Coexistence of similar species in a space-limited intertidal zone

The lower intertidal zone (0.0 to +1.0 m mean low water [MLW]) of rocky sho res in New England is a space-limited community occupied by two similar rho dophyte seaweeds, Chondrus crispus and Mastocarpus stellatus, that overlap broadly in their use of three essential resources: space, light, and nutrie nts. C. crispus coexists primarily with the prostrate-crust generation of M . stellatus lower on the shore (less than +0.35 m MLW) and with the alterna te upright-frond generation higher on the shore (greater than +0.35 m MLW). Our objectives were to determine (1) whether these two species compete and (2) if so, what process(es) enable their coexistence. Upright fronds of M. stellatus transplanted to the lowest intertidal zone (less than +0.25 m ML W) where C. crispus predominates grew faster and showed higher survivorship after 2 yr than those transplanted to areas where M. stellatus predominate s. However, the failure of upright fronds of M. stellatus to consistently r ecruit limits their abundance in the lowest zone and reduces the frequency of preemptive competition by C. crispus. Moreover, when M. stellatus was gr own in combination with fronds of C, crispus in this zone, the dominant com petitor C. crispus suppressed the growth and reproductive output of M. stel latus fronds. Neither species was significantly consumed by littorinid gast ropods in held and laboratory experiments, indicating that herbivory does n ot control patterns of coexistence. In the low zone, coexistence between C. crispus and M. stellatus appears to be mediated primarily by disturbances from winter storm waves that remove large, fast-growing C. crispus fronds a nd limit its abundance. Coexistence of C. crispus fronds and M. stellatus c rusts in this zone may also result from their different patterns of resourc e use. In contrast to the low zone, the slow growth of C. crispus in the mi d-low zone (approximately +0.5 m MLW) prevented the overgrowth of fronds of M. stellatus and, hence, prevented competition from occurring during the g rowing season (spring, summer, fall). Our evidence suggests that disturbanc es from herbivores or waves cause little mortality of fronds in this zone o r higher on the shore. Rather, in response to physiological stress, C. cris pus sloughs off metabolically unproductive apical tissue prior to resuming growth in the spring, minimizing the chance of overgrowth by these fronds i n the coming growing season. Together, these results suggest that the low p roduction, rather than high mortality, of C. crispus fronds enables these c ompetitors to coexist higher on the shore. On space-limited temperate rocky shores similar species often coexist because mortality from disturbances f alls most heavily on the dominant competitor. Our results imply that coexis tence of these species can be maintained by factors affecting the productiv ity, rather than the mortality, of the dominant competitor. But both sorts of processes may be important, as different processes may control coexisten ce of the same competitors in different environments.