The Biotic Environment

winter of 1940 resulted in a great shortage of grasshopper food and a decline in population 703

numbers. Only a few small areas of land remained moist enough to support growth of grass, and surviving grasshoppers congregated in these areas. But so did the birds that normally fed on the insects and they reduced the population density to an extremely low level (Andrewartha, 1961).

Lower than normal densities may also have a serious effect in species that modify their environment, for example, social insects. The temperature and humidity within the nest are normally quite different from those outside, being regulated by the activity of members of the colony. If a proportion of the population is removed or destroyed, those individuals that remain may no longer be able to keep the temperature and humidity at the desired level and the colony may die. Another example is the lesser grain borer, Rhizopertha dominica (Coleoptera), which is a serious pest of stored grain in the United States. In damaged (cracked) grain beetles can survive and reproduce even at low population density. However, in sound grain only cultures whose density is quite high will survive because the insects themselves, through their chewing activity, can cause sufficient damage to the grain that it becomes suitable for reproduction. The nature of this suitability is unknown.

Below a certain level of population, the so-called "threshold density," the chances of survival for a population are slim because of the unlikelihood of a meeting between insects of opposite sex and in reproductive condition. This is important in two areas of applied entomology, namely, quarantine service and biological control. Quarantine regulations are designed so that for a given pest the number of individuals entering a country over a period of time is sufficiently low that the chances of the pest establishing itself are very slim. In biological control programmes that use insects as the controlling agents, experience shows that it is better to release the insects in a restricted area, especially if they are limited in number, rather than distributing them sparsely, in order to improve the chances of establishing a breeding population.

Populations of many insect species may be considered self-regulating; that is, should the density of the population fall below normal (though not below threshold) it will, over time, return to its original level. There may be various reasons for this. As a species' density falls, its predators may experience greater difficulty in finding food so that they migrate elsewhere or produce fewer young. As a result of the decline in predator density, a larger proportion of the prey species may survive to reproduce. If this continues for several generations, the original population density may be reestablished. Another possibility is that with a decrease in density, there will be a greater choice of oviposition and, perhaps, resting sites. Selection of the best of these sites will again increase the chances of survival of an insect or its progeny and lead to a population increase. Some species have rather more specific mechanisms for overcoming the disadvantages of underpopulation. For example, females of some species use facultative parthenogenesis in the absence of males, which serves not only to maintain continuity of the population, but, as the progeny are generally all female, any offspring that do find a mate can make a substantial contribution to the next generation. In the desert locust adult females in the solitary phase (Chapter 21, Section 7) live longer, so that the chance of encountering a male is increased and, further, produce up to four times as many eggs compared to gregarious females.

4.1.2. Overpopulation

As population density rises beyond the normal level, members of a species will increasingly compete with each other for such resources as oviposition sites, overwintering

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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