12.2Interaction between Organisms and the Dynamics of Individual Groups


What Is an Individual Group?

It is not the case that individual organisms are living on their own separately from others. All organisms belonging to the same species have their habitats, methods to feed themselves, and breeding seasons in common, while maintaining close relationships between individuals. This type of group of interacting organisms inhabiting a certain area is called an individual group. Even though they are of the same species, two individuals separated from one another by a mountain, river, or valley, or by an urban area or farmland not suitable to their habitat belong to different individual groups as they do not have direct relationships in their lives. For example, DNA analyses have revealed that Asian black bears, distributed from the Chugoku to the Kinki regions, are divided into four individual groups. The genetic interaction between these individual groups appears to be obstructed by large rivers.

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Density Effect and the World Population Growth

Fig. 12-1. Time Change of Drosophila Population

Animal Demography Converging on the Carrying Capacity, NHK books, 1972: revised from Figure II-1 on page 29.

After breeding a small number of drosophilae in a container filled with a specific amount of agar medium, a combination of surviving flies and ones that hatched in the next generation was moved to a new container. When this was repeated periodically, the population of the flies increased steadily until it reached a point where intraspecific competition for limited resources and living space intensified, thereby decreasing the survival rate and fecundity (i.e., the density effect). The rate of increase then gradually tapered off until the population finally arrived at an approximately constant number (Fig. 12-1). The sigmoid pattern of population growth obtained under conditions such as this one is called a logistic curve, and most plants and animals conform to this pattern.
This effect however does not simply apply to the population of humankind (Fig. 12-2). Since the dawn of history, the world population has been increasing slowly over an extended period until around the 18th century. However, the past 100 years or so have witnessed a dramatic surge in the growth rate with the population amounting to approximately 6.6 billion in the year 2006. By altering the environment on their own, humankind has been augmenting the production of commodities required for comfortable living. We are therefore going to have to pay a high price in the near future just because density effect did not work with us.

Fig. 12-2. Transition of the World Population

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Interspecific Competition and Niches

Organisms of different species also frequently engage in competition for the sake of same resources (interspecific competition). Two species of drosophilae, Drosophila melanogaster and Drosophila hydei, were intermingled and introduced to 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 (Fig. 12-3A). This phenomenon is called competitive exclusion. When the flies were bred in a container filled with a 3-cm-thick deep medium for successive generations; however, D. hydei could coexist with D. melanogaster for a long period, albeit in a low proportion (approximately 5–10%) (Fig. 12-3B). The medium was cut apart into upper and lower layers with the thickness of 1.5 cm each to count the number of larvae of each species. As it turned out, although the overwhelming majority of inhabitants in the upper layer was D. melanogaster larvae, a comparatively large number of D. hydei larvae vis-à-vis those of the former species were found in the lower layer. This is attributable to the fact that D. hydei larvae have the ability to burrow into deep parts of the medium where partial oxygen pressure is low. A niche (ecological niche) refers to a "way in which an organism utilizes resources," e.g., habitats and food. As in the example above, a phenomenon in which two competing species divide their habitats and food resources (habitat isolation/food segregation) is referred to as niche differentiation.

Fig. 12-3. Examples of Competitive Exclusion and Coexistence by Niche Differentiation (Drosophila melanogaster and Drosophila hydei)

(A) Breeding for successive generations in a shallow medium. Whether the initial percentage of Drosophila melanogaster was 20% or 80%, it reached 100% after around 10 weeks. The numbers in the figure represent the number of vanished, repeated lineages.
(B) Breeding for successive generations in a deep medium. The initial frequency of D. melanogaster in this figure is 20%. Revised from The Niche in Competition and Evolution, (Arthur. W.), Wiley, 1987: p. 74. Fig. 5.5 (A), p. 73, Fig. 5.3 (B)

Fig. 12-4. Niche Differentiation of two Species of Red Swamp Crayfish Orconectes to Riverbed Habitats

A number of examples of coexistence between competing species owing to niche differentiation are observed in the wild (Fig. 12-4). Orconectes virilis and O. immunis are closely related North American crayfish species. The former tends to be distributed more in the lower reaches of a river and the later in the upper reaches. Both species prefer to inhabit rocks at the riverbed. However, in the middle reaches where the two species are distributed together, the less competitive O. immunis relocates its habitat from the originally preferred location, i.e., riverbed rocks, to bogs in order to coexist with its competitive counterpart.
The notion of "niche" is somehow comparable to the economic activities in human society. In the food service industry, for example, having many hamburger chains offering indistinguishable services will end up inviting fierce competition resulting in exclusion. Each franchiser will constantly attempt to differentiate their niche from competitors by launching new products with some added value. Also, there are many venture companies in pursuit of niches overlooked by major corporations that can set out to do businesses in niche industries. Since, etymologically, the words ecology and economics both derive from a Latin word "oikos" meaning a "house," both these fields have quite a few concepts in common. Ecology can conceivably be defined as the "economics of organisms" in the natural world.

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Predation Action

The action of animals to feed on other animals is called predation. A number of studies have been conducted on the population relationships between predators and prey as they display intriguing periodic fluctuations. A growth in prey population eventually leads to an increase in predator population, which in turn brings about a decrease in prey. The scarcity of prey then decreases the number of predators, thereby prompting the prey population to rise again. Both populations therefore fluctuate periodically. The human population, in contrast, does not decrease even if fish population is depleted by overfishing etc. In the end, the populations of herrings and sardines drop to the point where fishing is no longer viable.
In the natural world, there is a possibility that a number of other factors aside from predation action, such as climate and food conditions, bring forth fluctuations in populations. It would therefore be too rash to conclude predation as the factor responsible for any fluctuation in population. A notorious instance would be the population fluctuation of snowshoe hares and lynxes in 9-year cycles. The periodic outbreak of snowshoe hares had long been attributed to predator-prey interaction since the 1930s (Fig. 12-5). In the '70s, however, the population fluctuation of the snowshoe hare was observed in places where no lynx inhabited, thus leading some researchers to postulate that the fluctuation stemmed from a cycle between vegetation depletion by hares and its restoration. Studies conducted from the '80s to the '90s in several 100 ha research areas in Alaska finally put an end to this dispute. Artificially supplying food to hares in winter concurrently with excluding lynxes from the enclosed research areas finally brought the fluctuation to a closure. In other words, the fluctuation caused by the predatory carnivore eating the prey herbivore overlapped with the fluctuation in the plants eaten by the prey herbivore.

Fig. 12-5. Periodic Fluctuation of Snowshoe Hare and Lynx Population

Revised from MaCLurich, D.A., Univ. Toronto Stud. Biol. Ser. 43: 1-136, 1937


Blood Relationships and the Evolution of Sociality

The sociality of animals has evolved on the basis of kinship groups. In this regard, it differs considerably from the sociality of humans, who construct society with total strangers. It is requisite for avian and mammalian parents to build nests and look after their eggs or offspring. In light of their territory and kinship, chicks of some species of birds (e.g., Garrulus lidthi) remain in the proximity of their nests even after reaching the reproductive age to take care of their siblings being born subsequently; such individuals are called helpers. Even mammals such as African Wild Dogs and jackals that remain in packs where they were born after having grown up have helpers to look after their siblings. Partiality in the fecundity of adult individuals gives rise to helpers. The larger the number of helpers in a pack is, the higher the survival rate of the cubs (helpers' siblings) becomes. Helpers may have evolved on account of the advantages that they gain by taking over the territory from their parents and remaining there while taking care of their siblings.
Let us consider the worlds of lions and chimpanzees as examples to illustrate the kinship within packs. A pride of lions consist of a few male parents, several female parents, and their cubs, with the male parents being brothers. Once grown up, lion brothers are supposed to leave their pride and take over another in cooperation with each other. In the process, they kill all the cubs of other males and reproduce anew with the lionesses in the pride. Newly born cubs are all siblings and cousins. In case of chimpanzees, on the other hand, the females leave the troop while brothers remain there. The brothers protect the troop together and reproduce offspring with female chimps that enter their troop. The reproductive fitness of lions and chimpanzees is not equal even among brothers; there is partiality between individuals.
The worlds of insects such as honeybees and hornets are also composed of kinship groups. A queen gives birth to workers, which are all sisters that look after their mother and the youngest sister (the next queen).
As has been described above, social behavior within animal groups stems from the need to secure territories for reproduction, and contributes to the maintenance of kinship groups in those territories. Highly systematized sociality is thought to have evolved among animals through such social behavior as well as the impartiality in fecundity.

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Parasitism and Symbiosis

There are two kinds of relationships in which one species exploits the other through close interactions between individual organisms: parasitism, wherein one benefits at the expense of the other; commensalism, wherein one benefits while the other is neither harmed nor helped; and mutualism, wherein both species benefit reciprocally. Parasitism is classified into ectoparasitism, wherein leeches and ticks as well as plants such as mistletoe and Aeginetila indica absorb nutrients from the surface of their hosts, whereas endoparasitism, wherein roundworms, tapeworms, and many bacteria invade the body. Being generally far bigger than parasites, hosts do not usually die immediately after being parasitized unlike the cases of predation. Nevertheless, a disease or damage caused by the parasites can be fatal to the hosts.
Mutualism ranges from relationships in which both species live so close that each is essential to the other to ones where each can do without the other notwithstanding the benefit they receive (sometimes referred to as cooperation). The examples of two species living as an integrated unit include: termites and their intestinal bacteria that decompose the fibers of wood; cattle and bacteria in their rumen that decompose cellulose; and leguminous plants and root nodule bacteria.

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