BIOLOGY Volume 1 - A Guide to General Biology - 2004

10. ORGANISMS AND THE ENVIRONMENT

10.7. Population Ecology

10.7.5. Interspecific Interactions Affecting Population Size

Studies of population dynamics in natural environments can rarely be restricted to a single species, as species almost invariably interact with one another. Such interactions are referred to as interspecific. Organisms occupying different trophic levels engage in predator-prey and host-parasite relationships. More subtle relationships may also occur, benefiting one or both partners—such as commensalism and mutualism. At the same trophic level, interspecific competition can take place, for example, over food and space. This sometimes leads to resource partitioning.

Predator-Prey Relationships

A simple model of these interactions is well illustrated by a laboratory experiment involving two mite species: the predatory mite Typhlodromus and the phytophagous mite Eotetranychus. Fig. 10.21 demonstrates their cyclical population fluctuations, which are slightly out of phase with each other.

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Fig. 10.21. Population fluctuations of the predatory mite Typhlodromus and its prey, the phytophagous mite Eotetranychus, under laboratory conditions. (From M. K. Sands (1978) Problems in Ecology, Mills & Boon Limited.)

These curves are easily explained: an increase in the prey population (i.e., food supply) leads to a surge in the reproduction of their consumers—the predators (a "demographic boom"). In turn, this accelerates the mortality of the phytophagous mites, causing a "demographic crash" among them, which is inevitably followed by the starvation and predation of the predators themselves. A cycle is considered complete when the reduced number of predators allows the prey population to grow once again. Each such cycle evidently spans several generations.

Although predation is only one of the factors regulating population size, its significant role is unquestionable. Some data regarding its importance and long-term beneficial effects on prey populations come from the Kaibab Plateau in Arizona. In 1906, a game preserve was established there, and to protect the deer population, culling of their primary predators—pumas, wolves, and coyotes—was carried out over the next 30 years. Prior to 1906, the deer population was stable (approximately 4,000 individuals), but then, as shown in Fig. 10.22, a population explosion occurred, exceeding the carrying capacity of the available pastureland (estimated at around 30,000 individuals). The deer population grew exponentially until 1924, without leveling off, and reached an estimated 100,000 individuals. Subsequently, food shortages and disease led to a demographic crash. Moreover, overgrazing severely reduced pasture productivity, and despite a sharp drop in grazing pressure, the carrying capacity never returned to its 1906 level, eventually settling at 10,000 individuals.

Fig. 10.22. Growth curve of the deer population on the Kaibab Plateau following the eradication of predators.

Host-Parasite Relationships

Certain studied populations of parasites and their hosts (such as parasitoid insects infesting other insects) exhibit similar, though slightly out-of-phase, cycles.

Interspecific Competition

Competition between populations within an ecosystem can occur for any available resources—food, light, space, shelter, etc. If two species occupy the same trophic level and feed on the same organisms (plant or animal), they are highly likely to act as competitors. Over time, dietary specialization in one or both of these species may occur, allowing them to occupy different niches within the same trophic level. This process, which minimizes competition, is known as resource partitioning (see below). If competing species occupy the exact same niche, or if their niches heavily overlap, two outcomes are possible: either a steady stalemate is established, or one species proves less competitive and experiences a gradual decline in population size leading to complete local extinction. The latter scenario is termed competitive exclusion.

In natural environments, studying interspecific competition is challenging, but its existence was confirmed by the classical experiments of Russian biologist G. F. Gause, who worked with several species of the Ciliate genus Paramecium in 1934. Some of his results are illustrated in Fig. 10.23.

Fig. 10.23. Population growth of two Paramecium species: A — cultured separately; B — in mixed culture.

10.17. Having examined Fig. 10.23, answer the following questions:

a) What type of population growth curve is characteristic of species kept in isolation from one another?

b) What resources might these two species compete for in a mixed culture?

c) What factors give P. aurelia a competitive advantage over P. caudatum?

When P. aurelia and P. caudatum live together, the former feeds more actively, resulting in a decline in the population of P. caudatum after five days, and its complete disappearance ("extinction") from the mixed culture after roughly 20 days. This is the phenomenon known as competitive exclusion. However, the P. aurelia population reaches its stationary growth phase more slowly than when grown in isolation, meaning it also suffers from the effects of competition despite its advantages. This helps explain the selective pressure that drives both species to partition their niches. In natural conditions, the complete extinction of the less competitive species rarely occurs; the species simply becomes rare.

The competitive exclusion principle, or Gause's principle, has since been confirmed in experiments with various animals and plants, such as in mixed cultures of different duckweed species (Lemna).

Studying competitive exclusion in natural populations is complicated by the large number of interacting species, as well as by fluctuations in variables such as Temperature, humidity, and food availability, which inevitably affect the relative competitive abilities of the organisms in question and prevent a stable equilibrium from being established.

Considerable attention is drawn to a special form of competition associated with the release by certain species of organic substances that inhibit the growth of other organisms. In plants, this interaction is known as allelopathy. Such chemical warfare is particularly prevalent among microorganisms that secrete various Antibiotics and growth inhibitors to "monopolize" the nutrient substrate. A well-known example is penicillin, produced by Molds of the genus Penicillium, which suppresses the growth of Gram-positive Bacteria (Sec. 12.11.1).

Resource partitioning

The more specialized an Organism's ecological niche, the lower its chances of encountering serious competitors. In long-established, species-rich communities, evolution (coevolution) has led to resource partitioning (niche differentiation), i.e., the specialization of its members in utilizing resources that, for various reasons, are little needed by others. This enhances both the Stability of the Ecosystem and Its productivity.

Resource partitioning occurs in various ways, for example, through:

1) morphological and behavioral specialization in feeding exclusively on a specific type of food; thus, the beaks of various birds are shaped to adapt them for catching insects in flight, hammering tree trunks, cracking nuts, tearing meat, etc.;

2) vertical habitat segregation (stratification, see Fig. 10.15): some species live in the forest canopy, others in the forest floor litter;

3) horizontal habitat segregation, i.e., distribution among different microhabitats—depressions, hummocks, forest openings, etc.

However, despite the tendency of each species toward specialization, some niche overlap and the resulting interspecific competition still persist.