Biological communities: Change and adjustment in plant and animal communities
The approach in this paper is from the stand point of a systematic zoologist interested in verte brate groups. The illustrations are taken, and the ecological factors emerge, from a study of birds. Admitting that the rate and degree of change in the total environment of any organism may be negligible in relation to the short term of a single individual life history, no study of dis tribution nor hypotheses of speciation and phylogeny can disregard the factor of environ mental change. Some of the factors to be taken into account in varying degree are:
(a) Rates of growth, habits, and numerical status of other plants and animals in the area;
(b) the trend of vegetational succession;
(c) climatic changes and cycles; and
(d) the geo-chronology of the area.
In the case of the last two, methods of' assess ment and evaluation are indirect, and it is not proposed in this short paper to deal with them. Consideration of the first two is, however, prompted by field study of problems of increase and decrease, extinction, and the nature of eco logical barriers and isolating mechanisms. In particular, the significance of habits and temperament are considered as these are factors more readily assessed in birds than in most other animals.