Diel and tidal cycles regulate larval dynamics in salt marshes and mangrove forests

Variation in the release and recruitment of larvae of estuarine invertebrates affects the distribution and abundance of adults, as well as trophic interactions in both the plankton and the benthos. Larval release and supply are often timed to environmental cycles such as the diel and tidal cycles. Here, we determined using plankton tows whether the abundance of larvae spanning salt marsh and mangrove habitats across the intertidal landscape varied with diel and tidal cycles. Using three different sampling designs across two sites and within each of two estuaries over a 12-month period, we covered a range of spatial and temporal scales. This allowed us to test the general prediction that densities of meroplankton in the water column would be greater during nocturnal ebb tides than during other phases of the diel or tidal cycle. As predicted, nocturnal ebb tides yielded the highest densities of meroplanktonic larvae and were dominated by first-stage crab zoeae and this finding was most pronounced in the salt marsh. Throughout the course of the year, greater numbers of meroplankters consistently occurred during the ebb tide compared with the flood tide. The densities of other taxa (e.g. gastropods and polychaetes) showed no clear trends with diel or tidal cycles. This study highlights the effects of these pervasive physical cycles on the timing of larval release and supply in the salt marsh–mangrove complex, and emphasises their contribution to the trophic interactions and the dynamics of benthic populations within estuaries.     

Pedestrians' estimates of their own visibility: a simple and effective computer-based technique

INTRODUCTION: Research has shown that both pedestrians and drivers drastically overestimate pedestrians' nighttime visibility (NHSTSA, 2008a, 2008b; Owens & Sivak, 1996) and fail to appreciate the safety benefits of proven conspicuity aids. One solution is educational intervention (Tyrrell, Patton, & Brooks, 2004); however, the on-road assessment of its effectiveness is expensive and time consuming. METHOD: Experiment One introduces a computer-based alternative to the field-based approach, successfully replicating the previous study's trends among 94 students who either receive or do not receive an educational lecture. Experiment Two utilizes the simulation's portability to determine if professional roadway workers have a more accurate understanding of pedestrian conspicuity than students. Results: Results among 88 workers show they do not significantly appreciate the advantages of effective retroflective material configurations or vehicle headlamp settings, for example, any better than non-lectured students in Experiment One. Impact: The study's results demonstrate the need for education among all pedestrians and the benefits of efficient testing methods.     

Interannual variability of growth and reproduction in Bursera simaruba: the role of allometry and resource variability

Plants are expected to differentially allocate resources to reproduction, growth, and survival in order to maximize overall fitness. Life history theory predicts that the allocation of resources to reproduction should occur at the expense of vegetative growth. Although it is known that both organism size and resource availability can influence life history traits, few studies have addressed how size dependencies of growth and reproduction and variation in resource supply jointly affect the coupling between growth and reproduction. In order to understand the relationship between growth and reproduction in the context of resource variability, we utilize a long-term observational data set consisting of 670 individual trees over a 10-year period within a local population of Bursera simaruba (L.) Sarg. We (1) quantify the functional form and variability in the growth-reproduction relationship at the population and individual-tree level and (2) develop a theoretical framework to understand the allometric dependence of growth and reproduction. Our findings suggest that the differential responses of allometric growth and reproduction to resource availability, both between years and between microsites, underlie the apparent relationship between growth and reproduction. Finally, we offer an alternative approach for quantifying the relationship between growth and reproduction that accounts for variation in allometries.     

Do activity costs determine foraging tactics for an arctic seabird?

How energy costs affect foraging decisions is poorly understood for marine animals. To provide data relevant to this topic, we examined the relationship between activity levels and foraging behavior by attaching activity recorders to 29 chick-rearing wing-propelled diving birds (thick-billed murres, Uria lomvia) in 1999–2000. We connected the activity during the final dive bout with the prey item we observed being fed to the chicks. After accounting for changes in activity level with depth, activity was highest during the final dive of a dive bout, reflecting maneuvring during prey capture. Pelagic prey items, especially invertebrates (amphipods), were associated with higher depth-corrected activity, leading to shorter dives for a given depth (presumably due to higher oxygen consumption rates) and, thus, shorter search times (lower bottom time for a given depth). Pelagic prey items were likely captured during active pursuit, with the birds actively seeking and pursuing schooling mid-water prey. In contrast, benthic prey involved low activity and extended search times, suggesting that the birds slowly glided along the bottom in search for prey hidden in the sediments or rocks. We concluded that activity levels are important in determining the foraging tactics of marine predators. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Benchmarking a triplet of official estimates

The publication of official statistics at different levels of aggregation requires a benchmarking step. Difficulties arise when a benchmarking method needs to be applied to a triplet of related estimates, at multiple stages of aggregation. For ratios of totals, external benchmarking constraints for the triplet (numerator, denominator, ratio) are that the weighted sum of denominator/numerator/ratio estimates equals to a constant. The benchmarking weight, applied to the ratio estimates, is a function of the denominator estimates. For example, the United States Department of Agriculture’s National Agricultural Statistics Service’s county-level, end-of-season acreage, production and yield estimates need to aggregate to the corresponding agricultural statistics district-level estimates, which also need to aggregate to the corresponding prepublished state-level values. Moreover, the definition of yield, as the ratio of production to harvested acreage, needs to hold at the county level, the agricultural statistics district level and the state level. We discuss different methods of applying benchmarking constraints to a triplet (numerator, denominator, ratio), at multiple stages of aggregation, where the denominator and the ratio are modeled and the numerator is derived. County-level and agricultural statistics district-level, end-of-season acreage, production and yield estimates are constructed and compared using the different methods. Results are illustrated for 2014 corn and soybean in Indiana, Iowa and Illinois.     

No evidence for homeoviscous adaptation in intertidal snails: analysis of membrane fluidity during thermal acclimation, thermal acclimatization, and across thermal microhabitats

Many eurythermal organisms alter composition of their membranes to counter perturbing effects of environmental temperature variation on membrane fluidity, a process known as homeoviscous adaptation. Marine intertidal gastropods experience uniquely large thermal excursions that challenge the functional integrity of their membranes on tidal and seasonal timescales. This study measured and compared membrane fluidity in marine intertidal snail species under three scenarios: (1) laboratory thermal acclimation, (2) thermal acclimatization during a hot midday low tide, and (3) thermal acclimatization across the vertical intertidal zone gradient in temperature. For each scenario, we used fluorescence polarization of the membrane probe DPH to measure membrane fluidity in individual samples of gill and mantle tissue. A four-week thermal acclimation of Tegula funebralis to 5, 15, and 25°C did not induce differences in membrane fluidity. Littorina keenae sampled from two thermal microhabitats at the beginning and end of a hot midday low tide exhibited no significant differences in membrane fluidity, either as a function of time of day or as a function of thermal microhabitat, despite changes in body temperature up to 24°C within 8 h. Membrane fluidities of a diverse group of snails collected from high, middle, and low vertical regions of the intertidal zone varied among species but did not correlate with thermal microhabitat. Our data suggest intertidal gastropod snails do not exhibit homeoviscous adaptation of gill and mantle membranes. We discuss possible alternatives for how these organisms counter thermal excursions characteristic of the marine intertidal zone.     

Predator diversity and identity drive interaction strength and trophic cascades in a food web

Declining predator diversity may drastically affect the biomass and productivity of herbivores and plants. Understanding how changes in predator diversity can propagate through food webs to alter ecosystem function is one of the most challenging ecological research topics today. We studied the effects of predator removal in a simple natural food web in the Sierra Nevada mountains of California (USA). By excluding the predators of the third trophic level of a food web in a full-factorial design, we monitored cascading effects of varying predator diversity and composition on the herbivorous beetle Chrysomela aeneicollis and the willow Salix orestera, which compose the first and second trophic levels of the food web. Decreasing predator diversity increased herbivore biomass and survivorship, and consequently increased the amount of plant biomass consumed via a trophic cascade. Despite this simple linear mean effect of diversity on the strength of the trophic cascade, we found additivity, compensation, and interference in the effects of multiple predators on herbivores and plants. Herbivore survivorship and predator-prey interaction strengths varied with predator diversity, predator identity, and the identity of coexisting predators. Additive effects of predators on herbivores and plants may have been driven by temporal niche separation, whereas compensatory effects and interference occurred among predators with a similar phenology. Together, these results suggest that while the general trends of diversity effects may appear linear and additive, other information about species identity was required to predict the effects of removing individual predators. In a community that is not temporally well-mixed, predator traits such as phenology may help predict impacts of species loss on other species. Information about predator natural history and food web structure may help explain variation in predator diversity effects on trophic cascades and ecosystem function.     

Insects mediate the effects of propagule supply and resource availability on a plant invasion

Invasive species are a global threat to biodiversity and the functioning of natural ecosystems. Here, we report on a two-year experiment aimed at elucidating the combined and relative effects of three key controls on plant invasions: propagule supply, soil nitrogen (N) availability, and herbivory by native insects. We focus on the exotic species Lespedeza cuneata, a Rank 1 invasive species. Propagule supply and soil N-availability interacted to control the density and foliar cover of L. cuneata. In low N plots, density and foliar cover of L. cuneata were higher in the propagule addition plots than in the plots to which propagules were not added. Surprisingly, this interaction was significant only when the abundance of herbivores was experimentally reduced. This experiment provides evidence that native insect herbivores mediate the interactive effects of propagule supply and resources on invasion by a widespread invasive plant species.     

Nutrient input and removal trends for agricultural soils in nine geographic regions in Arkansas

Knowledge of the balance between nutrient inputs and removals is required for identifying regions that possess an excess or deficit of nutrients. This assessment describes the balance between the agricultural nutrient inputs and removals for nine geographical districts within Arkansas from 1997 to 2001. The total N, P, and K inputs were summed for each district and included inorganic fertilizer and collectable nutrients excreted as poultry, turkey, dairy, and hog manures. Nutrients removed by harvested crops were summed and subtracted from total nutrient inputs to calculate the net nutrient balance. The net balances for N, P, and K were distributed across the hectarage used for row crop, hay, pasture, or combinations of these land uses. Row-crop agriculture predominates in the eastern one-third and animal agriculture predominates in the western two-thirds of Arkansas. Nutrients derived from poultry litter accounted for >92% of the total transportable manure N, P, and K. The three districts in the eastern one-third of Arkansas contained 95% of the row-crop hectarage and had net N and P balances that were near zero or negative. The six districts in the western two-thirds of Arkansas accounted for 89 to 100% of the animal populations, had positive net balances for N and P, and excess P ranged from 1 to 9 kg P ha(-1) when distributed across row-crop, hay, and pasture hectarage. Transport of excess nutrients, primarily in poultry litter, outside of the districts in western Arkansas is needed to achieve a balance between soil inputs and removals of P and N.