Recruitment dynamics in a rainforest seedling community: context-independent impact of a keystone consumer

Abstract

The influence of keystone consumers on community structure is frequently context-dependent; the same species plays a central organising role in some situations, but not others. On Christmas Island, in the Indian Ocean, a single species of omnivorous land crab, Gecarcoidea natalis, dominates the forest floor across intact rainforest. We hypothesised that this consumer plays a key role in regulating seedling recruitment and in controlling litter dynamics on the island, independent of the type of vegetation in which it occurred. To test this hypothesis, we conducted crab exclusion experiments in two forest types on the island and followed the dynamics of seedling recruitment and litter processing for six years. To determine if these effects were likely to be general across the island, we compared land crab densities and seedling abundance and diversity at ten sites across island rainforest. Surveys across island rainforest showed that seedlings of species susceptible to predation by land crabs are consistently rare. Abundance and diversity of these species were negatively correlated to red crab abundance. Although red land crabs may be important determinants of seedling recruitment to the overstorey, differences in overstorey and seedling composition at the sites suggests that recruitment of vulnerable trees still occurs at a temporal scale exceeding that of this study. These "windows" of recruitment may be related to infrequent events that reduce the effects of land crabs. Our results suggest that unlike the context dependence of most keystone consumers in continental systems, a single consumer, the red land crab, consistently controls the dynamics of seedling recruitment across this island rainforest.

Effects of an Alien Ant Invasion on Abundance, Behavior, and Reproductive Success of Endemic Island Birds

“…The study was conducted during the transition from the dry to wet season (September 2001–January 2002) on Christmas Island (10°25′S, 105°40′E), an isolated oceanic island (135 km 2 , maximum elevation 360 m) dominated by structurally simple tropical rainforest (Du Puy 1993). We examined 3 states of primary forest: ant invaded, intact, and “ghosted.” These states result from the presence or absence of 2 key biotic drivers of forest structure and dynamics on the island: introduced the yellow crazy ant (O'Dowd et al 2003) and the red land crab (Green et al 1999, 2008). The yellow crazy ant is a tropical invader that has spread rapidly across the Indo‐Pacific region with human commerce and trade (Wetterer 2005).…”

“…Like many other invasive ants (Tsutsui & Suarez 2003), it can form expansive supercolonies with high, sustained densities of worker ants that extend from hectares to many square kilometers (Haines & Haines 1978; O'Dowd et al 2003). In the absence of yellow crazy ants, the forest floor is dominated by red crabs; removal experiments show that crabs consistently regulate seedling recruitment and litter breakdown across the island (Green et al 2008).…”

Carbohydrate supply drives colony size, aggression, and impacts of an invasive ant

“…Without G. natalis , seedling recruitment and litter decomposition are deregulated (Green et al. , , , O'Dowd et al. ), the abundance and species density of other non‐native ants and land snails increase (O'Dowd and Green , O'Loughlin and Green ), and secondary invasion by the giant African land snail is facilitated (Green et al.…”

The secondary invasion of giant African land snail has little impact on litter or seedling dynamics in rainforest

“…This naturally abundant land crab (~0.75 crabs m −2 ; Green ) plays a key functional role in shaping the forest understory structure by largely regulating seedling recruitment and litter decomposition (Green et al . , , ) and inhibits the entry of A. fulica into intact rainforest through opportunistic predation (Lake & O'Dowd ; Green et al . ).…”