Fickle Pollinators ‘Lured Away’ By Farms

UNITED KINGDOM–(ENEWSPF)–22 July 2011.  Pollinating insects are more likely to move between areas of native forest separated by pine plantations than those separated by small holder farms, scientists from Oxford University and Earthwatch have found.

The study challenges the long-held assumption that areas that provide resources to pollinators will act as ‘corridors’ between islands of native vegetation in fragmented landscapes, whereas areas that are resource-poor act as ‘barriers’ stopping pollinators from shuttling between islands of native plants.

The study found the opposite: resource-rich farms reduced movement of pollinators between native forest areas while resource-poor pine plantations increased movement of pollinators. However, the study also found that clearfelled areas acted as barriers to pollinators, suggesting that not all low-resource areas encourage pollinator movement.

The researchers believe that because the pollinating insects in the study are ‘fickle’ foragers, who don’t have an exclusive relationship with a single plant species, when they encounter the abundant food resources farmland provides they stop and forage, rather than travelling further to reach the next patch of native forest. The team has dubbed this ‘The Circe Principle’ after the sorceress from Homer’s Odyssey whose charms waylaid the hero on his voyage home.

A report of the research appears in this week’s Current Biology.

‘Looked at from the insect’s point of view it makes sense,’ said Dr David Boshier of Oxford University’s Department of Plant Sciences, co-author of the report. ‘These insects are not trying to pollinate a particular species of tree, they are just foraging, so if they leave a patch of native forest and fly across farmland which happens to be rich with resources they are likely to collect pollen and nectar there rather than carry on to another patch of native forest. Conversely, areas of sparse resources, such as plantations, have less to offer so the pollinators are more likely to continue their journey and eventually enter another patch of native forest. It’s important to note though that these particular pollinators tend not to cross areas without ground vegetation, so clearfells, although also resource-poor, acted as barriers to pollinators.’

The study focused on the pollination of Gomortega keule, an endangered tree species that only survives in patches of native forest in central Chile. This tree is mainly pollinated by hoverflies. By sampling seeds from all the trees in the study-area and conducting paternity analysis the team was able to map how pollen moved across the landscape and chart the movements of pollen-carrying insects.

‘It’s unusual to be able to study pollination in this kind of detail, but it was possible because this species of tree is so rare,’ said co-author Dr Tonya Lander, formerly of Oxford University’s Department of Plant Sciences. ‘Although we focused on hoverflies in Chile, we believe that the lessons learnt are very likely to apply to bees and other familiar pollinators found in gardens, forests and other habitats around the world.

’The team now hopes to extend their research to test whether the Circe Principle holds true for other kinds of pollinating insects in other environments. If it does approaches to landscape management may have to change with an emphasis on more sophisticated ways of categorising landscapes rather than as simply ‘habitat and non-habitat’. Ecological corridors could no longer be thought of as alleyways of native vegetation in otherwise ecologically value-less landscape. Rather the landscape as a whole, including areas not traditionally thought of as habitat, could be developed to provide ecological services while also continuing to be economically productive.

Image of Gomortega keule fruit [Credit: Tonya Lander]:

Image of Gomortega keule tree [Credit: Tonya Lander]: