TIERING AND ENCRUSTATION IN THE MODERN ROCKY INTERTIDAL – EFFECT OF ENVIRONMENTAL CHANGE AND IMPLICATIONS FOR PALEO-COMMUNITIES

The rocky intertidal is a classic natural laboratory for ecology due to the strong environmental gradient (tidal range) expressed in a relatively small area. To explore the effect of environmental change on encrustation patterns and vertical tiering, samples were collected along two environmental gradients (intertidal height and relative wave energy). By sampling 9 locations on the western coast of Vancouver Island, Canada near Bamfield Marine Science Centre, the dataset captures a gradient from calm waters within an inlet out to an exposed coastline. At each location two sites were selected, and at each sampling site 5 horizontal transects were censused from the lowest to the highest intertidal zone. Previous research has established patterns of species turnover along these environmental gradients, such that similar changes in the rate and complexity of encrustation and tiering might be expected.

Encrustation rate of the rocky shore is correlated with intertidal height; bare substrate is more common in upper intertidal (Spearman’s rho = 0.29, alpha 0.05 = 0.19). Similarly, both species richness (rho = -0.59) and the rate of tiering (encrustation of an organism on another organism, rho = -0.29) decreases with intertidal height; diversity and tiering are lower in the upper intertidal. Personal observations of the size and length of macroalgae also supports this trend; longer/broader forms are found lower in the intertidal. The only significant correlation between wave energy and encrustation/tiering is a negative relationship with the number of tiers (levels of encrustation, such as an epibiont on an epibiont on a host, rho = -0.21); fewer tiers are observed in the upper intertidal.

Given the strong relationship between species composition and intertidal height, it naturally follows that encrustation and tiering increase with diversity. However, it is surprising that the large-scale pattern (wave-exposure) did not correlate with encrustation patterns – except in that more complex tiers of encrustation are only observed in lower energy settings. These results provide interesting hypotheses to test in fossil ecosystems where encruster-host dynamics provide a unique window into in-situ ecological interactions.