Habitat fragmentation restricts insect pollinators and pollen quality in a threatened Proteaceae species

Disruption of pollination services has been investigated in many species; however, fragmentation induced effects on endemic plants that rely on species-specific invertebrate pollinators are still unclear. While honeybees can forage over long distances, many native bees with small body size are unable to extend their foraging range, making habitat fragmentation the largest threat to such systems. Here, we defined the pollinator assemblage of our target species, Conospermum undulatum (Proteaceae), among remnant populations characterized by different degrees of fragmentation. We explored the impact of fragmentation on flower visitation in eleven populations of C. undulatum. Lastly, we took advantage of the fragmentation gradient to tease apart the influences of pollen quantity and quality on pollen limitation. Small populations showed an impoverished pool of weakly effective pollinators. Flower visitation increased from 1.5% to 74.3% with increasing floral display index, while it increased from 1.3% to 5.6% with increasing connectivity levels of populations. We found evidence of pollen quantity limitation across all the populations; however, the overall pollen limitation was greater in small fragments where genetic factors limited the quality of pollen. Native bees were absent in small isolated remnants. Our study highlights the importance of such pollinators for plants that coevolved with them. The availability of compatible pollen is limited in small remnant populations and native pollinators appeared unable to maintain an adequate inter population pollen flow in heavily fragmented landscapes. The effects of habitat fragmentation can be exacerbated in small and isolated populations of plants that rely on species-specific pollinators for sexual reproduction.