Tag Archives: Gardening

All pollinators are equal, but some pollinators are more equal than others

The infamous line from George Orwell’s Animal Farm asserting that “All animals are equal, but some animals are more equal than others” nicely captures an ecological view of pollinators and their relationships with plants.  “Pollinators” by definition move pollen between flowers, but not all pollinators are equally good at transferring pollen of any particular plant: some are more effective than others. I’ll illustrate this with examples from the urban garden that Karin and I are developing, which I’ve discussed before.

As you can see from that link, the garden is modest in size, but nonetheless this year it contains a significant biodiversity of edible plants that require pollinators for some or all of the fruit and seed set, including: strawberries, apples, greengages, cherries, blackcurrants, squashes, courgettes, blackberries, fennel, runner beans, french beans, passion fruit, tomatoes, raspberries, and radishes.

Radishes?!”  I hear you ask.  “But they are grown for their edible swollen roots which don’t require pollination!”  True, usually.  But we let our radishes flower because we mainly grow them for their seed pods which, picked young, are delicious in salads and stir fries, like mustardy mange tout.  They look like this:

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The radish flowers are pollinated by a diversity of insects including butterflies, bees, and small flies:

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These insects will vary in their effectiveness as pollinators of radish, depending on the frequency of visits, how often they move between flowers, and the amount of pollen on their bodies.  This last factor is largely a function of size and hairiness (bigger, hairier insects carry more pollen as a rule), though cleanliness also plays a part: insects often groom the pollen from their bodies and, in the case of bees, may pack it into their pollen baskets where it’s not available for pollination.

The size and behaviour aspect is best illustrated by some recent photos that I took of visitors to the flowers of passion fruit (Passiflora caerulea var.).  We have a large, sprawling plant growing up a fence which is currently being visited by honey bees, hoverflies, solitary bees and bumblebees.  In comparison to the size of the flower and the position of the anthers (male, pollen producing parts) and stigmas (female, pollen receiving parts), the hoverflies, honey bees and solitary bees are relatively small.  These two images are of honey bees (Apis mellifera):

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Here’s an unidentified solitary bee:

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These bees were occasionally touching the anthers, mainly with their wings, so some pollen will be moved around.  But from what I observed it’s likely to be a relatively small amount in comparison to bumblebees, which are usually much larger and hairier, and don’t groom themselves as often as honey bees.  Here’s a Buff-tailed bumblebee (Bombus terrestris):

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They were actively collecting pollen as well as nectar.  Much of this pollen is packed into the pollen baskets on the rear legs and will go back to the nest to feed the developing larvae, but some will be involved in pollination:

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So it seems to be the bumblebees we have to mainly thank for the deliciously sweet-sour fruit we will enjoy later in the season. Of course to test this properly we would need to set up an experiment in which we excluded larger bumblebees from the flowers and only allowed smaller bees to forage, with appropriate experimental controls.  Would make a great project if any of my students are interested!  But it should give you a sense of just how complex the interactions between flowers and their pollinators are: the ecology of pollination is far from simple, despite what some would have us believe.

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What are YOU doing for our pollinators this year? (reduce, reuse, recycle part 6)

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Earlier this year I was asked to write a short article by my former PhD student, and still a current collaborator, Dr Sam Tarrant.  Sam works for the RSPB as the CEMEX UK-funded Biodiversity Advisor, and wanted something on pollinator conservation that could be circulated in the CEMEX company’s e-newsletter.  In the spirit of reworking and reusing odd bits of writing, I thought I’d post it here too.

 

Insects are vital for our country’s economy.  Don’t believe me?  Then read on….

Beneath a large black mulberry tree near the University of Northampton’s Newton Building there is a plaque that commemorates its planting “On Shakespeare Commemoration Day, 3rd May 1916”.  Despite its age this tree annually produces large crops of succulent berries, aided by the fact that wind eddies are sufficient to disperse its pollen, ensuring pollination and fruit set.  Each year it’s a scramble between students, lecturers and birds, to see who can eat the most.

In contrast, the old apple trees in the grounds possess a different strategy – pollination by insects that move from flower to flower each spring.  This form of pollination is both more sophisticated and less reliable than wind pollination, and is currently under considerable threat: whilst there will never be a shortage of wind currents in Britain, insect pollinators are in decline.

The apples trees are not alone in requiring insects to pollinate them, so to do other farm and garden crops, including oil seed rape, field beans, courgettes, runner beans, and strawberries and other soft fruit.  It’s worth at least £440 million annually to the British economy, and most of it is done by wild bees and hoverflies, rather than managed hives of honey bees.

But all is not well with these insects in Britain – they are in decline.  Although the extent of the “pollination crisis” is debated by scientists, long term records show us that these insects are under pressure: 23 species of bee and flower-visiting wasp have gone extinct since the mid 1800s, as have 18 species of butterflies.  Less obviously, other species have considerably reduced in abundance so that they are now found in only a small part of their previous distribution.

There are lots of gardeners who want to “do something” for the pollinators, and keeping honey bees is often mentioned.  By all means, if you wish to help the honey bees (which are suffering their own problems) then keep a hive or two.  That will not, however, help our wild, native pollinators; the analogy I use is that it’s the equivalent of trying to help our declining songbirds by opening a chicken farm!

If you want to make a real difference for pollinators in your own garden, here are a few ideas:

  • start by planting nectar and pollen rich flowers; there’s a useful list on the Royal Horticultural Society’s website (see below).
  • allow plants such as clover and dandelion to flower in your lawn, bees love them.
  • as well as food, pollinators also need nest and egg laying sites, so you could help by allowing some of the far corners of your plot to run a little wild.
  • wait until late Spring to cut back hollow stemmed perennials as they are used as hibernating places by some of our bees.
  • allow mason bees to nest in old walls and don’t worry about them, the wall won’t fall down.
  • And finally, stop using pesticides!

Changing some of our gardening habits can help a group of insects on which we rely and which supports our economy in a very real way.

 

Further reading and information:

Bees Wasps and Ants Recording Society:   http://www.bwars.com/

Bumblebee Conservation Trust:  http://www.bumblebeeconservation.org.uk/

Butterfly Conservation:  http://www.butterfly-conservation.org/

Hoverfly Recording Scheme:  http://www.hoverfly.org.uk/

Royal Horticultural Society’s list of plants for pollinators:  http://www.rhs.org.uk/Gardening/Sustainable-gardening/Plants-for-pollinators