Tiger shark (Galeocerdo cuvier) abundance and growth in a subtropical embayment: evidence from 7 years of standardized fishing effort

The tiger shark (Galeocerdo cuvier Peron and Lesueur 1822) is a widely distributed predator with a broad diet and the potential to affect marine community structure, yet information on local patterns of abundance for this species is lacking. Tiger shark catch data were gathered over 7 years of tag and release research fishing (1991–2000, 2002–2004) in Shark Bay, Western Australia (25°45′S, 113°44′E). Sharks were caught using drumlines deployed in six permanent zones (~3 km² in area). Fishing effort was standardized across days and months, and catch rates on hooks were expressed as the number of sharks caught h⁻¹. A total of 449 individual tiger sharks was captured; 29 were recaptured. Tiger shark catch rate showed seasonal periodicity, being higher during the warm season (Sep–May) than during the cold season (Jun–Aug), and was marked by inter-annual variability. The most striking feature of the catch data was a consistent pattern of slow, continuous variation within each year from a peak during the height of the warm season (February) to a trough in the cold season (July). Annual growth rates of recaptured individuals were generally consistent with estimates from other regions, but exceeded those for populations elsewhere for sharks >275 cm fork length (FL), perhaps because mature sharks in the study area rely heavily on large prey. The data suggest that (1) the threat of predation faced by animals consumed by tiger sharks fluctuates dramatically within and between years, and (2) efforts to monitor large shark abundance should be extensive enough to detect inter-annual variation and sufficiently intensive to account for intra-annual trends.     

Recruitment limitation constrains local species richness and productivity in dry grassland

Species coexistence and local-scale species richness are limited by the availability of seeds and microsites for germination and establishment. We conducted a seed addition experiment in seminatural grassland at three sites in southern Switzerland and repeated the experiment in two successive years to evaluate various circumstances under which seed limitation and establishment success affect community functioning. A collection of 144,000 seeds of 22 meadow species including grasses and forbs of local provenance was gathered, and seeds were individually sown in a density that resembled natural seed rain. The three communities were seed limited. Three years after sowing, single species varied in emergence (0-50%), survival (0-69%), and establishment rates (0-27%). One annual and 13 perennial species reached reproductive stage. Low establishment at one site and reduced growth at another site indicated stronger microsite limitation compared to the third site. Recruitment was influenced by differences in abiotic environmental conditions between sites (water availability, soil minerals) and by within-site differences in biotic interaction (competition). At the least water-limited site, sowing resulted in an increase in phytomass due to establishment of short-lived perennials in the second and third years after sowing. This increase persisted over the following two years due to establishment of longer-lived perennials. After sowing in a wetter year with higher phytomass, however, productivity did not increase, because higher intensity of competition in an early phase of establishment resulted in less vigorous plants later on. Due to the generally favorable weather conditions during this study, sowing year had a small effect on numbers of established individuals over all species. Recruitment limitation can thus constrain local-scale species richness and productivity, either by a lack of seeds or by reduced seedling growth, likely due to competition from the established vegetation.     

Crown ratio influences allometric scaling in trees

Allometric theories suggest that the size and shape of organisms follow universal rules, with a tendency toward quarter-power scaling. In woody plants, however, structure is influenced by branch death and shedding, which leads to decreasing crown ratios, accumulation of heartwood, and stem and branch tapering. This paper examines the impacts on allometric scaling of these aspects, which so far have been largely ignored in the scaling theory. Tree structure is described in terms of active and disused pipes arranged as an infinite branching network in the crown, and as a tapering bundle of pipes below the crown. Importantly, crown ratio is allowed to vary independently of crown size, the size of the trunk relative to the crown deriving from empirical results that relate crown base diameter to breast height diameter through crown ratio. The model implies a scaling relationship in the crown which reduces to quarter-power scaling under restrictive assumptions but would generally yield a scaling exponent somewhat less than three-quarters. For the whole tree, the model predicts that scaling between woody mass and foliage depends on crown ratio. Measurements on three boreal tree species are consistent with the model predictions.     

Dispersal per recruit: an efficient method for assessing sustainability in marine reserve networks.

Marine reserves are an increasingly important tool for the management of marine ecosystems around the world. However, the effects of proposed marine reserve configurations on sustainability and yield of populations are typically not estimated because of the computational intensity of direct simulation and uncertainty in larval dispersal and density-dependent recruitment. Here we develop a method for efficiently assessing a marine reserve configuration for persistence and yield of a population with sedentary adults and dispersing larvae. The method extends the familiar sustainability criteria of individual replacement for single populations based on eggs-per-recruit (EPR) to spatially distributed populations with sedentary adults, a dispersing larval phase, and limited carrying capacity in the settlement-recruit relationship. We refer to this approach as dispersal-per-recruit (DPR). In some cases, a single DPR calculation, based on the assumption that post-settlement habitat is saturated (i.e., at maximum recruitment), is sufficient to determine population persistence, while in other cases further iterative calculations are required. These additional calculations reach an equilibrium more rapidly than a full simulation of age- or size-structured populations. From the DPR result, fishery yield can be computed from yield-per-recruit (YPR) at each point. We assess the utility of DPR calculations by applying them to single reserves, uniformly distributed systems of reserves, and randomly sized and spaced systems of reserves on a linear coastline. We find that for low levels of EPR in fished areas (e.g., 10% or less of the natural, unfished EPR when post-settlement habitats are saturated by 35% of natural settlement), a single DPR calculation is sufficient to determine persistence of the population. We also show that, in uniform systems of reserves with finite reserve size, maximal fisheries yield occurs when the density of reserves is such that all post-settlement habitat is nearly saturated with settlers. Finally, we demonstrate the application of this approach to a realistic proposed marine reserve configuration.     

Species Imperilment and Spatial Patterns of Development in the United States

Abstract:  Conservation biologists and others hypothesize that humankind's "ecological footprint" is affected not only by the sheer intensity of human activity but also by its spatial arrangement. We used a multivariate statistical model and state-level data to evaluate correlations between species imperilment and the level and spatial distribution of human settlement and infrastructure development in the United States. The level of human activity—measured by the number of people and households, incidence of roads, and intensity of nighttime lights—was significantly correlated with the ecological imperilment of species. Our regression models consistently showed that a 1% increase in the level of human activity across the United States was associated with about a 0.25% increase in the proportion of plant and animal species considered at risk of extinction by The Nature Conservancy. The distribution of human activity did not affect species imperilment. Our results point to rising levels of human activity—and not some particular (e.g., sprawling) distribution of human activity—as the most relevant anthropogenic factor explaining biodiversity loss in the United States.     

Globalization of the Amazon Soy and Beef Industries: Opportunities for Conservation

Abstract:  Amazon beef and soybean industries, the primary drivers of Amazon deforestation, are increasingly responsive to economic signals emanating from around the world, such as those associated with bovine spongiform encephalopathy (BSE, "mad cow disease") outbreaks and China's economic growth. The expanding role of these economic "teleconnections" (coupled phenomena that take place in distant places on the planet) led to a 3-year period (2002–2004) of historically high deforestation rates. But it also increases the potential for large-scale conservation in the region as markets and finance institutions demand better environmental and social performance of beef and soy producers. Cattle ranchers and soy farmers who have generally opposed ambitious government regulations that require forest reserves on private property are realizing that good land stewardship—including compliance with legislation—may increase their access to expanding domestic and international markets and to credit and lower the risk of "losing" their land to agrarian reform. The realization of this potential depends on the successful negotiation of social and environmental performance criteria and an associated system of certification that are acceptable to both the industries and civil society. The foot-and-mouth eradication system, in which geographic zones win permission to export beef, may provide an important model for the design of a low-cost, peer-enforced, socioenvironmental certification system that becomes the mechanism by which beef and soy industries gain access to markets outside the Amazon.     

A simple approach to modeling microbial biomass in the rhizosphere

Microorganisms make an important contribution to the degradation of contaminants in bioremediation as well as in phytoremediation. An accurate estimation of microbial concentrations in the soil would be valuable in predicting contaminant dissipation during various bioremediation processes. A simple modeling approach to quantify the microbial biomass in the rhizosphere was developed in this study. Experiments were conducted using field column lysimeters planted with Eastern gamagrass. The microbial biomass concentrations from the rhizosphere soil, bulk soil, and unplanted soil were monitored for six months using an incubation–fumigation method. The proposed model was applied to the field microbial biomass data and good correlation between simulated and experimental data was achieved. The results indicate that plants increase microbial concentrations in the soil by providing root exudates as growth substrates for microorganisms. Since plant roots are initially small and do not produce large quantities of exudates when first seeded, the addition of exogenous substrates may be needed to increase initial microbial concentrations at the start of phytoremediation projects.