When Acorn Masts, Rodents, and Lyme Disease Collide

“‘Mast years’ is an old term used to describe years when beeches and oaks set seed. In these years of plenty, wild boar can triple their birth rate because they find enough to eat in the forestes over the winter… The year following a mast year, wild boar numbers usually crash because the beeches and oaks are taking a time-out and the forest floor is bare once again.” — The Hidden Life of Trees by Peter Wohlleben

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When a plant population’s annual production of seeds is highly variable and synchronous, it is considered a masting or mast seeding species. Why and how masting happens is a bit of a mystery, and efforts are underway to better understand this phenomenon. One thing is clear, boom and bust cycles can have dramatic effects on animals that use the fruits and seeds of these plants for food. Acorn production in oaks provides a stark example. As Koenig, et al. describe in Ecology (2015), a “variable acorn crop initiates a ‘chain reaction’ of responses that cascades through the ecosystem, affecting densities of deer, mice, ground-nesting birds, gypsy moths, and the tick vectors of Lyme disease.” The connection between mast seeding oaks and the prevalence of tick-borne pathogens is of particular interest considering the risks posed to humans.

Lyme disease is an infectious diesease caused by a bacterium vectored by ticks in the genus Ixodes. The life-cycle of a tick is generally 2 to 3 years, beginning after a larva hatches from an egg. From there the larva develops into a nymph and later an egg-laying adult, taking a blood meal each step of the way. Tick larvae feed on the blood of small rodents and birds, which is where they can pick up the bacterium that causes Lyme disease. After feeding, they develop into nymphs and go in search of another blood meal, perhaps another rodent or maybe something larger like a deer or a human. It is in their nymphal and adult stages that ticks transmit Lyme disease to humans. Nymphs tend to transmit the disease more frequently, partly because they go undetected more easily.

The risk to humans of being infected with Lyme disease varies year to year and is dependent largely on how many infected ticks are present. For this reason, it is important to understand the factors affecting the density of infected nymphs. In a study published in PLoS Biology (2006), Ostfeld, et al. collected data over a 13 year period in plots located in deciduous forests in the state of New York, a hotspot for Lyme disease. The predictors they considered included temperature, precipitation, acorn crop, and deer, white-footed mouse, and chipmunk abundances. Deer abundance and weather conditions had long been considered important in predicting the prevalence of ticks, but little attention had been paid to small mammals – the larval hosts for ticks – and the variability of acorn crops – an important food source for rodents.

deer tick (Ixodes scapularis) — via PhyloPic; user Mathilde Cordellier

The results of their study revealed a clear pathway – more acorns leads to more rodents which leads to more Lyme disease carrying nymphs. The process takes a couple of years. First, oak trees experience a mast year, flooding rodent populations with food. In the following year, the numbers of mice and chipmunks is unusually high. The year after that, there are lots and lots of nymphal ticks infected with Lyme-disease. The relationship is so direct that Richard Ostfeld claims, based on his research, that he can predict the incidence of Lyme disease among residents of New York and Connecticut based on when a mast year occurs. In a summer when there is an abundance of 2 -year-old oak seedlings in the surrounding forests, expect the infection rate of Lyme disease to be high.

Lyme disease also occurs in regions where oak trees are not present or are uncommon, so variability in acorn crops isn’t always the best predictor. The researchers acknowledge that acorn abundance is not going to be “a universal predictor of risk;” instead, anything that leads to an increase in rodent populations, whether it is some other food source or a lack of predators, may be a key indicator since rodents are reservoir hosts of Lyme disease.

A study published in Parasites and Vectors (2020) looked at the effects of rodent density on a number of tick-borne pathogens. They confirmed that an “increase in rodent density positively affects populations of nymphal ticks in the following year;” yet, they could not confirm that rodent density is the sole predictor of disease risk. Other factors come into play depending on the disease in question, and further research is needed to improve models that predict tick-borne diseases. They did, however, confirm that, by flooding the food supply with acorns, mast years can boost populations of a variety of rodents.

white-footed mouse (Peromyscus leucopus) — via wikimedia commons; USGS

A fear of ticks is justified. They suck your blood after all, and besides that, they can transmit some pretty serious diseases. Arm yourself by educating yourself. One place to do that is with The Field Guides podcast. Their tick two-parter is well worth the listen (part one and part two). Not only will it give you valuable information in protecting yourself against ticks, it may also give you an appreciation for their prowess. Just maybe. See also their You Tube video demonstrating how to sample for ticks.

To Fruit or Not to Fruit – The Story of Mast Seeding

Perennial plants that are able to reproduce multiple times during their lifetime don’t always yield the same amount of seeds each time they reproduce. For some of these plants, there is a stark difference between high-yield years and low-yield years, with low-yield years outnumbering the occasional high-yield years. In years when yields are high, fruit production can seem excessive. This phenomenon is called masting, or mast seeding, and it takes place at the population level. That is, during a mast year, virtually all individuals in a population of a certain species synchronously produce a bumper crop of seeds.

Plants of many types can be masting species. Bitterroot milkvetch (Astragalus scaphoides) and a tussock grass known as Chionochloa pallens are masting species, for example. However, this behavior is most commonly observed in trees, notably nut producing trees like oaks, beeches, and pecans. As you might imagine, the boom and bust cycles of mast seeding plant populations can have dramatic ecological effects. Animals that eat acorns, for example, are greeted with a veritable buffet in a mast year, which can increase their rate of reproduction for a spell. Then, in years when acorns are scarce, the populations of those animals can plummet.

How and why masting happens is not well understood. It is particularly baffling because masting populations can cover considerably large geographic areas. How do trees covering several square miles all “know” that this is the year to really go for it? While a number of possible explanations have been explored, there is still much to learn, especially since so many different species growing in such varied environments exhibit this behavior.

A popular explanation for mast seeding is predator satiation. The fruits and seeds of plants are important food sources for many animals. When a population of plants produces fruit in an unusually high abundance, its predators won’t possibly be able to eat them all. At least a few seeds will be left behind and can sprout and grow into new plants. By satiating their predators they help ensure the survival of future generations. However, even if a plant species has evolved to behave this way, it still doesn’t explain how all the plants in a particular population seem to know when it’s time for another mast year.

Predator satiation is an example of an economy of scale, which essentially means that individual plants benefit when the population acts as a whole. Another economy of scale that helps explain masting is pollen coupling. This has to do with the timing of flowering in cross pollinating species. If individuals flower out of sync with one another, the opportunities for cross pollination are limited. However, if individuals in a population flower simultaneously, more flowers will be pollinated which leads to increased fruit and seed production.  For this to happen, there are at least two factors that come into play. First, the plants have to have enough resources to flower. Making flowers is expensive, and if the resources to do so (like carbon, nitrogen, and water) aren’t available, it won’t happen. Second, weather conditions have to work in their favor. Timing of flowering depends, not only on daylength, but on temperature, rainfall, and other local weather conditions. If individuals across a population aren’t experiencing similar weather, the timing of their flowering may be off.

pollen-producing (male) flowers of pecan (Carya illinoinensis) — via wikimedia commons; Clemson University

Resource matching and resource budgeting are other proposed explanations for masting. Since plants can only use the resources available to them for things like growth and reproduction, they vary each year in how much growing or reproducing they do. Theoretically, if plants in a population are all going to flower in the same year, they all have to have access to a similar amount of resources. Often, the year following a mast year, there is a significant drop in fruit production, as though the plants have used up all of their available resources for reproduction and are taking a break. Some hypothesize that masting is a result of resource storage, and that plants save up resources for several years until they have what they need for yet another big year.

Another thing to consider is how plant hormones might play a role in masting. Gene expression and environmental cues both result in hormonal responses in plants. As Bogdziewicz, et al. write in Ecology Letters (2020), “if hormones and the genes that control them are hypersensitive to an environmental signal, masting can be at least partially independent of resource- and pollen-based mechanisms.” This and other potential explanations for masting are, at this point, largely theoretical. In their paper, Bogdziewicz, et al. propose a number of ways that theoretical predictions can be experimentally tested. If the “research agenda” outlined in their paper is carried out, they believe it will “take the biology of masting from a largely observational field of ecology to one rooted in mechanistic understanding.”

In her book, Braiding Sweetgrass, Robin Wall Kimmerer proposes an additional explanation for the mechanisms behind masting – the trees are talking to one another. Not in the way that you and I might converse, but rather by sending signals through the air via pheromones and underground via complex fungal networks. There is already evidence for this behavior when it comes to plants defending themselves from predators and in sharing resources, so why not in planning when to reproduce? As Kimmerer writes regarding masting, “the trees act not as individuals, but somehow as a collective.” The question now is how.

seedlings of European beech (Fagus sylvatica), a mast-seeding species — via wikimedia commons; user: Beentree

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