Experimental decoupling of the effects of hurricane disturbance on tropical gastropod populations and communities

Background/Question/Methods

Puerto Rico harbors ecosystems whose characteristics are molded by hurricane-dominated disturbance regimes.  Hurricanes relocate leaves and branches from the canopy to the soil surface, and alter understory microclimates (increase temperature and light; decrease humidity).  In this context, terrestrial gastropods are models for understanding disturbance dynamics.  Gastropods are taxonomically diverse, numerically abundant, ectothermic, desiccation-sensitive, and not particularly vagile.  They respond to small- and large-scale disturbances in species-specific fashions.  We used a two-by-two factorial design (canopy trimming versus debris deposition) deployed in each of three blocks to decouple the effects of canopy opening from those associated with debris deposition on short-term changes (value after minus value before manipulation) in population or community characteristics of gastropods in tabonuco forest of Puerto Rico.  The four combinations of treatment factors were (1) canopy not trimmed and debris not added (background condition without simulated hurricane effects); (2) canopy not trimmed but debris added (simulating resource redistribution but decoupling it from the consequences of canopy openings); (3) canopy trimmed but debris not added (simulating an altered microclimate, but decoupling it from the consequences of resource redistribution); and (4) canopy trimmed and debris added (simulating the combined effects of alteration of microclimate and resource redistribution).

Results/Conclusions

Six species were captured in sufficient abundance for powerful analyses at the population level.  Block effects were common (four species), indicating that considerable spatial heterogeneity characterized tabonuco forest, at least from the perspective of gastropod demographics.  These block effects were either consistent regardless of canopy trimming and debris addition (Pleurodonte caracolla), or emerged as interactions with main treatment factors (Alcadia striata, Platysuccinea portoricensis, Polydontes acutangula).  Moreover, block effects were pervasive for all aspects of biodiversity.  Despite variability in abundances induced by spatial heterogeneity, three species responded consistently to canopy removal (A. striata, P. caracolla, P. portoricensis) and P. acutangula responded consistently to debris addition.  In contrast, the effects of debris addition were predicated on canopy removal for P. portoricensis.  Two species did not respond to any treatments (Gaeotis nigrolineata, Nenia tridens).  Canopy removal affected richness, diversity, and rarity without interactive effects.  Debris addition affected richness and diversity without interactive effects.  Dominance and evenness were not affected by canopy removal, debris addition, or their interaction.  In the short-term, debris addition may ameliorate potential negative effects of canopy loss on terrestrial gastropods by insulating against increased temperatures, minimizing the loss of moisture, and enhancing substrate or forage availability.