Does restoration help the conservation of the threatened forest of Robinson Crusoe Island? The impact of forest gap attributes on endemic plant species richness and exotic invasions

Invasive plant species are major drivers of biodiversity losses, especially on islands which are prone to invasions and extinctions. In the “endemic montane forest” of Robinson Crusoe Island (Pacific Ocean, Chile) invasive exotic plant species threaten conservation efforts by establishing in gaps and outcompeting native tree species regeneration. We compared gap attributes and ground vegetation cover in three gap types: those dominated by native species (<5 % cover of invasive species), invaded gaps (>30 % cover by invasive species), and treated gaps (invasive species removed). We examined (a) which gap attributes favored native and exotic species, (b) the relationship between gap size and species richness, and (c) species responses to invasion and treatment. Gaps ranged in size from 46 to 777 m² caused mainly by uprooted and snapped trees. Multi response permutation procedures showed a different floristic composition between natural, invaded and treated gaps. The presence of Myrceugenia fernandeziana (native species) and Aristotelia chilensis (invasive species) as gap border trees was positively and negatively correlated with native species richness, respectively. New gaps had more native species than old gaps, and smaller gaps contained relatively more native species than larger ones. An increase in invasive species cover was related to a decline in native species cover and richness. 1–6 years after treatment gaps tended to recover their native floristic composition. Highly effective conservation management programs will concentrate on monitoring gap creation, early control of invasive species, and by treating smaller gaps first.