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Not sure what statistical test to use

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  • 18-01-2012 8:38pm
    #1
    seanknowsall
    Closed Accounts Posts: 65 ✭✭


    I am doing a project on a particular habitat and have two areas that I want to compare. Area A has been grazed by sheep and Area B has been grazed by cattle. The two types of grazing has had a different impact on the populations of wildflowers growing here. I have to represent this using some type of stats table... before grazing commenced I did a survey of the species and recorded 51....all 51 species were common to both areas. The area was then divided and grazing started for a set period of time. A year later I went back and found that Area A (sheep) had only 24 of the original 51 whereas Area B (cattle) had 40. My hypothesis is that cattle grazing is a more wildflower friendly regime on unimproved grasslands. I doing an ecology masters and so far have managed to get away without knowing anything about stats...until now!! Any suggestions gratefully accepted!!


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    #2
    Coles
    Registered Users Posts: 1,020 ✭✭✭Coles


    While you measured the number of individual species, did you also measure the incidences of each one?


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    #3
    seanknowsall
    Closed Accounts Posts: 65 ✭✭seanknowsall


    Coles wrote: »
    While you measured the number of individual species, did you also measure the incidences of each one?

    Ok the problem is the sites are both 1 hectare in size so I firstly established what was growing there and then randomly quadratted 20 times in each site. For example the species Lady's Bedstraw...it occurred in 12 out of 20 quadrats in Site A and 9 in Site B. 60% and 45% respectively. It would be impossible to accuratley count all the individual Lady's Bedstraw specimens in a hectare.

    After the grazing was complete I quadratted the sites randomly again and found a marked difference as you would expect, but more so in the site eaten by sheep (Site A). Ladys Bedstraw was down to 20% in Site A and 30% in Site B.


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    #4
    El Siglo
    Registered Users Posts: 3,803 ✭✭✭El Siglo


    For what you're doing a simple Chi-squared test might work for what you're trying to show, here's an example on wikipedia. You could also do a scatter plot of your data (no. of plant species in site A, no. in site B on both occasions) then fit a model to it and get an R2 value so you can say something like "grazing by sheep correlates with a reduction in plant species" etc...


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    #5
    seanknowsall
    Closed Accounts Posts: 65 ✭✭seanknowsall


    El Siglo wrote: »
    For what you're doing a simple Chi-squared test might work for what you're trying to show, here's an example on wikipedia. You could also do a scatter plot of your data (no. of plant species in site A, no. in site B on both occasions) then fit a model to it and get an R2 value so you can say something like "grazing by sheep correlates with a reduction in plant species" etc...


    Thanks very much for the help.... will do that!!


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