Are snail-insect interactions important in tropical streams?

Background/Question/Methods

Algal growth in temperate streams is typically governed by bottom-up or top-down processes (light or nutrient limitation versusgrazing by herbivores). Food availability for grazers may be influenced by riparian shading and, in such cases, dominant grazers may reduce algal biomass and lower the abundance of other insects through competition for food. These grazers may also compete for space on the preferred substrates of particularly sessile insects. However, little is known about the importance and spatiotemporal variability of such interactions in tropical streams.

The pachychilid snail Sulcospira (= Brotia) hainanensisis is an abundant grazer in Hong Kong streams, contributing to over 75% of invertebrate biomass in some sites. It is a feeding generalist and can efficiently consume both algae and detritus, thus making it a potential competitive dominant. Snail density was experimentally manipulated in four streams (two shaded, two unshaded), by allowing snail access or excluding them from algae-colonized tiles placed on the streambed. After 4 weeks, algal biomass and insect abundance and biomass on snail-exclusion and snail-inclusion tiles were quantified. The experiments were repeated in the wet season of 2011 and dry seasons of 2011 and 2012, to incorporate flow variations associated with Hong Kong’s monsoonal climate.

Results/Conclusions

Snail grazing pressure and algal biomass showed remarkable seasonal and annual variations. During the dry seasons, algal biomass in snail-exclusion treatments was 60%-150% higher than when snails were present, but was similar among unshaded sites in 2012. There was little or no evidence of any snail grazing effect during the wet season, when flows were higher.

Taxon-specific increases in certain insects, particularly chironomids, were evident in snail-exclusion treatments, during the dry seasons under baseflow conditions. This was especially apparent in 2011, when insect grazers were more abundant and algal biomass was more limited, implying more severe interspecific competition for food. Taxa with small body sizes and limited mobility showed stronger positive responses to snail reduction, suggesting “bulldozing” interference by snails. Nonetheless, snail effects on insects during the wet season were minor.

The strength of snail-insect competition varied seasonally and annually. It was higher during the dry season when intense insect grazing could override the positive effects of light on algae, and when snail grazing intensified food limitation for insects. However, given the marked temporal and spatial variations of algal biomass and flow regime, snail-insect interactions probably play a minor role in structuring benthic communities in tropical streams.