Invasion of the Earthworms

May 9, 2017

Photo by Rob Hille licensed under CC BY-SA 3.0

As an avid gardener, amateur fisherman, and a descendant of a long line of farmers, I have always held earthworms in high regard. These little ecosystem engineers are great for all of the above, right?

Not so fast! Things in life are never that simple! Let's start at the beginning. If you live in an area of North America where the glaciers once rested, there are no native terrestrial worms in your region. All of North America's native worm populations reside in the southeast and the Pacific northwest. All other worms species were wiped out by the glaciers. This means that, for millennia, northern North America's native ecosystems have evolved without the influence of any type of worms in the soil.

When Europeans settled the continent, they brought with them earthworms, specifically those known as night crawlers and red wigglers, in the ballasts of their ships. Since then, these worms have been spread all over the continent by a wide range of human activities like farming, composting, and fishing. Since their introduction, many forests have been invaded by these annelids and are now suffering heavily from earthworm activities.

As I said above, any areas that experienced glaciation have evolved without the influence of worms. Because of this, forests in these regions have built up a large, nutrient-rich, layer of decomposing organic material commonly referred to as "duff" or "humus." Native trees, shrubs, and forbs rely on this slowly decomposing organic material to grow. It is high in nutrients and holds onto moisture extremely well. When earthworms invade an area of a forest, they disrupt this rich, organic layer in very serious ways.

Worms break through the duff and and distribute it deeper into the soil where tree and forb species can no longer access it. Worms also pull down and speed up the decomposition of leaves and other plant materials that normally build up and slowly create this rich organic soil. Finally, earthworm castings or poop actually speed up runoff and soil erosion.

All of this leads to seriously negative impacts on native ecosystems. As leaves and other organic materials disappear into the soil at an alarming rate via earthworms, important habitat and food is lost for myriad forest floor organisms. In areas with high earthworm infestations, there is a significant lack of small invertebrates like copepods. The loss of these organisms has rippling effects throughout the ecosystem as well. It has been shown that, through these activities, earthworms are causing declines in salamander populations.

It gets worse too. As earthworms speed up the breakdown of the duff or humus, our native plant species are suffering. They have evolved to germinate and grow in these rich, organic soils. They rely on these soils for survival. As the nutrient rich layers get redistributed by earthworms, native plant and tree populations take a hit. Spring ephemerals have been hit the hardest by earthworm invasions for these reasons and more. There is very little recruitment and, in time, many species are lost. For small seeded species like orchids, earthworms can even consume seeds, which either destroys them outright or drags them down deeper into the soil where they cannot germinate. Earthworms have also been shown to upset the mycorrhizal fungi networks which most plant species cannot live without.

Top Left: Forest soil horizons without earthworms; Top Right: Forest soil mixed due to earthworms; Bottom Left: Forest understory diversity without earthworms; Bottom Right: Forest understory diversity with earthworms. Credits: [1]

So, what can we do about this? Well, for starters, avoid introducing new populations of earthworms to your neighborhood. If you are using earthworms as bait, do not dump them out onto land when you're done. If you must get rid of them, dump them into the water. Also, if you are using worm castings in your garden, it has been recommended that you freeze them for about a week to assure that no eggs or small worms survive the ride. If you are bringing new plants onto your property, make sure to check their root masses for any worm travelers. Remember, no worms are native if you live in a once glaciated region.

Sadly, there is not much we have come up with at this point for dealing with the current earthworm invasion. What few control methods have been developed are not practical on a large scale and can also be as upsetting to the native ecology as the earthworms. The best bet we have is to minimize the cases of new introductions. Earthworms are slow critters. They do not colonize new areas swiftly. In fact, studies have shown that it takes upwards of 100 years for earthworm populations to migrate 1/2 mile! Armed with new knowledge and a little attention to detail, we can at least slow their rampage.

Further Reading: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Invasive Beauty

September 15, 2015

The honeycomb ginger (Zingiber spectabile) produces one of the most stunning inflorescences I have ever seen. Sadly, it is this beauty that has led to its introduction far outside of its native range of Southeast Asia. As a result, it has become an aggressive invader in many tropical regions.

Invasive Ants Destroy Plant Sex Lives

July 28, 2015

Photo by Lalithamba licensed under CC BY 2.0

For all of the amazing symbioses ants and plants share, there is one thing ants seem to get in the way of... plant sex. That's right, plants have found a use for ants in pretty much every way except for when it comes to reproduction (with some exceptions of course). Ants being what they are, they can easily become a force to be reckoned with. For this reason, many plant species have co-opted ants as defense agents, luring them in with nectar-releasing glands, a resource that ants guard quite heavily.

When it comes to flowering, however, ants can become a bit overbearing. Research done at the University of Toronto shows that the invasive European fire ant has a tendency to guard floral nectar so heavily that they chase away pollinators. By observing fire ants and bumblebees, they found that ants change bumblebee foraging behaviors. The fire ants often harassed and attacked bumblebees as they visited flowers, causing them to spend significantly less time at each flower, a fact that could very well result in reduced pollination for the plant in question.

This reduction in pollination is made even more apparent for dioecious plants. Since ants are after nectar and not pollen, male flowers received more bumblebee visits than nectar-producing female flowers. This could become quite damaging in regions with heavy fire ant infestations.

As it turns out, the ants don't even need to be present to ward off bumblebees. The mere scent of ants was enough to cause bumblebees to avoid flowers. They apparently associated the ant smell with being harassed and are more likely to not chance a visit. Of course, this study was performed on using an invasive ant species. Because so many plant species recruit ants for things like protection and seed dispersal, it is likely that under natural conditions, the benefit of associating with ants far outweighs any costs to reproductive fitness. More work is needed to see if other ant specie exhibit such aggressive behavior towards pollinators.

Photo Credit: Lalithamba (https://www.flickr.com/people/45835639@N04)

Of Grass and Spiders

July 15, 2015

Spiders, toads, and grass. These three organisms seem kind of strange placed together in a single sentence. It would seem that the presence of each would have, if anything, marginal effects on the other. In healthy forests full of native species, this is the case. However, when new players enter the game, things are bound to change. As John Muir once said, “When we try to pick out anything by itself, we find it hitched to everything else in the universe.” As we grow to understand the natural world that we live in, the reality of this statement only becomes more apparent.

The new player in this case is a grass. Microstegium vimineum, commonly referred to as Japanese stiltgrass, was introduced to the US sometime around 1919. Since then it has spread to over 16 states and is especially abundant in the southeast. It invades disturbed habitats and forms dense mats, which can completely displace native vegetation. It quickly rises to monoculture status and it is on the move. It is only a matter of time before it spreads well into the north.

It’s not just vegetation that gets displaced either, most native insects don’t feed on M. vimineum. A monoculture of this grass is almost devoid of an insect community. However, there is one group of creepy crawlies that seems to have benefited from M. vimineum invasions. Wolf spiders are voracious predators. They eat a wide variety of insects and are certainly not above cannibalism. The dense carpets formed by M. vimineum offer security for wolf spiders. They can avoid one another and thus rise to abundance wherever this invasive grass grows. What few insects live in these stands quickly get gobbled up by the spiders. This is bad for yet another member of the forest community, the American toad.

It has been noticed that, in forests where M. vimineum dominates, toads are on the decline. It was long thought that the lack of prey insects was the cause but recent research has pointed to a different culprit, the wolf spiders themselves. Aside from eating what little food can be found in the carpet of grass, they are also dining on young toads. Spider depredation on toads seems to be rather routine among the grass, so much so that toad survival decreased by 65% in these areas. To make matters worse, the effects of the invasive grass seem to be at their worst in areas that were once the best forests for toad survival.

These findings are startling but by no means unique. The researchers are now going to look to see if this is happening to other amphibian species as well. At face value, it is not apparent how an invasive grass could affect toads but it is likely that instances like this are far more common than we even realize.

Photo Credit: JAYNA L. DEVORE