Niche differences and fitness differences drive competitive coexistence and exclusion in a serpentine annual community

Background/Question/Methods

Do niche differences contribute to the long-term coexistence of species? Answering this classic question has been difficult for two reasons. First, fitness differences, which drive competitive dominance, are difficult to disentangle from the niche differences that drive frequency-dependent advantages when rare (thereby buffering species from extinction). Second, there are a bewildering number of niche differences that can contribute to coexistence, making it empirically challenging to quantify their role in driving the diversity we see. To address this question, we combine recent developments in coexistence theory with demographic data and model fitting to quantify niche overlap and fitness differences between 10 focal serpentine annual plant species in Southern California. We use three years of demographic data, collected from plots seeded at varying relative abundances of component species, to estimate niche and fitness differences. We then compare parameter estimates to the long-term population dynamics of focal species growing in the presence and (experimentally manipulated) absence of the demographic effects of niche differences.

Results/Conclusions

We find that species pairs differed strongly in both fitness and niche differences. That is, 2-3 species were identified as clear competitive dominants (i.e. had high fitness compared to most other focal species), consistent with their high relative abundance in control plots, and increasing competitive dominance in experimental plots where the demographic advantages of niche differences were removed for all species. At the same time, several species were significantly niche differentiated (i.e. niche overlap less than 1) with these competitive dominants. These species persisted in long-term dynamics plots, but declined precipitously in abundance when the demographic advantages of those niche differences were removed. Thus, results suggest that niche differences are critical for the coexistence of approximately six of our ten focal species at our study site. In summary, our empirical data and parameter estimates suggest that both niche differences and competitive ability differences are important for community dynamics in this system – with the former buffering several species from extinction and the latter driving the competitive dominance of a few species and competitive exclusion of others.