Experimental Background to the Study of the Distribution and Abundance of Insects: I. The Influence of Temperature, Moisture and Food on the Innate Capacity for Increase of Three Grain Beetles

It is well known that in nature many species of insects vary in numbers from place to place at any one time, and in any one place their numbers fluctuate with the seasons. They tend to increase in numbers in favourable places or if we consider their numbers in one place they tend to increase during favourable seasons and to decrease during unfavourable seasons. Distribution from place to place and abundance at different times are two aspects of the one fundamental problem. Other things being equal species which can increase in numbers rapidly during the favourable periods will be more abundant than those which increase in numbers slowly. Thompson (1939) was aware of this and he pointed out that "the complex of factors which actually affect control in the case of any one species differs in composition from point to point and year to year in the area of distribution." Davidson and Andrewartha (1948) were able to identify the major factors determining numbers of Thrips imaginis in South Australia as they fluctuated from season to season. In any one year the number reached was a race against time, depending chiefly upon the extent of the favourable season. On the average the physical environment remained favourable for too short a period for the insects to increase to a point where competition became important. The complex of factors causing fluctuations in this and other natural populations can only be discovered by the ecologist who is prepared to study the natural popula-