Impacts of habitat connectivity on grassland arthropod metacommunity structure: A field-based experimental test of theory.

Metacommunity theory has advanced scientific understanding of how species interactions and spatial processes influence patterns of biodiversity and community structure across landscapes. While the central tenets of metacommunity theory have been promoted as pivotal considerations for conservation management, few field experiments have tested the validity of metacommunity predictions. Here, we tested one key prediction of metacommunity theory-that decreasing habitat connectivity should erode metacommunity structure by hindering species movement between patches. For 2 years, we manipulated an experimental old-field grassland ecosystem via mowing to represent four levels of habitat connectivity: (1) open control, (2) full connectivity, (3) partial connectivity, and (4) no connectivity. Within each treatment plot (10 × 10 m, n = 4 replicates), we measured the abundance and diversity (i.e., alpha and beta) of both flying and ground arthropods using sticky and pitfall traps, respectively. We found that the abundance and diversity of highly mobile flying arthropods were unaffected by habitat connectivity, whereas less mobile ground arthropods were highly impacted. The mean total abundance of ground arthropods was 2.5× and 2× higher in the control and partially connected plots compared to isolated patches, respectively. We also reveal that habitat connectivity affected the trophic interactions of ground arthropods, with predators (e.g., wolf spiders, ground spiders) being highly positively correlated with micro-detritivores (springtails, mites) but not macro-detritivores (millipedes, isopods) as habitat connectivity increased. Together these findings indicate that changes in habitat connectivity can alter the metacommunity structure for less mobile organisms such as ground arthropods. Because of their essential roles in terrestrial ecosystem functioning and services, we recommend that conservationists, restoration practitioners, and land managers include principles of habitat connectivity for ground arthropods when designing biodiversity management programs.

Revisiting two dogmas of conservation science.

Conservation science is a morally motivated field, with implicit and explicit values built into its practice. As such, conservationists must engage with conservation ethics to interrogate underlying values. We examine cutting-edge ecological science and contemporary ethics to revisit two conservation norms that have become dogmatic in the field: ecological collectives, but not individual animals, are valuable and anthropomorphism should be staunchly avoided. Emerging studies demonstrate that individuals and their intraspecific variation can be instrumentally valuable for conservation science, and there is an emerging consensus within environmental philosophy around the moral worth of individuals. Thus, we suggest conservation science should explicitly recognize the value of individuals. We also argue that avoiding anthropomorphism is detrimental to conservation because critical anthropomorphism enables a more nuanced scientific approach-allowing conservationists to ask enlightened questions with creativity and compassion. We provide evidence that both dogmatic norms are scientifically and morally outdated and propose new normative values to push conservation towards more robust science and ethical practice.

Revisión de dos dogmas de las ciencias de la conservación Resumen Las ciencias de la conservación son un campo con motivaciones morales y valores implícitos y explícitos integrados en su práctica. Por lo tanto, los conservacionistas deben trabajar con la ética de la conservación para interrogar los valores subyacentes. Analizamos la ecología de vanguardia y la ética contemporánea para revisar dos normas que se han convertido en dogmas dentro del campo: los colectivos ecológicos, pero no los animales individuales, son valiosos y el antropomorfismo debe evitarse a toda costa. Los estudios emergentes demuestran que los individuos y sus variaciones intraespecíficas pueden tener un valor instrumental para las ciencias de la conservación y que existe un consenso emergente dentro de la filosofía ambiental en torno al valor moral de los individuos. Por lo tanto, sugerimos que las ciencias de la conservación deberían reconocer de forma explícita el valor de los individuos. También discutimos que evitar el antropomorfismo daña a la conservación pues el antropomorfismo crítico permite una estrategia científica más matizada-lo que permite que los conservacionistas hagan preguntas informadas con creatividad y compasión. Proporcionamos evidencias de que ambos dogmas son científica y moralmente obsoletos y proponemos nuevos valores normativos para guiar a la conservación hacia una ciencia más sólida y una práctica más ética.

Differences in prey personality mediate trophic cascades.

Functional trait approaches in ecology chiefly assume the mean trait value of a population adequately predicts the outcome of species interactions. Yet this assumption ignores substantial trait variation among individuals within a population, which can have a profound effect on community structure and function. We explored individual trait variation through the lens of animal personality to test whether among-individual variation in prey behavior mediates trophic interactions. We quantified the structure of personalities within a population of generalist grasshoppers and examined, through a number of field and laboratory-based experiments, how personality types could impact tri-trophic interactions in a food chain. Unlike other studies of this nature, we used spatial habitat domains to evaluate how personality types mechanistically map to behaviors relevant in predator-prey dynamics and found shy and bold individuals differed in both their habitat use and foraging strategy under predation risk by a sit-and-wait spider predator. In the field-based mesocosm portion of our study, we found experimental populations of personality types differed in their trophic impact, demonstrating that prey personality can mediate trophic cascades. We found no differences in respiration rates or body size between personality types used in the mesocosm experiment, indicating relative differences in trophic impact were not due to variation in prey physiology but rather variation in behavioral strategies. Our work demonstrates how embracing the complexity of individual trait variation can offer mechanistically richer understanding of the processes underlying trophic interactions.

Woodlice change the habitat use of spiders in a different food chain.

BACKGROUND: In old field systems, the common woodlouse may have an indirect effect on a nursery web spider. Woodlice and nursery web spiders feed in different food chains, yet previous work demonstrated that the presence of woodlice is correlated with higher predation success by nursery web spiders upon their grasshopper prey. This finding suggested a new hypothesis which links two seemingly disparate food chains: when woodlice are present, the spider predator or the grasshopper prey changes their location in the vegetative canopy in a way that increases their spatial overlap and therefore predation rate. However, warming temperatures may complicate this phenomenon. The spider cannot tolerate thermal stress, meaning warming temperatures may cause the spider to move downwards in the vegetative canopy or otherwise alter its response to woodlice. Therefore, we would expect warming and woodlice presence to have an interactive effect on predation rate. METHODS: We conducted behavioral experiments in 2015, 2017, and 2018 to track habitat domains-the use of the vegetative canopy space by grasshoppers and spiders-in experimental cages. Then, we used three models of spider movement to try to explain the response of spiders to woodlice: expected net energy gain, signal detection theory, and individual-based modelling. RESULTS: Habitat domain observations revealed that spiders shift upward in the canopy when woodlice are present, but the corresponding effect on grasshopper prey survival was variable over the different years of study. Under warming conditions, spiders remained lower in the canopy regardless of the presence of woodlice, suggesting that thermal stress is more important than the effect of woodlice. Our modelling results suggest that spiders do not need to move away from woodlice to maximize net energy gain (expected net energy gain and signal detection theory models). Instead spider behavior is consistent with the null hypothesis that they move away from unsuccessful encounters with woodlice (individual-based simulation). We conclude that mapping how predator behavior changes across biotic (e.g. woodlouse presence) and abiotic conditions (e.g. temperature) may be critical to anticipate changes in ecosystem dynamics.

Nitrogen and carbon concentrations and stable isotope ratios: Data from a 15N tracer study in short-form Spartina alterniflora and Distichlis spicata.

We present four datasets that provide information on primary production, nitrogen (N) uptake and allocation in two salt marsh grasses, short-form Spartina alterniflora and Distichlis spicata. These four datasets were generated during a month-long stable isotope (15N) tracer study described in the companion manuscript (Hill et al., 2018). They include an allometry dataset containing mass and height data for individual plants harvested from Colt State Park, Bristol, Rhode Island and used to nondestructively estimate plant masses. A second dataset contains weekly stem height measurements collected over the course of the 15N tracer study. Also included are high resolution data from 49 vegetated compartments (leaves, stems, fine/coarse roots, rhizomes) and bulk sediment depth intervals, reporting the mass, carbon and N concentrations, and stable isotope ratios measured following the harvest of cores over time. Additionally, we provide a complementary dataset with estimates of microbial removal from potential and ambient denitrification enzyme assays. These data, along with source code used in their analysis, are compiled in the NitrogenUptake2016 R package available from the Comprehensive R Archive Network.

Nitrogen uptake and allocation estimates for Spartina alterniflora and Distichlis spicata.

Salt marshes have the potential to intercept nitrogen that could otherwise impact coastal water quality. Salt marsh plants play a central role in nutrient interception by retaining N in above- and belowground tissues. We examine N uptake and allocation in two dominant salt marsh plants, short-form Spartina alterniflora and Distichlis spicata. Nitrogen uptake was measured using 15N tracer experiments conducted over a four-week period, supplemented with stem-level growth rates, primary production, and microbial denitrification assays. By varying experiment duration, we identify the importance of a rarely-measured aspect of experimental design in 15N tracer studies. Experiment duration had a greater impact on quantitative N uptake estimates than primary production or stem-level relative growth rates. Rapid initial scavenging of added 15N caused apparent nitrogen uptake rates to decline by a factor of two as experiment duration increased from one week to one month, although each experiment shared the qualitative conclusion that Distichlis roots scavenged N approximately twice as rapidly as Spartina. We estimate total N uptake into above- and belowground tissues as 154 and 277 mg N·m-2·d-1 for Spartina and Distichlis, respectively. Driving this pattern were higher N content in Distichlis leaves and belowground tissue and strong differences in primary production; Spartina and Distichlis produced 8.8 and 14.7 g biomass·m-2·d-1. Denitrification potentials were similar in sediment associated with both species, but the strong species-specific difference in N uptake suggests that Distichlis-dominated marshes are likely to intercept more N from coastal waters than are short-form Spartina marshes. The data and source code for this manuscript are available as an R package from https://github.com/troyhill/NitrogenUptake2016.