Grazers on Benthic Seaweeds - Seaweed Biology - page 165

this may be due to relatively high phlorotannin contents (Amsler et al. 2005 ; Iken
et al. 2011 ). Chemical defenses in seaweeds may also play a role in the invasive
potential of a species, which typically have significant effects on the structure and
biodiversity of the native community. For example, the invasive, chemically
defended Caulerpa taxifolia is largely unpalatable and presents poor habitat to a
variety of native grazers in temperate Australia, making it unlikely that grazing
could control the abundance or spreading of the invasive species (Gollan and
Wright 2006 ). Chemical defense in seaweeds also can have community-wide
effects by altering trophic dynamics, and can even change the outcome of trophic
cascades. In Australia, the chemically defended red alga Delisea pulchra survived
intense urchin grazing in a kelp bed, which led to an intermediate stable community
state containing grazer-resistant foliose algae between the typical alternate states of
kelp beds and urchin barrens (Wright et al. 2005 ). Waterborne cues released by
herbivores grazing on macroalgae may function as a form of chemical communica-
tion with neighboring con-specific algae to increase their defense levels (Toth and
Pavia 2007 ). However, these cues may act in a broader sense as “infochemicals”
when signaling to other community members. Cues released from herbivore-algal
grazing may attract potential predators of the herbivores, which would benefit the
seaweed. Both a fish and a crab predator were significantly attracted to cues
released by Ascophyllum nodosum when grazed by periwinkles, thus inducing
top-down control of the herbivore by the two predators (Coleman et al. 2007 ).
This shows that such waterborne cues may act as a seaweed distress signal, a
mechanism well known from terrestrial plant-insect interactions (Dicke 2009 ). In
a different example, cues exuded by the chemically defended Delisea pulchra in
Australian kelp beds induced settlement of urchin larvae, although the chemically
defended alga is unpalatable to the adult urchins (Williamson et al. 2004 ). Adult
urchins will instead feed on other seaweeds within the community, thus providing a
competitive advantage to D. pulchra. Simultaneously, urchins gain associational
protection against fish predators from the chemically defended alga in their habitat.
8.6 Climate Change Effects on Seaweed-Herbivore Interactions
An emerging, yet timely, consideration is the effects that climatic changes will have
on the interactions between seaweeds and grazers. Metabolic theory predicts rising
temperatures to generally increase metabolic rates in both seaweeds and herbivores
(see also Chap. 3 by Eggert and Chap. 18 by Bartsch et al.). Manipulative tank
experiments, however, indicated that temperature rise has a proportionally larger
effect on grazers, which outweigh any increased algal growth. This resulted in an
overall reduction of algal biomass (O'Connor 2009 ). Conversely, no change in
herbivore impact at higher temperatures was seen in an in situ experiment in a rocky
intertidal in California (Morelissen and Harley 2007 ). It was hypothesized that
multiple abiotic drivers interacted with varying effects on biotic interactions,
partially offsetting and ameliorating each other, thus not leading to a clearly