Experiment Looks at Impact of European Brown Snail on Presidio Native Plants.

By: Jason Ericson

The European brown snail Helix aspersa was introduced to California from Europe sometime in the 1850s. It has since thrived and is considered a troublesome pest to agriculture, but little has been written about its effect on our native plants.

I worked with Mark Frey, an ecologist with the Presidio Trust, and two students from Oberlin College, Albert Davila and Rose Hartley, to design an experiment to peer into the understudied ecology of these small, but numerous herbivores. We wanted to ask if H. aspersa’s alien descent gave them some predilection for our Presidio natives--perhaps because our natives have not had the evolutionary time to develop adequate chemical and physical defenses to this relatively new invader--and to see if this army of little brown snails might be quietly subverting our restoration work.

H. aspersa is easy to distinguish from other snails in the area once you know two distinctive characteristics: It has a brown-speckled shell and a closed umbillicus. The umbillicus is the center of the shell's spiral—a closed umbillicus looks like a cinnamon roll, an open umbillicus like a donut. Together these characteristics are distinctive.

Our field experiment tested the effect of H. aspersa on the seedlings of three species of native annuals and three species of non-native annuals, selected for their taxonomic and physiological similarities to each other. We used seedlings because this stage of life is most vulnerable to snail herbivory, and we chose annuals because their survival strategy of rapid, above-ground growth, rather than conservative investment in roots like perennials, is a sort of all or nothing gambit. We planted these seedlings into a bare, sandy area of Presidio Hills within eight pairs of wire mesh enclosures. Each of these 16 enclosures contained an identical number and palette of plants, three of each species, for a total of 18 plants per enclosure. In half of the pairs of enclosures, we added 8-10 H. aspersa, the other half were controls free of snails.

When we began, we were prepared to measure the difference in the effects of snails on the growth of natives vs. non-native plants to within a volume of a half-centimeter cubed over the course of several months. The difference between the test and control plots turned out to be visible and unmistakable.

To our great surprise, after just three weeks, H. aspersa completely decimated every plant species except one in the test plots. As plants in all of the controls flourished, even poking out of the enclosures in places, snail plots were quickly cut back to a single row of G. dissectum standing alone in a patch of empty sand.

Preliminarily, our monitoring data seems to suggest that G. dissectum and another non-native, Vulpia myuros, were not preferred, and only consumed aggressively as alternatives dwindled. The third non-native, Stellaria media, was consumed at approximately similar rates to the natives we tested, Claytonia perfoliata, Daucus pusillus, and Plantago erecta.

For now, we can only confirm that snails have an effect and that this effect can be dramatic. Our results are suggestive, but we have too little data to say that H. aspersa shows any preference for native plants over non-natives. We're considering ways to refine our techniques, but this is an important step in understanding the ecology of one of the Presidio's most numerous and potentially most important herbivores.