Population interactions in primary succession: An example of contramensalism involving rock-colonizing bryophytes

Facilitation of later-arriving species by early colonists is one of several mechanisms that together drive the directional changes in species composition observed in primary succession. However, the other 'half' of this interaction - the effect of the facilitated species on its benefactor - is often unclear. If this effect is inhibitory, then the overall interaction is contramensalism (+, -). An example of a contramensal interaction is given. This involves the overgrowth of a complex taxonomic assemblage of several 'pioneer' bryophyte species by the liverwort Conocephalum conicum. The former facilitates the latter, as judged by a very highly significant enhancement of C. conicum's growth rate; while C. conicum inhibits (indeed kills) the overgrown pioneer assemblage. The mechanism underlying the facilitatory component of this interaction is not yet clear, but may involve elevation of microclimatic humidity. In contrast, the mechanism of inhibition is readily apparent: tissue death due to virtually complete blockage of light penetration. The directness of the link between small-scale localized interspecific interactions (such as this) and broad-scale community successional change depends on many factors; including the degree of physical disturbance.