Competition for safe real estate,not food,drives density‐dependent juvenile survival in a large herbivore

Density‐dependent competition for food reduces vital rates, with juvenile survival often the first to decline. A clear prediction of food‐based, density‐dependent competition for large herbivores is decreasing juvenile survival with increasing density. However, competition for enemy‐free space could also be a significant mechanism for density dependence in territorial species. How juvenile survival is predicted to change across density depends critically on the nature of predator–prey dynamics and spatial overlap among predator and prey, especially in multiple‐predator systems. Here, we used a management experiment that reduced densities of a generalist predator, coyotes, and specialist predator, mountain lions, over a 5‐year period to test for spatial density dependence mediated by predation on juvenile mule deer in Idaho, USA. We tested the spatial density‐dependence hypothesis by tracking the fate of 251 juvenile mule deer, estimating cause‐specific mortality, and testing responses to changes in deer density and predator abundance. Overall juvenile mortality did not increase with deer density, but generalist coyote‐caused mortality did, but not when coyote density was reduced experimentally. Mountain lion‐caused mortality did not change with deer density in the reference area in contradiction of the food‐based competition hypothesis, but declined in the treatment area, opposite to the pattern of coyotes. These observations clearly reject the food‐based density‐dependence hypothesis for juvenile mule deer. Instead, our results provide support for the spatial density‐dependence hypothesis that competition for enemy‐free space increases predation by generalist predators on juvenile large herbivores.     

Prior exposure to stress allows the maintenance of an ecosystem cycle following severe acidification

Ecosystem processes vary temporally due to environmental fluctuations, such as when variation in solar energy causes diurnal cycles in primary production. This normal variation in ecosystem functioning may be disrupted and even lost if taxa contributing to functioning go extinct due to environmental stress. However, when communities are exposed to the stress at sub-lethal levels over several generations, they may be able to develop community-level stress tolerance via ecological (e.g. species sorting) or evolutionary (e.g. selection for tolerant genotypes) mechanisms and thus avoid the loss of stability, as defined by the resistance of a process. Community tolerance to a novel stressor is expected to increase the resistance of key processes in stressed ecosystems. In freshwater communities we tested whether prolonged prior exposure to an environmental stressor, i.e. acidification, could increase ecosystem stability when the communities were exposed to a subsequent press perturbation of more severe acidification. As a measure of ecosystem stability, we quantified the diurnal variation in dissolved oxygen (DO), and the resistance of the DO cycle and phytoplankton biomass. High-frequency data from oxygen loggers deployed in 12 mesocosms showed that severe acidification with sulfuric acid to pH 3 could cause a temporary (i.e. two-week long) loss of diurnal variation in dissolved oxygen concentration. The loss of diurnal variation was accompanied by a strong reduction in phytoplankton biomass. However, the pre-exposure to acidification for several weeks resulted in the maintenance of the diurnal cycle and higher levels of phytoplankton biomass, though they did not return to as rapidly to pre-exposure functioning as non-exposed mesocosms. These results suggest that ecosystem stability is intrinsically linked to community-wide stress tolerance, and that a history of exposure to the stressor may increase resistance to it, though at the cost of some resilience.     

Statistical analysis of test-day milk yields using random regression models for the comparison of feeding groups during the lactation period

Abstract

Random regression models are widely used in the field of animal breeding for the genetic evaluation of daily milk yields from different test days. These models are capable of handling different environmental effects on the respective test day, and they describe the characteristics of the course of the lactation period by using suitable covariates with fixed and random regression coefficients. As the numerically expensive estimation of parameters is already part of advanced computer software, modifications of random regression models will considerably grow in importance for statistical evaluations of nutrition and behaviour experiments with animals. Random regression models belong to the large class of linear mixed models. Thus, when choosing a model, or more precisely, when selecting a suitable covariance structure of the random effects, the information criteria of Akaike and Schwarz can be used. In this study, the fitting of random regression models for a statistical analysis of a feeding experiment with dairy cows is illustrated under application of the program package SAS. For each of the feeding groups, lactation curves modelled by covariates with fixed regression coefficients are estimated simultaneously. With the help of the fixed regression coefficients, differences between the groups are estimated and then tested for significance. The covariance structure of the random and subject-specific effects and the serial correlation matrix are selected by using information criteria and by estimating correlations between repeated measurements. For the verification of the selected model and the alternative models, mean values and standard deviations estimated with ordinary least square residuals are used.     

Response of dark respiration to temperature in Eriophorum vaginatum from a 30-year-old transplant experiment in Alaska

Background: In the Arctic region, temperature increases are expected to be greater under anticipated climate change than the global average. Understanding how dark respiration (R_d) of common Arctic plant species acclimates to changes in the environment is therefore important for predicting changes to the Arctic carbon balance.

Aims: The aim of this study is to investigate the influence of genotype and growing environment on R_d, the temperature response (Q₁₀) of R_d, and foliar N (N_leaf) of the Arctic sedge Eriophorum vaginatum.

Methods: We measured R_d, and determined its Q₁₀ and N_leaf of E. vaginatum populations that were reciprocally transplanted 30 years previously along a latitudinal transect of 370 km in northern Alaska.

Results: R_d and Q₁₀ did not differ among populations (ecotypes) of E. vaginatum, but the local environment had a significant effect on both variables. R_d as well as N_leaf was higher in northern, colder sites, while Q₁₀ was lower there.

Conclusions: R_d in the different populations of E. vaginatum is a very plastic trait and controlled by growing environment, as is N_leaf. The lower Q₁₀ values in the northern sites were most likely a consequence of substrate inhibition of R_d at higher temperatures.     

The effect of plant identity and the level of plant decay on molecular gut content analysis in a herbivorous soil insect

Plant roots represent an important food source for soil‐dwelling animals, but tracking herbivore food choices below‐ground is difficult. Here, we present an optimized PCR assay for the detection of plant DNA in the guts of invertebrates, using general plant primers targeting the trnT‐F chloroplast DNA region. Based on this assay, we assessed the influence of plant identity on the detectability of ingested plant DNA in Agriotes click beetle larvae. Six different plant species were fed to the insects, comprising a grass, a legume and four nonlegume forbs. Moreover, we examined whether it is possible to amplify DNA of decaying plants and if DNA of decayed plant food is detectable in the guts of the larvae. DNA of the ingested roots could be detected in the guts of the larvae for up to 72‐h post‐feeding, the maximum digestion time tested. When fed with living plants, DNA detection rates differed significantly between the plant species. This may be ascribed to differences in the amount of plant tissue consumed, root palatability, root morphology and/or secondary plant components. These findings indicate that plant identity can affect post‐feeding DNA detection success, which needs to be considered for the interpretation of molecularly derived feeding rates on plants. Amplification of plant DNA from decaying plants was possible as long as any tissue could be retrieved from the soil. The consumption of decaying plant tissue could also be verified by our assay, but the insects seemed to prefer fresh roots over decaying plant material.     

Feeding performance of juvenile hatchery‐reared spotted seatrout Cynoscion nebulosus

The feeding performance of individual hatchery‐reared (HR) and wild juvenile spotted seatrout Cynoscion nebulosus was compared across a series of six 1·5 h feeding exposures over a 3 day period in a controlled experiment. The predation cycle served as a context for discerning feeding performance elements. The experimental design facilitated assessments of the effects of experience, motivation due to hunger or satiation and prey density and encounter frequency. Although feeding success improved significantly across successive trials for both groups of C. nebulosus, wild C. nebulosus successfully captured and consumed significantly more Palaemonetes spp. prey and completed most performance metrics more efficiently than HR C. nebulosus. Total exposure time decreased with experience for both groups of C. nebulosus; however, HR C. nebulosus took longer to complete feeding exposures. Underpinning this difference was the time spent by HR C. nebulosus in non‐search mode and for completing various foraging behaviours. Nevertheless, juvenile HR C. nebulosus exhibited sufficient foraging plasticity to switch from a pelleted diet to live novel prey.     

Radiotracer Experiments on Biological Volatilization of Organic Iodine from Coastal Seawaters

Biological volatilization of iodine from seawaters was studied using a radiotracer technique. Seawater samples were incubated aerobically in serum bottles with radioactive iodide tracer (¹²⁵I), and volatile organic and inorganic iodine were collected with activated charcoal and silver wool trap, respectively. Iodine was volatilized mainly as organic iodine, and inorganic iodine volatilization was not observed. Influence of light intensity on the volatilization was determined, but no significant differences were observed under light (70,000 lux) and dark conditions. The effect of the chemical form of iodine on the volatilization was determined, and the results suggested that volatilization preferentially occurs from iodide (I^?) but not from iodate (IO₃ ^?). Volatilization did not occur when the samples were autoclaved or filtered through a 0.22-μm pore size membrane filter. Incubation of the samples with antibiotics caused decreased volatilization. Conversely, enhanced volatilization was observed when the samples were incubated with yeast extract. Fifty-nine marine bacterial strains were then randomly isolated from marine environments, and their iodine-volatilizing capacities were determined. Among these, 19 strains exhibited significant capacities for volatilizing iodine. 16S ribosomal RNA gene comparisons indicated that these bacteria are members of Proteobacteria (α and γ subdivisions) and Cytophaga-Flexibacter-Bacteroides group. One of the strains, strain C-19, volatilized 1 to 2% of total iodine during cultivation, and the gaseous organic iodine was identified as methyl iodide (CH₃I). These results suggest that organic iodine volatilization from seawaters occurs biologically, and that marine bacteria participate in the process. Considering that volatile organic iodine emitted from the oceans causes atmospheric ozone destruction, biological iodine volatilization from seawater is of great importance. Our results also contribute to prediction of movement and diffusion of long-lived radioactive iodine (¹²⁹I) in the environment.     

No evidence for local adaptation in an invasive alien plant: field and greenhouse experiments tracing a colonization sequence

Background and Aims

Local adaptation enables plant species to persist under different environmental conditions. Evolutionary change can occur rapidly in invasive annual species and has been shown to lead to local adaptation. However, the patterns and mechanisms of local adaptation in invasive species along colonization sequences are not yet understood. Thus, in this study the alien annual Impatiens glandulifera was used to investigate local adaptation to distinct habitats that have been consecutively invaded in central Europe.

Methods

A reciprocal transplant experiment was performed using 15 populations from alluvial deciduous forests, fallow meadows and coniferous upland forests, and a greenhouse experiment was performed in which plants from these habitats were grown under treatments reflecting the main habitat differentiators (shade, soil acidity, competition).

Key Results

Biomass production, specific leaf area, plant height and relative growth rate differed between habitats in the field experiment and between treatments in the greenhouse, but not between seed origins. Overall, there was no indication of local adaptation in either experiment.

Conclusions

Since I. glandulifera is a successful invader in many habitats without showing local adaptation, it is suggested that the species is coping with environmental variation by means of high phenotypic plasticity. The species seems to follow a ‘jack-and-master’ strategy, i.e. it is able to maintain high fitness under a wide range of environmental conditions, but performs particularly well in favourable habitats. Therefore, the proposed colonization sequence is likely to be based primarily on changes in propagule pressure. It is concluded that invasive alien plants can become dominant in distinct habitats without local adaptation.