Effects of Habitat Fragmentation on Pollinator Diversity and Plant Reproductive Success in Renosterveld Shrublands of South Africa

Abstract

We examined the effects of habitat fragmentation on the pollinator diversity and reproductive success of seven perennial plant species in renosterveld shrublands in South Africa. We sampled pollinators in small (0.5–2 ha), medium (3–10 ha), and large (>30 ha) habitat fragments during the peak flowering period in spring and summer. We also compared fruit set and seed set in the seven plant species on different‐sized fragments. Hand‐pollinated controls were used to determine pollination deficits in three species. Seed‐germination studies were done on two species to determine the effect of reduced seed set on reproductive output. Overall, the species richness of bees, flies, and butterflies did not vary significantly among different‐sized fragments. However, the abundance of particular species of bees and monkey beetles (Scarabaeidae: Hopliini) was significantly affected by fragment size, together with other factors such as vegetation cover and the ratio of grass to shrubs. Fragment size and distance to large remnants of vegetation had a significant influence on seed or fruit set in four of the seven plant species examined. One species failed to set any seed in small‐ and medium‐sized fragments. Comparisons of fruit and seed set among hand‐pollinated and naturally pollinated plants confirmed that seed set was pollinator‐limited in the orchid Pterygodium catholicum. Our results suggest that perennial plant species respond in different ways to fragmentation, and that populations on small fragments do not always experience pollination deficits. Existing classifications of pollination systems, in relation to their vulnerability to fragmentation, do not adequately explain our results. We argue that there needs to be a greater focus on the habitat requirements of pollinators to predict the effects of habitat fragmentation on pollination systems and plant reproductive success.

Reproductive success of Cabralea canjerana (Meliaceae) in Atlantic forest fragments, Brazil

“…Other species either do not show any different response in reproductive output between fragmented and continuous forest or present even higher reproductive output in smaller fragments and open areas than in continuous forest (e.g., Costin, Morgan, & Young, 2001;Dick, 2001;Lopes, & Buzato, 2007;Dunley et al, 2009). Some authors argue that the influence of fragment size and quality depends on the habitat requirements of the plants and the pollinators (Aizen, & Feinsinger, 1994b;Kwak, et al 1998;Donaldson, Nänni, Zachariades, & Kemper, 2002;Quesada, 2003) and those different groups of species respond differently to habitat fragmentation (Kwak et al, 1998). Cabralea canjerana is an example of a species that shows a reduction in its reproductive success in fragmented habitats.…”

Global pollinator declines: trends, impacts and drivers

“…addressing habitat isolation effects independently of habitat area effects) [21,57,58]. Several studies have not found an effect of fragment area on overall community richness or abundance of bee pollinators, although they have detected differential responses among tribes or guilds, with some favoured by increased native habitat, and others favored by increased (non-native) matrix area [53,58,59]. Other studies report declining species richness and abundance with decreased fragment size for bees [57] and butterflies [60].…”

Honeybee health in Africa—a review

“…2 0 0 8 ; M u n y u l i 2 0 11 ) . A l l t h e abovementioned causes can result in habitat loss and fragmentation that may affect various pollinators, including honeybees (Donaldson et al 2002;Brown and Paxton 2009;Dietemann et al 2009;Munyuli 2011;Goulson et al 2015). In particular, the transition from natural to agricultural farmland and the planting of monocultures change floral diversity and may impact the nutritional requirements and fitness of honeybees (Johannsmeier and Mostert 2001;Murray et al 2009;Goulson et al 2015).…”