Competitive
interactions between native and exotic species have been long recognized as a
significant hurdle to efforts to restore native biodiversity in degraded
habitats. In the vast majority of grassland habitats in California, exotic annual grasses dominate
communities in terms of both cover and biomass. In interior regions of the
state, competition from annual grasses has been shown to reduce establishment
of native seedlings and also increase mortality of established native
individuals. In more mesic habitats such as those in
the northern and central coasts, the prospects for native species is more
favorable: competitive interactions more consistently favor native perennial
grasses. Once established, native stands are able to reduce exotic biomass and
resist subsequent invasion by exotic species. Recent research has turned to ways
of influencing competitive relationships between native and exotic species
through active management in an effort to achieve management goals such as
increased native biodiversity. Management
strategies – including grazing, prescribed fire and seed addition – have been
applied in an effort to influence competitive outcomes in favor of native
species at the expense of exotic annual grasses. A mixture of methods that
includes the addition of native seeds along with treatments such as grazing or
burning that are designed to reduce competitors' growth and survival will have
the greatest likelihood of increasing native establishment and survival. On a
pessimistic note, some non-native species, such as exotic perennial grasses in
coastal habitats, have shown the ability to respond positively to restoration
strategies designed to control exotic annual grasses. Restoration projects must
guard, therefore, against ushering in a new wave of exotic competitors that
present a new suite of challenges to native biodiversity.