capscale {vegan} | R Documentation |
Constrained Analysis of Principal Coordinates (CAP) is an ordination method
similar to Redundancy Analysis (rda
), but it allows
non-Euclidean dissimilarity indices, such as Manhattan or
Bray–Curtis distance. Despite this non-Euclidean feature, the analysis
is strictly linear and metric. If called with Euclidean distance,
the results are identical to rda
, but capscale
will be much more inefficient. Function capscale
is a
constrained version of metric scaling, a.k.a. principal coordintates
analysis, which also is based on Euclidean distance but still is more
useful with other dissimilarity measures.
capscale(formula, data, distance = "euclidean", comm = NULL, add = FALSE, ...)
formula |
Model formula. The function can be called only with the
formula interface. Most usual features of formula hold,
especially as defined in cca and rda . The
LHS must be either a community data matrix or a dissimilarity matrix,
e.g., from
vegdist or dist .
If the LHS is a data matrix, function vegdist
will be used to find the dissimilarities. RHS defines the constraints.
The constraints can be continuous or factors, they can be transformed
within the formula, and they can have interactions as in typical
formula . The RHS can have a special term Condition
that defines variables ``partialled out'' before constraints, just like
in rda or cca . This allows the use of
partial CAP. |
data |
Data frame containing the variables on the right hand side of the model formula. |
distance |
Dissimilarity (or distance) index in
vegdist used if the LHS of the formula is a
data frame instead of dissimilarity matrix. |
comm |
Community data frame which will be used for finding
species scores when the LHS of the formula was a
dissimilarity matrix. This is not used if the LHS is a data
frame. If this is not supplied, the ``species scores'' are the axes
of initial metric scaling (cmdscale ) and may be
confusing. |
add |
logical indicating if an additive constant should be
computed, and added to the non-diagonal dissimilarities such
that all eigenvalues are non-negative in underlying
Principal Co-ordinates Analysis (see cmdscale
for details). |
... |
Other parameters passed to rda . |
The Canonical Analysis of Principal Coordinates (CAP) is simply a
Redundancy Analysis of results of Metric (Classical) Multidimensional
Scaling (Anderson & Willis 2003). Function capscale uses two steps:
(1) it ordinates the dissimilarity matrix using
cmdscale
and (2) analyses these results using
rda
. If the user supplied a community data frame instead
of dissimilarities, the function will find the needed dissimilarity
matrix using vegdist
with specified
distance
. However, the method will accept dissimilarity
matrices from vegdist
, dist
, or any
other method producing similar matrices. The constraining variables can be
continuous or factors or both, they can have interaction terms,
or they can be transformed in the call. Moreover, there can be a
special term
Condition
just like in rda
and cca
so that ``partial'' CAP can be performed.
The current implementation differs from the method suggested by Anderson & Willis (2003) in three major points which actually make it similar to distance-based redundancy analysis (Legendre & Anderson 1999):
cmdscale
, whereas capscale
uses axes
weighted by corresponding eigenvalues, so that the ordination
distances are best approximations of original dissimilarities. In
the original method, later ``noise'' axes are just as important as
first major axes.
capscale
uses all axes with positive eigenvalues. The use of
subset is necessary with orthonormal axes to chop off some
``noise'', but the use of all axes guarantees that the results are
the best approximation of original dissimilarities.
capscale
adds species scores as weighted sums
of (residual) community matrix (if the matrix is available), whereas
Anderson & Willis have no fixed method for adding species scores.
With these definitions, function capscale
with Euclidean
distances will be identical to rda
in eigenvalues and
in site, species and biplot scores (except for possible sign
reversal).
However, it makes no sense to use capscale
with
Euclidean distances, since direct use of rda
is much more
efficient. Even with non-Euclidean dissimilarities, the
rest of the analysis will be metric and linear. The test for all axes
is available only for ordinations defined with formula
interface.
The function returns an object of class capscale
which is
identical to the result of rda
. At the moment,
capscale
does not have specific methods, but it uses
cca
and rda
methods
plot.cca
,
scores.rda
etc. Moreover, you
can use anova.cca
for permutation tests of
``significance'' of the results.
Warnings of negative eigenvalues are issued with most dissimilarity
indices. These are harmless, and negative eigenvalues will be ignored
in the analysis. If the warnings are disturbing, you can use argument
add = TRUE
passed to cmdscale
, or, preferably, a
distance measure that does not cause these warnings. In
vegdist
, method = "jaccard"
gives such an index.
Alternatively, after square root transformation many indices do not
cause warnings.
Function rda
usually divides the ordination scores by
number of
sites minus one. In this way, the inertia is variance instead of sum
of squares, and the eigenvalues sum up to variance. Many
dissimilarity measures are in the range 0 to 1, so they have already
made a similar division. If the largest original dissimilarity is less or
equal to 4
(allowing for stepacross
), this division is undone in
capscale
and original dissimilarities are used. The inertia is
called as squared dissimilarity
(as defined in the dissimilarity matrix), but keyword
mean
is added to the inertia in cases where division was
made, e.g. in Euclidean and Manhattan distances.
Jari Oksanen
Anderson, M.J. & Willis, T.J. (2003). Canonical analysis of principal coordinates: a useful method of constrained ordination for ecology. Ecology 84, 511–525.
Legendre, P. & Anderson, M. J. (1999). Distance-based redundancy analysis: testing multispecies responses in multifactorial ecological experiments. Ecological Monographs 69, 1–24.
rda
, cca
, plot.cca
,
anova.cca
, vegdist
,
dist
, cmdscale
.
data(varespec) data(varechem) vare.cap <- capscale(varespec ~ N + P + K + Condition(Al), varechem, dist="bray") vare.cap plot(vare.cap) anova(vare.cap)