Studying small populations using big experiments: Reflections from an LTER Fellow

Isabela Borges is a graduate student in Sarah Fitzpatrick’s lab in the Integrative Biology department at Michigan State University. Isabela won the J.S. Karling Graduate Student Research Award from the Botanical Society of America for her work on plant inbreeding on the legume-rhizobia mutualism. She is broadly interested in the feedbacks between community ecology and contemporary evolution, and their consequences for the persistence of small populations.

plants in greenhouse
Isabela’s 2,000 partridge pea plants growing in the greenhouses of the Kellogg Biological Station.

Summer 2021 was a busy one. When I first proposed to conduct an experiment on two thousand plants, that just seemed like a nice large number––I’d grown these plants before, conducted large experiments before, and thought I had some grasp on the magnitude of the number 2000. It’s just 500 x 4, or 400 x 5, or 200 x 10. Really, how hard could it be?

Ironically enough, small populations are the focus of my dissertation work. I am interested in what happens when habitat fragmentation and climate change cause natural populations to shrink. How does that affect their ability to interact with other species in their environments? How likely are they to completely disappear? One of the main threats to small, fragmented populations is inbreeding: mating with relatives often has detrimental effects on an individual’s ability to survive and reproduce––what is termed “inbreeding depression.” We know that inbreeding is common in small populations, where organisms have fewer mating options, and that it can affect how susceptible some species are to extinction. However, we still don’t know much about how inbreeding affects the interactions that species have with others.

Interactions between species are one of the main topics of research in ecology. Some interactions are negative, like predation and disease, but some are positive––a mutualism is an interaction in which both parties benefit. For example, when plants give nectar to a bee in exchange for pollination, that is a mutualism. Previous research has shown that inbred populations can suffer more from negative interactions when compared to healthy populations, but we know little about how inbreeding affects mutualisms. It is possible that the presence of a mutualist helps inbred populations persist, but it is also possible that the detrimental effects of inbreeding leads mutualisms to break down, as inbred partners can no longer give their fair share.

To test these ideas, I first had to generate some inbred and not-inbred populations. Plants are great study subjects for this: I can act as a pollinator and cross plants that are more or less related to make seeds that are more or less inbred. I did this in the greenhouse for two generations of plants, collecting over 3500 seeds that differed in how inbred they are. The specific plant species I chose, Chamaecrista fasciculata or the partridge pea, has a mutualism with soil bacteria that provide it with nitrogen in exchange for sugars. The next step, then, was to plant these seeds and let some of them interact with the bacteria while leaving some without it. My goal was to see how plants of all different inbreeding backgrounds interacted with the bacteria, so I needed seeds whose parents were siblings, half-siblings, cousins, seconds cousins, etc. Which is how I ended up with 2000 plants.

plants outdoors
Moving the partridge pea plants outdoors allowed them to interact with beneficial species like pollinators, but also put them at risk for attacks by voles and other herbivores.

With the help of an amazing NSF REU student, Luana Fenstemacher, I spent my summer planting, inoculating, and watering these plants. They were first planted in the greenhouse, then moved to an outdoor area so that they could interact with other species, such as herbivores and pollinators––making me constantly worried they would get fatally munched on by voles. Luana and I measured all of these plants multiple times and monitored them daily to know when they flowered. Once fall started, I collected every single seed that each plant produced, then dug whole plants up from their pots to measure their roots. From germination to harvest, these plants were a full time job from May to November.

Now, going into Spring of 2022, I finally have the data to test my ideas about mutualism and inbreeding. Counting the seeds that these plants produced will tell me if the presence of a mutualist was enough to offset the costs of inbreeding, or whether inbred plants failed to establish or benefit from this mutualism. Given how changes to habitat and climate have increased the prevalence of inbreeding while also disrupting mutualisms, we need to understand how inbred organisms interact with their environments to figure out how best to protect them. Science is hard, and sometimes unintuitive, but the hope is that my giant experiment can better help us look after small populations in an uncertain future.