Exercise 17.1: Co-occurrence in hunting spider data
Petrus would like to think about co-occurrence of hunting spider
species. He wants to know which species co-occur, and to what extent can
these co-occurrence patterns be explained by joint response to
environmental gradients.
What approach should he used to look at these questions?
He is clearly after methods for studying co-occurrence, so he wants
to do covariance modelling on \(\Sigma\), e.g. using latent variables or
graphical modelling.
Code Box 17.1: Estimating co-occurrence patterns using a copula
model
library(mvabund)
data(spider)
library(ecoCopula)
par(mfrow=c(1,2), mgp=c(1.75,0.75,0), mar=c(3,3,1,1))
spiderAbund = mvabund(spider$abund)
spider_glmInt = manyglm(spiderAbund~1,data=spider$x)
ord_spiderInt = cord(spider_glmInt)
plot(ord_spiderInt, biplot=TRUE) #for a biplot
# now plot a correlation matrix
library(gclus)
#> Loading required package: cluster
sortVars=order.single(ord_spiderInt$sigma)
library(corrplot)
#> corrplot 0.92 loaded
corrplot(ord_spiderInt$sigma[sortVars, sortVars], type="lower", diag=FALSE, method="square")
Code Box 17.2: Co-occurrence patterns explained by environmental
variables
par(mfrow=c(1,2), mgp=c(1.75,0.75,0), mar=c(3,3,1,1))
spider_glmX = manyglm(spiderAbund~.,data=spider$x)
ord_spiderX=cord(spider_glmX)
plot(ord_spiderX,biplot=TRUE)
corrplot(ord_spiderX$sigma[sortVars,sortVars],type="lower",diag=FALSE, method="square")
ss = c(sum(ord_spiderInt$loadings^2), sum(ord_spiderX$loadings^2))
c(ss, 1-ss[2]/ss[1])
#> [1] 7.4720372 4.1561882 0.4437677
absCor = c( sum(abs(ord_spiderInt$sigma)),
sum( abs(ord_spiderX$sigma)) ) - ncol(spider$abund)
c(absCor, 1-absCor[2]/absCor[1])
#> [1] 52.653880 21.917547 0.583743
Exercise 17.2: Spider presence-absence
Repeat the analyses of Code Box 17.1 on presence-absence data,
which you can construct using spiderPA=pmin(spiderAbund,1).
Estimate the correlation matrix.
par(mfrow=c(1,2), mgp=c(1.75,0.75,0), mar=c(3,3,1,1))
spiderPA=pmin(spiderAbund,1)
spiderPA_glmInt = manyglm(spiderPA~1,data=spider$x, family="cloglog")
ord_spiderPAInt=cord(spiderPA_glmInt)
plot(ord_spiderPAInt, biplot=TRUE) #for a biplot
# now plot a correlation matrix
sortVars=order.single(ord_spiderPAInt$sigma)
corrplot(ord_spiderPAInt$sigma[sortVars, sortVars], type="lower",
diag=FALSE, method="square")
How does the correlation matrix compare to Figure 17.2b?
Correlations tend to be smaller, they range from about -0.4 up to
0.4.
Calculate the sum of squared loadings, with and without
predictors, as in Code Box 17.2. Are the values smaller for the
presence/absence data?
par(mfrow=c(1,2), mgp=c(1.75,0.75,0), mar=c(3,3,1,1))
spiderPA_glmX = manyglm(spiderPA~.,data=spider$x, family="cloglog")
ord_spiderPAX = cord(spiderPA_glmX)
plot(ord_spiderPAX, biplot=TRUE)
corrplot(ord_spiderPAX$sigma[sortVars,sortVars], type="lower", diag=FALSE, method="square")
ss = c(sum(ord_spiderPAInt$loadings^2), sum(ord_spiderPAX$loadings^2))
c(ss, 1-ss[2]/ss[1])
#> [1] 4.5520766 1.3766597 0.6975754
absCor = c( sum(abs(ord_spiderPAInt$sigma)),
sum( abs(ord_spiderPAX$sigma)) ) - ncol(spiderPA)
c(absCor, 1-absCor[2]/absCor[1])
#> [1] 32.0031865 1.4122521 0.9558715
Is this what you expected?
The fact that correlations became weaker is expected. But
interestingly, predictors were able to explain most of the correlation,
with responses being close to uncorrelated after environmental variables
were included.
Are your conclusions any different to Code Box 17.2, in terms of
the extent to which patterns in co-occurrence can be explained by
environmental variables?
I guess you would conclude here that most (more than two thirds) of
co-occurrence in presence-absence can be explained by environmental
variables, whereas this number was about half when looking at
abundance.
Exercise 17.3: Co-occurrence in bird communities
Françoise visited 51 sites near Lyon, France, and estimated
abundance of 40 different species of birds (by listening to bird songs
for 15 minutes) (Tatibouët, 1981). He would like to understand bird
co-occurrence, and the extent to which it is explained by predictors
related to urbanisation. Load the data and fit ordinal regressions to
each response:
library(ade4)
data(aviurba)
abund=mvabund(aviurba$fau)
library(ordinal)
ft_birdsInt=manyany(abund~1, "clm", family="ordinal", data=aviurba$mil)
Use cord to fit a latent variable model and estimate the
correlation matrix.
par(mfrow=c(1,2),mgp=c(1.75,0.75,0),mar=c(3,3,1,1))
ord_birdsInt=cord(ft_birdsInt)
plot(ord_birdsInt, biplot=TRUE)
sortBirdVars=order.single(ord_birdsInt$sigma)
corrplot(ord_birdsInt$sigma[sortBirdVars, sortBirdVars], type="lower", diag=FALSE, method="square")
Looks like nothing much is happening here!
Add fields to the model, to study the extent to which presence or
absence of fields explains co-occurrence patterns. Calculate the sum of
squared loadings, for latent variable models with and without fields as
a predictor.
par(mfrow=c(1,2), mgp=c(1.75,0.75,0), mar=c(3,3,1,1))
ft_birdsX=manyany(abund~fields, "clm", family="ordinal", data=aviurba$mil)
ord_birdsX=cord(ft_birdsX)
plot(ord_birdsX,biplot=TRUE)
corrplot(ord_birdsX$sigma[sortBirdVars,sortBirdVars],type="lower",diag=FALSE, method="square")
ss = c(sum(ord_birdsInt$loadings^2), sum(ord_birdsX$loadings^2))
c(ss, 1-ss[2]/ss[1])
#> [1] 4.8906022 4.2383821 0.1333619
absCor = c( sum(abs(ord_birdsInt$sigma)), sum( abs(ord_birdsX$sigma)) ) - ncol(abund)
c(absCor, 1-absCor[2]/absCor[1])
#> [1] 87.2815406 73.0326620 0.1632519
What can you conclude about co-occurrence patterns of these
birds, and the extent to which they are explained by presence or absence
of fields?
The proportion of co-occurrence patterns that can be explained is
relatively small, so I guess we can say that fields is not
a major driver of co-occurrence patterns of birds.
The ordinal package has a bug in it (in version 2019.12)
so it conflicts with lme4 (specifically it overwrites the
ranef function), [issue posted on Github] (https://github.com/runehaubo/ordinal/issues/48). So if
you are running analyses using both packages, you need to detach the
ordinal package before continuing…
detach("package:ordinal", unload=TRUE)
#> Warning: 'ordinal' namespace cannot be unloaded:
#> namespace 'ordinal' is imported by 'ecoCopula' so cannot be unloaded
Code Box 17.3: A copula graphical model for Petrus’s spider
data
par(mfrow=c(1,2),mgp=c(1.75,0.75,0),mar=c(3,3,1,1))
graph_spiderInt = cgr(spider_glmInt)
plot(graph_spiderInt, vary.edge.lwd=TRUE)
graph_spiderX = cgr(spider_glmX, graph_spiderInt$all_graphs$lambda.opt)
#> Warning in cgr(spider_glmX, graph_spiderInt$all_graphs$lambda.opt): 'best' model
#> selected among supplied lambda only
plot(graph_spiderX, vary.edge.lwd=TRUE)
absCor = c( sum(abs(graph_spiderInt$best_graph$cov)),
sum( abs(graph_spiderX$best_graph$cov)) ) - ncol(spider$abund)
c(absCor, 1-absCor[2]/absCor[1])
#> [1] 45.25256 16.79820 0.62879
Exercise 17.4: Does soil dryness explain co-occurrence patterns in
Petrus’s data?
Recall that in Figure 16.1a, Alopecosa accentuata,
Alopecosa fabrilis and Arctosa perita decreased in
response to soil dryness, while all other species increased. Note that
the “unconstrained” correlation matrix of Figure 17.2b found negative
correlation patterns between these species and most others. To what
extent do contrasting responses to soil dryness explain the negative
correlations of Figure 17.2b? Answer this question by fitting a
covariance model of your choice to the spider data with and without soil
dryness as a predictor.
par(mfrow=c(1,2),mgp=c(1.75,0.75,0),mar=c(3,3,1,1))
spider_glmSoil = manyglm(spiderAbund~soil.dry,data=spider$x)
ord_spiderSoil=cord(spider_glmSoil)
corrplot(ord_spiderInt$sigma[sortVars,sortVars],type="lower",diag=FALSE, method="square")
corrplot(ord_spiderSoil$sigma[sortVars,sortVars],type="lower",diag=FALSE, method="square")
ss = c(sum(ord_spiderInt$loadings^2), sum(ord_spiderSoil$loadings^2))
c(ss, 1-ss[2]/ss[1])
#> [1] 7.4720372 5.9518021 0.2034566
absCor = c( sum(abs(ord_spiderInt$sigma)),
sum( abs(ord_spiderSoil$sigma)) ) - ncol(spider$abund)
c(absCor, 1-absCor[2]/absCor[1])
#> [1] 52.6538803 40.5948665 0.2290242
This explained less covariation than the model with all six
environmental variables, more like a fifth of it rather than half. But
anyway, the negative correlations we saw previously (above, left) do
involve Arctperi, Alopacce and
Alopfabr, the three species that have a negative
association with soil.dry. These three species are all
positively correlated with each other but negatively correlated with
most other species.
After including soil.dry in the model (above, right),
most of the negative associations have disappeared, and these species
become largely uncorrelated with all others. So while plenty of the
covariation does not seem to be explained by soil.dry, it
is doing a good job of capturing the negative co-occurrence patterns in
spider abundance.