  Ecologists and evolutionary biologists are relying on an increasingly
sophisticated set of statistical tools to describe complex natural systems. One
such tool that has gained increasing traction in the life sciences is
structural equation modeling (SEM), a variant of path analysis that resolves
complex multivariate relationships among a suite of interrelated variables. SEM
has historically relied on covariances among variables, rather than the values
of the data points themselves. While this approach permits a wide variety of
model forms, it limits the incorporation of detailed specifications. Here, I
present a fully-documented, open-source R package piecewiseSEM that builds on
the base R syntax for all current generalized linear, least-square, and mixed
effects models. I also provide two worked examples: one involving a
hierarchical dataset with non-normally distributed variables, and a second
involving phylogenetically-independent contrasts. My goal is to provide a
user-friendly and tractable implementation of SEM that also reflects the
ecological and methodological processes generating data.
