[R-sig-eco] Characterizing direction and magnitude of community change in response to a continuous predictor?

[Attila Lengyel]

Dear Brian and Pedro,

I think it is not a good idea to regress an unconstrained ordination axis,
especially a PCoA axis, on a linear predictor to test for the effect of an
external gradient on species composition. If the compositional turnover is
high enough, the gradient will need several PCoA axes to unfold
between-site distances in a metric space, so the first axis will not
represent the gradient in full length. This property of distance-based
ordinations is widely known as the horseshoe effect.
Why not use a constrained ordination (CCA, RDA, db-RDA) instead, with a
formula SPECIES ~ PREDICTOR separately for each region or SPECIES ~
PREDICTOR * REGION for the entire sample? Then you could partition the
total variation in species data into portions explained by the PREDICTOR
(or by REGION).

Another possibility is to relate between-site compositional distances with
environmental distances using either correlation or regression. See methods
for 'distance decay' modelling as a form of beta diversity.

Best regards,

Attila

Pedro Pequeno <pacolipe using gmail.com> ezt írta (időpont: 2022. jún. 7., K,
20:14):

> Hi Brian,
>
> a simple approach would be to summarize species composition using some
> ordination method (e.g. PCoA) and then use it as a response variable
> in a linear model where the predictors would be the environmental
> gradient and region. Therefore, you could explicitly test for an
> interaction between gradient and region, i.e. whether the response of
> species composition to the gradient changes across regions. The
> resulting slopes could be a measure of direction and magnitude of
> change.
>
> Cheers,
>
> Pedro
>
> Em ter., 7 de jun. de 2022 às 13:19, Brian A. Gill
> <gillbriana using gmail.com> escreveu:
> >
> > Hi R-sig-ecology.
> >
> > Sorry if this question is vague.
> >
> > How can we characterize the direction and magnitude of change of
> biological
> > communities (species by site matrix or derived matrix of distances) in
> > response to a continuous environmental predictor?
> >
> > I want to do this for several different regions and be able to compare
> the
> > directions and magnitudes of community changes among regions.
> >
> > Maybe this is as simple as using a correlation coefficient...
> >
> > Any help is much appreciated.
> >
> > Thanks.
> >
> >
> > Brian
> >
> >
> >
> > --
> > Brian A. Gill, Ph.D. (He/Him)
> >
> > Postdoctoral Research Associate
> > School of Natural Resources and the Environment
> > University of Arizona, Tucson, Arizona, USA
> > www.BrianGillPhD.com
> >
> >         [[alternative HTML version deleted]]
> >
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