Degradation of habitat disrupts plant–pollinator interactions for a rare self-compatible plant

Habitat destruction has immediate consequences on biodiversity, whereas the effects of habitat degradation are slower and more subtle. Habitat quality and structure influence reproduction in rare plant species because changes in the local environment can disrupt sensitive plant–pollinator interactions. We used the self-compatible rare species Synthyris bullii to examine pollination and reproduction in response to woody encroachment, a type of degradation that occurs in prairies and savannas in the absence of fire. Additionally, we determined if autonomous selfing occurred more frequently than pollinator-mediated fertilization in degraded habitats. Infructescences from populations in open, semi-shaded, shaded habitats (i.e., different levels of encroachment) were collected to assess reproductive output (e.g., fruit/seed set) and fitness (i.e., germination). In addition, a pollinator exclusion treatment was conducted in these habitats to estimate pollen quantity (i.e., stigma pollen load). Pollinators contributed 32–57 % of pollen loads on average. We observed a significant increase in reproductive output associated with the pollinator treatment, even when the relative pollen contribution was small. Further, fruit and seed set were negatively affected by pollinator exclusion regardless of habitat type. We found evidence that pollen quantity/quality was lower in shaded habitats, which also played a role in lower fruit/seed set and germination compared to other habitats. Autonomous selfing does not occur at a sufficiently high rate, even in shaded habitats, to compensate for pollinator absence. As habitats degrade, reduced pollen quantity/quality and low autonomous selfing rates may contribute to the loss of rare species.