Predator-prey relationship

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Population fluctuations in predator and prey according to the Lotka-Volterra model. Typically, the predator population chases after the prey population .

The predator-prey relationship or, more rarely, the predator-prey relationship describes the dynamic interaction between a predator and a prey population over longer periods of time. It is a simplified model of a section of the food webs that are analyzed in the biological department of ecology . Predator-prey relationships can be represented mathematically and can, to a limited extent, be used to predict future population developments.

Demarcation

Predator-prey relationships represent a section of a food chain from one of the food webs that are a subject of research in the Department of Ecology . The technical term of the robber in a broader sense includes not only the real predators and parasites and parasitoids and grazers in their role as consumers of plant food . The modeling described below can be applied to all four cases. With the model, however, no population trends can be viewed from species that feed on dead organic matter, because with population fluctuations of scavengers , detritus eaters and decomposers can be expected no change in size of a prey population.

In nature there are numerous complex reaction patterns in the relationship between predator and prey, and their explanation is an essential, central area of ​​ecological theory. Due to the variety of different relationships, transferring from one system to another is difficult. In some cases a predator decimates a prey population to a fraction of its unaffected density, in other cases the influence of a predator on a prey population is hardly detectable. On the one hand, it is important whether a predator specializes in a certain type of prey or whether it is a generalist with numerous equivalent types of prey. Between these two extremes there is a wide range of cases of varying preference. On the other hand, the effects of other species and interactions with environmental factors are always significant.

Systems in which the predator regulates the density of its prey or in which the density of both is subject to cyclical fluctuations are of particular interest for ecological analysis. As a rule, numerous other factors such as affect food , air, offer habitat - competitors , pathogens, parasites, stress and other predators also the population sizes (see population dynamics ).

With the aim of representing and investigating general dynamic properties of predator-prey relationships, various mathematical models have been created in theoretical biology . The simplest and best known is the Lotka-Volterra model. The basis is the work of the Austrian mathematician Alfred J. Lotka and the Italian mathematician and physicist Vito Volterra , who independently formulated the Lotka-Volterra equations that are named after them in 1925 and 1926 . These are mathematical differential equations in which the quantitative aspect of population development as a function of time was presented for the first time. They are based on the logistic equation . The biological applications of these equations are known today as the first, second, and third Lotka-Volterra rules .

A computer simulation that illustrates the predator-prey relationship is the Wator simulation by Alexander K. Dewdney and David Wiseman.

The Lotka-Volterra model

In the Lotka-Volterra model, predator and prey species show coupled frequency fluctuations. A rich supply of food and prey enables the predators to raise many offspring, so that the predator population grows. The greater population density of the predators leads to a decimation of the prey population and thus to an inadequate food supply for the predators, so that fewer or no young animals can be raised and weak adult predators starve to death. This decline in the predator population now allows the prey population to recover and the periodic process starts over.

In the model, however, there are so-called neutral stable cycles. That means: The cycles are created without external influences, the cycle length results from the choice of the variables (without timer), without external interference, these cycles would continue to run forever without any deviation. However, cycles that can actually be observed in natural systems cannot normally arise due to this mechanism; due to the inevitable and always acting fluctuations of the environmental variables, populations subject to the model dynamics would in reality fluctuate acyclically and erratically. There are probably no populations whose fluctuations could be explained solely by the model. Nevertheless, the model is useful as a first approximation to explain coupled fluctuations in population density.

The most famous case in which coupled, delayed cycles have actually been observed in nature for the population of a predator and its prey are the cycles of the snowshoe hare ( Lepus americanus ) and its predator, the Canadian lynx ( Lynx canadensis ). The species show a cycle of about ten years in length (actually observed: 9-11 years) over a huge area (a large part of the north of North America, from Alaska to Newfoundland). This example has even been used in school textbooks. Originally interpreted as a particularly striking example of an oscillation of the Lotka-Volterra type, according to more recent studies the situation here is much more complicated. Large populations of rabbits seem to collapse mainly from lack of food. However, food as such is not scarce here (the rabbits don't eat their habitat bare), but good food with a high nutritional value. The grazed plants can form food poisons (toxins) when they are heavily grazed and are therefore less easy for the rabbits to eat. However, they only form these (energetically expensive) toxins when there is high feeding pressure. The interaction of the "predator" snowshoe hare and its vegetable "prey" seems to drive the cycle here. The lynx therefore only follows passively. This example (which is by no means explained down to the last detail) clearly shows that one should beware of simple explanations of complex facts, even if they seem to fit well into the model used for the explanation.

Other models

The number of muskrats is not determined by the number of predators, but a density-dependent phenomenon

Investigations carried out by the American zoologist and ecologist Paul Errington (1946) into the predator-prey relationship between muskrats and minks show a completely different behavior. The mink is the most important predator of the muskrat, but the population size of the muskrat is less influenced by the number of their predators than by the population density of the territory. Above all roaming animals with no territory or injured animals are prey of the mink. The individuals who would have had the lowest chance of survival anyway are given preference. In this case, the population size of the prey is limited to a regulated density by the eco-factor predator , which is given by the eco-factors food and space for building . Similar cases were found very frequently in other studies.

literature

Web links

Individual evidence

  1. ↑ Paws without claws? Large carnivores in anthropogenic landscapes [1]
  2. ↑ Nils Chr. Stenseth, Wilhelm Falck, Ottar N. Bjørnstad and Charles J. Krebs: Population regulation in snowshoe hare and Canadian lynx: Asymmetric food web configurations between hare and lynx . In: Proceedings of the National Academy of Sciences . Volume 94, No. 10, 1997, pp. 5147-5152
  3. ↑ Uri Wilensky: NetLogo Models Library: Wolf Sheep Predation. In: NetLogo Models Library :. Retrieved November 27, 2018 .