Volker Bahn

Performance and Reproduction of An Exotic Invader Across Temperate forest Gradients

Widespread colonization by invasive species often obscures their underlying niche requirements. A robust inference into habitat requirements demands direct measures of invasive species performance linked with associated environmental conditions. In the context of general ecological theory, we investigated the niche requirements of Microstegium vimineum, an invasive grass in the U.S. that overruns native vegetation in forest understories. We examined M. vimineums performance and reproduction as a function of environmental drivers across forested and unforested habitats along a 100-km regional and climatic gradient in the southeastern U.S. from the southern Appalachian Mountains to the Georgia piedmont. We then measured M. vimineum performance and reproduction in response to direct environmental drivers (diffuse light, litter cover, soil moisture, herbaceous cover, soil pH, clay content and temperature) in paired invaded and uninvaded plots. Lastly, we experimentally investigated recruitment in the context of experimental and natural disturbances. We find that all habitats are not equally suitable for M. vimineum—even those within which it occurs—and that the environmental conditions associated with roadsides and waterways are most suitable for M. vimineum persistence and spread. Microstegium vimineums soil moisture, light and leaf litter requirements may delineate the boundaries of suitable habitat for the exotic invader. Significant decreases in M. vimineum recruitment, performance and reproduction along these environmental gradients suggest its potential niche limitations. Nevertheless, we also find significant dispersal limits on M. vimineum populations not subject to conspicuous overland water flow. We discuss our findings in the context of spread, impact and management of invasive species.

Inland habitat associations of Marbled Murrelets on southwest Vancouver Island, British Columbia

Abstract We used a combination of standardized audio-visual surveys, made over nine years, and vegetation analysis to determine habitat associations of Marbled Murrelets (Brachyramphus marmoratus) breeding in inland coniferous forests. Our study, in Carmanah-Walbran watersheds on southwestern Vancouver Island, British Columbia, covered extensive contiguous tracts (>16,000 ha) of apparently suitable habitat that supported a large breeding population of murrelets, a threatened species. Indicators of stand occupancy (circling and subcanopy flights) from audio-visual surveys were consistently associated with known nest habitat indicators (availability of platform limbs, cover and thickness of epiphytes on tree limbs, variable canopy structure). Both murrelet detections and nest microhabitat indicators were associated with a suite of macrohabitat variables, indicating that the most suitable habitat was low-elevation old-growth forest with widely spaced large trees. Biogeoclimatic productivity units, based on soil moisture and nutrient regimes, were useful proxies of habitat suitability, but tree species composition, timber volume, and tree height, variables commonly available in timber inventory maps, were not. Using two principle component factors derived from habitat characteristics, we clustered the 27 survey stations into three groups that differed significantly in occupied and subcanopy detections of murrelets and in nest habitat indicators. This is a useful method for combining multivariate measures for classifying and mapping habitat for murrelets.

COMPARISON OF COASTAL FRINGE AND INTERIOR FORESTS AS RESERVES FOR MARBLED MURRELETS ON VANCOUVER ISLAND

Abstract Much of the protected habitat available to the threatened Marbled Murrelet Brachyramphus marmoratus and other old-growth associated species in the Pacific Northwest is in narrow strips along the coast (e.g., parks and scenic fringes). Using data over two years from three watersheds on southwest Vancouver Island, we show that such shoreline strip forests represent suboptimal habitat for murrelets. Murrelet detections, including circling and subcanopy behaviors, were significantly lower at 30 coastal stations (20–250 m from the shoreline edge) than at 30 interior stations (1.5–21.0 km inland). Densities of predators were significantly higher at the coastal stations. The coastal trees were of similar mean height and diameter, but they had lower structural diversity and provided fewer and less suitable (thinner epiphyte cover on large boughs) nesting platforms than trees in the interior. When possible, reserves for Marbled Murrelets should be placed in interior and not shoreline forests.

Becoming pure: identifying generational classes of admixed individuals within lesser and greater scaup populations

Estimating the frequency of hybridization is important to understand its evolutionary consequences and its effects on conservation efforts. In this study, we examined the extent of hybridization in two sister species of ducks that hybridize. We used mitochondrial control region sequences and 3589 double‐digest restriction‐associated DNA sequences (ddRADseq) to identify admixture between wild lesser scaup (Aythya affinis) and greater scaup (A. marila). Among 111 individuals, we found one introgressed mitochondrial DNA haplotype in lesser scaup and four in greater scaup. Likewise, based on the site‐frequency spectrum from autosomal DNA, gene flow was asymmetrical, with higher rates from lesser into greater scaup. However, using ddRADseq nuclear DNA, all individuals were assigned to their respective species with >0.95 posterior assignment probability. To examine the power for detecting admixture, we simulated a breeding experiment in which empirical data were used to create F1 hybrids and nine generations (F2–F10) of backcrossing. F1 hybrids and F2, F3 and most F4 backcrosses were clearly distinguishable from pure individuals, but evidence of admixed histories was effectively lost after the fourth generation. Thus, we conclude that low interspecific assignment probabilities (0.011–0.043) for two lesser and nineteen greater scaup were consistent with admixed histories beyond the F3 generation. These results indicate that the propensity of these species to hybridize in the wild is low and largely asymmetric. When applied to species‐specific cases, our approach offers powerful utility for examining concerns of hybridization in conservation efforts, especially for determining the generational time until admixed histories are effectively lost through backcrossing.

Antlers on the Arctic Refuge: capturing multi-generational patterns of calving ground use from bones on the landscape

Bone accumulations faithfully record historical ecological data on animal communities, and owing to millennial-scale bone survival on high-latitude landscapes, have exceptional potential for extending records on arctic ecosystems. For the Porcupine Caribou Herd, maintaining access to calving grounds on the Arctic National Wildlife Refuge (ANWR, Alaska) is a central management concern. However, variability in calving ground geography over the 30+ years of monitoring suggests establishing the impacts of climate change and potential petroleum development on future calving success could benefit from extended temporal perspectives. Using accumulations of female antlers (shed within days of calving) and neonatal skeletons, we test if caribou calving grounds develop measureable and characteristic bone accumulations and if skeletal data may be helpful in establishing a fuller, historically integrated understanding of landscape and habitat needs. Bone surveys of an important ANWR calving area reveal abundant shed antlers (reaching 103 km–2) and high proportional abundance of newborn skeletal individuals (up to 60% neonate). Openly vegetated riparian terraces, which compose less than 10 per cent of ANWR calving grounds, yield significantly higher antler concentrations than more abundant habitats traditionally viewed as primary calving terrain. Differences between habitats appear robust to potential differences in bone visibility. The distribution of antler weathering stages mirrors known multi-decadal calving histories and highlights portions of the antler accumulation that probably significantly extends records of calving activity. Death assemblages offer historically integrated ecological data valuable for the management and conservation of faunas across polar latitudes.

Decoupling litter barrier and soil moisture influences on the establishment of an invasive grass

Background and aimsThrough recruitment, plants establish in novel environments. Recruitment also is the stage where plants undergo the highest mortality. We investigate the recruitment niche for Microstegium vimineum, an annual grass from East Asia spreading throughout eastern North American forests.MethodsCurrent observational and greenhouse research indicates that M. vimineum recruitment may be inhibited by leaf litter and promoted by soil moisture; we use field studies to experimentally test how these factors influence M. vimineum germination, seedling survival and reproduction. Specifically, we introduce M. vimineum seeds into forest microhabitats with experimentally varied levels of soil moisture and leaf litter.ResultsSoil moisture increases M. vimineum germination regardless of leaf litter thickness and ameliorates seedling mortality in deep leaf litter. Seed production per m2 increases with watering, reflecting higher germination and survival, whereas per capita seed production increases with leaf litter thickness, reflecting density-dependent limits on seed production.ConclusionsThe interactive effects of varied levels of soil moisture and leaf litter thickness on key M. vimineum life history stages highlight the need to consider multiple drivers, such as rainfall and local forest disturbance, when assessing how soil properties influence the establishment of invasive plants.

Ecological Differentiation Among Key Plant Mutualists from a Cryptic Ant Guild

As key dispersers of herbaceous seeds, Aphaenogaster ants strongly influence the distribution of woodland plants in eastern North America. Ants within this genus are difficult to distinguish and often are identified by subgroup, but emerging research suggests they occupy species-specific ecological niches. As such, distinct climatic requirements among Aphaenogaster spp. might result in transient plant interactions with climate change. We examine whether there are ecological and distributional differences among Aphaenogaster species that coincide with current taxonomic differentiations. We use occurrence records for six Aphaenogaster spp. that occur in deciduous forests in eastern North America. We associate the geographic patterning of species occurrence with temperature and precipitation data, and we examine whether unique climatic niches characterize each species. We then predict habitat suitability throughout eastern North America using species distribution models. For verification, we test how well the predicted ranges fit observed occurrences using novel data sets for each species. We find that Aphaenogaster species within this cryptic genus demonstrate unique ecological and geographic signatures. Each species within the subgroup generally responds differently to temperature, and somewhat differently to precipitation and seasonal variance, suggesting unique ecological niches for each species. Our results indicate that each ant species may respond uniquely to changes in climate. Such shifts could disrupt current community associations and biotic interactions with ant-dispersed plants.

Progressive Cl- channel defects reveal disrupted skeletal muscle maturation in R6/2 Huntington's mice.

Huntington’s disease (HD) patients suffer from progressive and debilitating motor dysfunction. Previously, we discovered reduced skeletal muscle chloride channel (ClC-1) currents, inwardly rectifying potassium (Kir) channel currents, and membrane capacitance in R6/2 transgenic HD mice. The ClC-1 loss-of-function correlated with increased aberrant mRNA processing and decreased levels of full-length ClC-1 mRNA (Clcn1 gene). Physiologically, the resulting muscle hyperexcitability may help explain involuntary contractions of HD. In this study, the onset and progression of these defects are investigated in R6/2 mice, ranging from 3 wk old (presymptomatic) to 9–13 wk old (late-stage disease), and compared with age-matched wild-type (WT) siblings. The R6/2 ClC-1 current density and level of aberrantly spliced Clcn1 mRNA remain constant with age. In contrast, the ClC-1 current density increases, and the level of aberrantly spliced Clcn1 mRNA decreases with age in WT mice. The R6/2 ClC-1 properties diverge from WT before the onset of motor symptoms, which occurs at 5 wk of age. The relative decrease in R6/2 muscle capacitance also begins in 5-wk-old mice and is independent of fiber atrophy. Kir current density is consistently lower in R6/2 compared with WT muscle. The invariable R6/2 ClC-1 properties suggest a disruption in muscle maturation, which we confirm by measuring elevated levels of neonatal myosin heavy chain (MyHC) in late-stage R6/2 skeletal muscle. Similar changes in ClC-1 and MyHC isoforms in the more slowly developing Q175 HD mice suggest an altered maturational state is relevant to adult-onset HD. Finally, we find nuclear aggregates of muscleblind-like protein 1 without predominant CAG repeat colocalization in R6/2 muscle. This is unlike myotonic dystrophy, another trinucleotide repeat disorder with similar ClC-1 defects, and suggests a novel mechanism of aberrant mRNA splicing in HD. These early and progressive skeletal muscle defects reveal much needed peripheral biomarkers of disease progression and better elucidate the mechanism underlying HD myopathy.

Reducing tourism's threats to biodiversity: effects of a voluntary sustainability standard and training program on 106 Latin American hotels, lodges and guesthouses

ABSTRACT The tourism industry can negatively affect wildlife, plants and natural ecosystems through habitat destruction, pollution, over-exploitation of natural resources and visitor impacts to sensitive ecosystems. One approach to mitigate such threats is the application of voluntary sustainability standards, supported by training of tourism enterprises and verified by external audits. The Rainforest Alliance standard defines 78 criteria (requirements) for sustainable environmental, social and business practices, and has been adopted by over 600 tourism enterprises – including hotels, lodges and tour boats – in 12 countries. We examined the performance of 106 hotels in six Latin American countries against 29 of the sustainable tourism criteria most directly related to biodiversity conservation. Independent audits were used to assess hotel performance at baseline followed by a repeat assessment after training, about two years later. Mean conformance with the 29 biodiversity criteria increased significantly during this interval, from 44% to 58%. Improvements were greatest for businesses in the lowest third of performance at baseline (laggards) and smallest for hotels in the highest third (leaders). The results indicate that a voluntary sustainability standard and training program can serve both to recognize existing good actors and to drive incremental improvement in enterprises that were previously less sustainable.