Photosynthetic resource-use efficiency and demographic variability in desert winter annual plants

We studied a guild of desert winter annual plants that differ in long-term variation in per capita reproductive success (lb, the product of per capita survival from germination to reproduction, l, times per capita reproduction of survivors, b) to relate individual function to population and community dynamics. We hypothesized that variation in lb should be related to species' positions along a trade-off between relative growth rate (RGR) and photosynthetic water-use efficiency (WUE) because lb is a species-specific function of growing-season precipitation. We found that demographically variable species have greater RGR and greater leaf carbon isotope discrimination (Delta, a proxy inversely related to WUE). We examined leaf nitrogen and photosynthetic characteristics and found that, in this system, variation in Delta is a function of photosynthetic demand rather than stomatal regulation of water loss. The physiological characteristics that result in low Delta in some species may confer greater photosynthetic performance during the reliably moist but low temperature periods that immediately follow winter rainfall in the Sonoran Desert or alternatively during cool periods of the day or early growing season. Conversely, while species with high Delta and high RGR exhibit low leaf N, they have high biomass allocation to canopy leaf area display. Such trait associations may allow for greater performance during the infrequent conditions where high soil moisture persists into warmer conditions, resulting in high demographic variance. Alternatively, high variance could arise from specialization to warm periods of the day or season. Population dynamic buffering via stress tolerance (low RGR and Delta) correlates negatively with buffering via seed banks, as predicted by bet-hedging theory. By merging analyses of population dynamics with functional trait relationships, we develop a deeper understanding of the physiological, ecological, and evolutionary mechanisms involved in population and community dynamics.     

Wildlife contact analysis: emerging methods, questions, and challenges

Recent technological advances, such as proximity loggers, allow researchers to collect complete interaction histories, day and night, among sampled individuals over several months to years. Social network analyses are an obvious approach to analyzing interaction data because of their flexibility for fitting many different social structures as well as the ability to assess both direct contacts and indirect associations via intermediaries. For many network properties, however, it is not clear whether estimates based upon a sample of the network are reflective of the entire network. In wildlife applications, networks may be poorly sampled and boundary effects will be common. We present an alternative approach that utilizes a hierarchical modeling framework to assess the individual, dyadic, and environmental factors contributing to variation in the interaction rates and allows us to estimate the underlying process variation in each. In a disease control context, this approach will allow managers to focus efforts on those types of individuals and environments that contribute the most toward super-spreading events. We account for the sampling distribution of proximity loggers and the non-independence of contacts among groups by only using contact data within a group during days when the group membership of proximity loggers was known. This allows us to separate the two mechanisms responsible for a pair not contacting one another: they were not in the same group or they were in the same group but did not come within the specified contact distance. We illustrate our approach with an example dataset of female elk from northwestern Wyoming and conclude with a number of important future research directions.     

A trophic cascade triggers collapse of a salt-marsh ecosystem with intensive recreational fishing

Overexploitation of predators has been linked to the collapse of a growing number of shallow-water marine ecosystems. However, salt-marsh ecosystems are often viewed and managed as systems controlled by physical processes, despite recent evidence for herbivore-driven die-off of marsh vegetation. Here we use field observations, experiments, and historical records at 14 sites to examine whether the recently reported die-off of northwestern Atlantic salt marshes is associated with the cascading effects of predator dynamics and intensive recreational fishing activity. We found that the localized depletion of top predators at sites accessible to recreational anglers has triggered the proliferation of herbivorous crabs, which in turn results in runaway consumption of marsh vegetation. This suggests that overfishing may be a general mechanism underlying the consumer-driven die-off of salt marshes spreading throughout the western Atlantic. Our findings support the emerging realization that consumers play a dominant role in regulating marine plant communities and can lead to ecosystem collapse when their impacts are amplified by human activities, including recreational fishing.     

The importance of atmospheric deposition, charge and atomic mass to the dynamics of minor and rare elements in developing, ageing, and wilted leaves of beech (Fagus sylvatica L.)

The amounts of sixty elements in developing, maturing, senescent and wilting leaves, and in the wintering dead leaves attached to the branches, are reported for a beech (Fagus sylvatica) forest on mor Podzol in south Sweden, a site with no local sources of pollution or geological anomalies. The amounts (contents per leaf) of K (potassium), Rb (rubidium), Cs (caesium), Cu (copper) and P (phosphorus) were highest in young leaves, decreasing throughout the growing season and usually in the subsequent winter. The entirely opposite pattern with a continuous, mostly even increase of the amounts was measured with Be (beryllium), Ba (barium), Hg (mercury), Al (aluminium), Tl (thallium), Pb (lead), Bi (bismuth), V (vanadium), W (tungsten), As (arsenic), Sb (antimony), and Se (selenium). Amounts of rare-earth elements and some transition metals, such as Co (cobalt), Ti (titanium), and the actinides Th (thorium) and U (uranium) were more stable during the growing season, after an initial increase in early summer, but increased greatly in the winter. This winter increase in dead attached leaves has to be accounted for by uptake from long-distance transported constituents in dry and wet deposition. It was similar to deposition rate estimates using moss carpets from the same locality. A passive uptake was positively related to ionic charge and atomic mass. However, the amounts of several, mainly non-essential elements, such as Ni (nickel), Sc (scandium), Zr (zirconium), Cr (chromium), Ag (silver), and Cd (cadmium) were not much lower in the young or maturing leaves than in the wintered dead leaves of this deciduous (hardwood) forest and a proportion apparently originated from internal translocation in the trees. Seasonal fluxes or cycling of many of the scarce or rare elements reported here have never been studied before in forest ecosystems.     

The need to consider time,level, and trends: A turnover perspective

We focus on the contributions to research generated by considering attitudes and behaviors as dynamic over time and across different levels. Using turnover research as an example, we demonstrate how a past, present, and future focus, across levels, may enhance both theory and methodology. Copyright © 2013 John Wiley & Sons, Ltd.     

A framework for evaluating regional-scale numerical photochemical modeling systems

This paper discusses the need for critically evaluating regional-scale (~200–2,000 km) three-dimensional numerical photochemical air quality modeling systems to establish a model’s credibility in simulating the spatio-temporal features embedded in the observations. Because of limitations of currently used approaches for evaluating regional air quality models, a framework for model evaluation is introduced here for determining the suitability of a modeling system for a given application, distinguishing the performance between different models through confidence-testing of model results, guiding model development and analyzing the impacts of regulatory policy options. The framework identifies operational, diagnostic, dynamic, and probabilistic types of model evaluation. Operational evaluation techniques include statistical and graphical analyses aimed at determining whether model estimates are in agreement with the observations in an overall sense. Diagnostic evaluation focuses on process-oriented analyses to determine whether the individual processes and components of the model system are working correctly, both independently and in combination. Dynamic evaluation assesses the ability of the air quality model to simulate changes in air quality stemming from changes in source emissions and/or meteorology, the principal forces that drive the air quality model. Probabilistic evaluation attempts to assess the confidence that can be placed in model predictions using techniques such as ensemble modeling and Bayesian model averaging. The advantages of these types of model evaluation approaches are discussed in this paper.     

Reproductive biology and biochemical composition of the brooding echinoid <Emphasis Type="Italic">Amphipneustes lorioli</Emphasis> on the Antarctic continental shelf

The bathyal West Antarctic Peninsula (WAP) shelf experiences intense seasonal variability in primary production, with summer phytoplankton blooms yielding intense pulse of phytodetritus to shelf sediments. Echinoderms form a conspicuous proportion of the deposit-feeding megabenthos on the shelf and of these Amphipneustes lorioli was the most abundant irregular echinoid. To explore the reproductive response of A. lorioli to this seasonal production cycle, A. Lorioli was sampled at one location on the WAP shelf during four separate cruises between March 2000 and March 2001. Reproductive patterns were determined by histological analyses of gonad tissue, and elemental (CHN) analyses were used to estimate the nutritional and energetic status of the body tissues. Histological analysis of the brooding echinoid A. lorioli suggested a quasi-continuous gametogenic pattern in both the ovaries and the testes. Biochemical analysis of the gonads and the gut tissues were consistent with a continuous gametogenic cycle, showing no significant changes in the biochemical composition of the tissues among seasons. Size-frequency distributions of the embryo and juvenile echinoids within the adults’ brood pouches revealed a synchronous recruitment of embryos and juveniles in specific cohorts between different adult specimens. Whilst this occurrence of different cohorts of the developing brood may be an adaptation to limited brood space, there may also be an external factor influencing the synchrony between adult individuals. Nonetheless, a continuous gametogenic cycle and the lack of seasonal variation in the biochemical composition of gonad and gut tissues suggest that this deposit-feeding irregular urchin is exploiting a persistent sediment food bank in WAP shelf sediments throughout much of the year.     

High accuracy in situ radiometric mapping

In situ and airborne gamma ray spectrometry have been shown to provide rapid and spatially representative estimates of environmental radioactivity across a range of landscapes. However, one of the principal limitations of this technique has been the influence of changes in the vertical distribution of the source (e.g. 137Cs) on the observed photon fluence resulting in a significant reduction in the accuracy of the in situ activity measurement. A flexible approach for single gamma photon emitting radionuclides is presented, which relies on the quantification of forward scattering (or valley region between the full energy peak and Compton edge) within the gamma ray spectrum to compensate for changes in the 137Cs vertical activity distribution. This novel in situ method lends itself to the mapping of activity concentrations in environments that exhibit systematic changes in the vertical activity distribution. The robustness of this approach has been demonstrated in a salt marsh environment on the Solway coast, SW Scotland, with both a 7.6 cm x 7.6 cm NaI(Tl) detector and a 35% n-type HPGe detector. Application to ploughed field environments has also been demonstrated using HPGe detector, including its application to the estimation of field moist bulk density and soil erosion measurement. Ongoing research work is also outlined.     

Quantitative differences in mitochondrial ultrastructure of a thiobiotic and an oxybiotic turbellarian

Specimens of the thiobiotic acoel Solenofilomorpha funilis Crezée, 1975 and the oxybiotic acoel Otocelis sp. were collected from Mount Desert Island, Maine, USA in spring and summer of 1982, and the ultrastructure and distribution of their mitochondria were compared. The volume density and the surface density of the mitochondria are significantly higher in the epidermis and parenchyma of S. funilis than in the corresponding tissues of Otocelis sp. Surface densities of mitochondrial cristae are higher in all tissues studied (epidermis, muscle, and parenchyma) of S. funilis than in Otocelis sp. The differences in mitochondrial ultrastructure of these two species suggest that mitochondria in anaerobic tissues differ in function from their aerobic counterparts.     

Some problems arising in analysis of large-scale animal irradiation experiments

This paper describes some of the problems that occur in experiments designed particularly to study the effects of irradiation. One of the purposes of such experiments is to provide data that allow an informed judgment on estimates of the risks for man after exposure to radiation.It is no trivial matter to establish whether exposure to agents, such as radiation, increases the probability of a tumor arising de novo or whether the effect is an advancement of the time of appearance of naturally occurring tumors. In many murine tumors, the problem of establishing which of these two possibilities is the case is increased by the high natural incidence. In order to interpret the data from carcinogenesis experiments, a knowledge of the natural history of the tumor of interest is necessary. In this paper we present data that help to elucidate the biology of lung tumors in hybrid B6CF₁/An1 mice.The rising incidence of many tumors late in life raises the question of whether a common systemic factor is involved. In experiments with female mice, relatively low doses of radiation may sterilize the ovary with subsequent hormonal imbalances that in turn may influence the appearance of tumors in a number of hormone-influenced tissues. We have found that radiation may increase not only the age-specific rates of tumors but also the probability of the number of different tumor types.The possibility of a lack of independence of the occurrences of different tumors complicates the data analysis. To answer some of the questions we have raised, it would be of considerable help to have data on the prevalence of tumors; however, such data usually involve serial killing which is costly. We have examined the possibility of obtaining prevalence data for lung tumors by the determination of the incidence of lung tumors in mice dying from causes other than lung tumors. There appears to be no difference in the prevalence of lung tumors determined by using either this approach or serial killing.