SYMP 17-1 - Reciprocal feedbacks between the evolution of seed dormancy and the structure of annual plant communities

Persistent seed banks play central roles in the ecology and evolution of plant communities. Seed banks form when seeds fail to germinate during the growing season but survive with the ability to germinate in subsequent growing seasons. Seed banks are thought to be adaptive as bet-hedging mechanisms in response to uncertain growing conditions. They also facilitate the coexistence of multiple competing species via the storage effect.

Despite the importance of seed dormancy for both adaptation and community dynamics, we know little about the evolution of dormancy under interactions with many species. Many studies assessing the adaptive value of seed dormancy assume no competitive interactions. The few that include competition only account for intraspecific competition. Therefore, we do not know whether models of dormancy assuming intraspecific competition (or no competition) are satisfactory for explaining evolution in speciose communities. To address this issue, I asked whether selection for seed dormancy differs in single species models versus multispecies models. To answer this question, I modeled the evolution of seed dormancy in the variable environment annual plant model. In this model, seed dormancy is a phenotypically variable and heritable trait, and the dynamics of phenotypic change follow standard assumptions of quantitative genetics. I considered two mechanisms that maintain multiple species in the community: fluctuation-dependent mechanisms and fluctuating-independent mechanisms.

I find that selection in species rich models differs from single species models. Moreover, the directional change in selection (whether stronger or weaker compared to single-species expectations) depends on the mechanism maintaining species diversity. When compared with expectations from single-species models, selection favors greater dormancy under fluctuation-independent mechanisms and favors lower dormancy under fluctuation-dependent mechanisms. An explanation for this result comes from recognizing the distribution of reproductive fitness in these alternative scenarios. Dormancy is advantageous when reproductive fitness varies, which increases when species richness is maintained by fluctuation-independent mechanisms but decreases when richness is maintained by fluctuation-dependent mechanisms. Specifically, reproductive variation declines with the covariance between environment and competition. This covariance is absent under only fluctuation-independent mechanisms and is necessary for fluctuation-dependent mechanisms to operate in the first place.

These results imply that the evolution of seed dormancy cannot be understood apart from interactions between species, for these interactions generate a broader selective environment that influences patterns of fitness over time. Moreover, the exact mechanism by which species coexist influences the distribution of fitness, and adaptive strategy sets.