Description
Herbivores play key roles in structuring communities, influencing both the species composition and individual traits of primary producers. In marine systems, much of what we know about induced trait changes in seaweeds and seagrasses is based on pairwise interactions between conspecific and heterospecific herbivores. However, multiple herbivores often co-occur in communities, feeding on primary producers simultaneously. If herbivores act antagonistically or synergistically, whether through direct interactions or induced seaweed traits, net herbivore effects in an assemblage may differ from individual species. We currently have a limited understanding of the role of induced defenses in more complex herbivore communities, and I attempt to increase this knowledge in Chapters 1 and 2. In addition to focusing on individual species, most studies in marine systems have tested for induced defenses under ambient or high herbivore abundances. However, herbivore abundances can vary greatly across sites and between populations. Investigating how this variation affects the strength of inducible seaweed defenses at different spatial scales will increase our knowledge on where these defenses are most important. I address these ideas in Chapter 3. In Chapter 1, I investigated how herbivore composition affects the strength of an inducible defense in the brown seaweed Silvetia compressa. Although terrestrial herbivore species can elicit different pathways that suppress or enhance inducible plant defenses, marine studies comparing the strength of induced defenses between herbivores interacting sequentially versus simultaneously are lacking. In addition, we know little about whether herbivore assemblage responses differ from individual species in any system. To test these ideas, I used laboratory mesocosm experiments to examine whether 1) herbivore elicitation of and response to Silvetia's induced defense were species-specific and 2) whether elicitation and response by an herbivore assemblage differed from the individual species. I found that although an herbivore assemblage can induce defenses in Silvetia, the strength of elicitation by an assemblage was weaker compared to Silvetia grazed by the strongest inducer (Tegula funebralis) only. Furthermore, individual herbivores had varied responses to Tegula grazing, avoiding, preferring, or not responding to grazed tissues. The net effect of these individual preferences appeared to reduce avoidance of Tegula-grazed Silvetia by the herbivore assemblage. Overall, these results suggest that strong pairwise interactions between herbivores may be weaker when seaweeds are grazed by multiple herbivores. In Chapter 2, I compared the relative strength of Silvetia's induced defense between two herbivores with different grazing modes. A variety of studies have quantified differences in herbivore-induced defenses by measuring induced compounds or indirectly comparing differences between grazed and non-grazed tissues. However, herbivore feeding preferences depend on food choice, and direct comparisons of grazed tissues may differ from those predicted by indirect comparisons. Using lab mesocosm experiments, I tested whether palatability differed between seaweeds grazed by snails and seaweeds grazed by isopods. I found that snail-grazed tissues were less palatable than isopod-grazed tissues, suggesting that snails induce stronger defenses than isopods. These differences were not due to grazing rate during the elicitation phase or a lack of response to isopod grazing, but may be due to chemical differences between snail and isopod-grazed tissues. These results suggest that in communities where herbivores elicit defenses of different strength, interspecific interactions between herbivores may be asymmetric, depending on which seaweed phenotypes have been induced. In Chapter 3, I investigated how variation in the abundance of a single herbivore species influenced the strength and sensitivity of Silvetia induced defenses in different populations. Geographic comparisons of herbivore-induced seaweed defenses have typically tested whether the strength of induced defenses differs between populations. However, seaweed populations may also vary in sensitivity to grazing, such that one population induces defenses at lower levels of grazing than another population. Little research in marine or terrestrial systems has incorporated tests of sensitivity into studies of induced defenses. Filling this gap is necessary to understand the importance of these defenses in populations with different abiotic or biotic conditions. To address whether the sensitivity of seaweed induced defenses varies geographically, I exposed Northern and Southern California populations of the seaweed Silvetia compressa to five different densities of local populations of the snail Tegula funebralis, under ambient environmental conditions for each region. I found that Southern California seaweeds required high levels of grazing to induce changes in palatability, while all levels of herbivory induced defenses in Northern California seaweeds. This suggests that these induced responses may play more important roles for Silvetia communities in Northern California than Southern California. Through a series of common garden experiments, I determined that geographic differences at low levels of grazing were not due to environmental conditions, herbivore source, or constitutive palatability. Instead, this variation in sensitivity between populations may be due to long-term differences in environmental histories, such as grazing pressure, that have caused local adaptation or left legacy effects from exposure to local conditions. Overall, our results indicate that geographically separated Silvetia populations can greatly vary in sensitivity to grazing by a common herbivore species. Collectively, my results highlight that single species studies may overestimate the role of induced defenses in communities. Although I found strong pairwise effects when Silvetia was grazed by a single herbivore species, these species-specific effects changed when I incorporated multiple herbivore species or herbivores that induced defenses of different strength. In addition, focusing on single herbivore densities may over or under estimate the role of induced defenses in some populations. Thus, my research emphasizes the need to move beyond testing whether or not induced defenses occur in marine primary producers and investigate how these defenses vary in space and time in diverse communities.
