No abstract

A central tenet of plant defense theory is that adaptation to the abiotic environment sets the template for defense strategies, imposing a trade-off between plant growth and defense. Yet this tradeoff, commonly found among species occupying divergent resource environments, may not occur across populations of single species. We hypothesized that more favorable climates and higher levels of herbivory would lead to increases in growth and defense across plant populations. We evaluated whether plant growth and defense traits covaried across 18 populations of showy milkweed (Asclepias speciosa) inhabiting an east-west climate gradient spanning 257 of longitude. A suite of traits impacting defense (e.g., latex, cardenolides), growth (e.g., size), or both (e.g., specific leaf area [SLA], trichomes) were measured in natural populations and in a common garden, allowing us to evaluate plastic and genetically based variation in these traits. In natural populations, herbivore pressure increased toward warmer sites with longer growing seasons. Growth and defense traits showed strong clinal patterns and were positively correlated. In a common garden, clines with climatic origin were recapitulated only for defense traits. Correlations between growth and defense traits were also weaker and more negative in the common garden than in the natural populations. Thus, our data suggest that climatically favorable sites likely facilitate the evolution of greater defense at minimal costs to growth, likely because of increased resource acquisition.

1. Propagule size and number often vary by several orders of magnitude among cooccurring plant species. Explaining the maintenance of this variation and understanding how propagule size contributes to coexistence remain a central challenge for community ecologists. The dominant paradigm is that a competition-colonizason.

Despite increasing evidence that herbivory on a focal plant may hinge upon the identity of its neighbors, it is not clear whether predictable mechanisms govern the nature and magnitude of such associational effects. Using a factorial field experiment replicated at 14 sites across 80,000 hectares, we evaluated the mechanisms driving associational effects between two plant species mediated by grasshopper herbivores. Our experiment manipulated local neighborhood plant density (two levels) and frequency (three levels), nested within two larger-scale habitat contexts (habitats that did or did not have past agricultural land use). We found that the more palatable plant species, Solidago nemoralis, experienced reduced herbivory (associational resistance) when rare due mainly to reduced grasshopper foraging activity. Damage to the less palatable plant species, S. odora, was affected by the interaction between plant frequency and the land-use history of the site: it experienced increased damage (associational susceptibility) in even-frequency neighborhoods, but only in sites with a history of agricultural use. Behavioral assays generally corroborated the results from the field, further supporting the importance of foraging behavior in generating associational effects. In finding that associational effects are contingent upon relative palatability among plants and events in the distant past that modify contemporary habitat structure (i.e., past agricultural land use), our work suggests that foraging decisions made at the neighborhood level are important for generating associational effects and that in some cases these neighborhood interactions also depend on the larger-scale environmental context resulting from legacies of past land-use events.

Past and present human activities, such as historic agriculture and fire suppression, are widespread and can create depauperate plant communities. Although many studies show that herbivory on focal plants depends on the density of herbivores or the composition of the surrounding plant community, it is unclear whether anthropogenic changes to plant communities alter herbivory. We tested the hypothesis that human activities that alter the plant community lead to subsequent changes in herbivory. At 20 sites distributed across 80 300 hectares, we conducted a field experiment that manipulated insect herbivore access (full exclosures and pseudo‐exclosures) to four focal plant species in longleaf pine woodlands with different land‐use histories (post‐agricultural sites or non‐agricultural sites) and degrees of fire frequency (frequent and infrequent). Plant cover, particularly herbaceous cover, was lower in post‐agricultural and fire suppressed woodlands. Density of the dominant insect herbivore at our site (grasshoppers) was positively related to plant cover. Herbivore access reduced biomass of the palatable forb Solidago odora in frequently burned post‐agricultural sites and in infrequently burned non‐agricultural woodlands and increased mortality of another forb (Pityopsis graminifolia), but did not affect two other less palatable species (Schizachyrium scoparium and Tephrosia virginiana). Herbivory on S. odora exhibited a hump‐shaped response to plant cover, with low herbivory at low and high levels of plant cover. Herbivore density had a weak negative effect on herbivory. These findings suggest that changes in plant cover related to past and present human activities can modify damage rates on focal S. odora plants by altering grasshopper foraging behavior rather than by altering local grasshopper density. The resulting changes in herbivory may have the potential to limit natural recovery or restoration efforts by reducing the establishment or performance of palatable plant species.