Cascading effect in ecosystems

We must be grateful to scientific journalists when they take up the ungrateful work - to retell the results of research in a popular form, originally published in publications intended for specialists. However, with such a retelling, inaccuracies and even errors are sometimes allowed, which creates legal occasions for criticism from scientists. In the TRV-HOUSENT No. 114 for October 9, 2012, we could read the note “Invertebrates on the illuminated streets” of Natalia Reznik. It was written based on the materials of the article published in the journal Biology Letters . It is about the fact that more different invertebrates living on the surface on the surface of the soil of invertebrates (spider -shaped, beetles and representatives of other groups) than at some distance are collected on city lawns. The result, I must say, expected, and, in my opinion, scientifically not so interesting. The authors of the article emphasize that the difference between illuminated and unlit areas is especially clearly manifested for two environmental groups of invertebrates - predators and scavengers. From this, the author of the newspaper publication concludes: increased concentration predators And Padalshchikov Indicates on What​​ because of​ Svetoy pollution ecosystems can slide on more short Trophic level . I note that for the initiates it sounds incomprehensible, but by the word “slide” they can guess what was understood by the authors of the original article.

Pavel Kvartalnov, an ornithologist (and this almost automatically means that a person is observant and ethical), in his “living magazine” drew attention to this phrase. There, he brought a quote from the source, from which it became clear that it was a “trophic stunt”. Here is its fragment: ... Ecosystem Services may be altern Through Cascading from Higher to Lower Trophic LEVELS. What are these “cascading effects” that affect the important properties of the ecosystem and acting “from top to bottom”, from predators located on the upper floors of the food pyramid to plants at the very bottom?

The concept of the concept dates back to the work of three prominent American ecologists published back in 1960 - Nelson G. Hairston, Frederick E. Smith) and Lawrence B. Slobodkin, which made readers to think about some well -known facts. The point is that most often limits the growth of the number of organisms located at different trophic levels. In short, the essence of the reasoning of these authors (what is called the HSS hypothesis) can be reduced to the following theses:

1) The entire surface of the land (with the exception of glaciers and the hottest deserts) is covered with vegetation. Accordingly, herbivorous animals (phytophages) are generally not limited by food. With rare exceptions in which a person is usually guilty, we do not observe such that the animals completely eat all the vegetation.

2) If herbivorous animals are usually not limited by food, then the question arises: what restrains the growth of their populations? Apparently, the factors acting “from above”, that is, the press of predators.

3) But what limits the growth of populations of predators themselves? They just may not have enough food. In addition, there is a real danger of undermining your own food base - a decrease in the number of victims to a level that is insufficient to feed on it. This does not happen primarily due to the mechanisms of regulating the density of their own populations, for example, such as territoriality - the protection of the hunting area from strangers. We can observe territoriality among many animals: from small insectivorous birds to lions and tigers. And the singing of the male finish, and the growl of Leo is a warning to the neighbors that this territory is already busy.

4) The last reasoning of HSS applies to saprotrophs, that is, animals, mushrooms, bacteria, feeding on dead "organic". A huge mass of plant fabrics dies annually on land, but nowhere this mass of plant opada forms a thick layer covering the whole earth. The forest litter is still very thin. The absence on the surface of the sushi deposits of dead organics is ensured exclusively by the activity of saprotrophs. Here they are limited by food, but they cannot undermine their feed base. Food falls on them literally from the sky, for example, leaves and branches of trees, and their task is to grab their piece as soon as possible. Hence the fierce intraspecific and interspecific competition, a selection, aimed primarily at increasing the speed of mastery of free resources.

But back to the trophic cascade, to a situation in which “predation” (in the broad sense of the word) restrains the growth of the number of organisms of the underlying level. The most clearly cascading effect is not manifested in ground ecosystems, which include the HSS hypothesis, but in the aquatic environment, in the ecosystems of the aquatic thickness of lakes and oceans. If on land the main producers are large multicellular organisms, higher plants, then in water it is phytoplankton-a combination of microscopic planktonic algae and cycle-bacteria. With a relatively small average biomass, phytoplankton is characterized by very high products (an increase in biomass). Due to the consumption of phytoplankton, there is zooplankton, represented mainly by small crustaceans. Zooplankton, in turn, serve as food for Plandonophages fish, which themselves become victims of larger predatory fish.

Until the 50-60s of the last century, it was believed that the number and biomass of organisms of any trophic level is determined exclusively “from below”, the number of existing foods. This was partly confirmed by practice. For example, mineral fertilizers were added to fish -breeding ponds, thereby increasing the products of phytoplankton, and therefore, improving the feed base of zooplankton. Well, since the fish grown in ponds (especially their young ones) is eaten by zooplankton, the growth of their mass increased.

However, more and more information was gradually accumulated that the restriction “from above”, that is, the existence of predators, is sometimes no less important than the “below” restriction. For example, it was previously believed that it is possible to deal with the “flowering of water” (the massive development of plankton algae and cyanobacteria) only by reducing the intake of mineral nutrition elements (primarily phosphorus), the lack of which is the increase in the number of phytoplankton. However, later it turned out that it was possible to weaken the “flowering of water” in a completely unexpected way-the instilling of predatory fish in a reservoir that feed on smaller Plandonophages. A decrease in the number of fish feeding on zooplankton leads to the fact that the latter becomes much larger. Accordingly, its press on phytoplankton intensifies: as a result, “flowering of water”, if it does not stop, then at least significantly weakens. This is the classic example of a trophic cascade - a series of reinforcements and weakening of the press of “predators” (consumers) turning from level (from top to bottom).