Ecosystem engineers alter the evolution of seed size by impacting fertility and the understory light environment

It has been hypothesized that the extinction of the dinosaurs, and later the Pleistocene megafauna, created a darker forest subcanopy benefiting large‐seeded plants. Larger seeds and their fruit, in turn, opened a dietary niche space for animals thus strongly shaping the ecology of the Cenozoic, including our fruit‐eating primate ancestors. In this paper, we develop a mechanistic model where we replicate the conditions of tropical forests of the early Paleocene, with small animal body and small seed size, and the Holocene, with small animal body and large seed size. We first calibrate light levels in our model using stable carbon isotope ratios from fossil leaves and estimate a decrease of understory light of c. 90 μmol m⁻² s⁻¹ (a 19% decrease) from the Cretaceous to the Paleocene. Our model predicts a rapid increase in seed size during the Paleocene that eventually plateaued or declined slightly. Specifically, we find a dynamic feedback where increased animal sizes opened the understory causing negative feedback by increasing subcanopy light penetration that limited maximum seed size, matching the actual trend in angiosperm seed sizes in mid/high latitude ecosystems. Adding the ability of larger animals to increase ecosystem fertility to the model, further increased mean animal body size by 17% and mean seed size by 90%. Our model is a drastic simplification and there are many remaining uncertainties, but we show that ecological dynamics can explain seed size trends without adding external factors such as climate change.