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.
