The relative importance of natural enemy types for distance-dependent mortality at the seed-to-seedling transition

Background/Question/Methods

Distance- and density-dependent mortality plays a key role in the maintenance of forest diversity, and the signature of this phenomenon has been observed in forests worldwide. Studies that assess the species interactions that cause it have implicated natural enemies including insects, mammals, and fungi. Most experiments have targeted – and found impacts of – fungal pathogens, leading some to suggest that fungi are the most important driver of the diversity-promoting mechanism. However, using existing studies to assess the relative importance of certain types of natural enemies may be misleading for several reasons. Uneven research effort across enemy types may result in underestimation of the impacts of agents that are less often tested. Further, nearly all of the existing experimental tests target only a single enemy type; in doing so they overlook other, potentially more important, enemy types. Instead, studies should simultaneously manipulate multiple enemy types to determine their relative importance. We did so using fungicide, insecticide, and physical exclosures to assess the role of fungal pathogens, insects, and mammals at the seed and early seedling stages for three tropical tree species on the Pacific island of Saipan.

Results/Conclusions

We found distance-dependent mortality in five of six species-stages, but the source and severity varied widely between species and between life history stages of individual species. Insects were the most important cause of distance dependence; our field experiments show they caused distance dependence in three species-stages, whereas mammals caused distance dependent mortality in two species-stages and fungal pathogens in just one species-stage. In both instances where mammals caused distance-dependence, another enemy type played a stronger role at that species-stage. These results do not support a key role of fungi. Instead, a broad set of natural enemies appears to act together, and in different combinations over a plant’s life, to cause this diversity-promoting phenomenon.