  Weakly collisional plasmas dynamically develop pressure anisotropies with
respect to the magnetic field. These anisotropies trigger plasma instabilities
at scales just above the ion Larmor radius \rho_i and much below the mean free
path \lambda_{mfp}. They have growth rates of a fraction of the ion cyclotron
frequency - much faster than either the global dynamics or local turbulence.
The instabilities dramatically modify the transport properties and, therefore,
the macroscopic dynamics of the plasma. Their nonlinear evolution drives
pressure anisotropies towards marginal stability, controlled by the plasma beta
\beta_i. Here this nonlinear evolution is worked out for the simplest
analytically tractable example - the parallel firehose instability. In the
nonlinear regime, both analytical theory and the numerical solution predict
secular growth of magnetic fluctuations. They develop a k^{-3} spectrum,
extending from scales somewhat larger than \rho_i to the maximum scale that
grows secularly with time (~t^{1/2}); the relative pressure anisotropy
(\pperp-\ppar)/\ppar tends to the marginal value -2/\beta_i. The marginal state
is achieved via changes in the magnetic field, not particle scattering. When a
parallel ion heat flux is present, the firehose mutates into the new
gyrothermal instability (GTI), which continues to exist up to firehose-stable
values of pressure anisotropy, which can be positive and are limited by the
heat flux. The nonlinear evolution of the GTI also features secular growth of
magnetic fluctuations, but the spectrum is eventually dominated by modes around
the scale ~\rho_i l_T/\lambda_{mfp}, where l_T is the scale of the parallel
temperature variation. Implications for momentum and heat transport are
speculated about. This study is motivated by the dynamics of galaxy cluster
plasmas.
