Alvarez and Cushman (2002)

Basic FindingsProposed Conclusions
Plots invaded by Cape Ivy in this study contained 36% fewer native plant species and 37% fewer nonnative taxa, a pattern that persisted across habitat types and seasons
richness (number of species) of grass and forb species was lower in invaded plots, fern and shrub richness was not lowerStudy authors hypothesize that the reason cape ivy decreased the number of grass/forb species but not shrub/fern species, is because of more resource overlap. Grass and ferns all have life-history characteristics that facilitate colonization after disturbances and can quickly grow after cape ivy is removed (rapid growth rate, clonal reproduction, copious seed production). Ferns are shade tolerant and might not be impacted by cape ivy’s smothering growth habit. Cape ivy might not be competing for the same resources as shrubs. Shrub root systems extract water and nutrients from deeper soils than cape ivy.
the number of native species lowered with increasing cape ivy, the number of nonnative species did not lower with increasing cape ivyRemoving cape ivy might lead to more nonnative species growing, leading to a question “Is a system dominated by many invaders preferable to one dominated by only one?”
a 2 year cape ivy reduction experiment resulted in a 10% increase in the number of native species and 43% increase in the number of nonnative species
cape ivy decreases the numbers of native and non-native species, so removing cape ivy from invaded areas may facilitate the proliferation of other nonnative species
“If a central objective of land managers is to maintain biodiversity, then one of the first management activities should be to identify those invasive species that are having the greatest negative effects on the composition of those plant communities.”

Community Level Consequences of Plant Invasion: Effects on Three Habitats in Coastal California - Maria E. Alvarez and J. Hall Cushman (Alvarez and Cushman 2002)