Latitudinal Variation in Palatability of Salt-Marsh Plants: Which Traits Are Responsible?

Abstract

Biogeographic theory predicts that intense consumer–prey interactions at low latitudes should select for increased defenses of prey relative to high latitudes. In salt marshes on the Atlantic coast of the United States, a community‐wide pattern exists in which 10 species of low‐latitude plants are less palatable to a diverse suite of herbivores than are high‐latitude conspecifics. Examination of proximate plant traits (toughness, palatability of polar and nonpolar extracts, nitrogen content) of high‐ and low‐latitude conspecifics of nine plant species suggested that all these proximate traits had the potential to contribute to latitudinal differences in palatability of some plant species. Southern plants were tougher than northern plants (five species), had less palatable polar extracts (four species), and had lower N content (six species). Experimental evidence linking traits to latitudinal differences in palatability was strongest for polar extracts and lacking for N content. For one plant species, none of the traits we studied correlated with latitudinal variation in palatability. Because palatability differences may change when moving from fresh plants to freeze‐dried plants to plant traits, studies of latitudinal variation in freeze‐dried plants or plant traits are likely to under‐ or overestimate latitudinal variation in palatability of fresh plants. This study has begun to identify the proximate plant traits responsible for latitudinal variation in plant palatability in Atlantic coast salt marshes, but the ultimate evolutionary factors responsible for variation in these traits remain to be determined.

Latitudinal variation in top-down and bottom-up control of a salt marsh food web

“…We have a good understanding of how the importance of top-down and bottom-up factors changes at small spatial scales to affect ecological processes at a number of sites (Hacker andBertness 1995, Denno et al 2005). We also have an emerging understanding of latitudinal variation in herbivore population dynamics (Denno et al 1996) and plant-herbivore interactions (Siska et al 2002, Pennings et al 2009) at large geographic scales. Here we report on experimental work examining latitudinal variation in top-down and bottom-up processes that control populations of herbivores associated with the high-marsh shrub Iva frutescens.…”

Importance of local vs. geographic variation in salt marsh plant quality for arthropod herbivore communities

“…While the effects of latitude were not as dramatic as that of fertilizer addition, low-latitude plants were consistently larger, but lower in nitrogen, at the end of the experiment. Although high-latitude plants are higher in nitrogen, lower in phenolics and more palatable than low-latitude plants (Pennings, Siska & Bertness 2001;Siska et al 2002;Salgado & Pennings 2005), and although these differences are sufficient to cause geographic variation in herbivore body size (Ho, Pennings & Carefoot 2010), they were nevertheless overwhelmed in our experiments by fertilizer-induced nutritional status and food web composition. These findings are similar to those in our previous work with the high-marsh shrub Iva frutescens (Marczak et al 2011), where we found that latitudinal variation in plant quality had much smaller effects on herbivore Table 3.…”

“…Moreover, the concentration of phenolics in Spartina is lower in plants from high (2.45 AE 0.13% dry mass) vs. low (3.16 AE 0.10%) latitudes (Siska et al 2002). Preference tests in the laboratory using a variety of herbivores demonstrated that both polar extracts (polar extracts containing allelochemicals in artificial diet) and live leaf tissue from high-latitude Spartina are far more palatable (more diet or leaf area consumed) than their low-latitude counterparts (Pennings, Siska & Bertness 2001;Siska et al 2002;Salgado & Pennings 2005). In sum, high-latitude plants in these marshes are known to be more nutritious, softer, less defended and more palatable than low-latitude plants.…”

“…Spartina plants from high latitudes (RI to ME, 41-43°) are more nutritious (% N) and tender than conspecific plants from low latitudes (GA to FL, 30-31°) (Salgado & Pennings 2005;McCall & Pennings 2012). Moreover, the concentration of phenolics in Spartina is lower in plants from high (2.45 AE 0.13% dry mass) vs. low (3.16 AE 0.10%) latitudes (Siska et al 2002). Preference tests in the laboratory using a variety of herbivores demonstrated that both polar extracts (polar extracts containing allelochemicals in artificial diet) and live leaf tissue from high-latitude Spartina are far more palatable (more diet or leaf area consumed) than their low-latitude counterparts (Pennings, Siska & Bertness 2001;Siska et al 2002;Salgado & Pennings 2005).…”

“…At the geographic scale, plants from high latitudes are again richer in nitrogen, lower in phenolics and more palatable to herbivores than conspecific plants from the same habitats at low latitudes (Pennings, Siska & Bertness 2001;Siska et al 2002;Salgado & Pennings 2005). Planthopper herbivores show little variation in density with latitude, but herbivorous snails are more abundant at low latitudes (Pennings et al 2009).…”