On the Relationship between r/K Selection and Environmental Carrying Capacity: A New Habitat Templet for Plant Life History Strategies

Abstract

We propose a revision of the habitat templet approach for modelling the relationship between r/K selection and selection related to the level of resource impoverishment within vegetation. The latter is referred to in our model as "I (Impoverishment)-selection" and is represented as a continuum on one axis of the templet by the mean annual environmental carrying capacity of the habitat. This is perpendicular to the second axis representing the traditional r/K selection continuum and defined by the mean annual distance below environmental carrying capacity that the vegetation is maintained at, usually as a consequence of different levels of disturbance. The model thus recognizes three broad categories of selection but their relationship is defined by the interaction of two independent two-way selection continua rather than by a ''three-axes'' selection continuum as in both the triangular C-S-R model and the Southwood-Greenslade habitat templet. Each of the three categories of selection (r, K and I) reaches its maximum intensity along a separate side of the quadrangular templet. Thus, two of these are at maximum intensity simultaneously in each of two corners: r- and I-selection in one corner and K- and I-selection in the other. This leads to four rather than three ''extreme'' types of stragey (one in each of the four corners) which are contrasted on our version of the templet in terms of predicted differences in the relative allocation of photosynthate to photosynthesizing, reproductive, structural and defensive plant parts. Unlike the triangular C-S-R model and the Southwood-Greenslade habitat templet, the model proposed here does not predict a general decrease in competition intensity with increasing resource impoverishment. This leads to predictions for secondary successional trajectories which differ from those derived from the triangular model. Based on this revised habitat templet the "species-pool" hypothesis is proposed as an alternative to the "hump-back" model of species density variation across habitat fertility gradients.