Pollination is a complex process. It’s not as easy as an insect simply visiting a flower.
This is important to remember when talking about which species are the ‘best’ or ‘most important’ pollinators. Simply observing an animal visiting a flower is not, on its own, evidence that the animal is pollinating the flower.
Many insects (and other animals) visit flowers, to feed on pollen, nectar or other flower parts. Many of these interactions result in pollination…although some visitors are better pollinators than others. Some of these visitors commit floral larceny – they are robbers or thieves (there’s a difference!) of either pollen or nectar, and they leave without pollinating. Some might even damage flower parts so much that they indirectly affect the flower’s capacity to be pollinated by other visitors.
To know for sure that an insect is pollinating a particular flower, we first need to know what kind of reproductive system that flower has. Is it male, female or bisexual (containing both male and female parts)? Can it self-pollinate, or does it need to be outcrossed to another flower or plant of the same species? Once we know this, we then need to watch the behaviour of the insect that visits that flower. Does the insect visit one flower and fly away, or many flowers in a row? Does it move between plants? Does it actually touch the reproductive parts of the flower when it visits each flower?
This study, just published this week, looks at plant-pollinator networks around the world.
Despite the media coverage, this study does not show that European honey bees are the most important pollinator worldwide.
What the study actually shows is that honey bees are the most frequent flower visitors recorded in the 80 plant-pollinator network studies that the authors analyse. This is not evidence of ‘worldwide importance as a pollinator’. Many countries are missing from the map of where these studies were conducted. Some studies are from countries where the honey bee has been introduced – and honey bees can have negative impacts on native plants and pollinators in their introduced range (there are still lots of unanswered questions about these interactions).
Visitation vs Pollination. Saying that a flower visitor is the most important pollinator of a plant is like saying that a window-shopper spends the most money in-store. This study found that Apis mellifera (the European honey bee) was the most frequent floral visitor in 17 of the 80 networks analysed. None of these networks measured pollination. Honey bees weren’t even recorded in all the networks – only 41 out of the 80 networks documented A. mellifera as a flower visitor in the system. And honey bees didn’t visit nearly half (49.38%) of the plant taxa in these networks. Describing plant-pollinator visitation networks is a standard method for sampling interactions between flowers and insects and this method is used frequently by pollination ecologists. But describing a plant-pollinator network rarely tells us anything about the pollination efficiency or effectiveness of the insects observed during sampling. The authors of this particular study looked at pollination effectiveness using a completely different set of studies to the ones they analyse for visitation rates. From this other group of studies, they found no difference in effectiveness between honey bees and non-honey bee pollinators, based on single visits to flowers. In fact, they found that A. mellifera was “generally less effective” than the most effective non-honey bee pollinator. This is not evidence that honey bees are ‘the most important’ pollinator.
Ecological ‘importance’ is contextual. We know very little about the reproductive system of most plant species in the world, let alone what pollinates them. An important pollinator for one plant species may never interact with a neighbouring plant species. Some plants are highly specialised to only be pollinated by one particular group of animals; other plants will attract many different animal species to spread their pollen far and wide. Some insect pollinators, including many bee species, also visit flowers of wind-pollinated plants to collect pollen. And here are some great studies that have found that honey bees are not always the most effective or efficient pollinator for many crop varieties: Wild pollinators enhance fruit set of crops regardless of honey bee abundance; Non-bee insects are important contributors to global crop pollination. And don’t forget the birds, mammals and even reptiles that also pollinate many plants around the world.
In a nutshell: it’s ecologically impossible for a single species of insect to be ‘the most important pollinator’ worldwide, in all climates, all ecosystems, and for all plant species.
© Manu Saunders 2017