Shifting bottom-up and top-down regulation of oak recruitment across a regional resource gradient
Questions about the relative importance of top-down and bottom-up regulation of populations have long occupied ecologists. Recent work has explored how the strength of consumer- versus resourced-based limitation can shift in strength across both time and space, and indicates the need for larger scales of study across spatial resource gradients. A better understanding of how climate gradients affect population regulation is integral to conservation ecology, particularly in the context of global change.
We examined the effects of bottom-up (water limitation) and top-down (small mammal herbivory) forces on valley oak (Quercus lobata) sapling recruitment. We used existing literature on valley oak planting experiments and field data on natural populations at 26 sites across the species' distribution to explore how the factors regulating oak populations shift along a regional precipitation gradient. This approach allowed us to study time-integrated ecological processes in a long-lived system that would be impossible to manipulate in a strictly experimental setting.
We found that the relative importance of top-down and bottom-up forces in limiting oak recruitment shifted across the precipitation gradient, with small mammalian herbivory (top-down) more important as long-term mean site precipitation increased, and factors related to moisture stress (bottom-up) increasingly important at lower precipitation levels.
Our findings emphasize the importance of expanding the spatial scale of studies on population and community dynamics in order to better understand patterned variation in the multiple factors regulating them across resource gradients.