Economic feasibility of no-tillage and manure for soil carbon sequestration in corn production in northeastern Kansas

This study examined the economic potential of no-tillage versus conventional tillage to sequester soil carbon by using two rates of commercial N fertilizer or beef cattle manure for continuous corn (Zea mays L.) production. Yields, input rates, field operations, and prices from an experiment were used to simulate a distribution of net returns for eight production systems. Carbon release values from direct, embodied, and feedstock energies were estimated for each system, and were used with soil carbon sequestration rates from soil tests to determine the amount of net carbon sequestered by each system. The values of carbon credits that provide an incentive for managers to adopt production systems that sequester carbon at greater rates were derived. No-till systems had greater annual soil carbon gains, net carbon gains, and net returns than conventional tillage systems. Systems that used beef cattle manure had greater soil carbon gains and net carbon gains, but lower net returns, than systems that used commercial N fertilizer. Carbon credits would be needed to encourage the use of manure-fertilized cropping systems.     

Age and growth rates of Hawaiian hawksbill turtles (Eretmochelys imbricata) using skeletochronology

The Hawaiian hawksbill population has fewer than 20 females nesting per year; hence, there is a need to monitor this population closely and basic biological information on individual growth and age to maturity is critical. We present a skeletochronology analysis of Hawaiian hawksbills using humeri recovered from 30 dead stranded hawksbills, plus 10 dead hatchlings. Growth mark morphology shows readily distinguishable marks similar in appearance to other species, though some animals displayed more diffuse marks. Growth rates remained high (average 2.24–4.77 cm year^?1) from 20 to 80 cm straight carapace length (SCL). Hawksbills larger than 80 cm SCL had average growth rates of 0.3 cm year^?1. There were few adult turtles in the sample; however, results indicate hawksbills have faster growth rates than loggerhead or green turtles, with probable average age to maturity (at size 78.6 cm SCL) occurring between 17 and 22 years.     

Ecological opportunity and phenotypic plasticity interact to promote character displacement and species coexistence

We investigated the roles of resource availability and phenotypic plasticity in promoting ecological character displacement (i.e., trait evolution stemming from resource competition between species). Because ecological character displacement generates new populations that differ in resource use, this process should only occur when exploitable resources are available. We tested this hypothesis in two species of spadefoot toads (Spea bombifrons and S. multiplicata) whose tadpoles use phenotypic plasticity to develop into either an omnivore morph, which specializes on detritus, or a physically distinctive carnivore morph, which specializes on shrimp. Both species grow best on shrimp, but when reared together, S. bombifrons outcompetes S. multiplicata for shrimp and S. multiplicata outcompetes S. bombifrons for detritus. We found that when each species occurred alone in the field, they produced similar proportions of omnivores and carnivores. When the two species occurred together, however, they underwent ecological character displacement in larval development, with S. multiplicata producing mostly omnivores, and S. bombifrons producing mostly carnivores. We combined observations of natural populations with experiments to evaluate whether such character displacement was only possible when both shrimp and detritus were relatively abundant. Mixed-species ponds contained abundant detritus and shrimp, in contrast with nearby pure-species ponds, which were deficient in one resource. Experiments revealed that S. multiplicata competed poorly when detritus was rare and that S. bombifrons competed poorly when shrimp was rare. In nature, when one of these two resources was scarce, one species was missing, perhaps through competitive exclusion by the species that was the superior competitor for the remaining resource. Thus, ecological character displacement and, therefore, coexistence of close competitors, was only possible when diverse resources were available. Finally, even if exploitable resources are available, character displacement is not guaranteed to transpire if species cannot utilize such resources expeditiously. Phenotypic plasticity provides a general and important mechanism for facilitating resource partitioning. Thus, by facilitating shifts in resource use, phenotypic plasticity and ecological opportunity may often interact to promote divergence and coexistence of competitors.     

Ultrastructure of oogenesis in the holopelagic polychaetes Rhynchonerella angelini and Alciopa reynaudii (Polychaeta: Alciopidae)

Ultrastructural features of oogenesis were examined in the pelagic polychaetes Rhynchonerella angelini and Alciopa reynaudii which were collected from Bahamian waters by a manned submersible during 1979 and 1980. No definitive ovary was detected in either species. Oogonia are released into the coelom as packets of cells, where they undergo mitotic division while surrounded by an envelope of sheath cells. Cytokinesis is incomplete, resulting in intercellular bridges between oogonia. Oocytes undergo early stages of meiosis characterized by the presence of synapsed chromosomes, followed by a period of rapid cytoplasmic and nuclear growth. Oocytes are released from the packets in the early vitellogenic phase into the coelom, where they undergo yolk synthesis as solitary coelomic cells. Vitellogenesis includes both autosynthetic and heterosynthetic processes. Autosynthesis involves the fusion of secretory vesicles formed by the combined activity of the rough endoplasmic reticulum and Golgi complex, with convoluted electron-dense tubular bodies of unknown origin. Heterosynthesis involves the intense uptake of exogenous precursors through endocytosis and their fusion into nascent yolk bodies which, in turn, are presumed to fuse with autosynthetically-derived yolk bodies. No nutrient stores were detected in somatic tissues. Early and middle stages of vitellogenic oocytes were absent from the coelom. This absence combined with the high level of endocytotic activity suggests that vitellogenesis occurs rapidly. These features, in combination with the presence of an exceptionally thin body wall and gut, might serve as related adaptations for predator avoidance by the maintenance of relatively low tissue-density. Alciopid, phyllodocid, and nereid polychaetes share some common reproductive features including the presence of dispersed ovaries, clusters of syncytial germ cells which undergo meiosis while enveloped by somatic cells and the release of oocytes from the clusters prior to vitellogenesis.     

Use of isotopic analysis of vertebrae in reconstructing ontogenetic feeding ecology in white sharks

We conducted stable 13C and 15N analysis on white shark vertebrae and demonstrated that incremental analysis of isotopes along the radius of a vertebral centrum produces a chronological record of dietary information, allowing for reconstruction of an individual's trophic history. Isotopic data showed significant enrichments in 15N with increasing sampling distance from the centrum center, indicating a correlation between body size and trophic level. Additionally, isotopic values verified two distinct ontogenetic trophic shifts in the white shark: one following parturition, marking a dietary switch from yolk to fish; and one at a total length of >341 cm, representing a known diet shift from fish to marine mammals. Retrospective trophic-level reconstruction using vertebral tissue will have broad applications in future studies on the ecology of threatened, endangered, or extinct species to determine life-long feeding patterns, which would be impossible through other methods.