Associational resistance and shared doom: effects of epibiosis on herbivory

Abstract

The potential for spatial associations between palatable and unpalatable plant species to reduce herbivore pressure on the palatable species has been described as associational resistance, associational refuge or associational defense for numerous terrestrial and marine communities. One of the closest associations between species-epibiosis-has not been thoroughly investigated in this regard. In this study we evaluated how different associations between host seaweeds and epibiotic plants and animals influenced the movement of an omnivorous sea urchin (Arbacia punctulata) to the host and subsequent feeding on the host. A. punctulata showed clear preferences when given pairwise choices between 12 prey species (3 animals, 9 algae). These preferences were consistent and allowed us to rank the six epibiont species and six host species linearly from least to most preferred by A. punculata. Most host-epibiont associations dramatically changed urchin preference, increasing or decreasing urchin grazing on fouled hosts as compared to clean conspecifics. Herbivory on the host increased when the epibiont was more preferred, and decreased when it was less preferred than the unfouled host alga. Taking the host species as a point of reference, we classified epibiosis-caused decrease in herbivory as associational resistance, while epibiont-caused increases in herbivory were defined as shared doom. These epibiont-host-herbivore interactions could select for hosts that facilitate the growth of certain low preference epibionts on their surfaces in situations where the resulting decreases in herbivory would offset the various negative effects of being fouled. In contrast, in situations where herbivores are common, the negative effects of being fouled by palatable epibionts may be much greater than is generally assumed. In our assays, unpalatable hosts fouled by palatable epibionts became much more attractive to urchins and rose several ranks on the urchins' preference hierarchy.

Effects of epibiosis on consumer-prey interactions

“…Details on methodology and an expanded presentation or data are available in Wahl & Hay (1995), but a short overview is provided below. Most prey algae were collected between 0.5 and 2 m below MLW from jetties and pilings around Morehead City and Beaufort (32 42 0 N, 76 41 0 W), North Carolina, USA.…”

Dynamics of transitions between population interactions: a nonlinear interaction alpha-function defined

“…Wahl & Hay's (1995) study of epibiotic associations between host seaweed and some plant and animal epibionts showed that associations included both positive and negative e¡ects. The outcome,`associated resistance' or`shared doom', was highly in£uenced by the relative densities of the species involved.…”

Vicious circle in the intertidal: Facilitation between barnacle epibionts, a shell boring polychaete and trematode parasites in the periwinkle Littorina littorea

“…L. littorea is one of the most common gastropods at North Atlantic shores (Bertness, 1999) with a diverse epibiont community (Wahl and Hay, 1995;Buschbaum and Reise, 1999). In our study area, epibionts of L. littorea are mainly barnacles with up to 70% of snails on lower intertidal mussel beds being strongly covered with barnacles (Buschbaum and Reise, 1999).…”