Attack of the Invasive Species: Garlic Mustard and Exotic Earthworms Affect Plant Diversity

Imagine easy-on-the-ears bluegrass melodies, an occasional summertime thunderstorm, a mama bear on the side of the road with her cubs, illuminating fireflies within the grasses at night, and vivid green forest scenery. This is a summer to remember for an Angelino city girl.

These experiences come from living at Harvard Forest, and traveling within Massachusetts and New York with my mentors Mercedes Harris and Erin Coates, two master students from the University of Massachusetts at Amherst.

All the sites that I have traveled to are characteristically similar second-growth forests that are invaded by the Eurasian herbaceous plant garlic mustard (Alliaria petiolata). Garlic mustard is an invasive plant from the Brassicaceae family. It was introduced to North America in the 1860’s. The Brassica produces chemicals within its roots and leaves. Through release of its secondary compounds, garlic mustard disrupts the relationships between fungi and water conducting native plants. Garlic mustard has no natural predators in North America, which allows for its successful high abundance. To reduce the effects of native plant displacement by invasive garlic mustard, actions to remove it have been developed, such as herbicide spraying and manually hand pulling the plant and its roots.

The effects and abundance of garlic mustard can’t be ignored, as its impacts on the native flora occur above ground, right in front our eyes. But what’s going on below the forest floor where our eyes can no longer see? What types of interactions are occurring?

Exotic earthworms are the underground invaders in North American forests. Native earthworms were removed from North American forests during the last glacial advance, 18,000 years ago. During the European settlement in the 1700’s, invasive earthworms were introduced. Invasive earthworms change the forests soils by increasing litter decomposition and thus increasing nutrient availability. In order to settle, invasive plants must be able to obtain resources such as water, light, and nutrients. Earthworms allow for invasive plant species to thrive in northern American forests by increasing nutrient availability and reducing important relationships between fungi and native vascular plants.

My questions for this summer arise from wanting to understand how the Eurasian invasive duo, garlic mustard and exotic earthworms, indirectly facilitate invasion by decreasing native plant diversity. Through my research I will seek to answer the following questions; is there a relationship between native plant diversity and earthworm density? Is there a relationship between non-native plant diversity and earthworm density?  Does earthworm density vary within invaded garlic mustard plots, non-invaded plots and hand pulled eradicated plots.

In the field, I measured an area of 50 cm X 50 cm in the north-west corner of 21, 3m x 3m  plots. After removing leaf litter from the 50cm X 50cm area I measured environmental factors such as soil moisture and temperature. To collect earthworms, I prepared a solution made out of water and mustard flour. The mustard irritates the worms’ skin causing them to escape towards the surface of the soil. I poured the mustard and water solution onto the measured area and collected the rising earthworms. The earthworms I collected were weighed and counted. To account for native plant diversity, I used previously collected abundance and cover data along with data I helped collect this summer.

Studying the relationship between above- and below- ground invasive communities is important as it can enhance a better understanding of exotic earthworm and garlic mustard effects on native plant diversity. In understanding interactions between invasive earthworms and garlic mustard, land managers can better apply eradication methods.

Karina Martinez is a senior at California State Dominguez Hills, where she is majoring in Environmental Ecology Biology. Her research interests include plant-pollinator interactions and impacts of invasive plants on native flora and fauna.