Ecology: biology of interaction. 4.06. Classification of relationships between populations

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4.05. Lotka-Volterra Model

D. Shabanov, M. Kravchenko. Ecology: Biology of Interaction. Chapter 4. Population Ecology

April 7. Mutualism

4.06. Classification of relationships between populations Predation, parasitism, competition… Which level of biosystems do these concepts describe? Usually we understand them as relationships between organisms, although it is more accurate to consider them as interactions between populations. It is populations that are potentially immortal biosystems capable of evolution. A “mouse‑catching” fox caught and ate a field vole. Is this chance or a manifestation of a regular process? If foxes constantly eat field voles, such an interaction is of interest to ecology. A single interaction can be described at the organism level (although after the vole is eaten it ceases to exist as an organism). A persistent relationship can be represented as a relation between two populations. Or perhaps this interaction is more correctly described at the species level? Most likely not — at least because in a large part of the foxes’ range there are no certain species of field voles; the interaction involves not the species Vulpes vulpes (Linnaeus, 1758), the red fox, but separate populations of that species. Classifying relationships between populations is difficult; they are very diverse and linked by many transitions. Different authors use different classifications. Classifying relationships between species by the influence they exert on each other was proposed, for example, by the well‑known American ecologist Eugene Odum. He distinguished three types of population relationships: positive influence (+), negative influence (–) and no influence (0). However, correctly defining what “positive” or “negative” influence means is not easy. For instance, the influence of a predator population on a prey population is in some sense negative, and in some sense positive. For example, one of the few animal species whose numbers are not controlled by predators are African elephants. Although cases of lions killing and eating elephants have been recorded, an adult elephant is such a large prey that it is practically inaccessible to any predator. However, this was not always so. In the recent geological past (when Africa hosted a larger number of large herbivorous mammals, including members of the order Proboscidea) elephant numbers were regulated by saber‑toothed cats. Later this role was taken over by indigenous African elephant‑hunting tribes. Even later the same function was performed by armed poachers. And at the end of the 20th century elephants finally received protection. Large national parks were created where elephants can feel safe. Surprisingly, it turned out that staff of such parks must carry out periodic culling of elephants! The reason is that elephants affect the environment so strongly that, once their population exceeds a certain threshold, they can destroy all woody vegetation in savannas and woodlands, thereby undermining the resource base of their own existence. By limiting elephant numbers, predators could thereby increase the stability of this species’ persistence. So how to answer: is the influence positive or negative when predators (or culling) eliminate elephants? In the short term — negative (elephant mortality), in the long term — positive (stabilization of their dynamics). In order not to become confused by such approaches, we propose using a formal yet reliable method of separating interaction types between populations, based on the Lotka‑Volterra model: by how the abundance of one population changes in response to a change in the abundance of another. As you recall, in the form of the Lotka‑Volterra model presented in the previous section, the coefficients α and β describe the influence of individuals of one species on individuals of another. In the competition case discussed above we subtracted from the carrying capacity of the environment for one species the abundance of the other species multiplied by the appropriate coefficient. One could place a “+” sign before the coefficients, but consider them positive when an increase in the abundance of one species leads to an increase in the abundance of the other, and negative when an increase in one species’ abundance is followed by a decrease in the other’s. [IMG_1] On the basis of the foregoing, six basic forms of interaction between species can be distinguished. In addition, some of these forms can be divided into sub‑forms, as shown in Table 4.6.1. The meaning of the terms presented in this table will be explained in more detail later. Table 4.6.1. Classification of relationships between populations and species

Sign

Subtype

—

Competition

exploitative (without energy cost for interaction)

—

Exploitation

gall‑feeding or true predation

parasitoidy

parasitism

Symbiosis

cooperation (facultative, non-obligatory interaction)

proto‑operation (facultative, non‑obligate interaction)

—

Amensalism

Commensalism

Neutralism

A few words should be said about the term "symbiosis," which etymologically means "living together." Different authors use it in different meanings. Sometimes it denotes any coexistence, sometimes only mutually beneficial, sometimes only inseparable. In this course, this term is used according to the meaning given in Table 4.6.1. Since the meaning of this term can be vague, it may be best to abandon its use altogether. However, for mutually beneficial relationships between organisms, one of which is the habitat for the other, the use of the term "endosymbiosis" is generally accepted. Since interactions between individuals and populations in natural ecosystems are infinitely diverse, other approaches can be used to classify them, each of which draws attention to one aspect of such interactions. First of all, interactions should be divided into direct and indirect. When a fox catches voles, the interaction of populations occurs through the interaction of individuals. This is a direct interaction between populations. When dung beetles feed their larvae with the dung of ungulates, there is no direct interaction of individuals, but the beetles are affected by the resource produced by the ungulates. This is an indirect interaction between populations mediated by the abiotic environment. Finally, as a result of vole reproduction, fox hunting activity may shift to them, which will reduce the level of exploitation of the hare population. This is an example of interactions mediated by other populations (or indirect interactions). We are unable to account for all the consequences of a particular event and the functioning of a particular population. Like ripples on water, changes caused by its activity will spread throughout the ecosystem. But, like ripples on water, in most cases, these consequences will become less and less pronounced. That is why direct and indirectly mediated by the abiotic environment interactions are most often considered in ecology courses. To account for interactions mediated by other populations or chains of such populations, mathematical models are most adequate. The original classification of relationships between species was proposed by the Russian zoologist V.M. Beklemishev. He distinguished topical connections (which are expressed in changes in the habitat; sphagnum acidifies the soil and makes it favorable for the sundew), trophic connections (feeding of individuals of one species by individuals of another, as well as their remains and waste products); factory connections (related to providing habitat or shelter; a woodpecker makes hollows in a pine tree, and fleas live in a dog's fur), and phoric connections (transport of individuals of one species by individuals of another).

4.05. Lotka-Volterra Model

D. Shabanov, M. Kravchenko. Ecology: Biology of Interaction. Chapter 4. Population Ecology

April 7. Mutualism