Plant growth can be limited by resource acquisition and defence against consumers, leading to contrasting trade‐off possibilities. The competition‐defence hypothesis posits a trade‐off between ...competitive ability and defence against enemies (e.g. herbivores and pathogens). The growth‐defence hypothesis suggests that strong competitors for nutrients are also defended against enemies, at a cost to growth rate. We tested these hypotheses using observations of 706 plant populations of over 500 species before and following identical fertilisation and fencing treatments at 39 grassland sites worldwide. Strong positive covariance in species responses to both treatments provided support for a growth‐defence trade‐off: populations that increased with the removal of nutrient limitation (poor competitors) also increased following removal of consumers. This result held globally across 4 years within plant life‐history groups and within the majority of individual sites. Thus, a growth‐defence trade‐off appears to be the norm, and mechanisms maintaining grassland biodiversity may operate within this constraint.
Wood-feeding insects face a host of physical, chemical, and nutritional barriers in their food resource. Wood-boring Hymenoptera in the family Siricidae are associated with mutualistic basidiomycete ...wood-rot fungi, which assist colonization and provide nutrition for their insect partner, though functional properties of this symbiosis are poorly described. In this study, we document the behavioral and morphological adaptations of the globally invasive woodwasp, Sirex noctilio F., for foraging using its fungal symbiont Amylostereum areolatum (Chaillet ex Fries) Boidin. Larvae concentrated foraging near the border of fungal symbiont growth in pine xylem. Foraging larvae do not ingest bulk xylem tissue, but rather use specialized asymmetric mandibles to press xylem shavings and extract liquid fractions. Fluids drain toward the oral cavity via a sulcus on the occlusal surface of the left mandible. Processed shavings are expelled from the oral cavity without ingestion and passed along the underside of the larvae via peristaltic undulation to the rear of the feeding gallery. Larval midguts lack elaborated chambers typical in insects reliant on microbial fermentation of cellulose, and no xylem tissue was recovered from gut dissections. Larval behavior and functional morphology indicate larval S. noctilio do not ingest xylem, but instead use the fungus as an external gut for digestion of recalcitrant lignocellulosic compounds.
Emerald ash borer (Agrilus planipennis Fairmaire (Coleoptera: Buprestidae)), an invasive phloem-feeding beetle native to Asia, has devastated North American ash forests since its detection in ...Michigan, United States in 2002. As the emerald ash borer has continued to spread, the potential for successful long-term management hinges upon the release, establishment, and spread of introduced larval and egg parasitoids for biological control. Here, we focus on the establishment and evidence for spatial spread of introduced larval parasitoid, Spathius agrili Yang and Spathius galinae Belokobylskij & Strazanac (Hymenoptera: Braconidae) in the state of Maryland. To assess each species, we analyzed historical release and recovery data and resampled previous release sites and nonrelease sites for establishment. We found little evidence of establishment or spread for S. agrili, despite a comparatively large number of release locations, events, and individuals. By contrast, despite much lower propagule pressure and shorter history of releases, we detected multiple established populations of S. galinae at release sites and at sites up to 90 km from the nearest release point approximately 3 yr after its most current release. Our findings show that S. galinae has established and spread rapidly following field releases whereas its congener, S. agrili has not. Although it may still be too early to evaluate the level of population control and ash protection afforded by S. galinae, these findings indicate the need for continued investment in S. galinae for emerald ash borer classical biological control efforts.
Numerous studies show that increasing species richness leads to higher ecosystem productivity. This effect is often attributed to more efficient portioning of multiple resources in communities with ...higher numbers of competing species, indicating the role of resource supply and stoichiometry for biodiversity–ecosystem functioning relationships. Here, we merged theory on ecological stoichiometry with a framework of biodiversity–ecosystem functioning to understand how resource use transfers into primary production. We applied a structural equation model to define patterns of diversity–productivity relationships with respect to available resources. Meta-analysis was used to summarize the findings across ecosystem types ranging from aquatic ecosystems to grasslands and forests. As hypothesized, resource supply increased realized productivity and richness, but we found significant differences between ecosystems and study types. Increased richness was associated with increased productivity, although this effect was not seen in experiments. More even communities had lower productivity, indicating that biomass production is often maintained by a few dominant species, and reduced dominance generally reduced ecosystem productivity. This synthesis, which integrates observational and experimental studies in a variety of ecosystems and geographical regions, exposes common patterns and differences in biodiversity–functioning relationships, and increases the mechanistic understanding of changes in ecosystems productivity.
Increases in nutrient availability and alterations to mammalian herbivore communities are a hallmark of the Anthropocene, with consequences for the primary producer communities in many ecosystems. ...While progress has advanced understanding of plant community responses to these perturbations, the consequences for energy flow to higher trophic levels in the form of secondary production are less well understood. We quantified arthropod biomass after manipulating soil nutrient availability and wild mammalian herbivory, using identical methods across 13 temperate grasslands. Of experimental increases in nitrogen, phosphorus, and potassium, only treatments including nitrogen resulted in significantly increased arthropod biomass. Wild mammalian herbivore removal had a marginal, negative effect on arthropod biomass, with no interaction with nutrient availability. Path analysis including all sites implicated nutrient content of the primary producers as a driver of increased arthropod mean size, which we confirmed using 10 sites for which we had foliar nutrient data. Plant biomass and physical structure mediated the increase in arthropod abundance, while the nitrogen treatments accounted for additional variation not explained by our measured plant variables. The mean size of arthropod individuals was 2.5 times more influential on the plot-level total arthropod biomass than was the number of individuals. The eutrophication of grasslands through human activity, especially nitrogen deposition, thus may contribute to higher production of arthropod consumers through increases in nutrient availability across trophic levels.
The symbiotic fungus
Amylostereum areolatum
is essential for growth and development of larvae of the invasive woodwasp,
Sirex noctilio
. In the nutrient poor xylem of pine trees, upon which
Sirex
...feeds, it is unknown whether
Amylostereum
facilitates survival directly through consumption (mycetophagy) and/or indirectly through digestion of recalcitrant plant polymers (external rumen hypothesis). We tested these alternative hypotheses for
Amylostereum
involvement in
Sirex
foraging using the innate dependency of all insects on dietary sources of sterol and the unique sterols indicative of fungi and plants. We tested alternative hypotheses by using GC-MS to quantify concentrations of free and bound sterol pools from multiple life-stages of
Sirex
, food sources, and waste products in red pine (
Pinus resinosa
). Cholesterol was the primary sterol found in all life-stages of
Sirex
. However, cholesterol was not found in significant quantities in either plant or fungal resources. Ergosterol was the most prevalent sterol in
Amylostereum
but was not detectable in either wood or insect tissue (<0.001 μg/g). Phytosterols were ubiquitous in both pine xylem and
Sirex
. Therefore, dealkylation of phytosterols (sitosterol and campesterol) is the most likely pathway to meet dietary demand for cholesterol in
Sirex
. Ergosterol concentrations from fungal-infested wood demonstrated low fungal biomass, which suggests mycetophagy is not the primary source of sterol or bulk nutrition for
Sirex
. Our findings suggest there is a potentially greater importance for fungal enzymes, including the external digestion of recalcitrant plant polymers (e.g., lignin and cellulose), shaping this insect-fungal symbiosis.
Exotic species dominate many communities; however the functional significance of species' biogeographic origin remains highly contentious. This debate is fuelled in part by the lack of globally ...replicated, systematic data assessing the relationship between species provenance, function and response to perturbations. We examined the abundance of native and exotic plant species at 64 grasslands in 13 countries, and at a subset of the sites we experimentally tested native and exotic species responses to two fundamental drivers of invasion, mineral nutrient supplies and vertebrate herbivory. Exotic species are six times more likely to dominate communities than native species. Furthermore, while experimental nutrient addition increases the cover and richness of exotic species, nutrients decrease native diversity and cover. Native and exotic species also differ in their response to vertebrate consumer exclusion. These results suggest that species origin has functional significance, and that eutrophication will lead to increased exotic dominance in grasslands.
Worldwide, native species increasingly contend with the interacting stressors of habitat fragmentation and invasive species, yet their combined effects have rarely been examined. Direct negative ...effects of invasive omnivores are well documented, but the indirect effects of resource competition or those caused by predator avoidance are unknown. Here we isolated and examined the independent and interactive effects of invasive omnivorous Black rats (Rattus rattus) and forest fragment size on the interactions between avian predators and their arthropod prey. Our study examines whether invasive omnivores and ecosystem fragment size impact: 1) the vertical distribution of arthropod species composition and abundance, and 2) the vertical profile of foraging behaviors of five native and two non-native bird species found in our study system. We predicted that the reduced edge effects and greater structural complexity and canopy height of larger fragments would limit the total and proportional habitat space frequented by rats and thus limit their impact on both arthropod biomass and birds' foraging behavior. We experimentally removed invasive omnivorous Black rats across a 100-fold (0.1 to 12 ha) size gradient of forest fragments on Hawai'i Island, and paired foraging observations of forest passerines with arthropod sampling in the 16 rat-removed and 18 control fragments. Rat removal was associated with shifts in the vertical distribution of arthropod biomass, irrespective of fragment size. Bird foraging behavior mirrored this shift, and the impact of rat removal was greater for birds that primarily eat fruit and insects compared with those that consume nectar. Evidence from this model study system indicates that invasive rats indirectly alter the feeding behavior of native birds, and consequently impact multiple trophic levels. This study suggests that native species can modify their foraging behavior in response to invasive species removal and presumably arrival through behavioral plasticity.
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