30
and native biodiversity in general. Bromus -dominated grasslands are depauperate in
diversity of plant functional types and animal communities (Rochester et al. 2010 ;
Molinari and D’Antonio 2014 ). An ongoing study suggests that it is the accumu-
lated litter of Bromus that suppresses native forb germination and not direct compe-
tition for soil nutrients between Bromus and native forbs (Molinari 2014 ). Molinari
and D’Antonio ( 2014 ) demonstrate that there are several native forbs that can coex-
ist with B. diandrus and that although native forb richness is depressed where this
species becomes dominant, several native forbs preferentially associate with this
grass. The infl uence of Bromus invasion on native animals has been little studied.
One study found that despite enhancing primary production, Bromus invasion into
coastal shrublands caused either a decrease in or no effect on arthropod assemblages
(Wolkovich et al. 2009b ). Key native ant species declined as litter of Bromus
increased (Wolkovich et al. 2009b ).
2.3.3 Management Strategies for Bromus
in Mediterranean California
Preventing further invasions of Bromus species is largely dependent on reducing the
incidence of disturbance and maximizing ecosystem resilience (Fig. 2.8 ). In grass-
lands, this may involve manipulating livestock grazing patterns or removal of feral
animals such as pigs. Where exotic grasses have invaded fi re-tolerant or even fi re-
dependent ecosystems , it is important to appreciate that these shrublands require a
particular fi re regime that typically involves long fi re-free intervals and high-
intensity fi re. Humans currently cause over 95 % of all fi res in the foothills and
coastal plain of California and have greatly increased the natural fi re frequency
(Safford and Van de Water 2014 ). Keeping unwanted ignitions out of these systems
is a major challenge. A more tractable problem is reducing planned ignitions and
resource managers in the region are beginning to appreciate that prescription burn-
ing, while possibly having some fi re-hazard reduction benefi t, is adding to the read-
ily ignitable fuel load on these landscapes.
Bromus dominance may decline under passive management for some landscapes
whereas other landscapes require active restoration. Sage scrub displaced by grass-
lands from various disturbances can return within a few decades if disturbance is
reduced and native seed sources are in close proximity (Freudenberger et al. 1987 ;
DeSimone and Zedler 1999 ; Callaway and Davis 1993 ), although some sites have
proved resistant to passive restoration (Stylinski and Allen 1999 ). The type of dis-
turbance associated with annual grassland appears important as soil disturbance that
affects biological soil crusts (if present) can favor exotic annuals such as Bromus
and inhibit native shrub seedlings (Hernandez and Sandquist 2011 ).
Native perennial grasslands invaded by exotic grasses show some promise of
being restored to a greater native grass and forb composition through active manage-
ment. In particular, winter/spring grazing has been shown to enhance dominance of
M.L. Brooks et al.