Agriculture Reference
In-Depth Information
T
HE
E
FFECTS OF
I
NVASION
Invasion has gained considerable attention only recently and therefore relevant literature
is extremely limited. The invasion of exotic species is one of the most serious threats for local
biodiversity and for ecosystem functioning (Mooney and Hobbs 2000). Due to plant-microbes
interactions (symbiosis, herbivory or pathogenicity), colonization by invasive plant species
could induce shifts in the structure of native soil microbial communities, ultimately resulting
in changes to the appearance of the entire ecosystem. Apart from invasive plants there are
also microbes invaders that could act as competitors to native microorganisms (Jules et al.
2002, Niwa et al. 2004, Wardle et al. 2004). Invasive microbes can change ecosystem
processes (e.g. nitrogen fixation) by disturbing local patterns of symbiosis or pathogenicity or
by disturbing local decomposition processes (van der Putten et al. 2007).
Among few studies focusing on how soil microbial communities respond to plant
invasion and how then they affect further plant invasion is that of Batten et al. (2006). The
aim of this study was to examine the impacts of invasive plants on the composition of soil
community around rhizosphere and to further investigate how these impacts are related to the
duration of invasion. Moreover, the spatial structure of invasion patches was taken into
account. Actually, they studied how the rhizosphere microbial community was shaped in the
center, the edge and outside the invasion patches. To cope with such a sudject, samples were
taken from the center and the edge of the invasive patches as well as from the surrounding
native plant community. Two invasive species (an annual forb and an annual grass named
starthistle and goatgrass respectively) and five native annual forbs were studied.
To a large extent the invasive plant species spread within areas of low fertility soils such
as serpentine grasslands in Northern California Coast Range. In such soils native species are
unsuccessful occupants and therefore only refuges of these species exist. In these marginal
areas, the overall microbial community composition in invasive edges was intermediate
between that in the center and those in native plants. Invasion by goatgrasses seemed to
change the community more rapidly than invasion by starthistle. In fact in patches of the
former species microbial community in edges was very similar to that in the center. In
starthistle sites, diversity was lower in the edges while the opposite occurred in goatgrass
sites. Several years after invasion the rhizosphere microbial communities became more
diversified in invasive patches than the native ones. In some cases, even in invaded areas the
variation in microbial communities from year to year exceeded that between invasive and
native plants.
The next question explored by Batten and his colleagues was how changes in microbial
properties were related to soil structure (Batten et al. 2005), based on the finding that the
rhizosphere of goatgrass consisted of fatty acids representative of arbuscular mycorrhizal
fungi (AMF). By producing glomalin AMF could affect soil aggregation. Indeed, soil
aggregation was enhanced in soil dominated by goatgrass compared to that dominated by
native vegetation.
