School of BioSciences - Research Publications
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ItemComplex interplay between intrinsic and extrinsic drivers of long-term survival trends in southern elephant seals.(Springer Science and Business Media LLC, 2007-03-27)BACKGROUND: Determining the relative contribution of intrinsic and extrinsic factors to fluctuations in population size, trends and demographic composition is analytically complex. It is often only possible to examine the combined effects of these factors through measurements made over long periods, spanning an array of population densities or levels of food availability. Using age-structured mark-recapture models and datasets spanning five decades (1950-1999), and two periods of differing relative population density, we estimated age-specific probabilities of survival and examined the combined effects of population density and environmental conditions on juvenile survival of southern elephant seals at Macquarie Island. RESULTS: First-year survival decreased with density during the period of highest population size, and survival increased during years when the Southern Oscillation Index (SOI) anomaly (deviation from a 50-year mean) during the mother's previous foraging trip to sea was positive (i.e., El Niño). However, when environmental stochasticity and density were considered together, the effect of density on first-year survival effectively disappeared. Ignoring density effects also leads to models placing too much emphasis on the environmental conditions prevailing during the naïve pup's first year at sea. CONCLUSION: Our analyses revealed that both the state of the environment and population density combine to modify juvenile survival, but that the degree to which these processes contributed to the variation observed was interactive and complex. This underlines the importance of evaluating the relative contribution of both the intrinsic and extrinsic factors that regulate animal populations because false conclusions regarding the importance of population regulation may be reached if they are examined in isolation.
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ItemEnvironmental Flows Can Reduce the Encroachment of Terrestrial Vegetation into River Channels: A Systematic Literature Review(SPRINGER, 2013-11-01)Encroachment of riparian vegetation into regulated river channels exerts control over fluvial processes, channel morphology, and aquatic ecology. Reducing encroachment of terrestrial vegetation is an oft-cited objective of environmental flow recommendations, but there has been no systematic assessment of the evidence for and against the widely-accepted cause-and-effect mechanisms involved. We systematically reviewed the literature to test whether environmental flows can reduce the encroachment of terrestrial vegetation into river channels. We quantified the level of support for five explicit cause-effect hypotheses drawn from a conceptual model of the effects of flow on vegetation. We found that greater inundation, variously expressed as changes in the area, depth, duration, frequency, seasonality, and volume of surface water, generally reduces riparian vegetation abundance in channels, but most studies did not investigate the specific mechanisms causing these changes. Those that did show that increased inundation results in increased mortality, but also increased germination. The evidence was insufficient to determine whether increased inundation decreases reproduction. Our results contribute to hydro-ecological understanding by using the published literature to test for general cause-effect relationships between flow regime and terrestrial vegetation encroachment. Reviews of this nature provide robust support for flow management, and are more defensible than expert judgement-based approaches. Overall, we predict that restoration of more natural flow regimes will reduce encroachment of terrestrial vegetation into regulated river channels, partly through increased mortality. Conversely, infrequent deliveries of environmental flows may actually increase germination and subsequent encroachment.
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ItemExperimental comparison of aerial larvicides and habitat modification for controlling disease-carrying Aedes vigilax mosquitoes(JOHN WILEY & SONS LTD, 2012-05-01)BACKGROUND: Microbial and insect-growth-regulator larvicides dominate current vector control programmes because they reduce larval abundance and are relatively environmentally benign. However, their short persistence makes them expensive, and environmental manipulation of larval habitat might be an alternative control measure. Aedes vigilax is a major vector species in northern Australia. A field experiment was implemented in Darwin, Australia, to test the hypotheses that (1) aerial microbial larvicide application effectively decreases Ae. vigilax larval presence, and therefore adult emergence, and (2) environmental manipulation is an effective alternative control measure. Generalised linear and mixed-effects modelling and information-theoretic comparisons were used to test these hypotheses. RESULTS: It is shown that the current aerial larvicide application campaign is effective at suppressing the emergence of Ae. vigilax, whereas vegetation removal is not as effective in this context. In addition, the results indicate that current larval sampling procedures are inadequate for quantifying larval abundance or adult emergence. CONCLUSIONS: This field-based comparison has shown that the existing larviciding campaign is more effective than a simple environmental management strategy for mosquito control. It has also identified an important knowledge gap in the use of larval sampling to evaluate the effectiveness of vector control strategies.