Mixed effect of habitat fragmentation on pollinator visitation rates but not on seed production in renosterveld of South Africa

Abstract

Habitat fragmentation – in terms of a combination of habitat loss and separation – is a major threat to the renosterveld of the Cape Floristic Region (CFR) of which 85% has been transformed, mostly for agriculture. The effects thereof has not been well studied, in particular in the Eastern inland areas. In this study we seek to understand how habitat patch size influence visitation rate of pollinators of winter flowering plants in the Eastern-Rûens Shale Renosterveld of the Overberg region. We compared pollinator visitation rates to flowers of Relhania garnotii, Clutia sp. nov and Oxalis purpurea between small (0–4.9 ha) and medium (5–15 ha) sized fragments, and compared these to the last large intact protected area, the Haarwegskloof Renosterveld Reserve (>500 ha), termed mainland. We classified degree of isolation among fragments into high (>100 m) and low (<100 m) based on the distance between the outer edge of a fragment to its surrounding fragments and compared pollinator visitation rates between the two levels of isolation. Furthermore, we compared seed set of Bobartia longicyma and Trichodiadema strumosum between fragments and the mainland. We recorded 442 plant-pollinator interactions over a total of 75 h of observation. Pollination rate was significantly higher in fragments compared to the mainland. Within fragments, pollination rate and species richness were significantly higher in medium-sized fragments compared to small fragments. The degree of isolation had a negative effect on pollinator visitation rates among habitat patches, with lower visitation rates and pollinator species richness recorded in fragments with higher degree of isolation, supporting the Habitat Amount Hypothesis (HAH). Plant species exhibited a strong reliance on pollinators for reproduction but were not pollen-limited in either the fragments or mainland. These results indicate that the effect of fragmented habitats do not necessarily affect pollination rates of a certain size of habitat patches, but beyond an undefined threshold, pollinator rates decrease significantly. While in very small fragments ecosystem functions might be declining, in this study insect species have been observed in the smaller fragments that only occurred there, which indicates that heterogeneity of habitat fragments could offer ecological benefits. This highlights the importance of preserving fragments, as an important part of maintaining the remaining renosterveld. Therefore, conservation efforts should be enhanced to also preserve the small fragments that may serve as stepping stones for pollinators in the fragmented renosterveld.