23.3Biological Interaction and Population Dynamics
In the natural world, no single species lives independently in the environment; instead, all individuals interact with individuals of the same or a different species. The collection of individual organisms actually living together is called a population. Living organisms interact with each other, such as predators feeding on prey and competition over the same food or habitat. This section discusses such interactions between organisms.
Intraspecific competition and density effects
An experiment was performed in which some Drosophila were placed in a container filled with a specific amount of culture medium. Surviving flies of the first generation and the newly hatched generation of flies were then put together in a new container containing the same amount of culture medium. When this process was repeated periodically, the population of flies increased steadily until it reached a point at which the rate of increase gradually tapered off, until the population finally arrived at an approximately constant number (Figure 23-4). The sigmoid pattern of population density thus obtained is termed the logistic equation, and the population converges on the carrying capacity, which is the saturation value of the population density. The logistic equation is formulated in terms of two parameters, namely, the increase rate per individual r and the carrying capacity K, where the population density N is a variable.
Generally, if population density increases, competition for food and living space will intensify among individuals of the same species (intraspecific competition). This impedes individual growth, leading to a reduced birth rate, increased mortality rate, and an increase in the number of emigrating individuals. In other words, the increase in population density becomes a factor that inhibits further increase. This process is termed population regulation, while the effect depending on density is called the density effect. In this way, under uniform conditions of food supply and living space, the population size also remains at a constant level.
However, in environments where the food supply and habitat size change dramatically by climate, population regulation depending on density is difficult to maintain; therefore, the population density fluctuates sharply.
Interspecific competition and niche segregation
Organisms of different species also frequently compete over the same resources (interspecific competition). For example, two species of Drosophila—Drosophila melanogaster and Drosophila hydeiss—were intermingled and placed together in a container filled with a 1.5-cm-thick shallow medium. When the flies were bred for successive generations, the latter species almost completely vanished from the container within 10 generations (Figure 23-5A). This phenomenon of one species becoming extinct when two different species compete for the same resource is called competitive exclusion. However, when the same two flies were bred in a container filled with a 3-cm-thick deep medium for successive generations, D. hydei was able to coexist with D. melanogaster for a long period, but at a low incidence rate of approximately 5–10% (Figure 23-5B). When the medium was cut apart into upper and lower layers, each with a thickness of 1.5 cm, the upper layer was found to be inhabited by an overwhelming number of D. melanogaster larvae, while the lower layer showed only a few D. melanogaster larvae, but a comparatively large number of D. hydei larvae. This phenomenon can be explained by the fact that D. hydei larvae have the ability to burrow into deep parts of the medium where the partial pressure of oxygen is low.
Figure 23-5 Examples of competitive exclusion and coexistence by niche segregation (Drosophila melanogaster and Drosophila hydei)
The way in which an organism utilizes resources such as habitats and food is called niche. The phenomenon in which two competing species divide their habitats and food resources (habitat and food segregation) is referred to as niche segregation.
Many examples of coexistence between competing species based on niche segregation can be observed in the wild, particularly among closely related species. Orconectes virilis and O. immunis are closely related North American crayfish species. The former tends to be distributed in the lower reaches of rivers, while the latter tends to be distributed in the upper reaches. However, there are areas where the two species coexist. In areas where only one of the two species inhabits (allopatric), both species prefer to inhabit riverbed rocks. Whereas in areas where the two species coexist (sympatric), the less competitive O. immunis inhabits bogs rather than the originally preferred riverbed rocks to avoid competition (Figure 23-6).
The action whereby animals feed on other animals is called predation. A number of studies on the population relationships between predators and prey have demonstrated intriguing periodic fluctuations by means of mathematical models (Lotka-Volterra equation)*1 . Growth in the prey population eventually leads to an increase in the predator population, which in turn brings about a decrease in the prey population. The scarcity of prey then decreases the number of predators, thereby prompting the prey population to rise again. In this way, both populations fluctuate periodically. Such periodic fluctuations have been confirmed with experimental systems (microcosm) consisting of microorganisms and protozoa, and also by using Callosobruchus chinensis and its predator, Heterospilus prosopidis (Figure 23-7).
*1 Mathematical model expressing predator-prey relationships using simultaneous differential equations with two variables. Proposed separately by Alfred Lotka (1925) and Vito Volterra (1928).
Niche segregation and character displacement
The water snails Hydrobia ulvae and H. ventrosa live in river mouths in North Europe, eating algae on the surface of sand particles with their mouthparts. In allopatric habitats where only one of the two species inhabits, both species have the same size of shells and feed on the same size of sand particles. In contrast, in sympatric habitats where both of the two species inhabit, the shell sizes of H. ulvae snails become larger than those of H. ventrosa. Accordingly, the respective sizes of the sand particles they feed on also become differentiated. Such sympatric differentiation in the size and morphology of two species that have the same size and morphology under allopatric conditions is called character displacement. This is a phenomenon brought about through years of adaptation in environments shared by competitive species.
Column Figure 23-2 Niche segregation and character displacement of Hydrobia estuary snails
Major population fluctuations in the natural world
In the natural world, many factors other than predation can cause periodic population outbreaks. Thus, it would be rash to propose predation as the main factor responsible for any population fluctuation. For example, in the Hudson Bay area of Canada, the capture records of the fur association from the 19th century showed that snowshoe hare and lynx populations had fluctuated in a cycle of 10 years for over 100 years. According to the proposed Lotka-Volterra equation applied from the 1920s, this was believed to be a periodic fluctuation caused by predator-prey interaction. However, a 15-year, large-scale quadrat (60 ha) study in the Yukon territory of Canada between 1960s and 1980 revealed a 10-year cycle of the snowshoe hares in quadrats from which lynxes had been eliminated. This gave rise to a theory that the 10-year cycle fluctuation stemmed from a cycle of vegetation depletion by hares and its restoration.
Subsequently, an 18-year, large-scale quadrat (100 ha) study was conducted in Alberta, Canada, between 1980s and 1990s to investigate the effects of predator elimination and vegetation depletion. In this study, identically sized quadrats were set up and some were shielded from predators using nets to verify the effect of predator elimination. On the other hand, food was artificially supplied to hares during the winter in some quadrats to negate the effect of food depletion. Different combinations of these two conditions were investigated, and as a result, the 10-year cyclical fluctuations in the number of snowshoe hares disappeared only in the quadrats combining both predator elimination and artificial feeding during winter, and persisted in all other quadrats. This indicated that the cyclic fluctuation was indeed caused by the combination of the two factors.
In these areas, there are predators other than the lynx, including foxes, owls, and martens, while preys other than the snowshoe hare include field mice and grouse. Recently, various animal species forming a predator-prey complex have been revealed to follow a similar 10-year-cycle in Canada and North Europe.