A test of optimal defense theory vs. the growth-differentiation balance hypothesis as predictors of seaweed palatability and defenses

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Heckman, Melanie L.
Hay, Mark E.
Kubanek, Julia
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Because organisms have limited resources to allocate to multiple life history traits, the Optimal Defense Theory (ODT) and the Growth-Differentiation Balance Hypothesis (GDBH) were developed by terrestrial plant ecologists to predict intraindividual defense allocation based on the cost of defense and these life history trade-offs. However, these theories have garnered equivocal experimental support over the years and are rarely experimentally extended from predictions of plant physiology to the palatability of the tissues an herbivore experiences. We therefore examined tissue palatability, nutritional value, and defense mechanisms in multiple Dictyotalean seaweeds in two Caribbean locations, using two herbivores. Relative palatability of tissues varied greatly with algal species, grazer species, and location. Because older bases were not consistently defended, GDBH did not predict relative palatability. We could not reject ODT without intensive measures of tissue fitness value and herbivore risk, and this theory was therefore not useful in making broad predictions of tissue palatability. In testing the physiological predictions of these theories, we found the young, growing apices of these seaweeds to be generally more nutritionally valuable than the old, anchoring bases and found organic-rich apices to be more chemically deterrent, thus supporting ODT. However, the combined chemical, nutritional, and structural traits of these algae all influenced herbivore choice. As a result, these patterns of apical value and chemical defense reflected palatability of live tissues for only one of five algal species, which rendered ODT and GDBH poor predictors of relative palatability for most algae.
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