The aim of Connell and Slatyer (1977) was to propose some alternative mechanisms for successional change to one proposed by Clements (1916). He had proposed that earlier colonizing species had a net positive effect on later colonists, facilitating their establishment. We simply suggested that the other two possible net effects, negative and null, should be included as alternatives. Thus earlier colonists could either inhibit later ones, have no effect, or facilitate them. We also suggested various field experimental tests of the three hypotheses. the 10 years since the Connell and Slatyer (1977) paper was published, a great deal of research on successional mechanisms has been done. From it we have come to realize that instead of alternatives, these mechanisms represent the extremes of a continuum of effects of earlier on later species. Also, the strengths and directions of these interactions could vary within a successional sequence. commenting on these models, Walker and Chapin (1987) suggest that all studies fit neatly into these three relation to their own studies they state (pp. 131): In summary, stochastic, life history, facilitative, competitive, and herbivory processes all affect the interaction between alder and spruce during succession, and no single successional process or model (sensu Connell and Slatyer 1977, Tab. 1) adequately describes successional change on the Alaskan floodplain. Particularly important was the observation that alder increased soil nitrogen content and reduced the degree of nutrient limitation of spruce growth (suggesting facilitation), although its net effect on spruce growth was inhibitory (Walker and Chapin 1986). The models of Connell and Slatyer (1977) do not adequately describe such successional changes for a very simple reason: they were never designed to do so. They were intended to model only one aspect of succession: the net effect of an earlier species on a later one. Other aspects, e.g. variations in seed availability, weather, floods, abundances of herbivores, etc., were simply not included in these models. Variations in density of the species were implicity included, since interactions, either between plants or with their herbivores, occur only when the density of one or both species is sufficiently high to produce a significant interaction. If we confine our attention to the net effects of the interactions for which the models of Connell and Slatyer (1977) were designed, then they appear to fit some of the successional changes on the Alaskan floodplain. Although Walker and Chapin (1986) found that spruce seedlings growing in alder soil suffer less nitrogen limitation, the net effect was that alder inhibited spruce seedling growth in the field. This inhibition could be removed by herbivory, since snowshoe hares prefer to browse alder over spruce (Bryant and Chapin 1986: Fig. 15.3). Connell and Slatyer (1977: 1123) describe their inhibition model as follows: once early colonists secure the space and/or other resources, they inhibit the invasion of subsequent colonists or suppress the growth of those already present. The latter invade or grow only when the dominating residents are damaged or killed, th s r leasing resources. This statement seems to describe quite well the alder-spruce-snowshoe hare interaction o the Alsakan floodplain described by Walker and Chapin (1986). While no species simultaneously facilitated and inhibited another in the study by Walker and Chapin (1986), such contrasting effects remain a possibility in this and other successions. While the possibility of such simultaneous contrasting mechanisms was not foreseen by Conn l and Slatyer (1977), the net effect of such interactions must be either positive, negative or zero. the latter case, the tolerance model applies; then life history att ibutes may determine how one species replaces another.
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Jun 1, 2013
Seyed Mahdi Mousavi Toroujeni + 2
