Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm


20.2.1  Energy Ecology

The psychology of sentient extraterrestrials is closely intertwined with the details of the local ecology. But the most fundamental global ecological factor is energy. A more energetic environment normally can support either a larger number of similar-sized creatures or a similar number of larger-sized creatures. We know that all organisms require energy to survive, and that on most worlds virtually all of this must come from the local sun. In a sense, the star "feeds" the planetary inhabitants.

How much bioenergy is available to drive a global ecosystem? Clearly the first factors to consider are planet type and biochemistry type. Jovian planets and terrestrials will differ in the energy available to their native lifeforms. (These planetological problems are dealt with in chapters 4 and 5 in some detail, and will not be repeated here.) Also, cold ammonia-solvent lifeforms may require less energy to maintain than a similar population of hot liquid-sulfur creatures.

But assuming a terrestrial Earthlike world and a biocarbon biota, we begin our analysis by noting that the total energy available at the top of the atmosphere should be about 2 x 1017 watts. About 90% of this is lost due to reflection, absorption and direct conversion to heat, or because it consists of unusable infrared radiation. The remaining 10% is available for photosynthesis -- about 2 x 1016 watts.

The theoretical maximum efficiency for the chemical conversion of photon energy into organic matter (food) is about 36%.3220 However, the net observed efficiency of Earthly chlorophyllic plants generally runs from 1-5% in the field. The global average is even lower -- about 0.2% -- since the large open oceans are essentially lifeless aqueous deserts.48 While other worlds may evolve more efficient photochemistries, or have a larger biologically active land area, it is doubtful that the 0.2% rating will be much improved by natural evolution alone. So, in the case of Earth, this leaves 4 x 1013 watts.

The energy pyramid (also "food chain" or "food web") shown in Table 20.2 illustrates how ecologies are powered by sunlight.3221 At the base of the pyramid are the "primary producers" of food -- on Earth, the green plants. These producers are eaten by "primary consumers," or herbivores. The herbivores, in turn, are eaten by carnivores, who them selves are eaten by still larger carnivores. Ecologists customarily refer to each successive stage of predation as a "trophic level." Thus plants are at the first trophic level, the smallest carnivores at the third trophic level, etc.


Table 20.2 Energy Flow at Various Trophic Levels in the Terrestrial Food Chain

Biomass Maintenance
Energy Budget
for Maintenance
1 Man
80 watts
Middle Carnivores
(Tertiary consumers)
800 watts
Small Carnivores
(Secondary Consumers)
8000 watts
(Primary Consumers)
80,000 watts
(Primary Producers)
1000 tons
800,000 watts


The Energy Pyramid (Figure 20.1) describes the flow of useful bioenergy through an ecological system. Plants are eaten by herbivores, which in turn are eaten by higher-level carnivores. Typical food chains have 3-5 stages, called ‘trophic levels." Only about 10% of the latent bioenergy in each level is passed along to the next -- about 90% is wasted as heat or in respiration.

Omnivores, such as humans, may eat at all consumer levels. This permits larger populations to be supported. If men ate frogs instead of trout in the above example, 30 people could be supported. If he ate grasshoppers, 900 people could live. If he could consume grass, 2000 persons would survive.997


Figure 20.1 The Energy Pyramid


Also, in ecology there is something called the Diversity-Stability Rule: Ecosystem stability tends to be correlated with food web complexity. The four equal-population food web structures shown in Figure 20.2 illustrate various possibilities. In (A), there are too few herbivores; in (B), too few carnivores. Predators and prey are too specialized in (C), which consists of simple linear food chains. Because of the multiplicity of intertrophic links in (D), it has the greatest potential for adaptive stability.297

Alpine and polar environments tend to have fewer links and less stability, while tropical and oceanic environments are generally more stable.


Figure 20.2 Illustrations of the Diversity-Stability Rule


Experiments conducted in a wide range of different environments have measured ecological efficiency -- the energy gained by an organism when it eats a member of the next lowest trophic level. A good rule of thumb is that at each level 90% of the available bioenergy is lost. Only 10%, on the average, is passed along to consumers at the next highest level. Due to the staggering amount of waste, few ecosystems on this planet have more than five trophic levels. Xenologists expect these generalizations to hold true for extraterrestrial ecologies as well.

As Table 20.3 demonstrates, the effects of limited bioenergy on size of population are striking. If all humans were purely herbivorous, Earth theoretically could support 50 billion of them. But if people tap into the food web as Level-5 carnivores (e.g., man eats trout, trout eat frogs, frogs eat grasshoppers, and grasshoppers eat grass), the terrestrial ecology could support only 50 million humans worldwide. If this happened, each person would have to patrol a home range (personally or by proxy) of about 10 km2 in order to find his daily meal.*


Table 20.3 Maximum Supportable Large-Organism Population
in a Typical Terrestrial Global Ecology (Earth)
Trophic Level
Maximum Supportable
Population of 70 kg
Warmblooded Animals
(e.g., ~100-watt humans)
 4 x 1013
4 x 1012
Carnivores I
4 x 1011
Carnivores II
4 x 1010
Carnivores III 4 x 109


Similar bioenergy assays may be made of smaller ecosystems, say, on the continental, regional, or local levels. But the conclusions are almost always the same: Herbivores maintain the highest population densities and the smallest home ranges, while carnivores are usually fewest in numbers and utilize the largest home ranges. Omnivores, who can tap into the food web at any trophic level beneath, fall somewhere in between. They are the most versatile and adaptive, and thus most likely to survive in both the best and worst of times.

Xenopsychologists are interested in these results for a number of reasons. The motives, instincts, and personality traits of a sentient ET are likely to be strongly influenced by its hereditary feeding habits. An herbivorous race might be more socially-minded and less disposed to kill or commit acts of overt physical aggression. An intelligent carnivorous race might instinctively live in rather small groups, and value individuality and personal courage above all else.** A species with a large home range tends to be solitary and "antisocial."

Of equally great importance is the fundamental lesson of environmental finiteness. This turns out to be one of the central driving forces behind all animal and sentient behavior -- whether psychological, social, or political. It is easy to see why this is so.

All lifeforms that dwell on planets, regardless of their shape, size, or biochemistry, must "consume negentropy" to live. This requires a flux of energy. But if biological order and information are to increase -- a process which most xenologists regard as the basic "goal" of life -- then energy flow through the total ecosystem must also increase.

But planetary bioenergy is strictly limited. The natural supply of usable energy will be in short supply on any world.

Scarcity is inevitable.


* The population density of human beings on Earth today ranges from 0.0003 km2/person in Hong Kong to 1.0 km2/person in Mongolia, with a worldwide average of about 0.04 km2/person. Humanity, it would appear, is already herbivorous (trophic level 2, on global average).

** Except where large herbivores have evolved without an associated predator (e.g., elephants, 0.3 km2/animal1725), carnivores are normally larger than herbivores because a predator must be more powerful than its prey. Larger bodies can support larger brains, and predation is a more active lifestyle than grazing and thus requires more alertness; xenologists expect carnivores to be more intelligent as a general rule. This conclusion has been tentatively confirmed by modern paleoneurologists.2910


Last updated on 6 December 2008