When describing an environment, you might not necessarily think to look right below your feet at the ground and the soil that it is made of. But it is that very soil that influences which plants can grow, where water collects, and which animals are able live there. For a fossorial animal (one that is adapted to dig), soil is an especially important predictor of where it can choose to live. The soil has to be sturdy enough to support a burrow but be of the right composition to be able to dig through without too much difficulty. Depending on the size and typical behavior of the animal, these needs can be very species-specific.
During the spring and summer, I worked on a team to collect soil samples from grassland habitats around San Diego County to create a habitat suitability model for California ground squirrels. Why do we care about ground squirrels and where they live? Ground squirrels serve as the “ecosystem engineers” of grassland environments by clearing vegetation and serving as anti-predator sentinels for some species (and dinner for others!). Their burrows also can serve as homes for other animals including the western burrowing owl, a species of concern in San Diego County. By better understanding the type of grassland habitat that ground squirrels prefer, we can also gain insight into what constitutes suitable burrowing owl habitat (see previous post Squirrels: There’s No Place Like Home for more information).
To date, we have hundreds of soil samples that are either from places where there are ground squirrel burrows or where ground squirrel burrows aren’t present. Our study focuses on the differences in soil characteristics between sites to determine if ground squirrels have a preference for particular soil types. After collecting our samples in the field, we took them to our project collaborators at the U.S. Fish & Wildlife offices in Carlsbad, who have been kind enough to train us and let us use their lab.
Components we look at include:
1) Bulk density – Using a soil corer, we collected a standard volume of soil from a standard depth below the surface. By taking the weight of the entire sample after it has been dried in an oven, we can calculate its density.
2) Percent gravel – After the density is determined, we take the whole sample and run it through a sieve (grated filter) to separate out gravel that is 2 millimeters (0.08 inches) or larger from the rest of the soil. Weighing the gravel and comparing it against the total weight of the sample gives us the percentage of the sample that is composed of gravel.
3) Soil texture – Soil is made up of essentially three components: sand, silt, and clay. Conducting a soil texture analysis allows us to classify soil samples into different categories based upon the percent composition. To figure out what percent of sand, silt, and clay make up our sample, we create a solution that contains a sample of our soil, water, and a chemical called sodium hexametaphosphate (a chemical common in most dishwashing detergents) that helps suspend the particles of soil longer than if we used water alone. We mix the resultant solution in a blender and put it in a graduated cylinder. Then we use a mini-plunger to mix the solution until it feels consistent and use a hydrometer to measure the specific gravity (relative density) of the liquid solution at different points in time: 40 seconds, 2 hours, and 3 hours after mixing.
The idea is that sand, silt, and clay settle to the bottom of the mixed solution at different rates. Sand settles to the bottom first, then silt, then clay. Using a calculation related to the ratios of measurements, we can get the percent composition of the sample.
It takes about 3.5 hours to process a single sample, so we try to have as many running simultaneously as possible. It’s a hectic day of making solutions, mixing them up, taking hydrometer and temperature readings, and keeping track of which stage in the process we are at overall for each sample. We have hundreds of samples to process, so we are excited to see what trends we find in the data after we are finished.
Susanne Marczak is a research technician with the San Diego Zoo Institute for Conservation Research.