08 November 2006

Papers read this week

Steven H. Ferguson. 2004. Effects of poisoning nonindigenous slugs in a boreal forest. Can. J. For. Res. 34: 449–455. Abstract

This is a poorly designed study. The author’s purpose "was to determine the abundance changes of soil arthropod groups associated with removal of a significant invasive group from the forest habitat and to relate abundance changes to soil arthropod diversity." So far, so good. But what invasive organism did the author pick? The "exotic Deroceras leave", a species that is usually considered to be native to North America. And he gives no explanation of why he thought D. leave may have been introduced to his study site. It may very well have been an introduced species there, but how do we know that?

Next, what method of removal of the "invasive" slug did the author pick? The poison metaldehyde, tablets of which were left at study plots. And when his results showed something he was perhaps not expecting, he was left without an explanation: "significant removal of one of these invasive groups, slugs, resulted in a decline in abundance of three other arthropod groups, Collembola, mites, and nonindigenous isopods. Reasons for these changes were not obvious." But later, he adds "Without further experimentation, metal poisons may have been toxic to these invertebrates." Duh! Shouldn’t you have tested that before the study? Wouldn’t it have made more sense to hand pick and remove the slugs periodically rather than use a poison with uncertain effects on the rest of the biota?

It is hard to believe this paper got accepted for publication. And it is harder to believe I wasted my time not only reading it but also writing about it.

Mark Blaxter, Ben Elsworth, Jennifer Daub. 2004. DNA taxonomy of a neglected animal phylum: an unexpected diversity of tardigrades. Proceedings: Biological Sciences Vol. 271, Biology Letters Supplement 4, Pages: 189 – 192. Abstract

A few years ago, Finlay1 argued in Science that that "there is little support for the idea that microbial eukaryote species are geographically restricted" and, therefore, there were probably no endemic microbial species. He concluded that "free-living microbial eukaryotes, all of which have body sizes less than about 2 mm, are probably sufficiently abundant to have worldwide distribution."

The authors of this study applied a DNA barcode system to tardigrades from urban and rural sites in southern Scotland and defined molecular operational taxonomic units (MOTU). Tardigrades are microscopic, aquatic animals that live in various semi-terrestrial habitats, such as mosses and lichens (but also in the sea). Because of their small sizes (most less than 1 mm in size) and their ability to survive drying that increases their chances of transportation by winds, most species are expected, following Finlay’s reasoning, to have a global distribution.

The authors claim that they did not observe a "global" distribution pattern when they mapped their MOTU, but instead found a disjunct distribution pattern between urban and rural sites.

What is the problem with this interpretation? They are comparing apples to oranges, that's what the problem is. The distribution of a group of microbial eukaryotes within Scotland doesn’t represent a global sampling effort. If the distribution of a group of microbial eukaryotes, especially between urban and rural sites, over an area (in this case, Scotland) that is relatively small compared to the overall global land mass is disjunct, one cannot extrapolate from this to the distribution of the group over the entire earth. But that’s exactly what the authors claim: "…our data from just three closely spaced sites suggest that tardigrades do have a biogeography." I rest my case.

1. Bland J. Finlay. 2002. Global Dispersal of Free-Living Microbial Eukaryote Species. Science : 1061-1063. Abstract



Comment sent by Tim Pearce:

I am grateful for your reviews of the interesting literature, so I don't have to read it myself!

The global microbes one sounds intriguing. I didn't look at the abstract, just your review. But as I was reading it, things like Punctum were coming to mind. Punctum minutissimum does seem to be rather widespread, at least in the USA east of the Cascade mountains out west. However, Punctum smithi seems to have a much narrower distribution. It would be interesting to look at the DNA to address whether P. smithi is indeed a separate species, or if it might be environmental variation or a local genetic variation that causes them to grow the calcium "tooth".

budak said...

Molluscs from Vanuatu for your persusal :)