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

The extinction of the dinosaurs and later, the Pleistocene Megafauna, has
been hypothesized to have 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 ~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 a negative feedback by increasing subcanopy
light penetration that limited maximum seed size which matched the actual
trend in angiosperm seed sizes in mid/high latitude ecosystems. 
Adding that larger animals can 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 like climate changes.