Abstract from Research Report No. 293

D.E. Chamberlain, S.N. Freeman, G.M. Siriwardena, J.A. Vickery & R.B. Bradbury (2002)

The Potential Value of the Field Scale Evaluation in Assessing the Impact of GMHT Crops on Birds and Mammals ISBN 978-1-906204-14-3

SUMMARY

1. The effects of genetically modified herbicide-tolerant (GMHT) crops on farmland biodiversity are being assessed by comparing the abundance of various taxa between conventional and GMHT halves of experimental fields in a large-scale experiment termed the Farm-Scale Evaluation (FSE).
2. A bird and mammal survey was carried out in spring and summer 2000 in order to assess the feasibility of monitoring birds and mammals as part of the FSE. Specifically, this project was designed to assess the power of the FSE experimental design to detect differences in the utilisation of conventional and GMHT crop. This was achieved through a power analysis of various measures of bird and mammal occurrence for selected species, including measures of foraging activity in the crop.
3. A total of 24 sugar beet sites, 11 maize sites and 10 rape sites were surveyed, but many species were absent from several sites. Three different survey approaches were used: territory mapping of all birds in the experimental and surrounding field boundaries; point counts of birds in fields (to record foraging events); and foraging observations of aerial feeders. Mammals were recorded when located during any bird surveys. Observers had no prior knowledge of how each half of a field had been treated.
4. The power analysis was based on Poisson and binomial models incorporating site and treatment effects. Three different comparisons were made: GMHT vs conventional crop over the whole survey period, GMHT crop before herbicide application vs conventional crop in the same time period (i.e. early in the season) and GMHT crop after herbicide application vs conventional crop in the same time period (i.e. late in the season).
5. Ten bird species were selected for analysis based on their abundance and detectability given the survey methods used and also to represent a range of dietary preferences. The species were Red-Legged Partridge Alectoris rufa, Skylark Alauda arvensis, Wren Troglodytes troglodytes, Dunnock Prunella modularis, Whitethroat Sylvia communis, Robin Erithacus rubecula, Blackbird Turdus merula, Song Thrush T. philomelos, Chaffinch Fringella coelebs and Yellowhammer Emberiza citrinella. For aerial feeders four species were considered: Swallow Hirundo rustica, Sand Martin Riparia riparia, House Martin Delichon urbica and Swift Apus apus, but only Swallow occurred in sufficient numbers for analysis.
6. Most analyses had low power to detect significant differences in the data collected. The most powerful analyses involved a comparison of GMHT treated (i.e. sprayed) crop with conventionally treated crop later in the breeding season. Point counts collecting foraging data were most likely to yield analyses of high power in this data set.
7. Simulated data for additional sites showed surveys of Red-Legged Partridge, Swallow, Dunnock, Wren, Whitethroat, Blackbird, Chaffinch and Yellowhammer were predicted to achieve 90% power given the combined sample sizes for 2000 and those to be part of the FSE in 2001. Results for maize (an expected total of 39 sites) were of relatively high power in comparison with results from sugar beet (50 sites). Oilseed rape (37 sites) had the analyses of lowest power with no species predicted to achieve adequate power with additional sites.
8. In the case of mammals, only Hare Lepus europeaus and Rabbit Oryctolagus cuniculus were recorded in sufficient numbers for analysis and these were of lower power relative to the bird analyses. Rabbit presence on sugar beet crops was the most powerful analyses and 90% power was predicted with a total of <24 sites. Caution should be exercised when interpreting these data as the methods used were not designed specifically for surveying mammals.
9. A general power model based on all bird species combined was carried out to determine the differences in bird abundance between GMHT and conventional crop required to achieve 90% power, given typical abundances and average differences detected during this study. This predicted that to achieve a statistical comparison with 90% power, a difference of approximately 125% in numbers (i.e. more than double) between treatments would be required for sample sizes of 40-60 sites (the range of sample sizes expected with additional sites in future years) for Poisson models. For binomial models, the odds ratio would need to be approximately 9 times greater on one treatment for 40-60 sites to achieve 90% power.
10. In summary, the power analyses presented here indicate that 8 species have the potential power to detect significant differences in bird abundance or occurrence between treatments with the additional sites proposed for 2001. Point counts recording foraging individuals on maize crops in the later half of the breeding season, (i.e. after herbicides application to GMHT crops) are likely to yield the most powerful analyses. Detection of significant differences in mammal abundance was less likely.
11. There is an important caveat relating to these results. Crucially the findings are dependent on acceptance of the assumption that derived parameter estimates are likely to remain similar when further sites are surveyed (e.g. would the mean difference from 5 sites be the same as the mean difference from 50 sites). Caution is therefore needed when interpreting these power analyses due to the small sample sizes involved.
12. Due to the above caveat, we suggest that the analysis presented in this paper should be used as a baseline to direct future studies rather than be used as a definitive statement of the power of the current GMHT trials to detect differences in bird abundance.

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