A forwarded reply from a slug specialist to this request follows; unfortunately Mark Wilson didn't leave a contact e-mail address in his post, otherwise I would send it directly rather than to the whole listserv.
Thanks, Markus Eichhorn > -----Original Message----- > From: Ecological Society of America: grants, jobs, news > [mailto:[email protected]] On Behalf Of Mark Wilson > Sent: 14 January 2011 17:08 > To: [email protected] > Subject: [ECOLOG-L] Monitoring > > Hi folks, > I ve collected count data (once monthly) from eight trees (one trap > per tree) for a species of protected slug in the UK over the past > year. A fellow student here mentioned in passing that I could > potentially use power statistics on my data to calculate the amount of > time it would take to detect a red alert (50% or greater decrease in > population size over 25 years) or amber alert (25% decrease over 25 > years). Would this be possible and can anybody advise me on what > statistics program I could use for my analysis? > Looking forward to hearing from you > Mark > PS I did not remove specimens from the traps on my monthly visits. I > just counted and left them in place. Dear Mark, I saw your appeal for help and I hope that some of the things I say may be of use. There are not many of us slug watchers in the UK and even fewer people who record, or have recorded, slug abundance. I am one of those few. I am an amateur with no formal qualification in mollusc matters; indeed no formal, academic qualification in anything related to natural history. I am just someone who has spent much of his spare time over the last 40 years observing and recording things in the natural world. You can email me at [email protected] if you think I can be of any help to you. I do have some observations which may be relevant but you may not be happy with what they appear to imply. I did some standardised slug counting experiments over a period of 2 years. There was an annual cycle of abundance which was fairly consistent between the two years. However, the absolute abundance was rather different between these years. I did no statistics on the data but, if I had, I am pretty sure there would have been a significant difference between years. It was not because of climate change, or because some species were increasing or decreasing in the long term. I think it was simply that the weather was different. This is the first problem with detecting long-term changes. Slug populations can be volatile and a significant difference between two successive years, however great, cannot be used to indicate a long-term decline. For that, you need a long time series. How long? We have found some of our data in a bird monitoring operation, that have been able to detect long-term changes with data sets as short as 10 years long - but you do need annual data, not just two snapshots 10 years apart. There is another problem with slugs. Abundance of slugs counted does not necessarily go with abundance of slugs. When times are hard (too hot, too dry, too cold, too windy) the cunning beasts go underground or under shelter and are not easily countable. What you count depends on both what is there and the immediate weather conditions. And this recorded abundance can change massively from one day to the next (and I have example data sets to demonstrate this). A systematic data collection system which did not take into account wind, rain, temperature would be misleading. I am not sure what species you study but we do have a tree-living species here, Lehmannia marginata. It spends a good deal of time up trees grazing on algae. However, like species which remain close to ground level, L. marginata is very dependent on weather. In dry conditions it is more likely to be inactive or operating at ground level. In frosty conditions it does not operate at all. If I had slug traps in trees, the number trapped would reflect both abundance and weather. Overall then, there are big problems with slug abundance data and its analysis. But do not let that deter you - it is well worth the challenge. -- Mr C R du Feu 66 High Street, Beckingham, Nottinghamshire, DN10 4PF Telephone 01427 848 400 [email protected] This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.
