Autumn is harvest time and this is true for animals as well as for people. Local oak trees, from majestic live oaks to many small stature scrub oaks, are heavy with acorns, and you’ve probably noticed enterprising squirrels burying those acorns. Acorns are an important food for wildlife; bears, birds, and deer all eat acorns. In some cultures, acorns also are an important food source for people; acorns are easy to collect and rich in calories thus are an efficient food. Once on the ground, acorns are protected from deterioration by chemicals called tannins and their thick husk. Because they store so well, many animals (including humans) store them to eat later, especially over the winter when other types of food are less abundant.
Like squirrels, Florida Scrub-Jays store acorns underground, a behavior called caching. Archbold Biological Station scientists observed that single scrub-jays cached between six and eight thousand acorns collected from several species of Florida scrub oaks. Every fall, each acorn is individually buried and hidden in the sand. Although Florida Scrub-Jays have a diverse diet of insects, small animals like frogs and lizards, and plant foods, acorns are their most important food during the winter and early spring.
Because acorns are an important resource to Florida Scrub-Jays, researchers studying the jays at Archbold are interested in the number and distribution of the acorns that are produced each year. Florida Scrub-Jays defend home territories and the resources on them. Every year scientists conduct acorn surveys within these territories to see how many acorns are available in a given year, and estimate how many acorns each territory holds.
From August through September, when the acorns are mature and still on the oaks, members of the Avian Ecology program don their plant ecologist hats and conduct acorn counts. At each plot, they measure a sample of permanently tagged oaks from 5-6 species, recording their shrub height of approximately 2 to 10 feet. Scientist also record how many stems they have, and the number of acorns each bears. This provides information about oak shrub survival, growth, and reproduction, as well as estimates of the acorn resources available to scrub-jays and other animals.
In 2018, researchers recorded over 7,000 acorns on just under 200 plots. A typical year is approximately 10,000 and the best year on record was 26,500. The variation in acorn abundance from year to year can pose problems. Dr. Reed Bowman, Director of the Avian Ecology Program at Archbold Biological Station notes that, “Acorn abundance varies tremendously from year to year. During cool, wet winters when other foods, such as insects, are scarce, a poor acorn crop can lead to lower overwinter survival in the jays.”
Just as the oaks provide an important resource for scrub-jays and other animals, animals that cache acorns provide a service to the oaks. Jays retrieve only about 30% of the acorns they cache in the sand and acorns that are not eaten may eventually sprout and grow. By dispersing acorns away from the parent plant, animals can help plants colonize new areas and ensure that seeds are not shaded out by their parent plant.
By studying systems like caching, we gain insights into both plants and animals and how the environment influences both, and their interactions. Long-term data are a hallmark of biological field stations like Archbold, and provide insights into how relatively small and slow changes,, affect not only populations, but communities and ecosystems. By studying these interactions, we can help preserve scrub oaks, the jays, and all the other plants and animals in the Florida scrub that interact and rely on both.
Written by Dr. Angela Tringali