Agriculture Reference
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A positive selection effect occurs when species that are highly productive in
monoculture are the ones over-yielding in mixture. Taken together, these two
processes can increase the growth of mixtures above that which would be
expected based on that expected from monocultures [48]. Loreau and Hector
[48] used this technique on native species assemblages in Europe and found
that the complementarity effect accounted for the higher yields in mixtures.
The selection effect varied from negative to positive across sites and was less
important than complementarity.
In contrast to the results of Loreau and Hector [48], we have found strong
selection effects in native-exotic mixtures that are caused by exotic grasses.
Polley et al. [49] found that both the selection and “complementarity” effects
were negative in three species mixtures with annuals. The exotic
Lolium
perenne
was the most important species in explaining negative selection
effects. Wilsey and Polley [40] found a large positive selection effect that
increased with species richness; a smaller complementarity effect was found
only when mixtures were planted with high evenness. The strong selection
effect, especially during the first year, was driven mostly by the exotic grass
Panicum coloratum
. Local species extinctions, which started in the second
year of the study, were highest in species with low aboveground growth rates
and in plots planted with low species evenness [40]. Taken together, these stud-
ies suggest that the dynamics of native-exotic mixtures may differ from mix-
tures with only native species. However, further research with a greater num-
ber of study systems is needed to determine how general this phenomenon is.
Further monitoring of our experimental plots over many years will be helpful
in determining whether local extinction rates are higher in plots as a function
of the proportion of exotic species.
Conclusions
I found that results from exotic-native comparisons differed depending on
whether a single exotic-native species pair was compared or if replicated
groups of exotics and native species were compared. Comparing groups of
exotics to natives gave more consistent results, and supported the hypothesis
that exotic species have higher aboveground growth rates than natives.
Comparing single species gave more widely varying results because of the
large amount of variation that exists among species. Of course, this variation
among species is important and should be taken into account by land managers
dealing with a new invasive species. Management plans will have to be some-
what species-specific in these cases. However, if the goal of a manager or pol-
icy maker is to develop a comprehensive general management plan for exotic
species at a given site, or to develop ecological theories on exotic species
effects, studies with replicated species will be highly useful.
There are other problems with studies that focus on native-exotic species
pairs. For one, native species are not always picked randomly. Introduced spe-
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