The interplay between local biodiversity floral odours and reproductive success varies across different population sizes of Cypripedium calceolus.

Abstract

To attract pollinators, individuals of the threatened and food-deceptive orchid Cypripedium calceolus (L.) employ visual and olfactory signals, notably volatile organic compounds (VOCs). However, habitat fragmentation has disrupted its population into patches of still relatively large sizes, but also small sizes. We hypothesized that small orchid populations inhabit low-biodiversity areas with fewer pollinators, potentially leading to extinction. To investigate whether local biodiversity and olfactory signals vary with population size, we analysed site-specific vegetation characteristics, insect and flower visitor diversity, plant growth traits and floral VOC profiles in small and large C. calceolus populations. Our results revealed that smaller populations occupy habitats with lower biodiversity, consist of smaller plants with fewer flowers and exhibit reduced flowering success compared to larger populations. VOC profiles also varied between population sizes. However, fruit production and reproductive success did not differ, indicating that these chemical and ecological differences do not necessarily affect reproductive output. These results highlight that population size is linked to variation in plant traits and floral scent but does not directly predict reproductive success under current conditions.