J F Derry | University of Edinburgh (original) (raw)
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Papers by J F Derry
Agricultural Systems, 1998
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 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
Trends in Ecology & Evolution, 2009
African Journal of Range & Forage Science, May 9, 2008
Current Research by J F Derry
Agricultural Systems, 1998
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 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
Trends in Ecology & Evolution, 2009
African Journal of Range & Forage Science, May 9, 2008