Phenological sequences reveal aggregate life history response to climatic warming

Climatic warming is associated with organisms breeding earlier in the season than is typical for their species. In some species, however, response to warming is more complex than a simple advance in the timing of all life history events preceding reproduction. Disparities in the extent to which different components of the reproductive phenology of organisms vary with climatic warming indicate that not all life history events are equally responsive to environmental variation. Here, we propose that our understanding of phenological response to climate change can be improved by considering entire sequences of events comprising the aggregate life histories of organisms preceding reproduction. We present results of a two-year warming experiment conducted on 33 individuals of three plant species inhabiting a low-arctic site. Analysis of phenological sequences of three key events for each species revealed how the aggregate life histories preceding reproduction responded to warming, and which individual events exerted the greatest influence on aggregate life history variation. For alpine chickweed (Cerastium alpinum), warming elicited a shortening of the duration of the emergence stage by 2.5 days on average, but the aggregate life history did not differ between warmed and ambient plots. For gray willow (Salix glauca), however, all phenological events monitored occurred earlier on warmed than on ambient plots, and warming reduced the aggregate life history of this species by 22 days on average. Similarly, in dwarf birch (Betula nana), warming advanced flower bud set, blooming, and fruit set and reduced the aggregate life history by 27 days on average. Our approach provides important insight into life history responses of many organisms to climate change and other forms of environmental variation. Such insight may be compromised by considering changes in individual phenological events in isolation.     

Scale-dependent responses of plant biodiversity to nitrogen enrichment

Experimental studies demonstrating that nitrogen (N) enrichment reduces plant diversity within individual plots have led to the conclusion that anthropogenic N enrichment is a threat to global biodiversity. These conclusions overlook the influence of spatial scale, however, as N enrichment may alter beta diversity (i.e., how similar plots are in their species composition), which would likely alter the degree to which N-induced changes in diversity within localities translate to changes in diversity at larger scales that are relevant to policy and management. Currently, it is unclear how N enrichment affects biodiversity at scales larger than a small plot. We synthesized data from 18 N-enrichment experiments across North America to examine the effects of N enrichment on plant species diversity at three spatial scales: small (within plots), intermediate (among plots), and large (within and among plots). We found that N enrichment reduced plant diversity within plots by an average of 25% (ranging from a reduction of 61% to an increase of 5%) and frequently enhanced beta diversity. The extent to which N enrichment altered beta diversity, however, varied substantially among sites (from a 22% increase to an 18% reduction) and was contingent on site productivity. Specifically, N enrichment enhanced beta diversity at low-productivity sites but reduced beta diversity at high-productivity sites. N-induced changes in beta diversity generally reduced the extent of species loss at larger scales to an average of 22% (ranging from a reduction of 54% to an increase of 18%). Our results demonstrate that N enrichment often reduces biodiversity at both local and regional scales, but that a focus on the effects of N enrichment on biodiversity at small spatial scales may often overestimate (and sometimes underestimate) declines in regional biodiversity by failing to recognize the effects of N on beta diversity.     

Ontogenetic changes in small-scale movement by recruits of an exploited mussel: implications for the fate of larvae settling on algae

Many South African populations of the brown mussel Perna perna have been depleted through over-exploitation by subsistence harvesters. This is problematic because recovery after disturbance is very slow, partly because recruits are largely associated with adult mussels. However, unlike large recruits of 3.5–10 mm that exhibit spatial structure related to that of adults, a very high proportion of settlers and small recruits (0–3.5 mm) occur on foliose algae. We tested the hypothesis that recruits on algae move to adult mussel beds after a period of growth, with the null hypothesis that they die at a certain size. We conducted an indirect field study comparing the ratios of large to small recruits in 100% cover mussel patches at locations with high and low algal cover. A second laboratory experiment analysed whether the size of recruits on algae affects their active movement behaviour in response to nearby mussel patches. Large/small recruit ratios were slightly, but not significantly greater in high than low algal cover locations. Both small (2–2.5 mm) and medium (4.5–5.5 mm) recruits remained on algae and moved very short distances throughout the laboratory experiment, while larger recruits (9–10 mm) moved significantly further distances and more often into mussel patches. The results suggest that very large recruits are able to migrate actively to nearby mussel patches, indicating ontogenetic shifts in this behaviour. However, the absence of a significant difference in ratios between field locations with high and low algal cover suggests many large recruits are accidentally dislodged from the algae and presumably die. Thus settlement of P. perna onto algae is likely to be wasted, with consequences for sustainable management of the mussel resource.     

Flux capacities and acclimation costs in Trichodesmium from the Gulf of Mexico

Phytoplankton function and acclimation are driven by catalytic protein complexes that mediate key physiological transformations, including generation of photosynthetic ATP and reductant, and carbon and nitrogen fixation. Quantitation of capacities for these processes allows estimation of rates for key ecosystem processes, and identification of factors limiting primary productivity. We herein present molar quantitations of PSI, PSII, ATP synthase, RuBisCO and the Fe protein of nitrogenase of Trichodesmium collected from the Gulf of Mexico, in comparison to determinations for a range of cyanobacteria growing in culture. Using these measurements, estimates were generated for Trichodesmium capacities for carbon fixation of 1–3.4 g C g chl a ⁻¹ h⁻¹ and nitrogen fixation of 0.06–0.17 g N g chl a ⁻¹ h⁻¹, with diel variations in capacities. ATP synthase levels show that ATP synthesis capacity is sufficient to support these levels of carbon and nitrogen fixation, and that ATP synthase levels change over the day in accordance with the ATP demands of nitrogenase and RuBisCO activity. Levels of measured complexes indicate that Trichodesmium manifests n-type diel light acclimation through rapid changes in RuBisCO:PSII, supported by significant investment of cellular nitrogen. The plasticity in the levels and stoichiometry of these core complexes show that changes in the abundance of core protein complexes are an important component of acclimation and regulation of metabolic function by Trichodesmium populations. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Spatial–temporal pattern of bluefin tuna (Thunnus thynnus L. 1758) gonad maturation across the Mediterranean Sea

Atlantic bluefin tuna (BFT; Thunnus thynnus) is a migrating species straddling the North Atlantic Ocean and Mediterranean Sea. It is assumed that this species is divided into a western and an eastern stock, which spawn in the Gulf of Mexico and the Mediterranean Sea, respectively. To learn more about the reproductive behavior of the eastern BFT stock, we tracked gonadal development in adult fish that were sampled between April and July during three consecutive years (2003–2005). Sampling campaigns were carried out using common fishing methods at selected locations within the Mediterranean Sea, namely Levantine Sea, Malta, and Balearic Islands. An additional sampling point, Barbate, was situated northwest of the Straits of Gibraltar along the Atlantic coast. Morphometric parameters such as the total body mass (M _B) and the weights of the gonads (M _G) were recorded, and the respective gonadosomatic index (GSI) values were calculated. The data collected revealed two important trends: (1) GSI values are higher in fish caught in the eastern rather than the western locations across the Mediterranean Sea, and (2) the GSI reaches maximum values between late May and early June in Levantine Sea (eastern Mediterranean Sea), and only 2 and 4 weeks later in the central (Malta) and western (Balearic Islands) locations, respectively. The advanced gonadal development in BFT correlates well with higher sea surface temperatures. Our findings also distinguish the northern Levantine Sea BFT population (mean M _B 78.41 ± 4.13 kg), and the Barbate BFT population with the greatest M _B (all fish sampled > 100 kg). These data reflect a situation in which the eastern Mediterranean basin may function as a habitat for young BFT, until they gain a larger M _B and are able to move to the Atlantic Ocean. However, the existence of genetically discrete BFT populations in the Mediterranean Sea cannot be ruled out.     

Temporal and spatial variability in stable isotope ratios of SPM link to local hydrography and longer term SPM averages suggest heavy dependence of mussels on nearshore production

Temporal changes in hydrography affect suspended particulate matter (SPM) composition and distribution in coastal systems, potentially influencing the diets of suspension feeders. Temporal variation in SPM and in the diet of the mussel Perna perna, were investigated using stable isotope analysis. The δ¹³C and δ¹⁵ N ratios of SPM, mussels and macroalgae were determined monthly, with SPM samples collected along a 10 km onshore–offshore transect, over 14 months at Kenton-on-Sea, on the south coast of South Africa. Clear nearshore (0 km) to offshore (10 km) carbon depletion gradients were seen in SPM during all months and extended for 50 km offshore on one occasion. Carbon enrichment of coastal SPM in winter (June–August 2004 and May 2005) indicated temporal changes in the nearshore detrital pool, presumably reflecting changes in macroalgal detritus, linked to local changes in coastal hydrography and algal seasonality. Nitrogen patterns were less clear, with SPM enrichment seen between July and October 2004 from 0 to 10 km. Nearshore SPM demonstrated cyclical patterns in carbon over 24-h periods that correlated closely with tidal cycles and mussel carbon signatures, sampled monthly, demonstrated fluctuations that could not be correlated to seasonal or monthly changes in SPM. Macroalgae showed extreme variability in isotopic signatures, with no discernable patterns. IsoSource mixing models indicated over 50% reliance of mussel tissue on nearshore carbon, highlighting the importance of nearshore SPM in mussel diet. Overall, carbon variation in SPM at both large and small temporal scales can be related to hydrographic processes, but is masked in mussels by long-term isotope integration.     

A Markov model for assessing ecological stability properties

Ecological systems are frequently modeled as dynamic systems. It is natural then to use the techniques of dynamic systems theory to analyze such models. However, the methods and results that are produced by dynamic systems theory do not always capture the aspects of ecological systems that are of greatest interest to managers and decision-makers. We identify some of the challenges of using dynamic systems theory to explore ecological systems and propose an alternative approach that emphasizes the understanding of transient effects in light of uncertainty and variability. We illustrate this method by an examination of a model for phosphorus levels in fresh water lakes.     

Inferring linear feature use in the presence of GPS measurement error

Global Positioning System (GPS) collars are increasingly used to study animal movement and habitat use. Measurement error is defined as the difference between the observed and true value being measured. In GPS data measurement error is referred to as location error and leads to misclassification of observed locations into habitat types. This is particularily true when studying habitats of small spatial extent with large amounts of edge, such as linear features (e.g. roads and seismic lines). However, no consistent framework exists to address the effect of measurement error on habitat classification of observed locations and resulting biological inference. We developed a mechanistic, empirically-based method for buffering linear features that minimizes the underestimation of animal use introduced by GPS measurement error. To do this we quantified the distribution of measurement error and derived an explicit formula for buffer radius which incorporated the error distribution, the width of the linear feature, and a predefined amount of acceptable type I error in location classification. In our empirical study we found the GPS measurement error of the Lotek GPS_3300 collar followed a bivariate Laplace distribution with parameter ρ = 0.1123. When we applied our method to a simulated landscape, type I error was reduced by 57%. This study highlights the need to address the effect of GPS measurement error in animal location classification, particularily for habitats of small spatial extent.     

A method for assigning species into groups based on generalized Mahalanobis distance between habitat model coefficients

Habitat association models are commonly developed for individual animal species using generalized linear modeling methods such as logistic regression. We considered the issue of grouping species based on their habitat use so that management decisions can be based on sets of species rather than individual species. This research was motivated by a study of western landbirds in northern Idaho forests. The method we examined was to separately fit models to each species and to use a generalized Mahalanobis distance between coefficient vectors to create a distance matrix among species. Clustering methods were used to group species from the distance matrix, and multidimensional scaling methods were used to visualize the relations among species groups. Methods were also discussed for evaluating the sensitivity of the conclusions because of outliers or influential data points. We illustrate these methods with data from the landbird study conducted in northern Idaho. Simulation results are presented to compare the success of this method to alternative methods using Euclidean distance between coefficient vectors and to methods that do not use habitat association models. These simulations demonstrate that our Mahalanobis-distance-based method was nearly always better than Euclidean-distance-based methods or methods not based on habitat association models. The methods used to develop candidate species groups are easily explained to other scientists and resource managers since they mainly rely on classical multivariate statistical methods.     

NEAR‐STREAM LANDUSE EFFECTS ON STREAMWATER NUTRIENT DISTRIBUTION IN AN URBANIZING WATERSHED1

ABSTRACT: We investigated spatial and temporal relationships among surface and subsurface watershed attributes and stream nutrient concentrations in urbanizing Johnson Creek watershed in northern Oregon. We sampled stream water at eight urban and five nonurban locations from March 1998 through December 1999. We sampled eight wells distributed over the two primary aquifers in the watershed. Using a Geographic Information System (GIS), percentages of landuse attributes within a radius of 30, 91, and 152 m from each sample site were quantified. We analyzed relationships between (1) nutrient concentrations and percentage cover of different landuse attributes, and (2) nutrient concentrations and underlying hydrologic units. We did not find a significant relationship between ground water chemistry and stream water chemistry. We found elevated levels of phosphorus (P) concentrations correlated with urban landuse, while higher nitrogen (N) concentrations were correlated with nonurban (primarily agricultural) landuse. We concluded that elevated levels of N in nonurban areas of Johnson Creek watershed were associated with agricultural practices. We further concluded that urban development factors such as increases in storm drains, dry wells, and impermeable surfaces may be responsible for higher input of P to the stream in urbanizing areas of the Johnson Creek watershed.