or trophic chains; this is how we talk about the trophic levels of producers, consumers and decomposers.

In turn, each and every one of these trophic levels interacts with the physicochemical environment that provides the conditions for life and, at the same time, acts as a source and sink for energy and matter.

fundamental concepts

Gross and net primary productivity

First, we must define primary productivity, which is the rate at which biomass is produced per unit area.

It is usually expressed in units of energy (Joules per square meter per day), or in units of dry organic matter (kilograms per hectare per year), or as carbon (mass of carbon in kg per square meter per year).

In general, when we refer to all the energy fixed by photosynthesis, we usually call it gross primary productivity (PPG).

Of this, a proportion is spent in the respiration of the autotrophs themselves (RA) and is lost in the form of heat. Net primary production (PPN) is obtained by subtracting this amount from PPG (PPN = PPG-RA).

This net primary production (PPN) is what is ultimately available for consumption by heterotrophs (these are bacteria, fungi and the rest of the animals that we know).

Secondary productivity

Secondary productivity (PS) is defined as the rate of production of new biomass by heterotrophic organisms. Unlike plants, heterotrophic bacteria, fungi and animals, they cannot make the complex, energy-rich compounds they need from simple molecules.

They always obtain their matter and energy from plants, which they can do directly by consuming plant material or indirectly by feeding on other heterotrophs.

It is in this way that plants or photosynthetic organisms in general (also called producers), comprise the first trophic level in a community; primary consumers (those that feed on producers) make up the second trophic level and secondary consumers (also called carnivores) make up the third level.

Transfer efficiencies and energy pathways

The proportions of the net primary production that flow along each of the possible energy pathways ultimately depend on the transfer efficiencies, that is, on the way in which the energy is used and passes from one level to another. other.

Energy transfer efficiency categories

There are three categories of energy transfer efficiency and, with these well defined, we can predict the pattern of energy flow at trophic levels. These categories are: consumption efficiency (EC), assimilation efficiency (EA) and production efficiency (EP).

Let us now define these three categories mentioned.

Mathematically we can define the consumption efficiency (EC) as follows:

EC = I _n / P _n-1 × 100

Where we can see that the EC is a percentage of the total available productivity (P _n-1) that is effectively ingested by the upper contiguous trophic compartment (I _n).

For example, for primary consumers in the grazing system, EC is the percentage (expressed in units of energy and per unit of time) of the PPN that is consumed by herbivores.

If we were referring to secondary consumers, then it would be equivalent to the percentage of productivity of herbivores, consumed by carnivores. The rest die without being eaten and enter the decay chain.

On the other hand, the assimilation efficiency is expressed as follows:

EA = A _n / I _n × 100

Again we refer to a percentage, but this time to the part of the energy that comes from food, and ingested in a trophic compartment by a consumer (I _n) and that is assimilated by their digestive system (A _n).

This energy will be that available for growth and for the execution of work. The remainder (the part not assimilated) is lost with the faeces and then enters the trophic level of the decomposers.

Finally, the production efficiency (EP) is expressed as:

which is also a percentage, but in this case we refer to the assimilated energy (A _n) that ends up being incorporated into new biomass (P _n). All the unassimilated energetic remnant is lost in the form of heat during respiration.

Products such as secretions and / or excretions (rich in energy), which have participated in metabolic processes, can be considered as production, P _n, and are available, as corpses, for decomposers.

Global transfer efficiency

Having defined these three important categories, we can now ask ourselves about the "global transfer efficiency" from one trophic level to the next, which is simply given by the product of the aforementioned efficiencies (EC x EA x EP).

Expressed colloquially, we can say that the efficiency of a level is given by what can be effectively ingested, which is then assimilated and ends up being incorporated into new biomass.

Where does the lost energy go?

The productivity of herbivores is always lower than that of the plants on which they feed. We could then ask ourselves: Where does the lost energy go?

To answer this question, we must draw attention to the following facts:

1. Not all plant biomass is consumed by herbivores, as much of it dies and enters the trophic level of decomposers (bacteria, fungi and the rest of detritivores).
2. Not all the biomass consumed by herbivores, nor that of herbivores consumed in turn by carnivores, is assimilated and is available to be incorporated into the consumer's biomass; a part is lost with the faeces and thus passes to the decomposers.
3. Not all the energy that is assimilated is actually converted into biomass, since some of it is lost as heat during respiration.

This happens for two basic reasons: Firstly, due to the fact that there is no energy conversion process that is 100% efficient. That is, there is always a loss in the form of heat in the conversion, which is perfectly in line with the Second Law of Thermodynamics.

Second, since animals need to do work, which requires energy expenditure and, this in turn, implies new losses in the form of heat.

These patterns occur at all trophic levels, and as predicted by the Second Law of Thermodynamics, part of the energy that one tries to transfer from one level to another is always dissipated in the form of unusable heat.

References

1. Caswell, H. (2005). Food Webs: From Connectivity to Energetics. (H. Caswell, Ed.). Advances in Ecological Research (Vol. 36). Elsevier Ltd. pp. 209.
2. Curtis, H. et al. (2008). Biology. 7th Edition. Buenos Aires-Argentina: Editorial Médica Panamericana. pp. 1160.
3. Kitching, RL (2000). Food Webs and Container Habitats: The natural history and ecology of phytotelmata. Cambridge University Press. pp. 447.
4. Lindemann, RL (1942). The trophic – dynamic aspect of ecology. Ecology, 23, 399-418.
5. Pascual, M., and Dunne, JA (2006). Ecological Networks: Linking Structure to Dynamics in Food Webs. (M. Pascual & JA Dunne, Eds.) Santa Fe Institute Studies in the Sciences of Complexity. Oxford University Press. pp. 405.