Seeing the recent discussion of Ovenbirds and invasive non-native earthworms, I thought that I should really weigh-in, given that my graduate research at the University of Minnesota highlighted the link between earthworm invasions and Ovenbirds. I am also a sometimes birder in Ohio, as I am in the Cleveland area almost monthly.


I have a few additional comments regarding the earthworm impact. First, the impacts of non-native earthworms DO appear to be most dramatic in previously glaciated areas that had no earthworms (this would include northern Ohio). However, there is plenty of research showing that non-native earthworms can cause major negative effects to the forest floor even in more southern areas that also have native earthworm species present. In many cases, the non-native species still make up the bulk of the earthworm "community". Even outside my office in Washington DC's Rock Creek Park, the forest floor is carpeted with the "middens" of Night Crawlers, all of which originate from Europe.

Second, our knowledge of the harmful effects of European earthworms (~20 species in the US) is fairly high; however, we are less certain about impacts of the Asian species (unknown # of species in US) that do not yet seem to be widespread beyond the eastern seaboard. There is reason for concern because the Asian species fill the same "niche" as the most damaging of the European species (the night crawler and smaller "red worm", also called "leaf worm"). There is still time to prevent the widespread invasion of Asian species in much of the US. Many European species are already widespread; even in remote national forests of northern Wisconsin and Minnesota, roughly 85% of the landscape has multiple non-native earthworm species present, reflecting a long history of human-assisted introduction. Finding areas without earthworms requires hiking more than a mile from the nearest paved road or logging road - basically a place that fishermen have not yet reached
 and which earthworms haven't spread to on their own. Even in these remote areas, earthworm-free patches are hard to find, because a history of logging also is related to earthworm spread.

Third, other human-related factors have been documented to negatively effect Ovenbirds in addition to earthworms, most notably, fragmentation of forest habitat. Across the eastern US, Ovenbirds are also among the most frequently killed birds from collisions with buildings, windows, and communication towers. Determining the relative importance of different threats in causing a decline is exceptionally difficult, and determining whether a decline is real or perceived is also difficult. So it's hard to say whether any anecdotal reductions in Ovenbird numbers are attributable to earthworms without designing a study to test for an earthworm effect (see next).

Fourth, any benefit of earthworms as food to ground-nesting birds is far outweighed by the dramatic changes to the forest floor habitat, which leads to reduced density of Ovenbirds and Hermit Thrushes and increased predation on nests. An effect of earthworms on abundance of insect prey is possible, but has yet to be clearly shown. To give an idea of the magnitude of the earthworm effect, I observed 25% fewer singing Ovenbirds in heavily invaded forests compared to earthworm-free forests. My study was specifically designed to test for an earthworm effect (sites were similar to each other except for earthworm abundance) so the results are likely "cleaner" than what one notices when making anecdotal comparisons. However the same pattern of decline held up when comparing invaded and earthworm-free points across two entire national forests. This suggests that the earthworm-bird link could be widespread. It would be interesting to see whether the same negative
 effect holds across the entire eastern US and if it holds for other ground-nesters (including the Worm not-eating Warbler).


Fifth, with some practice, it is easy to tell whether an area has invasive earthworms or not just by looking at the forest floor. The above-mentioned middens are a dead give-away, but characteristics of the litter layer and soil are also telling. As mentioned by John Pognacik, there is a systematic method for assessing invasions by sight, but this does require a bit of training. Our method will be published sometime next winter and others at Minnesota are currently working on training future users (land managers, botanists, field biologists, etc.), and this will improve knowledge of invasive earthworm distributions.

Finally, if you are interested in contributing to our knowledge of invasive earthworms, I invite you to visit the website of Great Lakes Worm Watch (linked below). This site administered by the University of Minnesota has a goal of documenting worm invasion distributions and impacts throughout the Great Lakes region, including Ohio. Sampling earthworms (typically using a liquid-mustard solution) is easy, extremely fun, and requires no ecological expertise...It can be done in your backyard or in your favorite birding location with no damage to the environment.

Here is the website of Great Lakes Worm Watch:
http://www.nrri.umn.edu/worms/


Specific information on contributing information about earthworm distributions:
http://www.nrri.umn.edu/worms/team/index.html

And, finally, a link to coverage of the research on earthworms and Ovenbirds (the story broke back in March):
http://smithsonianscience.org/2012/02/earthworms-to-blame-for-decline-of-ovenbirds-in-northern-midwest-forests-study-reveals/


Scott Loss
Washington DC (occasionally Cleveland Heights)

Post-doctoral Research Fellow
Smithsonian Migratory Bird Center
National Zoological Park
Washington DC

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