  Networks with absolute concentration robustness (ACR) have the property that
a translation of a coordinate hyperplane either contains all steady states
(static ACR) or attracts all trajectories (dynamic ACR). The implication for
the underlying biological system is robustness in the concentration of one of
the species independent of the initial conditions as well as independent of the
concentration of all other species. Identifying network conditions for dynamic
ACR is a challenging problem. We lay the groundwork in this paper by studying
small reaction networks, those with 2 reactions and 2 species. We give a
complete classification by ACR properties of these minimal reaction networks.
The dynamics is rich even within this simple setting. Insights obtained from
this work will help illuminate the properties of more complex networks with
dynamic ACR.
