Papers by J F Derry
Agricultural Systems, 1998
We examined a range of tracking policies, designed to tackle climatic variation, using a simulati... more We examined a range of tracking policies, designed to tackle climatic variation, using a simulation model of a semi-arid grazing system. These compared annual sales designed to limit stocking rate, pre-emptive sales triggered by insufficient rainfall, and variable sales and stocking rate regimes determined by the current season's rainfall. Although the flexible stocking strategies did reduce mortality losses, compared with fixed stocking, they did not increase average annual sales. The main reasons for this are that major losses of stock are associated less with 1-year than with 2-year droughts, which are difficult to track, and that destocking can be really effective only if the productive potential of the herd can be re-established more rapidly than is possible from depleted herd resources. Tracking policies did have a considerable advantage in terms of reduced inter-annual variability of sales, which would be of economic benefit to the commercial livestock sector. For subsistence pastoralists, the traditional policies of maintaining the maximum number of breeding stock, and of hoping that most of them will survive drought, may be as close as 'opportunistic' management can get to dealing with drought.
Double normal distributions can be used to resolve many sward height frequency distributions into... more Double normal distributions can be used to resolve many sward height frequency distributions into two components representing the 'short' (patches) and 'tall' (non~patches) areas in the sward. The effect of sample size on the precision and accuracy of parameters of sward height distributions was examined by drawing sub-samples (n=10) of increasing sample size (50 to 1 000) from simulated height data (n=10 000) from three different, typical height distributions, viz. normal (ungrazed), bimodal (leniently grazed) and positively skewed (intensely grazed). The coefficient of variation of components of all three distributions decreased sharply with increasing sample size and CVs for all means were <15% with 200 height measures, and <10% or all means, with the exception of the 'tall' mean in the
bimodal distribution, at a sample size of 100. At a given
sample size, proportions in the two components were
less-precisely measured than the means, especially
when the components are equally represented in the
population (i.e. bimodal), where 500 measurements are
required for a precision of 15%. Accuracy also increased with sample size, and with 400 samples,
deviations were within 10% of the true values for most parameters of the three distributions. A sample size of 200 is recommended for quantifying the mean height of 'short' and 'tall' components of the sward whereas 400-500 samples are required to precisely estimate their relative proportions.
We describe a computer program that tracks the three-dimensional coordinates of a snail (moving w... more We describe a computer program that tracks the three-dimensional coordinates of a snail (moving within a tank) from images captured by a single video camera. The tank and a mirror placed at 45o to one side are viewed to provide direct and reflected images of a pond snail, Lymnaea stagnalis, mov- ing around the tank. The output of the camera is captured with an IBM-compatible PC using a frame- grabbing VideoBlaster board. The main features of our Windows program is the frame-by-frame analy- sis of the captured video, enhancement of contrast, location of dark “snails,” discrimination by size, trigonometric determination of the x,y,z coordinates, and the final export of data in a spreadsheet- ready format. After the user has outlined the position of the tank on the screen and set the discrimina- tion thresholds, no further user activity is required. This makes the analysis of slowly moving mollusks much less operator intensive and less tedious. The method is applicable to other faster moving or- ganisms (e.g., fish) because the analysis is performed on captured video sequences.
A very brief summary of the purpose of the project, the research activities, the outputs of the p... more A very brief summary of the purpose of the project, the research activities, the outputs of the project, and the contribution of the project towards DFID's development goals. (Up to 500 words).
A piosphere is an ecological system of interactions between a watering point, its surrounding veg... more A piosphere is an ecological system of interactions between a watering point, its surrounding vegetation and the grazing animal. In the simplest case of an isolated watering point in one uniform rangeland type, a gradient of utilisation pressure develops which is greatest near the watering point and decreases as a function of distance from it. Water-dependent herbivores are forced to congregate within a maximum distance of about 10 km from watering points in the dry season. Piosphere patterns are always detected in herbaceous species composition. Range condition, grass production, biomass, understory cover, standing crop and basal cover have also all been found to be directly proportional to distance from water. In areas with large populations of elephants the density and canopy cover of trees is directly proportional to distance from watering points. In the absence of elephants an increase in woody plant density and canopy cover tends to occur in a zone just beyond the sacrifice ar...
In an investigation into ways to manage browse resources, we showed that lopping branches in the ... more In an investigation into ways to manage browse resources, we showed that lopping branches in the late dry season can provide valuable forage, particularly as a source of N, at a critical time of year. Lopping stimulates re-growth and appears not to affect the survival of tree. We showed that cattle and goats use different forms of browse, with cattle
Endeavour, 2009
The long-term marital dance of Emma and Charles Darwin was set to the routine beat of an almost d... more The long-term marital dance of Emma and Charles Darwin was set to the routine beat of an almost daily piano recital. Emma was a proficient pianist, and so a quality instrument was a welcome and appropriate housewarming present for their first marital home in London. That same piano accompanied the Darwins on their move to Downe before being upgraded for a newer model, which is still there, whilst another, cheaper piano may have played in Charles Darwin's work, particularly on earthworms. Whilst he lamented his own lack of musicality, Darwin revelled in his wife's prowess, a capacity that he recognised could be inherited, not least through observation of his own children. The evolution of musicality, he reasoned, was rooted in sexual attraction as a form of communication that preceded language.
Trends in Ecology & Evolution, 2009
Darwinism appears in many more academic areas than just evolutionary biology. New disciplines are... more Darwinism appears in many more academic areas than just evolutionary biology. New disciplines are created out of its fusion with existing fields of study. However, this practise is criticised for a lack of evidence-based justification, and for adopting gene-oriented reductionism in the social sciences. This article briefly considers seven examples of new disciplines for which Darwinism has been appropriated. In each case, succinct background information precedes quotes provided for this purpose by a leading researcher.
African Journal of Range & Forage Science, May 9, 2008
The main radiation of large mammalian herbivores in Africa took place in the Pliocene-Pleistocene... more The main radiation of large mammalian herbivores in Africa took place in the Pliocene-Pleistocene, when a long-term trend towards aridification promoted grasslands and the diversification of ruminant grazers. Traditional models of this evolution identify habitat fragmentation in response to climate change as the primary cause of speciation and diversification. However, with their adaptation to poorer and drier diets, these animals incurred a cost: an increase in water requirements to aid digestion and to thermoregulate their larger body size. Water requirements are included in the current model, relating this foremost physiological need to an environmental resource: habitat fragments would have contained sources of drinking water for the persistence of water-dependent species. However, the location of drinking water and its influences are subsumed within the extent of those fragments; their roles in habitat fragmentation and the subsequent evolutionary processes are less clear. The current model neglects to consider the interaction of water dependency with the location of drinking water; increased aridification would not only have involved shifts in the spatial distribution and species composition of forage resources, but it would also have increased the patchiness of surface water distributions. Animal foraging distributions would have been constrained within species-specific distances of their drinking water over an evolutionary timescale, distances that were not necessarily in accord with the extent of their potential habitat as solely defined by forage distributions, particularly as animals became more dependent on water with the increase in grazing and its associated traits. Drinking-water location exerts a dominant influence on animal survival and reproduction, suggesting that these roles of drinking-water location need to be incorporated into existing models of large mammalian herbivore evolution in Africa.
Current Research by J F Derry
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Papers by J F Derry
bimodal distribution, at a sample size of 100. At a given
sample size, proportions in the two components were
less-precisely measured than the means, especially
when the components are equally represented in the
population (i.e. bimodal), where 500 measurements are
required for a precision of 15%. Accuracy also increased with sample size, and with 400 samples,
deviations were within 10% of the true values for most parameters of the three distributions. A sample size of 200 is recommended for quantifying the mean height of 'short' and 'tall' components of the sward whereas 400-500 samples are required to precisely estimate their relative proportions.
Current Research by J F Derry
bimodal distribution, at a sample size of 100. At a given
sample size, proportions in the two components were
less-precisely measured than the means, especially
when the components are equally represented in the
population (i.e. bimodal), where 500 measurements are
required for a precision of 15%. Accuracy also increased with sample size, and with 400 samples,
deviations were within 10% of the true values for most parameters of the three distributions. A sample size of 200 is recommended for quantifying the mean height of 'short' and 'tall' components of the sward whereas 400-500 samples are required to precisely estimate their relative proportions.