Floral display and habitat fragmentation: Effects on the reproductive success of the threatened mass‐flowering Conospermum undulatum (Proteaceae)

Abstract

Fragmentation of natural vegetation is currently one of the largest threats to plant populations and their interactions with pollinators. Plant reproductive susceptibility to habitat fragmentation has been investigated in many species; however, the response of wild mass‐flowering species is poorly known, with research limited to mainly boreal plant species. Here, we studied twelve remnant populations of the threatened mass‐flowering shrub Conospermum undulatum in the southwest Australian biodiversity hotspot, each presenting different population size, level of isolation, and floral display. We assessed the impact of fragmentation on (a) fruit and seed production; and (b) seed germination. To gain a deeper understanding of factors influencing the reproductive success of C. undulatum, we performed pollinator exclusion and self‐pollination treatments to experimentally assess the mating system of this threatened shrub. We found C. undulatum to be strictly self‐incompatible and totally reliant on pollinators visiting with an outcrossed pollen load to complete the reproductive cycle. Further, we found that fruit production dropped from 35% to <20% as a result of decreasing floral display. A reduction in population size from 880 to 5 plants and from ~700 to 0.21 in the floral display index led to a decrease in seed output, while a similar reduction in seed output, from 6% to 3%, was observed as a result of increasing isolation index from −21.41 to −0.04. Overall, seed germination was positively related to population size, and a negative relationship was found between germination and isolation. Synthesis and applications. Our results demonstrate the important relationship between pollinators and floral morphology in plants of southwest Australia that have coevolved with native pollinators and developed characteristic flower morphologies over long time frames. Indeed, due to its characteristic pollination mechanism, the self‐incompatible C. undulatum can only rely on specialized native pollinators for pollen flow and cannot rely on its mass‐flowering trait to attract generalist pollinators from coflowering species; neither can it compensate for the lack of visitors by promoting geitonogamy. Consequently, fragmentation has a significant effect on the reproductive output of C. undulatum, and size, isolation, and floral display of populations are important factors to be considered when planning conservation actions for the species.

Pollen adaptation to ant pollination: a case study from the Proteaceae

“…The flowering period usually ranges from late August to late October. In a recent study, Delnevo et al (2019 b ) found that the pollination mechanism in C. undulatum is an effective physical barrier against autogamous selfing, and also found that this species possesses a strongly developed self-incompatibility system that prevents the development of the embryo following geitonogamous selfing. The hermaphroditic flowers are small, measuring ~7 mm in length, with the tube being ~4 mm.…”

Non‐native plants and nitrogen addition have little effect on pollination and seed set in 3‐year‐old restored woodland

“…However, there is also contrasting evidence to suggest that increased pollinator abundance or diversity, high visitation rate and/or having generalist pollinators in a plant community can substantially lower the occurrence of pollen limitation (Phillips et al 2010;Wolowski et al 2014). Equally, factors other than pollen limitation, such as size of floral display, can be important determinants of reproductive success (Delnevo et al 2019). It is likely that non-specialised pollination syndrome of our study species has buffered them from pollen limitation.…”

“…The native plants at our site were visited by a wide diversity of insect pollinators, and the main pollinator, A. mellifera, acted as a generalist pollinator. In addition, the lack of a significant difference in the number of viable seeds between the B. sessilis bagged treatment and control would usually indicate self-pollination, as is typical of the family (Delnevo et al 2019). However, this result needs to be interpreted with caution due to limited power to detect a difference.…”

“…Equally, factors other than pollen limitation, such as size of floral display, can be important determinants of reproductive success (Delnevo et al . 2019). It is likely that non‐specialised pollination syndrome of our study species has buffered them from pollen limitation.…”

“…In addition, the lack of a significant difference in the number of viable seeds between the B. sessilis bagged treatment and control would usually indicate self‐pollination, as is typical of the family (Delnevo et al . 2019). However, this result needs to be interpreted with caution due to limited power to detect a difference.…”