Analysis of community ecology data in R

CA & DCA (unimodal unconstrained ordination)

R functions

cca (library vegan) - if the environmental matrix is not specified, cca function in vegan package calculates unconstrained correspondence analysis¹⁾.
decorana (library vegan) - this function implements the original DECORANA program, written by Mark Hill in Fortran, into R. For historical reasons²⁾, only four DCA ordination axes are returned, and this is not easy to change.
CA (P. Legendre, Numerical Ecology with R, called CA.newr in the 2nd edition) - code can be found here.

¹⁾

Alternatively to library vegan, there is also a package ca with function ca calculating correspondence analysis, with plot, plot3d and summary functions. Package ade4 offers function dudi.coa.

²⁾

The function decorana is implementing the original Hill's algorithm for DCA introduced by Fortran program DECORANA, which itself can calculate only four ordination axes; CANOCO behaves in the same way.