Proceedings of the Third International Conference on Invasive ...

<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 1: Spartina Biologyenvironment over five clonal generations and two growingseasons, latitudinal differences in palatability, toughness andnitrogen content persisted undiminished (Salgado andPennings 2005). (Palatability <strong>of</strong> extracts was not examinedin this study.) This study did not rule out <strong>the</strong> possibility thatS. alterniflora has induced defenses against herbivores, butdid indicate that plastic responses alone, whe<strong>the</strong>r to <strong>the</strong>biotic or abiotic environment, would not be sufficient toexplain <strong>the</strong> latitudinal patterns in palatability. However,constitutive differences in defense are strong, and couldexplain part or all <strong>of</strong> <strong>the</strong> latitudinal gradient in palatability.The conclusion that <strong>the</strong>re is latitudinal variation in geneticresistance to herbivory is consistent with o<strong>the</strong>r studiesshowing constitutive latitudinal variation in S. alternifloratraits (Seneca 1974; Seliskar et al. 2002) and geneticdifferentiation <strong>of</strong> S. alterniflora populations across latitude(O'Brien and Freshwater 1999).The ultimate factors producing <strong>the</strong>se latitudinaldifferences in plant traits are yet to be determined; however,because all ten plant species studied showed similarlatitudinal differences in palatability (Pennings et al. 2001),it is probable that <strong>the</strong> ultimate explanations lie in generalecological and biogeographic processes ra<strong>the</strong>r thanidiosyncratic aspects <strong>of</strong> Spartina biology. Because <strong>the</strong>sestudies were all done at sites exposed to full-strengthseawater, latitudinal differences in soil edaphic conditionswere minor compared to <strong>the</strong> differences that occur acrosselevation within single marshes, and thus probably notimportant. It is possible that latitudinal differences insubstrate type (peat at high latitudes; mineral soils at lowlatitudes) might select for differences in plant palatability insome way. The most likely hypo<strong>the</strong>ses, however, are that<strong>the</strong> differences in plant traits are produced by latitudinaldifferences in herbivore pressure or in plant phenology.Considerable evidence indicates that herbivore pressureon S. alterniflora and o<strong>the</strong>r marsh plants is greater at lowversus high latitudes. In low-latitude marshes, gastropodscan strongly suppress growth <strong>of</strong> S. alterniflora (Silliman andZieman 2001), but gastropods have no effect on S.alterniflora growth in high-latitude marshes (Pennings andSilliman 2005). Omnivorous grasshoppers are larger and aremore likely to eat leaves ra<strong>the</strong>r than seeds or arthropods inlow- versus high-latitude marshes (Pennings and Silliman2005; Wason and Pennings 2008). Grazing damage to S.alterniflora from snails and grasshoppers (but not Prokelisiaspp.) is greater in low- versus high-latitude marshes(Pennings, unpublished data). Thus, it is possible thatgreater herbivore pressure at low versus high latitudes couldselect for increased plant defenses at low latitudes, creating ageographic difference in plant palatability. Alternatively,<strong>the</strong> shorter growing season at high latitudes might select forleaves that are higher in nitrogen but less tough (Reich andOleksyn 2004; Wright et al. 2004). Differences in <strong>the</strong>se leaftraits driven by phenology would likely affect palatability toherbivores even if herbivore pressure did not select for <strong>the</strong>traits.CONCLUSIONSSpartina alterniflora varies in palatability to herbivoresat both local and geographic scales. We have a basicunderstanding <strong>of</strong> how variation in leaf traits correlates withvariation in palatability at both spatial scales, but how muchdifferent leaf traits affect <strong>the</strong> feeding preferences <strong>of</strong> differen<strong>the</strong>rbivores remains to be determined. Similarly, we havesome understanding <strong>of</strong> how variation in soilbiogeochemistry, herbivore pressure and phenology may be<strong>the</strong> ultimate factors mediating local and geographic variationin leaf traits, but much remains to be learned about <strong>the</strong>relative importance <strong>of</strong> <strong>the</strong>se ultimate factors at differentspatial scales.Given that local and geographic patterns in palatability<strong>of</strong> S. alterniflora exist, <strong>the</strong>se patterns are important for atleast three reasons. First, <strong>the</strong>y provide a unifying frameworkto link studies <strong>of</strong> plant-herbivore interactions done indifferent zones <strong>of</strong> different marshes. Although differentresults may be obtained in different studies, much <strong>of</strong> <strong>the</strong>variation among studies may be reconciled by understanding<strong>the</strong> underlying differences in plant palatability at local andgeographic scales. Second, this variation allows tests <strong>of</strong>general <strong>the</strong>ories <strong>of</strong> plant-herbivore interactions, which positthat plant-herbivore interactions will vary predictably withphysical stress (Goranson et al. 2004; Huberty and Denno2004) and latitude (Pennings et al. 2001). <strong>Third</strong>, <strong>the</strong>sepatterns may lend insight into some aspects <strong>of</strong> introductions<strong>of</strong> S. alterniflora into new geographic locations. Forexample, plants introduced from relatively high-latitude sitesare likely to be more vulnerable to herbivores than plantsintroduced from low-latitude sites.ACKNOWLEDGMENTSMy work on local and geographic variation ininteractions between S. alterniflora and its herbivores hasbeen supported by <strong>the</strong> National Geographic Society, <strong>the</strong>National Science Foundation (DEB-0296160 and 0638796,OCE99-82133), and <strong>the</strong> Environmental Institute <strong>of</strong> Houston.This work is part <strong>of</strong> <strong>the</strong> Georgia Coastal Ecosystems LTERprogram.REFERENCESAnderson, C.M. and M. Treshow. 1980. A review <strong>of</strong> environmentaland genetic factors that affect height in Spartina alternifloraLoisel. (salt marsh cordgrass). Estuaries 3:168-176.Bortolus, A. and O. Iribarne. 1999. Effects <strong>of</strong> <strong>the</strong> SW Atlantic burrowingcrab Chasmagnathus granulata on a Spartina salt marsh.Marine Ecology Progress Series 178:79-88.Bowdish, T.I. and P. Stiling. 1998. The influence <strong>of</strong> salt and nitrogenon herbivore abundance: direct and indirect effects. Oecologia113:400-405.-11-