Norris Z. Muth

We assessed variation in feeding preference of Cambarus bartonii (Appalachian Brook Crayfish) for stocked Salmo trutta (Brown Trout) or wild Salvelinus fontinalis (Brook Trout) across 8 central Pennsylvania headwater streams with different recent exposures to trout stocking. In separate trials, we allowed crayfish to interact with and choose between fillets of stocked Brown Trout or wild Brook Trout in either a flowing Y-apparatus or a standing-water aquarium. In the Y-apparatus trials and standing water aquarium trials, crayfish exhibited no detectable feeding preference based on proximity to stocking sites from which they were collected. In the Y-apparatus trials, crayfish were more likely to move towards a fillet type than not. Overall, our results provide evidence that crayfish do not distinguish between stocked Brown Trout and wild Brook Trout as food sources, and history of exposure to stocked trout does not influence feeding preference. Specifically, crayfish accept stocked trout as a food source, even if this resource is new to the environment (i.e., with the additions of new stocking sites in previously unstocked streams). Acceptance of stocked trout as a food resource suggests crayfish may play an important role in the local incorporation of this significant allochthonous nutrient input. Future research should address if local uptake of stocked trout by crayfish is able to mitigate downstream nutrient loads related to stocking.

Theoretically, induced defenses should be prevalent within low resource environments like the forest understory where constitutive defenses would be costly. Also, the induced response should be stronger when the herbivore is a generalist rather than a specialist, which often have mechanisms to avoid or overcome plant defenses. These ideas have been previously tested for herbaceous species, and we examined these predictions in Lindera benzoin (spicebush), a common woody shrub of the eastern deciduous forest. Lindera benzoin plants in contrasting light environments served as control plants or were subjected to one of four treatments: application of jasmonic acid, clipping, herbivory by the specialist Epimecis hortaria (tulip tree beauty) and herbivory by the generalist Spodoptera exigua (beet armyworm). Following treatment, we assessed induced responses by measuring leaf chemistry (C/N ratio, protein content, and peroxidase activity), and by using insect bioassays with E. hortaria larvae. We found no difference in peroxidase activity between light environments in controls, plants treated with clipping or jasmonic acid. In plants subject to insect herbivory, peroxidase activity was greater in shade plants than in sun plants. The magnitude of this increase in the shade varied between the herbivores, with a 32 fold increase in plants exposed to the generalist S. exigua and a 9 fold increase in plants exposed to the specialist E. hortaria . Leaves from shade plants had more protein and lower C/N ratios than leaves from sun plants, regardless of induction treatment. In control plants, E. hortaria larvae consumed more leaf biomass and achieved greater final weights in the sun than in the shade, but these differences disappeared with induction treatments were applied. These results are among the first to show rapid induction in a woody plant, and different levels of induction with light environments and with specialist versus generalist herbivores.

Leaf chemistry and physiology vary with light environment and are often thought to directly affect herbivory patterns. Biotic (e.g., parasitoids and predators) and abiotic (e.g., temperature, relative humidity) factors known to influence herbivory also co-vary with light environment. Irrespective of mechanism, light-based differences in herbivore damage must be the result of variable herbivore abundance, per capita effects, or both. We examined the effect of light environment on leaf defence and leaf nutritional quality in Lindera benzoin (Lauraceae) and relate this to the abundance and impact of its lepidopteran herbivore Epimecis hortaria (Lepidoptera: Geometridae). In this system we consistently observe greater natural field herbivory in shade habitats relative to high light habitats, despite similar herbivore abundances; differences in herbivory are therefore most likely attributable to different per capita impacts of herbivores across environments. Potential herbivore behaviours responsible f... RésuméLa chimie et la physiologie de la feuille varient avec l'environnement lumineux et on considère généralement qu'ils influencent directement les patrons d'herbivorie. Les facteurs biotiques (par exemple les parasites et les prédateurs) et abiotiques (par exemple la température et l'humidité relative) influençant l'herbivorie covarient aussi avec l'environnement lumineux. Quel que soit le mécanisme en cause, les différences de dommages causés par l'herbivorie et attribuables à la lumière doivent etre le résultat d'une variation de l'abondance des herbivores, des effets individuels ou des deux. Nous avons examiné l'effet de l'environnement lumineux sur les défenses de la feuille et ses qualités nutritionnelles chez Lindera benzoin (Lauraceae) et l'avons relié à l'abondance et l'impact de son herbivore lépidoptère Epimecis hortaria (Lepidoptera, Geometridae). Dans ce système en milieu naturel, nous observons toujours un taux d'herbivorie plus élevé dans les habitats ombragés que dans ceux très lumineux,...

Invasion biologists often suggest that phenotypic plasticity plays an important role in successful plant invasions. Assuming that plasticity enhances ecological niche breadth and therefore confers a fitness advantage, recent studies have posed two main hypotheses: (1) invasive species are more plastic than non‐invasive or native ones; (2) populations in the introduced range of an invasive species have evolved greater plasticity than populations in the native range. These two hypotheses largely reflect the disparate interests of ecologists and evolutionary biologists. Because these sciences are typically interested in different temporal and spatial scales, we describe what is required to assess phenotypic plasticity at different levels. We explore the inevitable tradeoffs of experiments conducted at the genotype vs. species level, outline components of experimental design required to identify plasticity at different levels, and review some examples from the recent literature. Moreover, we suggest that a successful invader may benefit from plasticity as either (1) a Jack‐of‐all‐trades, better able to maintain fitness in unfavourable environments; (2) a Master‐of‐some, better able to increase fitness in favourable environments; or (3) a Jack‐and‐master that combines some level of both abilities. This new framework can be applied when testing both ecological or evolutionary oriented hypotheses, and therefore promises to bridge the gap between the two perspectives.

In attempting to determine the traits associated with invasive plant species, ecologists have often used species native to the invaded range as “control species.” Because many native species themselves are aggressive colonizers, comparisons using this type of control do not necessarily yield information relevant to distinctions between invasive and noninvasive species. Here we implement an alternative study design that compares phenological, architectural, size, and fitness traits of several introduced invasive species to introduced noninvasive species within two genera of Asteraceae (Crepis and Centaurea). While there were many significant differences between the genera, there were few shared attributes among invasive or noninvasive congeners, even for traits as seemingly important as the number of inflorescences produced and the size of seed heads. Instead, the results suggest that differences in invasiveness between closely related species is better explained as the result of complex trait interactions and specific introduction histories.

Biological invasion is one aspect of ecosystem function that may be controlled by the biological diversity of the invaded community, and there have been a number of recent studies that investigated relationships between diversity and invasibility. Most experimental studies report that higher species or functional group diversity increases resistance to invasion, but the role of genetic diversity is unknown. We used a model organism, Arabidopsis thaliana (Brassicaceae), to investigate relationships between genotypic richness and community invasibility by creating communities with 1, 2, 4, and 8 genotypes of A. thaliana at constant low (417 plants m ⁻² ) and high (834 plants m ⁻² ) densities, that once established, were invaded with a congener, Arabidopsis suecica . To reduce the potential effects of methodological confounding related to “sampling effects,”“variance reduction effects,” or confounding of abundance with diversity, we (1) created random communities from a relatively large pool of functionally and phenotypically similar genotypes, (2) evaluated individual and community traits across richness treatments, and (3) analyzed similarity of communities within treatments (for “quasi‐ replication”) and between adjacent treatments (for “nestedness”). Genotypic richness had no effect on A. suecica demography (emergence, survivorship), size (biomass, rosette area), or reproductive potential (rates of bolting and fruiting or number and size of bolts). In contrast, the density of A. thaliana genotypes had strong effects on the size and reproductive potential of A. suecica , which suggests that characteristics of the recipient community other than genotypic richness (e.g. light) form the most important determinant of community invasibility. Individual‐ and community‐level traits of community members (cover, biomass, survivorship) did not differ among richness treatments, and within‐ and between‐treatment similarity was reduced (relative to other recent experiments) but not eliminated. We evaluate our results vis‐a‐vis recent analyses of diversity‐invasibility experiments, and provide directions for future investigations of genetic diversity.