The Biotic Environment

mating and oviposition. Among Odonata, where mating immediately precedes egg laying, 705

males also protect females as they oviposit.

Territoriality with respect to food availability may be seen in some species, especially parasitoids and social insects. For example, female ichneumons, braconids, chalcidids, and scelionids (Hymenoptera) may mark the host either chemically or physically as they oviposit so that other females of the species do not lay in the same host. Such behavior ensures that the offspring will have adequate food for complete development. (The marks may also be the means by which hyperparasites locate a host!) Social insects defend both their nest and foraging sites against members of other colonies.

4.2. Interspecific Interactions 4.2.1. Competition and Coexistence

An important form of interaction is when an insect competes with other organisms for the same resources. Grasshoppers, sheep, and rabbits all eat grass, and if this is in short supply the presence of the mammalian herbivores will have a very obvious effect on the distribution and abundance of grasshoppers living in the same area. However, as noted earlier, food is seldom a limiting factor as far as the abundance of animals is concerned, and the other requirements of these three species are so different that the species can coexist perfectly well. The collection of requirements that must be satisfied in order for a species to survive and reproduce under natural conditions is described as a niche. Thus, a niche includes both physical and biotic requirements, and its complexity varies with the environment in which a species finds itself. For example, as noted in Chapter 22, Section 3.2.3, the critical day length for induction of diapause in a species may vary with latitude. Equally, with reference to biotic requirements, the complexity of a niche will differ according to the number and nature of other species utilizing the same resources. The more closely two species are related, the more nearly identical will be their requirements (i.e., their niche), and the greater will be the degree of competition between them where the two species coexist. Normally, in this situation the less well-adapted species becomes extinct or restricted to areas where it can again compete favorably with the other species as a result of different environmental conditions, a phenomenon known as competitive exclusion or displacement. In the absence of competition, a species' niche will be broader (less complex); that is, a species' requirements will be less stringent and form the so-called "fundamental" niche. Conversely, the niche occupied by a species that coexists with others is known as the "realized" niche.

A well-documented example of competitive exclusion in insects involves three species of chalcidid, belonging to the genus Aphytis, which are parasitoids of the California red scale, Aonidiella aurantii, found on citrus fruits. In the early 1900s, the golden chalcidid, Aphytis chrysomphali, was accidentally introduced into southern California, probably along with red scale on nursery stock imported from the Mediterranean region, though it is a native of China. During the next 50 years, A. chrysomphali spread along with its host throughout the citrus-growing area and exerted a reasonable degree of control over red scale, particularly in the milder coastal areas. However, in 1948, a second species, also Chinese, A. lingnanenis, was introduced in the hope of obtaining even better control of the pest. During the 1950s, A. lingnanensis gradually displaced A. chrysomphali so that, by 1961, the latter was virtually extinct, being restricted to a few small areas along the coast. However, A. lingnanensis was ineffective as a control agent of red scale in the inland citrus-growing areas around San

FIGURE 23.5. Changes in the distribution of Aphytis chrysomphali, A. lingnanensis, and A. melinus in southern California between 1948 and 1965. [After P. DeBach and R. A. Sundby, 1963, Competitive displacement between ecological homologues, Hilgardia 34:105-166. By permission of Agricultural Sciences Publications, University of California; and P. DeBach, D. Rosen, and C. E. Kennet, 1971, Biological control of coccids by introduced natural enemies, in: Biological Control (C. B. Huffaker, ed.). By permission of Plenum Publishing Corporation and the authors.]

Fernando, San Bernadino, and Riverside, where annual climatic changes are greater. It was found, for example, that periods of cool weather (18° C or less for 1 or 2 weeks) or several nights of hard frost caused high mortality of all stages. Even light overnight frosts (—1°C for 8 hours) killed sperm in the spermathecae of females, which did not mate again, and rendered males sterile. Also, exposure of females to a temperature of 15°C for 24 hours led to an increase in proportion of male progeny. These factors caused a reduction in "effective progeny production" (number of female offspring produced per female) from 21.4 to 4.5 and a resultant inability of the species to control red scale populations. Consequently, a third species, A. melinus, was introduced from India and Pakistan in 1956 and 1957. This species rapidly displaced A. lingnanensis from these inland areas, and by 1961 virtually the entire population of chalcidids in these areas was made up of A. melinus (DeBach and Rosen, 1991). Changes in the distribution of the three Aphytis species between 1948 and 1965 are summarized in Figure 23.5.

Competitive displacement does not always occur, however, because closely related organisms have evolved mechanisms that enable them to occupy almost but not quite the same niche. These mechanisms include habitat selection (spatial selection), microhabitat selection, temporal (diurnal and seasonal) segregation, and dietary differences. Two or more of these mechanisms may operate simultaneously to prevent competition between species.

Spatial segregation is shown by the distribution of A. lingnanensis and A. melinus in southern California, which DeBach and Rosen (1991) considered to have stabilized, with A. lingnanensis occupying the milder (less climatically extreme) coastal districts and A. melinus the interior. Spatial separation is also seen in larval damselflies (Zygoptera), which

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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