06 December 2005

Do overpopulated deer influence forest snail populations?

Last Sunday's post by Nuthatch at bootstrap analysis reviewed a recent paper by Allombert et al.,1 on the impact of overabundant deer on forest invertebrates, mainly arthropods, but also on land mollusks (gastropods). Nuthatch's emphasis was on the potential indirect and negative effects of overabundant deer on migratory songbirds. I cannot comment on that aspect, nor on the conclusions of the study regarding arthropods. Instead, I will criticize only the results obtained with snails and slugs.

The subject study1 was carried out on the Haida Gwaii archipelago (Queen Charlotte Islands) 80 km off British Columbia, Canada, where Sitka black-tailed deer (Odocoileus hemionus sitkensis) were introduced in 1878. The authors determined the abundances of various arthropods and land mollusks on 6 islands, 2 of which did not have deer. Invertebrates were collected with pitfall traps consisting of "two plastic cups fitted together with a light gray ceramic tile supported by small sticks as cover". To kill anything that fell in them, the traps were filled with a 1:1 mixture of water and ethylene glycol with a few drops of detergent.

The authors determined that "gastropod species density (significantly) and abundance (markedly but not significantly) decreased with increasing [deer] browsing history". I have 2 objections to this conclusion.

1. The most commonly used, and perhaps the most effective, land snail collection method is leaf litter and soil sieving. (This is also the most labor intensive and time-consuming method.) I have never used pitfall traps for snails and I don't know how efficient they are, compared to soil sieving, to determine the land snail fauna of an area. Arthropods presumably do fall into pitfall traps, but snails and slugs, on the other hand, may simply crawl out of the trap before they contact the liquid on the bottom, especially if they are repelled by ethylene glycol. (I don't know how snails and slugs react to ethylene glycol.)

The point is that the study may have missed some of the snail and slug species present on the islands. I am assuming nobody had surveyed before for land snails on the Haida Gwaii archipelago and so there is probably no other survey data to compare with. (The islands sound like wild, isolated places. I would love to visit them one day).

2. They only identified 9 gastropod species, but, unfortunately, they don't give a full list of names. Nor do they tell which species were found on which islands. I would like to know if any of the gastropod species were themselves introduced to the islands by humans, because if they were, then this would be a factor in determining their presence or absence on a particular island.

My main problem is that 9 species is a low baseline. If one island without deer had 9 species of snails and another with deer had, say, 5 species, could one really be confident that the difference is due to deer? Maybe, maybe not.

The authors cite a study by Suominen2 that obtained somewhat similar results. In the cited study, Suominen studied the effects of reindeer and moose grazing on land mollusks in the Finnish Lapland in 23 paired plots each up to 30x30 m. In each pair, one plot was fenced to keep large animals out. One confounding factor is that the fences had been placed at different dates, one dating from the 1940s, while others from the late 1980s. Suominen too used pitfalls to collect gastropods, although he was aware that pitfall trapping “is not the optimal method for sampling terrestrial gastropods”. Thirteen species of gastropods were found, but less than 20 individuals were collected for each of 6 species3. Of the remaining 7 species, the abundances of 2 did not differ significantly between grazed and ungrazed plots, 4 species were more abundant on ungrazed plots and one species (Zoogenetes harpa) was actually more abundant on grazed plots. From these results, I would hesitate to derive any generalizations as to whether or not grazing by large mammals is impacting land mollusk populations.

Nevertheless, what these studies demonstrated may indeed be true as far as the interactions of large grazing mammals and the snail faunas of high latitude forests are concerned. But I don't think they are relevant, as far as snails and slugs are concerned, for lower latitude forests, in which I am mostly interested.

In 2nd growth Maryland forests, where the white tailed deer (Odocoileus virginianus) is native and overabundant, I routinely find more than 20 species of snails and slugs with the total approaching 30 in good places. Of course, we have no way of knowing which species lived in the virgin forests that were there 200 years ago. But based on the distribution patterns of all species and some old records, I would guess that the land mollusk species compositions of virgin forests were probably close to what we find today in 2nd growth forests. In other words, deer overabundance hasn't probably had any effect on snails and slugs. In any case, even if we knew that there were differences in the faunal compositions of 2nd and old-growth forests, it would be difficult to single out the responsible factors.

Moreover, most snail and slug species of 2nd growth forests concentrate on and around deadwood (large fallen trees or snags), where they find both shelter and food (rotting wood and fungi). I can't think of a mechanism how deer overabundance could affect the quality and quantity of large dead trees.

Note added 7 December 2005: While I was writing this post yesterday, I sent an e-mail to Otso Suominen and asked for a copy of his 1999 paper. Later I went ahead and posted what I had written without waiting for his response. Suominen has since kindly sent me pdf copies of not one, but 2 of his relevant papers. Therefore, today I have revised parts of my post dealing with Suominen's study. My conclusions from yesterday remain unchanged.

For another update, please read this post.

Appreciations to Nuthatch at bootstrap analysis for bringing the paper by Allombert et al., to my attention and Tim Pearce for quickly sending me a pdf copy of it and Otso Suominen for copies of his papers.

1. Sylvain Allombert, Steve Stockton & Jean-Louis Martin. A Natural Experiment on the Impact of Overabundant Deer on Forest Invertebrates. Conservation Biology 19:1917-1929
2. Suominen, O. 1999: Impact of cervid browsing and grazing on the terrestrial gastropod fauna in the boreal forests of Fennoscandia. Ecography 22: 651-658.
3. I am hesitant to attach any significance to results obtained with less than about 20 individuals, which is obviously an arbitary cut-off point.


pascal said...
This comment has been removed by a blog administrator.
pascal said...

Oops - didn't see you already had that the Suominen '99 paper already.

pascal said...

I've got a copy of that '99 Suominen paper, but no scanner - I could mail you a paper copy if you'd like.


I sent an e-mail to Suominen this morning & asked for a copy of it. If I don't get it from him, I'll bother you. Thanks.

bnorden said...

I can add a comment on the use of pitfalls for snails. I am well into a survey of the molluscs of a large tract at Cove point in Calvert County, Maryland. Must previous survey work has been done at the same site. The small mammal survey made extensive use of pitfall traps and no snails or slugs were taken, even though they are abundant in the area.