There goes the neighborhood: apparent competition between invasive, and native orchids

Background/Question/Methods

Apparent competition occurs when one species is negatively affected by the presence of another species because they share a common predator. In Puerto Rico, the exotic Asiatic orchid, Spathoglottis plicata, has naturalized and the self-pollinating species has spread rapidly over the last two decades, becoming locally abundant.  Two forms exist: one with white flowers and the other with magenta flowers.  In parts of the island, they occupy the same habitat as the native xenogamous orchid, Bletia patula, which is vegetatively similar to S. plicata and bears magenta flowers. The two species are hosts to the same endemic florivorous weevil, Stethobaris polita, a specialist on orchid flowers. We ask whether the presence of Spathoglottis plicata affects the reproductive success of Bletia patula by elevating the abundance of the weevil. We measured local densities of the orchids where the two species grow together and also where B. patula grows in the absence of S. plicata.  Female reproductive success (fruit production) in Bletia was recorded at all sites. We also monitored the abundance of weevils and the extent of floral damage they cause to B. patula. In addition, we experimentally tested in vitro whether weevils preferred one species over the other. 

Results/Conclusions

We found that the number of weevils per individual of B. patula was significantly higher where S. plicata was also present. Associated with this increment in weevil presence, floral damage to B. patula was also greater where S. plicata was present.  Consequently, reproductive effort and success was higher for B. patula in the absence of S. plicata. The weevil choice experiments showed that the beetles preferred B. patula flowers over S. plicata white flowers but prefer S. plicata magenta flowers over similarly colored B. patula flowers. Thus, the invasive S. plicata has detrimental effects on native B. patula by elevating populations of native weevils that specialize on orchid flowers resulting in dire consequences for natural fruit and seed production of the native orchid.  Assuming that variation in fruit production in B. patula has demographic consequences as has been shown for other orchids, we suggest that apparent competition between the two species exists through the activity of a shared florivorous weevil.