Some ecological characteristics of the primary trophic level of a secondary succession

Abstract

Simultaneous studies have been made of the macroscopic organisms of a series of sites in a secondary succession to determine the extent to which the bionomic strategies of the organisms and the features of the com­munities show consistent patterns or trends. In the estimation of primary productivity allowance was made for turnover. Although standing crop and litter biomass increased through succession, the level of primary production remained within the range 750–980 g m -2 . The following trends, with successional age, were recorded: leaf life expectancy in­creased, the amount of biomass allocated to reproduction fell, as did the ratio of flowering to reproductive structures in the herb layer, average germinule size increased and seedling survival fell. The flowering struc­tures of the dicotyledons were predominantly hermaphrodite in the field seral stages, but monoecious in the woodland trees; dioecious plants were relatively rare. The early successional plants were often self pollinated; this mechanism is characteristic of very few of the plant species found in the other seral stages. Insect pollination was the most frequent mechanism in the forbs of the old field and wind for the woodland trees. In the assessment of these characters, from direct observation or from existing published information, frequency was expressed not only simply as species presence, but also by weighting the different species for their relative abundance in the vegetation of that site: the patterns revealed by these approaches differed in emphasis. The seed bank closely predicted the composition of the early successional flora. The pattern of succession in these sites would seem to conform most closely to the initial floristic composition model of Egler ( Vegetatio 4, 412 (1954)), with a tolerant stage followed by an inhibitory stage, the latter in the old field. Here the conditions provided by the thick grass vegetation maintain large popu­lations of small mammals and molluscs that increase the mortality of seedlings, including tree seedlings, and so retard the rate of succession. Non-native (immigrant) plant species contributed up to 16% of the vegetation cover in the first two years of the succession, but thereafter they were rare or absent. Because the comprehensive nature of the study often precluded elaborate replication, the general conclusions could be limited by the extent that the findings were site specific, therefore com­parisons have been made with other studies of particular features to evaluate the biological robustness of the findings. Within the trends and patterns summarized above there were interesting exceptions, but the life-history traits observed conformed to previously published predic­tions for habitats with similar levels of productivity, but varying levels of disturbance.