Result Object from Constrained Ordination

Ordination methods cca, rda, dbrda and capscale return similar result objects. All these methods use the same internal function ordConstrained. They differ only in (1) initial transformation of the data and in defining inertia, (2) weighting, and (3) the use of rectangular rows $\times$ columns data or symmetric rows $\times$ rows dissimilarities: rda initializes data to give variance or correlations as inertia, cca is based on double-standardized data to give Chi-square inertia and uses row and column weights, capscale maps the real part of dissimilarities to rectangular data and performs RDA, and dbrda performs an RDA-like analysis directly on symmetric dissimilarities.

Function ordConstrained returns the same result components for all these methods, and the calling function may add some more components to the final result. However, you should not access these result components directly (using $): the internal structure is not regarded as stable application interface (API), but it can change at any release. If you access the results components directly, you take a risk of breakage at any vegan release. The vegan provides a wide set of accessor functions to those components, and these functions are updated when the result object changes. This documentation gives an overview of accessor functions to the cca result object.

Usage

ordiYbar(x, model = c("CCA", "CA", "pCCA", "partial", "initial"))
# S3 method for class 'cca'
model.frame(formula, ...)
# S3 method for class 'cca'
model.matrix(object, ...)
# S3 method for class 'cca'
weights(object, display = "sites", ...)

Arguments

object, x, formula: A result object from cca, rda, dbrda, or capscale.
model: Show constrained ("CCA"), unconstrained ("CA") or conditioned “partial” ("pCCA") results. In ordiYbar the value can also be "initial" for the internal working input data, and "partial" for the internal working input data after removing the partial effects.
display: Display either "sites" or "species".
...: Other arguments passed to the the function.

Details

The internal (“working”) form of the dependent (community) data can be accessed with function ordiYbar. The form depends on the ordination method: for instance, in cca the data are weighted and Chi-square transformed, and in dbrda they are Gower-centred dissimilarities. The input data in the original (“response”) form can be accessed with fitted.cca and residuals.cca. Function predict.cca can return either working or response data, and also their lower-rank approximations.

The model matrix of independent data (“Constraints” and “Conditions”) can be extracted with model.matrix. In partial analysis, the function returns a list of design matrices called Conditions and Constraints. If either component was missing, a single matrix is returned. The redundant (aliased) terms do not appear in the model matrix. These terms can be found with alias.cca. Function model.frame tries to reconstruct the data frame from which the model matrices were derived. This is only possible if the original model was fitted with formula and data arguments, and still fails if the data are unavailable.

The number of observations can be accessed with nobs.cca, and the residual degrees of freedom with df.residual.cca. The information on observations with missing values can be accessed with na.action. The terms and formula of the fitted model can be accessed with formula and terms.

The weights used in cca can be accessed with weights. In unweighted methods (rda) all weights are equal.

The ordination results are saved in separate components for partial terms, constraints and residual unconstrained ordination. There is no guarantee that these components will have the same internal names as currently, and you should be cautious when developing scripts and functions that directly access these components.

The constrained ordination algorithm is based on QR decomposition of constraints and conditions (environmental data), and the QR component is saved separately for partial and constrained components. The QR decomposition of constraints can be accessed with qr.cca. This will also include the residual effects of partial terms (Conditions), and it should be used together with ordiYbar(x, "partial"). The environmental data are first centred in rda or weighted and centred in cca. The QR decomposition is used in many functions that access cca results, and it can be used to find many items that are not directly stored in the object. For examples, see coef.cca, coef.rda, vif.cca, permutest.cca, predict.cca, predict.rda, calibrate.cca. See qr for other possible uses of this component. For instance, the rank of the constraints can be found from the QR decomposition.

The eigenvalues of the solution can be accessed with eigenvals.cca. Eigenvalues are not evaluated for partial component, and they will only be available for constrained and residual components.

The ordination scores are internally stored as (weighted) orthonormal scores matrices. These results can be accessed with scores.cca and scores.rda functions. The ordination scores are scaled when accessed with scores functions, but internal (weighted) orthonormal scores can be accessed by setting scaling = FALSE. Unconstrained residual component has species and site scores, and constrained component has also fitted site scores or linear combination scores for sites and biplot scores and centroids for constraint variables. The biplot scores correspond to the model.matrix, and centroids are calculated for factor variables when they were used. The scores can be selected by defining the axes, and there is no direct way of accessing all scores of a certain component. The number of dimensions can be assessed from eigenvals. In addition, some other types can be derived from the results although not saved in the results. For instance, regression scores and model coefficients can be accessed with scores and coef functions. Partial component will have no scores.

Distance-based methods (dbrda, capscale) can have negative eigenvalues and associated imaginary axis scores. In addition, species scores are initially missing in dbrda and they are accessory and found after analysis in capscale (and may be misleading). Function sppscores can be used to add species scores or replace them with more meaningful ones.

Note

The latest large change of result object was made in release 2.5-1 in 2016. You can modernize ancient stray results with modernobject <- update(ancientobject).

References

Legendre, P. and Legendre, L. (2012) Numerical Ecology. 3rd English ed. Elsevier.

Author

Jari Oksanen