Acorns aren’t only for squirrels. They serve as a food source for a variety of wildlife, such as mice, deer, and turkeys. This presents somewhat of a problem for oak trees – acorn producers – because their future depends on acorns surviving and germinating to become the next generation.
A recent study by USDA Forest Service research ecologist Cathryn Greenberg and mathematical statistician Stan Zarnoch investigated a number of questions about relationships between acorns and predators. Their results were published in the Canadian Journal of Forest Research.
Greenberg and Zarnoch used scarlet oak and white oak acorns to test the predator satiation hypothesis (PSH). The hypothesis says that “masting,” or boom and bust seed production shown by some plant species – in this case oaks – is a strategy that allows at least some seeds to escape predation.
In effect, this is a “safety in numbers” tactic, since the predators can only eat so many acorns. “If the trees have a bad acorn producing year, the mice and predators will have a hard time surviving or reproducing successfully, and so the next year there would be fewer acorn predators,” says Greenberg. “Then, if there’s a good acorn crop the following year, there would be fewer predators and more surviving acorns.”
The researchers set up their experiment to answer four additional questions:
- Are primary vertebrate acorn predators the size of mice, squirrels, or deer?
- Does the abundance of mice rely on prior-year acorn production?
- Are red or white oak acorns more popular with vertebrate predators?
- Does acorn removal rate between years depend on prior- or current-year acorn production?
The experiment took place in the Bent Creek Experimental Forest in western North Carolina. There, the scientists used nine randomly placed study blocks. Each block contained exclosures that only allowed in mice, exclosures that allowed in mice and mid-sized vertebrates such as squirrels or jays, and completely open controls that allowed vertebrates of any size to enter, including deer and turkeys. The scientists used different mesh sizes for the exclosure types to control the size of the vertebrates entering them.
Next, the scientists marked and placed equal numbers of acorns from the red and white oak groups – in this case, from the scarlet oak and white oak trees, respectively – in the exclosures. Throughout the study, they removed unmarked, “natural” acorns.
In order to obtain acorn counts and removal rates, they counted the number of acorns in each block at regular intervals every year. Each fall, when new acorns started falling, the cycle started again with new experimental acorns.
The scientists also measured the natural acorn yield for oak trees throughout the research site. They collected acorns in traps and counted each year’s total to determine the average crop size. Mouse populations were also estimated annually using traps placed in grids at several locations throughout the forest.
The scientists found that acorns were removed from the exclosures faster during years of low production than in years of ample acorn production. These results generally supported the predator satiation hypothesis.
The study yielded insight on the other research questions as well. “Of the exclosures that only mice could enter, all of the acorns were eventually removed, but much more slowly compared to the other exclosure type or open controls. We found no difference in removal rates between the exclosures with medium (squirrel-sized) mesh and the open controls,” explains Greenberg. “That meant the large acorn predators, such as deer and turkey, weren’t the biggest source of predation pressure. Mice plus medium sized acorn predators, such as squirrels and chipmunks, were the most important acorn predators.”
“In our study, mouse populations were negatively affected by the size of acorn crops only after a complete crop failure, not if crop sizes were good or moderate,” continues Greenberg. “In addition, we also saw no predator preference for white versus red oak acorns; both types were removed at similar rates in all but two years, in which red oak acorns were removed more quickly. This was somewhat surprising, since red oak acorns are generally higher in tannins – a bitter-tasting organic substance – and are therefore thought to be less tasty to wildlife.” However, she noted that the study couldn’t determine whether red or white oak acorns were eaten or cached after being removed from the exclosures. It could only determine that they were removed.
Lastly, the study found a relationship between acorn crop size and removal rate the following year. A larger crop size one year would likely be associated with a faster removal rate the following year – presumably because acorn predator populations were higher – and vice versa.
Studying acorns and the relationships between them and their predators is worth the effort. “Acorns are considered by some to be a keystone forest species: they feed animals like mice and squirrels; in turn, hawks, owls, foxes, and coyotes eat the mice!” adds Greenberg. “In a way, acorns are a foundation for the food chain, at least in mixed-oak forests.”
For more information, email Cathryn Greenberg at firstname.lastname@example.org.