Geographic variation in the sensitivity of an herbivore‐induced seaweed defense

Abstract

Intraspecific variation in primary producer induced defenses may affect how these defenses contribute to population and community regulation. For instance, inducible defenses may play larger roles in primary producer populations that are sensitive to low levels of grazing than populations that only respond to high levels of grazing. Although the incidence of herbivore-induced defenses is well documented, and there are examples of geographic variation in these defenses, we have limited knowledge of the factors that affect sensitivity to grazing (i.e., the minimum grazer density needed to elicit these responses) within and among populations. Filling this gap is necessary to understand the importance of these defenses under different conditions. To address whether the sensitivity of seaweed induced defenses varies geographically, we exposed Northern and Southern California populations of the seaweed Silvetia compressa to five densities of the snail Tegula funebralis, under ambient, regional environmental conditions. Southern seaweeds required high levels of grazing to induce defenses, while all levels of herbivory decreased Northern seaweed palatability. To better understand these different responses to low levels of grazing, we conducted common garden experiments to directly test the roles of experimental environment and herbivore source. Consistent with initial experiments, Northern Silvetia responded to low levels of grazing, regardless of environmental conditions, while Southern Silvetia never responded to low levels of grazing, even after being acclimated to Northern environmental conditions for 24 d. Additionally, Southern Silvetia did not respond to grazing by Northern snails, suggesting that herbivore source did not explain this geographic pattern in inducible seaweed defenses. Together, these observations suggest that the seaweed source explains this geographic pattern in sensitivity. Trade-offs with constitutive defenses did not help explain this pattern, as Southern Silvetia had weaker constitutive defenses than Northern seaweeds. This may be due to a combination of low grazing pressure and low predictability of attack in the South. Thus, population variation in sensitivity may be due to long-term differences in environmental histories, resulting in local adaptation or legacy effects from exposure to local conditions. Overall, our results indicate that these herbivore-induced responses may be more important in Northern Silvetia populations than Southern ones.

Local and regional geographic variation in inducible defenses

“…It is well established that invasive predators often have strong impacts on recipient ecosystems (Grosholz et al, 2000; Simberloff & Von Holle, 1999). Because many invasions occur over broad geographic scales, examining geographic variation in antipredator defenses can provide key insights into the adaptive capacity of native prey species (Jones & Long, 2018; Kishida et al, 2007; Long et al, 2011; Nunes et al, 2014). In this study, we compared inducible defense expression in snails from two regions having substantially different contact histories with the invasive predatory green crab.…”

“…The frequency of predator–prey interactions likely varies considerably across spatial scales, potentially leading to local and broad‐scale (regional) geographic variation in the expression of inducible defenses. It is well known that the expression of inducible defenses can vary regionally (Edgell et al, 2009; Jones & Long, 2018; Kishida et al, 2007; Long et al, 2011; Nunes et al, 2014; Trussell & Nicklin, 2002; Trussell & Smith, 2000), but what remains less clear is how regional (≥100s of kilometers) patterns in inducible defense expression vary with more local‐scale (≤10s of kilometers) patterns. Attention to scale‐dependent patterns in the expression of inducible defenses will help identify the factors shaping local versus regional patterns of adaptation as well as enhance our understanding of the role that inducible defenses may play in shaping local and regional variation in community structure and dynamics via trait‐mediated indirect interactions (Peckarsky et al, 2008; Schmitz & Trussell, 2016; Trussell & Schmitz, 2012; Werner & Peacor, 2003).…”

Season‐specific impacts of climate change on canopy‐forming seaweed communities

“…Additionally, because invertebrate grazers can alter algal‐algal interactions such as by controlling epiphyte growth (Hoffmann et al., 2020 ; Rogers & Breen, 1983 ), they were included in all mesocosms. To add ecologically realistic densities of these grazers relative to Silvetia biomass, we scaled field densities (# of grazer individuals per gram of Silvetia ) reported in a previous study (Jones, 2016 ) to our mesocosms. As a result, we added six Tegula funebralis , six Lottia strigatella , six Lottia scabra , 10 Littorina scutulata , and one Cyanoplax hartwegii to each mesocosm.…”

Season-specific impacts of climate change on canopy-forming seaweed communities.

“…Additionally, because invertebrate grazers can alter algal-algal interactions (Rogers & Breen 1983, Hoffmann et al 2020, they were included in all mesocosms. To add ecologically realistic densities of these grazers relative toSilvetia biomass, we scaled field densities (# of grazer individuals per gram of Silvetia ) reported in a previous study (Jones 2016) to our mesocosms. As a result, we added six Tegulafunebralis , six Lottia strigatella , six Lottia scabra , ten Littorina scutulata , and one Cyanoplax hartwegii to each mesocosm.…”