  We construct a theory of holant clones to capture the notion of
expressibility in the holant framework. Their role is analogous to the role
played by functional clones in the study of weighted counting Constraint
Satisfaction Problems. We explore the landscape of conservative holant clones
and determine the situations in which a set $\mathcal{F}$ of functions is
"universal in the conservative case", which means that all functions are
contained in the holant clone generated by $\mathcal{F}$ together with all
unary functions. When $\mathcal{F}$ is not universal in the conservative case,
we give concise generating sets for the clone. We demonstrate the usefulness of
the holant clone theory by using it to give a complete complexity-theory
classification for the problem of approximating the solution to conservative
holant problems. We show that approximation is intractable exactly when
$\mathcal{F}$ is universal in the conservative case.
