Effects of habitat isolation on pollinator communities and seed set

Abstract

Destruction and fragmentation of natural habitats is the major reason for the decreasing biodiversity in the agricultural landscape. Loss of populations may negatively affect biotic interactions and ecosystem stability. Here we tested the hypothesis that habitat fragmentation affects bee populations and thereby disrupts plant-pollinator interactions. We experimentally established small "habitat islands" of two self-incompatible, annual crucifers on eight calcareous grasslands and in the intensively managed agricultural landscape at increasing distances (up to 1000 m) from these species-rich grasslands to measure effects of isolation on both pollinator guilds and seed set, independently from patch size and density, resource availability and genetic erosion of plant populations. Each habitat island consisted of four pots each with one plant of mustard (Sinapis arvensis) and radish (Raphanus sativus). Increasing isolation of the small habitat islands resulted in both decreased abundance and species richness of flower-visiting bees (Hymenoptera: Apoidea). Mean body size of flower-visiting wild bees was larger on isolated than on nonisolated habitat islands emphasizing the positive correlation of body size and foraging distance. Abundance of flower-visiting honeybees depended on the distance from the nearest apiary. Abundance of other flower visitors such as hover flies did not change with increasing isolation. Number of seeds per fruit and per plant decreased significantly with increasing distance from the nearest grassland for both mustard and radish. Mean seed set per plant was halved at a distance of approximately 1000 m for mustard and at 250 m for radish. In accordance with expectations, seed set per plant was positively correlated with the number of flower-visiting bees. We found no evidence for resource limitation in the case of mustard and only marginal effects for radish. We conclude that habitat connectivity is essential to maintain not only abundant and diverse bee communities, but also plant-pollinator interactions in economically important crops and endangered wild plants.

Enhancing floral resources for pollinators in productive agricultural grasslands

“…This loss of floral diversity has been linked to the increased use of inorganic fertilisers, reseeding, improved drainage and a greater frequency of cutting and grazing (Blackstock, et al, 1999;Bullock, et al, 2011). For those areas that have escaped the impact of modern practices and remain under traditional extensive management systems, the diversity and cover of flowering plants represents a key resource for many insect pollinators (Forup and Memmott, 2005;Noordijk, et al, 2009;Steffan-Dewenter and Tscharntke, 1999).…”

“…those containing both grassland and arable) the retention of such areas can play a key role in the maintenance of insect pollinators, particularly for taxa such as bees that forage at a landscape scale (Decourtye, et al, 2010 ;Steffan-Dewenter and Tscharntke, 1999). As insect pollinators are estimated to support 9.5% of the worldwide production of key vegetables, fruits and oil producing crops (worth €153 billion), this loss of species-rich grasslands may have economic consequences for the provision of this ecosystem service (Gallai, et al, 2009).…”

Depressed pollination in habitat fragments causes low fruit set

“…Recent research on experimentally fragmented plant populations has demonstrated that this process does indeed pose a plausible threat (Ste¡an-Dewenter & Tscharntke 1999). To establish this link between declining pollination and declining reproduction, evidence is required that the reproductive output of plants growing in fragments is pollen limited.…”

Comparison of phenological, aerobiological and melissopalynological patterns of Olea europaea

“…Previous studies have showed that there is a timing correlation between the pollen collection by honeybees and the flowering period (Andrada and Telleria, 2005;Dimou and Thrasyvoulou, 2007). Moreover, honeybees collect food only from a restricted area around the hive (Visscher and Seeley, 1982;Ekert, 1993;Steffan-Dewenter and Tscharntke, 1999). Thus, compared to the air pollen traps, the pollen collected by traps adjusted to beehives strictly represents the taxa only from the area around the apiary.…”