by Sean Sterrett
Animals that live in the soil of forest floors are particularly challenging to study. Think about it – in order to study soil profiles and the intricate details of everything that lives in relation to soil (i.e. ecology), and given current technology, we usually have to disturb the soil profile, giving up our ability to “see” the relationships. Truly, a Catch-22 situation. But ecologists have drawn inference and learned about organisms that live below our feet by observing, sampling and experimenting.
Terrestrial worms (Order Opisthopora; often called earthworms) are a large group of soil-dependent organisms that can be found across the earth. Among them is the family Lumbricidae, which includes many of the common and recognizable earthworms that have spread across the globe by human activity (for example, “Night Crawlers”, Lumbricus terrestris). Usually, you can go into any forest and flip a log or rock and there you have it – a slimy earthworms big enough that you might mistake it for a snake. You might be thinking that earthworms living in soil, minding their own business, wouldn’t have much of an impact on natural ecosystems. But you’d be very wrong according to wildlife and ecosystem ecologists who have been slowly understanding how measly little earthworms can impact entire forest ecosystems with effects felt all the way up into vertebrate animal populations. Firstly, non-native earthworms have the ability to outcompete native earthworms for food and other soil resources (Hendrix et al. 2008). Second, by feeding through annual leaf litter at a pace that is too rapid to replenish that critical layer of forest floors, we now know that non-native earthworms influence microbial soil processing and thus nutrient cycling (Bohlen et al. 2004). And third, and most relevant to SPARCnet, non-native earthworms may play a role as a novel food source for red-backed salamanders (hereafter RBS; Maerz et al. 2005), but also negatively affect their distribution as well by altering critical soil habitat (Maerz et al. 2009). That leaves us thinking – are most of the earthworm impacts to RBS bad?
Despite being among the most cosmopolitan of salamanders in the Eastern U.S., RBS are reliant upon a fairly specific habitat; intact, contiguous forests. Why? Because intact (and presumptively “healthy”) forest keeps the soil cool and moist for much of the year, especially in the Spring and Fall, when salamanders are at the ground surface (instead of underground) doing salamander activities (courting, breeding, eating, laying eggs, taking in the sights, etc.). More fundamentally, RBS are lungless, fully terrestrial organisms and need to stay cool and moist to respire through their skin. The cyclical deciduous trees provide an annual layer of leaves that slowly breaks down by the actions of insects, bacteria and fungi; eventually creating a layer of proto-soil (called the humus layer), which many organisms, including RBS, rely on for critical habitat. And this, in fact, is how earthworms play their biggest role to influence RBS (and everything else that relies on natural forest floors). Worms make their living by eating through soil and quickly digesting organic materials and ultimately modifying soil structure. This wouldn’t be much of a problem in a forest with a “normal” density of native worms. However, when non-native earthworms invade and begin to reproduce, they’re able to become incredibly dense. So, tons of earthworms ends up leaving the forest floor with drastically altered soil habitat. The direct and indirect effects of earthworms on vertebrates, like RBS, are still being pursued by ecologists. So, that’s a bad thing that earthworms do to RBS…are there any upshots to having worms around? There have been a series of studies that have investigated how RBS may benefit from the presence of earthworms, an organism often referred to as an ecosystem engineer, because it makes its living by burrowing through soil and completely modifying it (Caceres-Charneco and Ransom 2010, Ransom 2011, Ransom 2012). Dr. Tami Ransom and her colleague have suggested that salamanders benefit from the ecosystem engineering of earthworms because the big underground paths created by earthworms create refuge for RBS. While this is a cool and intuitive finding, there are likely nuances to these relationships, which will require further study.
So, how did European earthworms get into your backyard and local park in the first place? The same way most widely distributed, non-native species moved around the earth; by humans actively or accidentally moving them there. If you grew up fishing, it’s likely you’ve seen small plastic containers full of worms that your mom, dad or uncle bought for you to fish with. You go out fishing for the day; catch a few, lose a few. But as the day ends, you’re done fishing and the rest of those worms get tossed in the forest. This is one of the primary ways that a single species, (Lumbricus terrestris; Hendrix et al. 2008) has successfully been introduced to and is thriving in all types of areas outside its native range. Invasive species are one of the top threats to native biodiversity. There likely isn’t much you can do to influence non-native earthworms now, but there is a possibility for you to be a better steward of forested ecosystems by learning about and educating others on the impacts of non-native, and invasive, species on our native biodiversity.
- Hendrix, P.F., M.A. Callaham Jr., J.M. Drake, C. Huang, S.W. James, B.A. Snyder and W Zhang. 2008. Pandora's Box contained bait: The global problem of introduced earthworms. Annual Review of Ecology Evolution and Systematics 39:593-613.
- Maerz, J.C., J.M. Karuzas, D.M. Madison and B. Blossey. 2005. Introduced invertebrates are important prey for a generalist predator. Diversity and Distributions 11:83-90.
- Maerz, J.C., V.A. Nuzzo and B. Blossey. 2009. Declines in woodland salamander abundance associated with non-native earthworm and plant invasions. Conservation Biology 23:975-981.
- Cáceres-Charneco, R.I. and T.S. Ransom 2010. The influence of habitat provisioning: use of earthworm burrows by the terrestrial salamander, Plethodon cinereus. Population Ecology 52:517-526.
- Ransom, T.S. 2011. Earthworms, as ecosystem engineers, influence multiple aspects of a salamander’s ecology. Oecologia 165:745-754.
- Ransom, T.S. 2012. Comparison of direct, indirect, and ecosystem engineering effects of an earthworm on the red-backed salamander. Ecology 93:2198-2207.