Importance of species replication in understanding plant invasions into North American grasslands - Invasive Plants: Ecological and Agricultural Aspects - page 53

Thus, widely different conclusions would have been reached at this site if we
had not replicated species in this study. By including the major native and
exotic species from this system in our design, we were able to more accurate-
ly estimate the overall impact of exotic species in this system. Exotic species
ecology will greatly benefit by developing a more community-level approach
that replicates species [24].
Do exotic species affect species diversity-NPP relationships?
There has been much recent interest in whether exotic species are lowering
species diversity, and in turn, whether lowered species diversity will affect
ecosystem process rates. Although 25% of the earth's surface is reported to be
grassland, much of it is human-derived grassland, or grassland that assembled
from previously farmed areas. Classical succession theory would predict that
a high diversity native system would develop in these systems given enough
time. However, this has not been the case. Many areas no longer have an ade-
quate seed source of native species, or even if a seed source is available, native
species have a multitude of exotic species to contend with during early com-
munity development. A few studies have shown that exotic grasslands tend to
have lower diversity than do native grasslands, even many years after aban-
donment from agriculture [3, 11, 41]. However, in many cases, it is unknown
whether this low diversity is caused solely by higher resource availabilities due
to fertilizer carry-over [42-45], or if part of the effect is due to characteristics
of the species themselves. Christian and Wilson [3] found that former
Saskatchewan croplands planted with Agropyron cristatum had lower species
diversity than adjacent unplanted areas even after many years of abandonment.
Foster et al. [41] found that 34 species were largely unable to establish from
seed in low diversity grassland patches with strong dominance by exotic grass-
es such as Bromus inermis in Kansas. I have found essentially zero seedling
emergence in Bromus inermis plots within western Iowa (unpublished data).
Less interest has been focused on whether exotic species affect species diver-
sity-ecosystem functioning relationships [46, 47].
Although most researchers focus on the individual plant or population lev-
els, invasive species establish and grow in communities. Higher aboveground
growth rates by exotic species might lead not only to higher productivity, but
to a greater rate of local species extinctions [40]. Loreau and Hector [48] out-
lined a powerful technique for partitioning the net biodiversity effect (i.e.,
yield of a plant species in mixtures compared to expectations from monocul-
tures) into a selection and complementarity effect. The overall net biodiversi-
ty effect compares yield in mixtures to yield in monocultures. The partitioning
method then breaks this overall effect into a selection and complementarity
effect. These two effects can hypothetically range from negative to positive,
and are combined to account for the net biodiversity effect. The complemen-
tarity effect combines the effects of niche differentiation and facilitation [48].