Road Trip for Soil: Examining How Bacteria Influence Locoweed Distributions

Have you ever been on a road trip and noticed a new type of tree appear outside your car window, and then minutes later, you’re surrounded by an entire forest? As you continue driving, the number of trees dwindles, eventually giving rise to a new species. For example, in Joshua Tree National Park in California, you’ll find patches of Joshua Trees found nowhere else in the desert, despite similar conditions throughout the southwestern United States.

As a graduate student studying ecology at Loyola University Chicago, I wondered why these plants “choose” their specific locations. To find out, I hit the road and headed West to find answers.

Fit for Research

A healthy habitat for a particular plant depends on the climate (sun, rain, temperature, etc.) and the local soil ecosystem (pollinators, bacteria, decomposers, etc.). Under the right conditions, a plant “distributes across” or inhabits an area, but the boundaries that confines each distribution is unique to each plant. Sometimes, plant populations stop at obvious, natural barriers like mountains, city streets or large bodies of water, but more frequently populations seem to arbitrarily dissolve almost as quickly as they initially appear. My research explores the subtle changes that control plant distributions.

Specifically, I study a plant group called Astragalus, commonly referred to as “locoweed”. Locoweeds are in the legume family, which are common food sources for insects, but they’re also ingested by many unsuspecting species. The common name “locoweed”— derived from the Spanish word for crazy — comes from ranchers who noticed grazing livestock experiencing neurological diseases after consuming the toxic plants. Indeed, ranchers suffer $100 million in losses annually due to locoweed poisoning!

Locoweeds are generalists— they inhabit all sorts of landscapes including mountains, deserts and coasts. However, one species of locoweed, known as the freckled milkvetch (Astragalus lentiginosus) is rare to find and is so diverse in appearance that it causes botanists to disagree about just how many varieties there are. The more than 40 confirmed varieties of freckled milkvetch grow sometimes with overlapping distributions and sometimes in distinct locations, making them the perfect subject to study how environment and ecosystem shape habitat suitability.

Westward Bound

With a question and a species, I hit the road to find answers: a crazy road trip in search of a crazy plant.

Each day, I would find populations of the freckled milkvetch, and collect soil and seed samples. These small soil samples were taken at incremental distances to discover differences that influenced where plants lived. Thus, samples were collected at the base of the plants, fine-scale (about a football field away from the plants), and broad scale (over 5 miles away).

I found plants and soil samples over a huge range— from sea level in San Diego to the tops of mountains overlooking Los Angeles. I recorded the GPS coordinates of each site on a species distribution model, a map that shows where we predict there to be suitable habitat for the locoweeds based on environmental variables.

Over just one week, I logged over 1,700 miles in the car and collected over 250 pounds of soil from 360 locations— and then had to figure out how to get it all home! Lugging over 250 pounds of soil through the airport was a nightmare for TSA and for my lower back. But similar to how the locoweed’s seeds are dispersed by the wind, I too had to go with the flow—in the name of science!

Experimenting with Soil

Across the range I sampled, the soil varies in nutrient content, but also has different species of bacteria living within it. Locoweed forms a symbiotic, or mutually beneficial, relationship with some of these bacteria that helps both the plant and bacteria grow. Plants and bacteria exchange nutrients that each can exclusively produce, which enables them to thrive when together.

The presence of certain bacteria might, then, influence where locoweeds choose to live. For example, a narrow-ranged plant is likely to be restricted to small vicinities or specific locations such as mountains or dirt roads because it is confined to soil where the bacteria live.

To explore this possibility, I settled down back home in Chicago for some greenhouse work.

Using the soil and seeds I collected, I am now growing varieties of locoweed on different soils to see how plant growth depends on the composition of the soil versus their environment. This common garden experiment can test the ability of the plant to grow in environments that are within and outside its anticipated distribution. It can also show how soil bacteria from different soils influence the distribution of locoweed varieties.

As this work yields new results, we can hope to better understand our road trip scenery.