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 BiologyLOCAL AND GEOGRAPHIC VARIATION IN SPARTINA-HERBIVORE INTERACTIONSS.C. PENNINGSDepartment <strong>of</strong> Biology and Biochemistry, University <strong>of</strong> Houston, Houston TX 77204; spennings@uh.eduSpartina alterniflora is consumed by a variety <strong>of</strong> herbivores, and <strong>the</strong> nature <strong>of</strong> <strong>the</strong>se plant-herbivoreinteractions varies on both local and geographic scales. The palatability <strong>of</strong> S. alterniflora toherbivores varies within single marshes as a function <strong>of</strong> elevation. Tall-form plants, which occur atlow elevations close to creek banks, are more palatable to herbivores than are short-form plants,which occur at middle elevations on <strong>the</strong> marsh platform. The proximate causes <strong>of</strong> this localvariation in palatability include variation in leaf nitrogen content and chemical defenses.Differences in <strong>the</strong>se leaf traits are driven by variation in sediment biogeochemistry across <strong>the</strong>elevational gradient. The palatability <strong>of</strong> S. alterniflora to herbivores also varies geographically.High-latitude (New England) plants along <strong>the</strong> Atlantic Coast <strong>of</strong> <strong>the</strong> United States are more palatableto herbivores than are low-latitude (South Atlantic Bight) plants. The proximate causes <strong>of</strong> thislatitudinal variation in palatability include variation in leaf nitrogen content, toughness, andchemical defenses. Differences in palatability and leaf traits persisted over 5 clonal generations in acommon-garden greenhouse environment, and thus are probably genetically determined. A number<strong>of</strong> processes may contribute to driving this variation in leaf traits. The most likely ultimate causesare latitudinal variation in herbivore pressure and latitudinal variation in plant phenology.Understanding spatial variation in interactions between herbivores and S. alterniflora within itsnative range may shed insights into how <strong>the</strong>se interactions develop when S. alterniflora isintroduced to new regions, and may inform biocontrol efforts.Keywords: common-garden experiment, elevation, herbivory, latitude, leaf traits, salt marsh,Spartina alternifloraINTRODUCTIONEarly salt-marsh studies discounted <strong>the</strong> importance <strong>of</strong>herbivory to salt marsh ecology because herbivores did notappear to play a major role in energy flow through <strong>the</strong> marshfood web (Smalley 1960; Teal 1962). More recent work,however, has shown that herbivores can affect <strong>the</strong> biomass,distributions and reproduction <strong>of</strong> a variety <strong>of</strong> salt marshplants (Srivastava and Jefferies 1996; Bortolus and Iribarne1999; Pennings and Bertness 2001; Silliman and Zieman2001; Rand 2003; Silliman and Bortolus 2003; Ho andPennings 2008). Thus, a general understanding <strong>of</strong> <strong>the</strong>ecology <strong>of</strong> salt-marsh plants requires consideration <strong>of</strong> plan<strong>the</strong>rbivoreinteractions.In order to obtain a general understanding <strong>of</strong> plan<strong>the</strong>rbivoreinteractions, it is necessary to consider spatialvariation. Even within <strong>the</strong> limited confines <strong>of</strong> salt marshhabitats, interactions between plants and herbivores are notexactly <strong>the</strong> same everywhere. They vary with marshelevation (Silliman and Bertness 2002; Goranson et al.2004), local plant community composition (Rand 1999,2003, 2004), and latitude (Pennings et al. 2001; Penningsand Silliman 2005; Pennings et al. 2007). Thus, a generalunderstanding <strong>of</strong> plant-herbivore interactions in salt marshesmust incorporate spatial variation on a variety <strong>of</strong> scales.Here, I address variation in interactions betweenherbivores and Spartina alterniflora at local and geographicscales. At <strong>the</strong> local scale I focus on <strong>the</strong> intertidal gradientacross <strong>the</strong> marsh. Along this gradient, soil biogeochemistryvaries markedly, producing strong variation in plantmorphology and palatability to herbivores. At <strong>the</strong>geographic scale I focus on latitudinal variation along <strong>the</strong>Atlantic Coast <strong>of</strong> <strong>the</strong> United States. Latitudinal differencesin palatability <strong>of</strong> salt marsh plants to herbivores are strikingand general across <strong>the</strong> plant and herbivore community.Understanding variation in plant-herbivore interactions atboth <strong>the</strong>se scales provides a framework for syn<strong>the</strong>sizingresults <strong>of</strong> different studies done in different locations, allowstests <strong>of</strong> general ecological <strong>the</strong>ory, and may shed insights intoecological processes obtained when Spartina is introducedinto new geographic regions.LOCAL VARIATIONSpartina alterniflora varies in palatability to herbivoreswithin individual marshes because <strong>of</strong> variation in soilbiogeochemistry that mediates plant traits. Salt marshhabitats are physically stressful for plants. Periodic flooding<strong>of</strong> marsh soils leads to high levels <strong>of</strong> sulfides, low redoxlevels, low levels <strong>of</strong> oxygen, and low bioavailability <strong>of</strong>nitrogen, a suite <strong>of</strong> factors that are inimicable to vigorous-9-