Biotropica 45(6) Editor’s Choice: Snails, Slugs, & Rain Forest Nutrient Cycling
I’m pleased to announce the November 2013 Editor’s Choice article: Wallace M. Meyer III, Rebecca Ostertag, and Robert H. Cowie. 2013. Influence of Terrestrial Molluscs on Litter Decomposition and Nutrient Release in a Hawaiian Rain Forest. 45(6): 719-727.
I was really impressed by the article’s experimental approach and the insights it provides into how decomposition and nutrient cycling are influenced by some of the less charismatic (sorry, Wallace) organisms in tropical forests…slugs and snails. As you will read below, they may not be cuddly and captivate the public’s interest in ways other tropical animals do, but they play a critical and underappreciated role in the functioning of these ecosystems. Dr. Meyer to explains in his essay below. EB
Terrestrial molluscs (snails and slugs) of the islands of the Pacific are recognized for their spectacular diversity and high levels of endemism but also because they are under extreme threat, with many species already extinct. For example, the native Hawaiian fauna used to be extremely diverse (over 750 species) with over 99 % of the species endemic. Tragically, the majority of these species are now extinct, with estimates of extinction ranging from 65 to 90 % (see Figure 1). These native/endemic species are being replaced by a smaller number of widely distributed non-native species. For example, many of the non-native molluscs in our study are now established on many other Pacific islands. Starting from this perspective, we designed an experiment to quantify the role of the most abundant species (native and non-native) in litter decomposition and nutrient cycling in a Hawaiian rainforest in order to begin to assess how these changes are influencing the functioning of ecosystems.
Studying the influence of these animals on ecosystem processes in Hawaii is not only important because their communities are undergoing major compositional changes. Recent studies suggest that decomposer communities, and macro-invertebrates (invertebrates > 2.0 mm) in particular, have a significant influence on rates of decomposition in the tropics. However, the effects of macro-invertebrates on litter decomposition are often ignored because the experimental bags containing the litter are typically constructed with mesh that prevents macro-invertebrate access to the litter. Our litter bags were constructed using plastic berry containers that provided a three-dimensional frame facilitating mollusc access (see Figures 2-3). Previous studies examining the role of snails and slugs in litter decomposition and nutrient release were conducted in temperate regions, so our study in a tropical forest is also an important contribution to understanding how these forests function.
We used 42 field mesocosms each containing 10 litter bags (see Figure 4). We selected five mollusc species (one native and four non-native) and added individuals of one, at its estimated field density, to each of seven mesocoms. The remaining seven were controls with no snails. We chose this design to be able to assess the influence each species separately.
We tested three hypotheses: (1) presence of abundant terrestrial mollusc species increases rates of leaf litter decomposition; (2) different mollusc species influence the rates of nutrient release differently, and (3) terrestrial molluscs facilitate recruitment of meso-invertebrates (invertebrates between 100 μm and 2 mm in width). This third hypothesis was examined since ingestion of litter by molluscs is generally thought to be less important than indirect effects (modification of the habitat to facilitate either micro-arthropod or microbial recruitment and/or activity; see Figure 5).
Our study demonstrates the importance of these molluscs in litter decomposition and nutrient release. Only one of the 35 experimental mesocosms had less litter mass loss than the control – molluscs increased rates of litter decomposition. In addition, our estimate of the effects of molluscs on litter decomposition is conservative because the total mollusc biomass in each mesocosm (which had only one species at its mean field density) was well below natural levels of overall molluscan biomass. Although significant differences in nutrient concentration and quantities for a few individual nutrients were observed, overall, our findings suggest that different mollusc species have largely similar effects on nutrient concentrations and quantities. However, future studies that explicitly control for mollusc biomass are needed to understand these processes in more detail.
Although we hypothesized that molluscs may modify the habitat in ways that increase recruitment of other invertebrates that can also facilitate litter decomposition, none of our experimental treatments had significantly higher abundances of non-mollusc invertebrates than the controls. Because there were no increases in abundance of other invertebrate groups or any evidence of consumption of the litter, the differences among treatments in the mesocosms were probably a result of molluscs facilitating microbial growth. Because all mollusc species, regardless of trophic position (one species was a facultative predator) or biogeographical origin, facilitated litter decomposition, it seems possible that commensalism among molluscs and microbes is driving increases in decomposition rates.
To what extent then does changing the mollusc composition have on the functioning of a Hawaiian rainforest? In our study site, the most abundant species are all introduced, whereas native snails are comparatively rare; for example, the only native snail in our study had the lowest density of the five species. This then raises the awkward conservation question: are these abundant non-native mollusc species benefitting native ecosystems by maintaining important ecological processes that were once carried out by native species?
Unfortunately, we are missing important pieces of information to answer this question (e.g. species composition, richness and densities of molluscs in historical communities). Nonetheless, litter decomposition and nutrient release were correlated with mollusc biomass and were not species specific. As such, maintaining adequate mollusc biomass may be essential for maintaining a healthy functioning ecosystem.
Wallace Meyer, Department of Biology, Pomona College, Claremont, CA, U.S.A