‘Tolerable’ hillslope soil erosion rates in Australia: Linking science and policy

This paper reviews water-borne soil erosion in Australia in the context of current environmental policy needs. Sustainability has emerged as a central tenet of environmental policy in Australia and water-borne hillslope soil erosion rates are used as one of the indicators of agricultural sustainability in State of the Environment reporting. We review attempts to quantify hillslope erosion rates over Australia and we identify areas at risk of exceeding natural baseline rates. We also review historical definitions of sustainable, or ‘tolerable’ erosion rates, and how to set these rates. There are many ways to estimate hillslope erosion and these can create confusing results. Moreover their application for land management purposes requires nuanced interpretations that ultimately depend on the desired objective of decision-makers. Soil is the earth surficial material that serves as a medium for plant growth and the notion of tolerable soil erosion arose historically to assess the impact of soil loss on agricultural uses. However now that the impact of erosion on aquatic ecosystems been recognized as a major concern for Australia, the concept of tolerable erosion needs to be revised. Here we discuss three definitions of tolerable soil erosion, following recent literature. We derive estimates of long-term agricultural sustainability based on natural soil production rates and discuss this in relation to other defined land-management objectives such as aquatic ecosystem health. We conclude that the desired objectives of land managers must be clearly articulated before questions of ‘where to invest to control erosion’ and ‘how to assess return-on-investment’ can be answered.     

Echolocation beam shape in emballonurid bats, Saccopteryx bilineata and Cormura brevirostris

The shape of the sonar beam plays a crucial role in how echolocating bats perceive their surroundings. Signal design may thus be adapted to optimize beam shape to a given context. Studies suggest that this is indeed true for vespertilionid bats, but little is known from the remaining 16 families of echolocating bats. We investigated the echolocation beam shape of two species of emballonurid bats, Cormura brevirostris and Saccopteryx bilineata, while they navigated a large outdoor flight cage on Barro Colorado Island, Panama. C. brevirostris emitted more directional signals than did S. bilineata. The difference in directionality was due to a markedly different energy distribution in the calls. C. brevirostris emitted two call types, a multiharmonic shallowly frequency-modulated call and a multiharmonic sweep, both with most energy in the fifth harmonic around 68?kHz. S. bilineata emitted only one call type, multiharmonic shallowly frequency-modulated calls with most energy in the second harmonic (~46?kHz). When comparing same harmonic number, the directionality of the calls of the two bat species was nearly identical. However, the difference in energy distribution in the calls made the signals emitted by C. brevirostris more directional overall than those emitted by S. bilineata. We hypothesize that the upward shift in frequency exhibited by C. brevirostris serves to increase directionality, in order to generate a less cluttered auditory scene. The study indicates that emballonurid bats are forced to adjust their relative harmonic energy instead of adjusting the fundamental frequency, as the vespertilionids do, presumably due to a less flexible sound production.     

Spatio-temporal vigilance architecture of an Australian flying-fox colony

The social structure of animal aggregations may vary considerably in both space and time, yet little is known about how this affects vigilance. Here, we investigate the vigilance architecture of a colony of wild-living grey-headed flying-foxes (Pteropus poliocephalus) in Australia and examine how spatial as well as temporal variation in social organization influences social and environmental vigilance. We sampled color-marked individuals at different stages of the reproductive cycle and the year and at different locations in the colony to examine the effects of temporal and spatial factors on social and environmental vigilance. We found that vigilance architecture reflected the social structure of the colony, with the highest environmental vigilance being displayed by bats at the periphery of the colony, and the highest social vigilance by bats that roosted at intermediate distances from the colony’s edge. Furthermore, we found that vigilance levels reflected changes in reproductive state, with social vigilance increasing toward the mating season, particularly in males. Our findings show that spatial and temporal variation in social structure can have differential effects on social and environmental vigilance. This highlights the necessity to differentiate between functions of vigilance to understand fully vigilance architecture in aggregations of social animals.