Ecology: Biology of Interaction. IV-06. Classification of Relationships Between Populations
The well-known American ecologist Eugene Odum proposed classifying relations between species (or populations) by their effects on one another. He distinguished three types of effects: positive (+), negative (−), and neutral (0). However, correctly defining positive and negative effects is not always straightforward.
IV-6. Classification of relationships between populations. Predation, parasitism, competition... What level of biological systems do these concepts describe? Usually, we mean 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. When hunting, a fox caught and ate a vole. Is this an accident or a manifestation of a regular process? If foxes constantly eat voles, such an interaction is of interest to ecology. A single interaction can be described at the organism level (although after the vole was eaten, it ceased to exist as an organism). A constant relationship can be represented as a relationship between two populations. Or perhaps this interaction is better described at the species level? Most likely, no. If only because in a significant part of the fox's range, certain species of voles are absent; it is not the species Vulpes vulpes (L., 1758), the common fox, that interacts with them, but individual populations of this species. Classifying relationships between populations is not easy; they are very diverse and have many transitions. Different authors use different classifications. For example, the famous American ecologist Eugene Odum proposed classifying relationships between species (or populations) based on their impact on each other. He identified three types of population relationships with each other: positive impact (+), negative impact (-), and no impact (0). However, it is not easy to correctly define what a "positive" or "negative" impact is. For example, a predator population affects a prey population negatively in some ways and positively in others. Thus, one of the few animal species whose numbers are not controlled by predators is the African elephant. And although cases of lions killing and eating elephants have been recorded, an adult elephant is such a large prey that it is inaccessible to almost any predator. However, it was not always so. In the recent geological past (when Africa was inhabited by a significantly larger number of large herbivorous mammals than today, including proboscideans), the elephant population was regulated by saber-toothed cats. Later, this role was taken over by indigenous African elephant hunters. Even later, armed poachers performed the same function. And at the end of the 20th century, elephants were finally protected. Large national parks were created where elephants could feel safe. However, strangely enough, it turned out that the staff of such national parks have to carry out periodic culling of elephants! The fact is that elephants affect their environment in such a way that, by multiplying above a certain limit, they can destroy all tree vegetation in the savanna and woodlands, thereby undermining the resource base for their own existence. Thus, by limiting the number of elephants, predators could thereby increase the sustainability of this species. So, how to answer: is the impact of predators (or culling) positive or negative in this case? In the short term, it is negative (death of elephants); in the long term, it is positive (stabilizes their population dynamics). Precisely to avoid confusion in approaches, we propose to use a formal but reliable method for dividing types of interactions between populations, based on the Lotka-Volterra model: by how the number of one population changes in response to changes in the number 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 impact of individuals of one species on individuals of another. In the case of competition discussed above, we subtracted the number of individuals of another species multiplied by the corresponding coefficient from the carrying capacity for one species. We can put a "+" sign before the coefficients, but consider them positive if an increase in the number of one species leads (in the short term!) to an increase in the number of another, and negative if an increase in the number of one species leads to a decrease in the number of another. Based on this, we can identify 6 main forms of interaction between species. In addition, some forms can be further divided, as shown in Table IV-6.1. The meaning of the concepts presented in this table will be explained in more detail later. Table IV-6.1. Classification of relationships between populations and species.
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Table 4.6.1. Classification of relationships between populations and species |
Sign |
Interaction type |
|
|
α |
β |
||
|
— |
— |
— |
Competition |
|
exploitative (without energy cost for interaction) |
|||
|
+ |
— |
— |
Exploitation |
|
gall‑feeding or true predation |
|||
|
merophagy or pasture predation (feeding on parts) |
|||
|
parasitoidism |
|||
|
+ |
+ |
+ |
Symbiosis |
|
proto‑operation (facultative, non‑obligate interaction) |
|||
|
— |
0 |
0 |
|
|
+ |
0 |
0 |
|
|
0 |
0 |
0 |
|
A few words should be said about the term "symbiosis," which etymologically means "living together." Different authors use it with different meanings. Sometimes it denotes any coexistence, sometimes only mutually beneficial coexistence, and sometimes only inseparable coexistence. In this course, the term is used according to the meaning shown in Table IV-6.1. Since the meaning of this term can be vague, it might be better 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 very diverse, other approaches can be used to classify them, each considering one aspect of such interactions. First of all, interactions should be divided into direct and indirect. When a fox catches voles, the interaction between 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 between individuals, but the beetles are affected by the resource produced by the ungulates. This is an interaction between populations mediated by the abiotic environment. Finally, as a result of vole reproduction, fox hunting activity may shift to them, reducing the level of exploitation of the hare population. This is an example of indirect interactions mediated by other populations (or indirect interactions). We cannot account for all the consequences of a particular event or the functioning of a particular population. Like ripples on water, the 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 ecology courses most often consider direct interactions and interactions mediated by the abiotic environment. To account for interactions mediated by other populations or chains of such populations, it is best to use mathematical models. The original classification of interspecies relationships was proposed by the Russian zoologist V.M. Beklemishev. He distinguished topical connections (expressed in changes in habitat: sphagnum acidifies the soil and makes it favorable for sundew), trophic connections (feeding of individuals of one species by individuals of the opposite sex, as well as their remains and waste products); fabric connections (related to providing habitat or shelter: a woodpecker makes a hollow in a pine tree), and phoric connections (transport of individuals of one species by individuals of another).