Publisher : Ecological Society of America
Place of publication :
Publication year : 2002
Thematic : Invasive Alien Species
Language : English
Note
A venerable generalization about community resistance to invasions is that
more diverse communities are more resistant to invasion. However, results of experimental
and observational studies often conflict, leading to vigorous debate about the mechanistic
importance of diversity in determining invasion success in the field, as well as other ecosystem
properties, such as productivity and stability. In this study, we employed both field
experiments and observational approaches to assess the effects of diversity on the invasion
of a subtidal marine invertebrate community by three species of nonindigenous ascidians
(sea squirts). In experimentally assembled communities, decreasing native diversity increased
the survival and final percent cover of invaders, whereas the abundance of individual
species had no effect on these measures of invasion success. Increasing native diversity
also decreased the availability of open space, the limiting resource in this system, by
buffering against fluctuations in the cover of individual species. This occurred because
temporal patterns of abundance differed among species, so space was most consistently
and completely occupied when more species were present. When we held diversity constant,
but manipulated resource availability, we found that the settlement and recruitment of new
invaders dramatically increased with increasing availability of open space. This suggests
that the effect of diversity on invasion success is largely due to its effects on resource
(space) availability. Apart from invasion resistance, the increased temporal stability found
in more diverse communities may itself be considered an enhancement of ecosystem function.
In field surveys, we found a strong negative correlation between native-species richness
and the number and frequency of nonnative invaders at the scale of both a single quadrat
(25 3 25 cm), and an entire site (50 3 50 m). Such a pattern suggests that the means by
which diversity affects invasion resistance in our experiments is important in determining
the distribution of invasive species in the field. Further synthesis of mechanistic and observational
approaches should be encouraged, as this will increase our understanding of
the conditions under which diversity does (and does not) play an important role in determining
the distribution of invaders in the field.
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Keywords : marine invasion
Encoded by : Pauline Carmel Joy Eje
