Plants in ecological webs

Images: spider web © Kenneth Allen, CC BY-SA 2.0; ants tending aphids © Judy Gallagher CC BY 2.0

Note: This is a science outreach piece belonging to a series I wrote for the newsletter of the Fredericton Botanic Garden.  I’d be happy to see it modified for use elsewhere and so am posting the text here under a CC BY-NC 4.0 license. If you use it, though, I’d appreciate hearing where and how.

A visit to any Botanic Garden surely means attention paid to plants – that’s what “Botanic” means, after all.  When you visit our Fredericton Botanic Garden, for example, your attention will probably first be drawn to our flowerbeds and forests; to the primulas in the Hal Hinds Garden and the daylilies in our newly expanded Daylily Bed; to the reeds by our ponds and the ferns along our Woodland Fern Trail.  All these beautiful plants are worth your time – but we hope you’ll look beyond them, too.  That’s because each of our plants is also part of a larger ecological web.

Take the aphids on the green stem at right.  A gardener might be horrified by their arrival, and if aphids are devastating your prize rose you might pardonably reach for a spray bottle and some dish soap.  But look closely, because there’s a lot going on in, and behind, that picture.  The aphids are there, of course, for lunch, with their mouthparts inserted into the plant’s phloem tissue for sap.  That sap has lots of sugar, but only very, very small quantities of amino acids, vitamins, and other nutrients that the aphids need.  In order to get enough of those rarer constituents, they process a lot of sap – passing most of it right through their bodies and producing a copious, sugary exudate called honeydew.  And that’s why the ants are there: harvesting that honeydew.  Ants can’t access harvest plant saps directly, but aphid honeydew is a conveniently tapped keg.

That’s three species – plant, aphid, and ant – but the ecological web extends far beyond that.  The ants aren’t just taking honeydew; they’re farming the aphids.  They tend them, and protect them from enemies to preserve their honeydew supply (so if you thought the 10,000 year record of human agriculture was something to be proud of, think again).  The aphids’ enemies include lacewings, assassin bugs, and the larvae of ladybird beetles, all of which are voracious predators.  They also include parasitic wasps that, if not repelled, will inject their eggs so that their larvae can develop feasting on living aphid tissues, devouring their unfortunate victims from the inside out.  Not pleasant for the aphids, one imagines; but essential for the wasp species.

That’s not the end of the web, either.  Wasps and lacewings and aphids alike can be prey for insectivorous birds such as warblers that glean their meals from the surfaces of plants.  Furthermore, the plant that supports this whole aphid-based community interacts in other ways with its neighbours.  If the aphid load is high enough – and, of course, the ants are trying to keep it that way – then the plant’s growth will be suppressed, to the advantage of competing vegetation nearby.  We need to look down, to, belowground.  The plant couldn’t grow in the first place without its “mycorrhizal” partners: fungi that grow in association with the plant’s roots, helping it harvest nutrients from the soil in exchange for a share of the sugars the plant synthesizes from water, carbon dioxide, and sunlight.  Ants, then, might disadvantage mycorrhizae by protecting the aphids that extract phloem sap that otherwise supplies the roots with sugars.  These connections don’t stop.  Whether you zoom in to the microscopic (“endophytic” fungi living inside stem and leaf tissue) or zoom out to larger scales (hawks preying in their turn on the bird predators of the insects), the web can be traced ever further from the cluster of aphids where we began.

So in a Garden, or anywhere else, think webs (and not just spider).  An aphid on a plant might be a gardening annoyance, but it might also be a thread that, pulled upon, leads to fascination.  Nature is like that: there’s always more to reward the curious eye.

© Stephen Heard  June 7, 2018; CC BY-NC 4.0

Other Botanic Garden newsletter outreach pieces:



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