\chapter[An Introduction to Food Webs]{An Introduction to Food
Webs, and Lessons from Lotka--Volterra Models} A food web is a
real or a model of a \emph{set of feeding relations among
species or functional groups}. This chapter has two foci, (i) a
very brief introduction to multi-species webs as networks, and
(ii) a re-examination of old lessons regarding the effect of
food chain length on a web's dynamical properties.
\begin{figure}[ht] \centering \begin{minipage}[c]{.35\linewidth}
\centering \includegraphics[width=3cm]{web1} \end{minipage}
\begin{minipage}[c]{.35\linewidth} \centering \begin{tabular}{c|
c c c c } & \multicolumn{4}{c}{\emph{~As predators~}}\\
\emph{~As prey~} & ~R~ & ~B~ & ~A~ & ~P~\\ \hline
R&0&$-$&$-$&$-$\\ B&$+$&0&0&$-$\\ A&$+$&0&0&0\\ P&$+$&$+$&0&0\\
\end{tabular}\\ \end{minipage} \caption{Two representations of a
food web with 4 nodes and 8 directed links. The link label ``v''
indicates the proportion of $P$'s diet comprised of $R$.}
\label{fig:web} \end{figure} \section{Food Web Charac