Abstract
High-latitude plants are often more palatable to herbivores than low-latitude conspecifics. Does increased plant palatability lead to better herbivore performance? Our field and laboratory work investigated (A) whether high-latitude plants have traits indicating that they should be higher-quality foods for herbivores; (B) whether geographic differences in plant quality are more important than local adaptation of herbivores. We studied 3 plant species and 6 invertebrate herbivores in U.S. Atlantic Coast. Past studies had shown high-latitude individuals of these plants are more palatable than low-latitude conspecifics. We documented plant traits and herbivore performance (body size) in the field across latitude. We collected individuals from different latitudes for factorial (plant region x herbivore region) laboratory experiments, examining how herbivore performance was affected by plant region, herbivore region, and their interaction (i.e., local adaptation). Field surveys suggested high-latitude plants were likely of higher quality to herbivores. Leaf nitrogen content in all plant species increased toward high latitudes, consistent with lower leaf C/N and higher leaf chlorophyll content at high latitudes. Furthermore, leaf toughness decreased toward higher latitudes in 1 species. The body size of 4 herbivore species increased with latitude, consistent with high-latitude leaves being of higher quality, while 2 grasshopper species showed the opposite pattern, likely due to life-history constraints. In the laboratory, high-latitude plants supported better performance in 4 herbivore species (marginal in the 5th). The geographic region where herbivores were collected affected herbivore performance in all 6 species; however, the pattern was mixed, indicating a lack of local adaptation by herbivores to plants from their own geographic region. Our results suggest that more-palatable plants at high latitudes support better herbivore growth. Given that geographic origin of either plants or herbivores can affect herbivore performance, the nature of plant-herbivore interactions is likely to change if climate change “reshuffles” plant and herbivore populations across latitude.
Highlights
Ecologists have long been interested in latitudinal gradients in diversity and species interactions because these are likely to reveal fundamental effects of geography and climate on ecological processes [1,2,3,4]
Most plant traits changed in ways that would suggest that highlatitude plants were a higher quality food than low-latitude plants for herbivores
Leaf C content did not vary across latitude in all three plant species, but leaf N content increased toward higher latitudes in all three species (P = 0.007, 0.0001 and 0.01 for Solidago, Iva and Spartina respectively)
Summary
Ecologists have long been interested in latitudinal gradients in diversity and species interactions because these are likely to reveal fundamental effects of geography and climate on ecological processes [1,2,3,4]. In the case of plant-herbivore interactions, this theory predicts higher plant palatability, due to weaker defenses against herbivores, at high versus low latitudes (Fig. 1). The most extensive tests of this theory come from coastal salt marshes, systems with relatively few plant species that have large geographic distributions, allowing intraspecific or intrageneric comparisons across a wide range of latitude. A community-wide study in salt marshes on the U.S Atlantic Coast found that herbivores (13 species) preferred to eat plants (10 species) from high versus low latitudes, and this result occurred regardless of the geographic location of the assay, herbivore species, year, or season of plant collection [9]. Comparisons of herbivore density and herbivore damage to leaves found that both were greater at low latitudes, suggesting that selection driven by herbivore pressure might help explain differences in plant palatability across latitude [12]
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