Island Invasion by a Threatened Tree Species: Evidence for Natural Enemy Release of Mahogany (Swietenia macrophylla) on Dominica, Lesser Antilles

Abstract

Despite its appeal to explain plant invasions, the enemy release hypothesis (ERH) remains largely unexplored for tropical forest trees. Even scarcer are ERH studies conducted on the same host species at both the community and biogeographical scale, irrespective of the system or plant life form. In Cabrits National Park, Dominica, we observed patterns consistent with enemy release of two introduced, congeneric mahogany species, Swietenia macrophylla and S. mahagoni, planted almost 50 years ago. Swietenia populations at Cabrits have reproduced, with S. macrophylla juveniles established in and out of plantation areas at densities much higher than observed in its native range. Swietenia macrophylla juveniles also experienced significantly lower leaf-level herbivory (∼3.0%) than nine co-occurring species native to Dominica (8.4–21.8%), and far lower than conspecific herbivory observed in its native range (11%–43%, on average). These complimentary findings at multiple scales support ERH, and confirm that Swietenia has naturalized at Cabrits. However, Swietenia abundance was positively correlated with native plant diversity at the seedling stage, and only marginally negatively correlated with native plant abundance for stems ≥1-cm dbh. Taken together, these descriptive patterns point to relaxed enemy pressure from specialized enemies, specifically the defoliator Steniscadia poliophaea and the shoot-borer Hypsipyla grandella, as a leading explanation for the enhanced recruitment of Swietenia trees documented at Cabrits.

Ticking time bombs - current and potential future impacts of four invasive plant species on the biodiversity of lowland tropical rainforests in south-east Viti Levu, Fiji

“…Although species of Swietenia are often considered non-invasive (Richardson, 1998), we have observed saplings of the species in primary rainforest, several kilometres away from plantations. This parallels reports about the potential invasiveness in another island rainforest on Dominica in the Caribbean (Norghauer et al, 2011). Flowers of mahogany are pollinated by insects (bees, moths and thrips) and are mostly outcrossed (Bawa et al, 1985).…”

Invasive Plants and Enemy Release: Evolution of Trait Means and Trait Correlations in Ulex europaeus

“…This release from natural enemies has been proposed as one of the most important ecological factors contributing to the invasiveness of numerous species [11], [12], [13]. Arguments on this Enemy Release Hypothesis (ERH) have been reviewed by Keane and Crawley [14] and Liu and Stiling [15].…”

Herbivores limit the population size of big‐leaf mahogany trees in an Amazonian forest

“…This inference is also supported by recent observations of ‘enemy release’ of Swietenia in invasive Caribbean populations from which Steniscadia is absent, and where the density of trees > 25 cm diam. was ca 125 stems ha −1 (Norghauer et al ). The widespread notion that the reliance on gaps to attain canopy size by non‐pioneer, light‐demanding tree species precludes their codominance at the stand level presupposes that their juvenile growth and recruitment rates are optimal in gaps, and that no JC effects suppress their performance across them.…”