Ella is a PhD student at the University of Toronto, Canada. She is broadly interested in species interactions and plant ecology and evolution, and is currently studying urban eco-evolutionary dynamics. Here, Ella shares her perspective about the study of global patterns while living a global pandemic.
Ella, at her desk at home in April 2020.
Institute. University of Toronto.
Academic life stage. PhD.
Recent JBI paper. Martin, E., & Hargreaves, A. L. (2023). Gradients in the time seeds take to germinate could alter global patterns in predation strength. Journal of Biogeography, 50(5), 884–896. https://doi.org/10.1111/jbi.14582
Big questions, small world: studying global patterns while living a global pandemic. I began this project in March 2020. I was three months into my Master’s degree at McGill University and was stuck at home learning to accept the growing likelihood that my plans for fieldwork in the Galapagos islands were not going to happen any time soon. While awaiting news of university verdicts on international fieldwork, one of my co-supervisors, Dr. Anna Hargreaves pitched me the idea: a synthesis study exploring latitudinal gradients in the time seeds take to germinate. The idea came out of her work on latitudinal gradients in the strength of species interactions, especially her recent-at-the-time “B.I.G. Experiment”, a large-scale standardized seed predation experiment. This project, and several others like it were testing the hypothesis that species interactions should be stronger (and therefore more ecologically and evolutionarily important) at lower latitudes and elevations, originally proposed by Charles Darwin nearly two hundred years before.
An illustration of the biotic interactions hypothesis. Tropical latitudes are usually warmer, more biodiverse, and have higher productivity. In these more climatically favourable environments, species interactions are expected to be stronger, in this case: higher rates of daily seed predation.
To test this hypothesis, researchers used standardized prey: artificial or commercial versions of early life stages that don’t belong to any particular environment, and so avoid any cases of local adaptation. This method allows researchers to set out prey, and return at a later time to count how many had been eaten, obtaining comparable measures of daily predation rates at locations around the world. What it ignores, however, is exposure time: the duration of time a prey is exposed to predators which determines its risk of being predated over its lifetime, and thus the actual ecological or evolutionary importance of predation. This is where I came in. Using seeds as our study system and published literature as our data source, we set out to try to answer whether seeds’ exposure time (the length of time between dispersal and germination) varied geographically, potentially altering the predicted global patterns of seed predation strength. To do so, I set out to collect as much data on germination times from as many species and locations as I could, in hopes of revealing large-scale patterns.
I adapted to the work of conducting a synthesis project at the same time as I adapted to the work-from-home lifestyle. Rather than spending my summer working outdoors in an exotic location, I spent it at home, travelling the world from the couch, the back porch, the kitchen table, or my bed, through the papers I was reading and extracting data from. I filled spreadsheets with germination times as I spent time watching the plants grow back in my garden and on my walks around the neighbourhood. By the end of the summer, I had filtered through thousands and read over a hundred papers on germination times (often in the middle of the night as I had also lost all concept of time and become semi-nocturnal).
We had realized that compiling data on germination times was not so straightforward due to the large variability in methods. Some studies tested germination timing in the natural field conditions, some in outdoor pots, some in greenhouses, and some in growth chambers under a variety of conditions. Some studies pretreated seeds to induce germination, whereas others did not. Some studies reported time to germination as a mean, or a time to 50% germination, or a maximum or a minimum. I spent the next year doing analyses: excluding some data, adding new data, adding and removing model terms, trying different modelling approaches, looking at relationships between time to germination and latitude, elevation, temperature, precipitation, seed size, and phylogeny. I began to realize and use the wealth of data that exists online free for public use. I could instantly download climate data from all over the world, I could find databases of seed sizes, and plant phylogenies that could help me answer global-scale questions spanning over a thousand species.
An example of a seed depot used in standardized seed predation experiments, this one in my backyard with sunflower seeds, for a different project.
Across all of our analyses, the results remained consistent, if somewhat difficult to explain. We observed that, in natural environments, seeds germinated faster at high latitudes, but low elevations, despite our expectations that these two gradients would be analogous. In terms of climate, seeds in nature germinated faster in warmer, drier environments with high temperature seasonality. What this tells us is that it is unlikely that seeds in high predation (low latitude, low elevation, warm, wet, consistent) environments are unlikely to have universally evolved faster germination to escape predation. In fact, tropical seeds tended to have longer germination times, meaning that they not only have a higher daily risk of predation, but they are exposed to predators over a longer period, resulting in a much greater lifetime risk of predation than seeds at high latitudes, which appear to experience low predation rates over short time periods. For elevational gradients, however, faster exposure times in low predation environments (low elevations), means that seeds would experience similar predation risks across elevations.
Clearly, there is still much to learn about how seeds respond to predation. Syntheses projects have their limits, but being able to take on this type of large-scale biogeographic question, to try to inform our understanding of global patterns in species traits, without leaving the house in the midst of a pandemic, was still a fascinating experience. My perception of the world simultaneously condensed to my home and immediate surroundings, and expanded as I learned about species from around the world and thought about these global patterns, with only a laptop and an internet connection.
An emerging white clover (Trifolium repens) in a growth chamber.