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.

All the Plant Shows, part one

Podcasts are among the most accessible and powerful mediums through which we can tell and hear the stories of plants. The popularity of podcasts is evidence that if we want to share our love of plants with the world and get others to love them too, we have to be using podcasts to do it. They are essential tools in the communication of plant science and, when used effectively, they may even help the plant-indifferent gain a lifelong appreciation for the botanical world.

As a longtime listener of podcasts and a lover of plants, I have been on a constant search for podcasts about plants. I’ve even included reviews of some of those podcasts here on this blog (see reviews for Gastropod, In Defense of Plants, Native Plant Podcast, The Field Guides, Botanical Mystery Tour, and Plants and Pipettes). I’m not sure if it’s just me, but it seems that in the past few years, plant podcasts have experienced a boom. There are definitely more plant-themed podcasts out there now than I recall seeing when I first went in search of them nearly a decade ago, and I imagine there are more out there than I’m even aware of. Seeing that, I figured it was time to collect all those podcasts into a single post (or series of posts). Each podcast is deserving of a post of its own, but in the meantime, a few sentences will have to do.

When I say plant podcasts, I realize that could include gardening podcasts. Why shouldn’t it? After all, what’s gardening without plants? However, this isn’t a gardening blog, and even as an avid gardener (and a professional one), I don’t really listen to many gardening podcasts. A few gardening or gardening-adjacent podcasts are included here either because I particularly enjoy them or because they tend to go beyond the act of gardening and are particularly known for giving plants the center stage.

In Defense of Plants – Long-running and consistent, this is the go-to podcast (and website) for learning about plants and plant science. It’s adamant about telling the stories of plants for plant’s sake. A typical episode features the host, Matt, interviewing experts and plant science professionals about their specific area of study or work.

Native Plant Podcast – Going strong for 5 years now, this podcast is exactly what it says it is – a podcast about native plants. There is a major focus on gardening and landscaping with native plants, which the main host, John, has been doing since before it was cool. Every episode ends with a pet story and a toast.

The Field Guides – Easily one of my favorite podcasts, largely because the hosts are so affable and are clearly having fun, but also because the format is so unique. Each episode, Steve and Bill pick a natural history topic and then walk around in a natural area talking about it – the sounds of footsteps and the wildlife around them included. Not specifically a plant podcast, but plants come up in every episode even if they aren’t the main topic of discussion.

Plants and Pipettes – A podcast focused mainly on what’s going on inside of plants – molecular plant biology, in other words. If that doesn’t sound like your thing, give it a shot anyway. The hosts are fun and funny, good at explaining things, and find lots of other plant and plant-adjacent things to talk about in addition to molecular biology. Plus, you are probably more interested in cellular-level interactions than you think you are.

Plant Crimes – True crime stories involving plants. Well-researched and well-crafted tales about things like missing water lilies, redwood poaching, and how lemons and the mob are related. Ellen interviews people involved in or knowledgeable about the incidents and weaves excerpts from those conversations into her storytelling. I’m anxiously awaiting the second season.

Plant Book Club – Ellen (of Plant Crimes) and Tegan and Joram (of Plants and Pipettes) read a plant-themed book and then talk about it. Everything you love about their individual podcasts combined into one. It’s a tour de force!

Botanize! – An audio series produced by Encyclopædia Britannica. Each episode is a brief exploration of a plant, group of plants, or some other plant-centric topic. It’s way more entertaining than reading an encyclopedia entry. Melissa is a charismatic host who is clearly excited about plants and nature. Her and her occasional guests add personal experiences to the science of plants.

Cultivating Place – This is a perfect example of a more-than-just-gardening gardening podcast. In Jennifer’s words, “gardens encourage a direct relationship with the dynamic processes of the plants, animals, soils, seasons, and climatic factors that come to bear on a garden, providing a unique, and uniquely beautiful, bridge connecting us to our larger environments — culturally and botanically.” Each episode features a conversation with a grower, gardener, naturalist, scientist, artist, or otherwise and, while many of the episodes are garden-focused, others go beyond the garden to discuss other plant-y things like seed banking (see this recent episode with Dr. Naomi Fraga).

A Way to Garden – This is perhaps a more typical gardening podcast, but easily one of the best ones out there. My belief is that gardens ought have a purpose that goes beyond their aesthetic qualities. They should be ecologically functional, acting as habitat rather than destroying it. Margaret seems to think so too. Plus, she loves birds and is a great conversationalist, and who can resist her regular check-ins with Ken Druse?


This is part one of (at least) two. There are many more podcasts to highlight here. In the meantime, is there a particular plant-themed podcast (or podcast episode) that you enjoy and would like to recommend? If so, share it with us in the comment section below.

Podcast Review: The Field Guides

Who doesn’t love nature walks and scientific journal articles? Luckily there is a podcast that combines the two. The Field Guides is hosted by two guys who are obsessed with the natural world and the science behind it. For each episode the hosts pick a nature topic to study in depth, then they head out to a natural area to talk about what they learned. The discussion takes place outside as they hike around, giving listeners the experience of being “out in the field, in the woods, and on the trail.”

field-guides

The discussion is conversational as the two hosts (and occasional guests) share things from different studies they have read, inserting personal anecdotes and thoughts as they arise. Observations on what they are seeing as they walk and talk also enter into the conversation. The nature sounds in the background make for a great score, and surprises along the way add a little suspense and intrigue to the experience.

The Field Guides is a young podcast – about a year and a half old – and has averaged around one episode per month. Episodes vary in length from as few as 20 minutes to an hour, so catching up on past episodes is not an insurmountable task. And it’s worth it. The guides have already explored some great topics that shouldn’t be missed, including hibernation, birds in the winter, salamanders, spruce grouse, and ice spikes. A bonus episode takes the listener along on a Christmas bird count, which, speaking for myself, is an inspiration to get involved in this 117 year old tradition. As a plant nerd, the botanically themed episodes are particularly interesting, and have so far included fall foilage, witch hazel, pokeweed, staghorn sumac, and others.

This is a ball gall on a tall goldenrod (Solidago altissima). The first episode of The Field Guides is all about the fascinating world of goldenrod galls. (photo credit: wikimedia commons)

A ball gall on a tall goldenrod (Solidago altissima). The first episode of The Field Guides is all about the fascinating world of goldenrod galls. (photo credit: wikimedia commons)

The notes that accompany each episode are often extensive and include things like references to the journal articles discussed and other resources cited, answers to questions that came up during the episode, and corrections to any mistakes that were made. Clearly the hosts are thorough in their research and passionate about the subjects they cover, but they are not without a sense of humor. The information presented is great, but what makes this podcast so listenable is the way that it is presented. It’s approachable, fun, and light-hearted – drawing the listener in to the conversation and out in to nature.

Check out individual interviews with the hosts of The Field Guides on these two episodes of In Defense of Plants podcast: Environmental Action with Bill Michalek and Reflections on Science with Steven Fleck

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