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I am a biologist by training so please excuse my naivety. I generated the ordination diagrams presented here using NMDS on Bray-Curtis dissimilarities. The two diagrams represent communities of moths and birds along an elevational gradient and the labels on the plot are site elevations. I used ANOSIM (R:Vegan) to assess the significance of dissimilarity across the "differently colored" assemblage clusters in the moth data and the result was significant (R = 0.88). Using the same grouping factor (as denoted by the colors in moth NMDS) I checked for dissimmilarity in the bird data and it gives me significant differences (R = 0.82) while I can clearly see the communities along a more or less smooth arc with no apparent abrupt clustering? I read that the NMDS axes are arbitrary and cannot be compared across two independent datasets.

There is clearly something fundamentally wrong in the way I am interpreting these plots, or the ANOSIM results. Any comments would be appreciated.

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The R-statistic in ANOSIM is a ratio between within-group and between-group dissimilarities. The steps in the analysis are: 1. calculate a matrix of dissimilarity scores for every pair of sites 2. convert the dissimilarities to ranks 3. calculate the R statistic as the ratio between dissimilarities between sites within a group and the dissimilarities between sites that are in different groups. The closer this value is to 1, the more the sites within a group are similar to each other and dissimilar to sites in other groups. 4. The significance of the R-statistic is determined by permuting the membership of sites in groups.

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  • $\begingroup$ You could try to interpret the sets individually, then compare the interpretations $\endgroup$ – Dr. Eldard Mukasa Jun 22 '18 at 15:55
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Similar pattern showed by the two groups depicted that the presence of one group could have influence the other, if by nature we assume moths to be a source of food for the birds (Majerus, 2002; Hill, Randle, Fox & Parsons,2010)). We can then conveniently say distribution of birds in place and number is determine by the occurrence of moths.

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