Patterns of frequency and density dependence are highly variable in diverse annual flowering plant communities

Applications of ecological theory to natural communities often assume that
competitive, negative density-dependent processes are the only type of
interaction important for diversity maintenance. Recent advances suggest
that positive interactions within trophic levels (e.g. plant-plant) may
also affect plant coexistence. Though positive plant-plant interactions
theoretically might result in positive or nonmonotonic frequency or
density dependence (FD/DD), less is known about how commonly these
patterns occur, or which ecological processes might result in such
patterns in natural plant communities. In this study, we test for signals
of variable frequency and density dependence in annual flowering plant
communities in Western Australia and search for evidence that interactions
among plants during flowering might induce positive or nonmonotonic FD/DD
in flowering plants. Using four common annual wildflower species, we ask
if plant fecundity exhibits positive or nonmonotonic FD/DD and if
pollinator-mediated plant-plant interactions during flowering change
patterns of FD/DD relative to pollinator-independent plant interactions.
Three species exhibited nonmonotonic (hump-shaped) density dependence, and
only one species experienced strictly negative density dependence. Each
species exhibited a different pattern of frequency dependence (positive,
negative, weakly nonmonotonic, and no detectable frequency dependence).
Pollinator-mediated plant-plant interactions during flowering induced both
nonmonotonic density dependence and negative frequency dependence in one
species. Importantly, the extent of variation in FD/DD observed in our
study brings into question the dominance of negative density and frequency
dependence in theory, suggesting instead that demographic responses of
plants to their communities fall along a continuum of possible density-
and frequency-dependent patterns.