Regeneration of Rhizophora mangle in a Caribbean mangrove forest:
interacting effects of canopy disturbance and a stem-boring beetle
Wayne P. Sousa · Swee P. Quek · Betsy J. Mitchell

Discussion
In discussions of the relative roles of propagule predation and microsite limitation in determining the numbers of adult plants, the suitability of a microsite for recruitment is often assumed to be independent of the risk of predation for propagules that disperse to it. Our results clearly demonstrate an interaction between the quality of microsites for recruitment and growth and the risk of Rhizophora propagules and seedlings being attacked and killed by Coccotrypes. While Rhizophora seedlings establish across the forest floor each year, beetle-caused mortality prevents their recruitment to the sapling sizeclass except within lightning-created gaps and the areas immediately surrounding them, which provide a spatial refuge from beetle attack. This pattern of herbivory accounts for observed spatial variation in stand structure: within the forest, sapling stages of Rhizophora are largely restricted to areas of recent canopy disturbance. Such microsites also provide the resources necessary for juveniles to grow into the adult canopy. One might conclude, therefore, that Rhizophora density is limited by these recruitment microsites rather than by the number of propagules consumed by herbivores. However, the beetles are effectively eliminating a key regeneration strategy of Rhizophora, a long-lived canopy dominant: the filling of gaps by the growth of saplings that have been released from suppression by the adult canopy. This reduces the competitive impact of Rhizophora and may enhance the opportunity for shade intolerant Laguncularia juveniles to survive and grow into the canopy when a gap is formed, leading to co-dominance of low intertidal forests by these species. Anderson (1989, p 310) argued that “even if predators prevented altogether the location of safe sites by seeds, they would still have a negligible impact on population size (i.e. they would reduce potential population size by only a small fraction) unless they interfered with the establishment of a seed bank capable of exploiting future changes in safe site abundance.” In effect, this is precisely what Coccotrypes predation does, but to a seedling bank rather than seed bank. An understory shade tolerant seedling bank acts much like a soil seed bank in providing a mechanism of “storing” future recruits (Warner and Chesson 1985; Chesson 1986). Just as seed predators can control plant recruitment by depleting the soil seed bank, so too can herbivores that prevent the establishment of shade tolerant juveniles beneath the adult canopy. Incorporating the influence of herbivores on the contribution of advance regeneration to forest dynamics, as demonstrated in this study, will significantly enhance the generality of theories concerning the impact of seed and seedling predators on plant populations. At present, we do not know why seedling mortality due to beetles is lower in light gaps than in the shaded understory. If mated female beetles disperse diurnally, they may actively avoid the potentially stressful high light and temperature conditions inside gaps. Alternatively, seedlings growing in the gap environment may develop morphological or chemical characteristics that deter beetles from burrowing into their stems. In our experiments, the difference in rates of attack in the two microenvironments developed within a few days after the seedlings were planted, probably more rapidly than could be accounted for by morphological changes in plant tissues. However, some form of light-activated, toxic secondary metabolite, or phototoxin, could be involved (L. Bjostad, personal communication). Such compounds are rapidly excited by absorption of light, accumulate relatively quickly under high light conditions, and can effectively defend against insect herbivores (Berenbaum 1987; Downum 1992). To our knowledge, phototoxins have not been looked for in Rhizophora, but have been found in more than 40 families of plants (Downum and Wen 1995; K. Downum, personal communication). An observation that argues against the involvement of phototoxins is that beetle populations continued to thrive in infested seedlings that were grown under high light conditions beneath a 2 m high, translucent fiberglass awning at the Galeta laboratory (Sousa et al. 2003). However, these beetles had already penetrated the epidermis of the propagules’ hypocotyls by the time they were collected from shady understory sites prior to planting. The epidermis contains an abundance of chlorophyll, is photosynthetically active (Smith and Snedaker 2000), and is the likely location of any phototoxic compounds.

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