[R-sig-eco] V-ratio in bipartite

[Carsten Dormann]

Dear Enrico,

the V.ratio function had an error, my serious apologies!
I just uploaded the new version of bipartite (1.03) to CRAN and attach 
the correct code below (how can an error slip into so little, so simple 
code?).

Two further comments:
* Why would anyone be interested in an index devised in '84 and even in 
the original publication regarded as "difficult to interpret"? Ah, but 
then I really am ignorant ...
* By sending the email to r-sig-ecology rather than to me as package 
maintainer directly, you expose my incompetence to the world. 
Weaker-willed scientists than I could be offended by that approach, and 
it deviates from the recommendations of the R mailing list webpage: 
questions about specific packages should go to the package maintainer. 
It certainly made me move quickly ;-)

Best wishes,

Carsten

V.ratio <- function(web){
  # invented by Schluter (1984, Ecology)
  # variable names follow this paper (to avoid having to document the code)
  # be careful with the interpretation!!!

  web <- web>0

  N <- ncol(web) #number of species
  M <- nrow(web) #number of "samples"
  Tj <- colSums(web) #species "density" per sample j
  ni <- rowSums(web) #nr. of individuals per species i
  p <- ni/N

  sigma.i.2 <- p*(1-p)
  S.T.2 <- 1/N*sum((Tj-mean(Tj))^2)
  V <- S.T.2 / sum(sigma.i.2)
  V
}




Enrico Barbone wrote:
> Dear all
> I am using the r package bipartite to calculate c-scores and v-ratios of incidence matrices. checking the results agains Gotelli's Ecosim output, I have foung considerable discrepancies regarding v-ratios. I cannot figure out where the problem is, as I have run both packages using very simple 4x4 incidence matrices, calculating v-ratios by hand using schluter 1984, and veryfing that, while ecosim results matched mine, bipartite provided unpredictable values. 
> Is there some kind of normalization, as for the c-score?
> thank you so much for your kind help.
>
>
> Enrico Barbone PhD
> University of Salento
> Di.S.Te.B.A.; Ecology laboratory
> Centro Ecotekne - S.P. Lecce Monteroni, 73100 Lecce - ITALY
> Work Phone: +39.0832.298600; Cell: +39.349.5097805
>
> 	[[alternative HTML version deleted]]
>
> _______________________________________________
> R-sig-ecology mailing list
> R-sig-ecology at r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
>
>   

-- 
Dr. Carsten F. Dormann
Department of Computational Landscape Ecology
Helmholtz Centre for Environmental Research-UFZ
Permoserstr. 15
04318 Leipzig
Germany

Tel: ++49(0)341 2351946
Fax: ++49(0)341 2351939
Email: carsten.dormann at ufz.de
internet: http://www.ufz.de/index.php?de=4205