Tuesday, November 26, 2013

Increased Litter Decomposition Rates by Terrestrial Gastropods in Hawaii

Terrestrial gastropods are often a major component of various terrestrial ecosystems.  It is thought that litter-dwelling terrestrial gastropods contribute to the cycling of nutrients either directly or indirectly through metabolism and modifying habitat to enhance micro-arthropod or microbial activity, respectively.  However, their role in ecosystem processes is poorly known particularly in tropical forests.  In a recent issue of Biotropica, Wallace M. Meyer III, Rebecca Ostertag, and Robert H. Cowie shed some light on this very issue in a paper entitled “Influence of Terrestrial Molluscs on Litter Decomposition and Nutrient Release in a Hawaiian Rain Forest.”  Meyer et al. (2013) used a field mesocosm approach to examine (1) whether the presence of terrestrial gastropod species increased rates of leaf litter decomposition, (2) whether different terrestrial gastropod species influence the rates of nutrient release differently, and (3) whether terrestrial gastropods facilitate recruitment of mesoinvertebrates.  The results of the experiments showed that the presence of gastropods increased litter decomposition rates and that the highest decomposition rates were those with the greatest gastropod biomass.  Furthermore, although there were differences in the rates of release of some nutrients among treatments, the different gastropod species appeared to influence nutrient release in a similar way.  Finally, there was no evidence that terrestrial gastropods facilitated mesoinvertebrate recruitment.

The authors have shown empirically that terrestrial gastropods can play a major role in litter decomposition.  One interesting aspect of the study is that it was done using the five most abundant species of gastropods in the Hawaiian rain forest: the native Succinea cepulla and four non-native species (Arion intermedius, Deroceras leave, Oxychilus alliarius, and Limax maximus).    The native species had the lowest density among the gastropods studies and is comparatively rare.  Indeed, Hawaii presents a particularly compelling case because some 65-90 percent of the 750+ species (over 99% endemic) are now considered extinct (Solem, 1990; Cowie et al. 1995; Cowie, 2001; Lydeard et al., 2004) so there is the distinct possibility that invasive gastropod species are now conducting important ecological processes that were once carried out by native species and potentially benefitting otherwise native ecosystems.  Regrettably, important information is lacking to fully address this issue such as species richness and densities in historical, native communities.

Literature Cited
Cowie, R. H., N. L. Evenhuis, and C. C. Christensen.  1995.  Catalog of the native aland and freshwater molluscs of the Hawaiian Islands.  Backhuys Publishers, Leiden, The Netherlands.
Cowie, R. H.  2001.  Invertebrate invasions on Pacific islands and the replacement of unique native faunas: a synthesis of land and freshwater snails.  Biol. Invasions 3:119-136.
Meyer III, W. M., R. Ostertag, and R. Cowie.  2013.  Influence of terrestrial molluscs on litter decomposition and nutrient release in a Hawaiian rain forest.  Biotropica 45(6):719-727.
Lydeard, C., R. H. Cowie, W. F. Ponder, A. E. Bogan., P. Bouchet, S. A. Clark, K. S. Cummings, T. J. Frest, O. Gargominy, D. G. Herbert, R. Hershler, K. E. Perez, B. Roth, M. Seddon, E. E. Strong, and F. G. Thompson.  2004.  The global decline of nonmarine mollusks.  Bioscience 54:321-330.
Solem, A.  1990.  How many Hawaiian land snail species are left? And what we can do for them.  Bishop Museum of Occasional Papers 30:27-40.