Earthworms and maple seeds   Andrew Joslin
  Aug 21, 2006 12:07 PDT 

Hello ENTS,
I wanted to run this past the experts. Since I'm somewhat isolated
from the tree biology community I may be noting something that has
already been well documented. Your feedback is appreciated.

I've observed that earthworms (nightcrawlers) are prolific planters
of maples and probably any deciduous tree species that have "winged"
seeds. It goes like this... The silver, red and norway maples in my
urban neighborhood seasonally carpet the yards and gardens with
winged seeds, the classic maple seed form. A few years back I looked
at my flower garden and thought "I have to clean up all those maple
seeds out or I'll be weeding the seedlings next spring". I was busy
the next couple of days and finally had a chance to get to the garden
and pick them up. I was surprised to find that what was a carpet of
leafy seeds was reduced to a scattered handful. Where did they go? A
closer look revealed that foraging nightcrawlers had been collecting
seeds. Each nightcrawler burrow had a clump of maple seeds pulled
into the top, the wings in first. Along with the maple seeds were
leaves also collected during the night. Over the years I've continued
to check out these clumps of seed and leaf detritus at the top of
nightcrawler holes. If you pull a clump out you can see that the worm
has been consuming the results of its nightly forays at leisure from
the safety of it's burrow below. The leafy material is stripped off
of the seed wing. I suspect that during the day predators like Robins
listen for the sound of nightcrawlers tugging and chewing on their
food store at the top of the burrow. If you put your ear down close
and listen you can sometimes hear it.

So the worm gets the seed into the ground. Then it provides a perfect
nursery setting for the maple seed to sprout. In the clump at the top
of the hole the seed is nestled in with gathered leaves and castings
from the worm's digestion. The gathered dead leaves hold moisture and
the castings provide ready nutrient for the new roots of the maple
seedling. It appears that the winged seed does more than disperse the
seed from the parent tree, it provides a meal in exchange for
planting and nursery services.

Andrew Joslin
Jamaica Plain, MA
Re: Earthworms and maple seeds   Lee Frelich
  Aug 21, 2006 16:11 PDT 

Andrew:

Its just a coincidence that these invasive earthworms happen to be able to
eat the wings of maple seeds, but not the seeds themselves. They just don't
have the appropriate mouth parts or size necessary to eat the seed itself.
Most seed predators of maple seeds eat the seed and throw away the wing.
But European earthworms are detritivores--they eat leaves and similar
materials. This is not a case like ants and many woodland wioldflowers
(violets, Trilliums, etc.), which have an elaiosome, or fat globule
attached to the seed, which the ants take and eat, and discard the seed
itself. There is true coevolution involved there, as opposed to coincidence
with European earthworms and maple seeds.

European earthworms are fundamental ecosystem engineers, and the few maple
seedlings they may be promoted are a drop in the bucket compared to the
billions of maple seedlings in the forest that have no chance of survival
because these earthworms make forest soils more nutrient poor, drier and
harder. Our research shows that a cascade of impacts throughout the
ecosystem starting from earthworm invasion leads to regeneration failure of
sugar maple.

Lee
Re: Earthworms and maple seeds   Andrew Joslin
  Aug 22, 2006 06:04 PDT 

Thanks for the response Lee. Sorry if I wasn't clear, the earthworms
are indeed not eating or trying to eat the seeds. They are only
interested in the leafy wing. I believe that nightcrawlers may have
coevolved with Norway Maples in Europe and are instrumental in
planting them. Note the very large wing on the Norway Maple, an
attractive morsel for an earthworm. The planting of indigenous maple
species (I've only observed Silver Maple planted by earthworms) is
coincidental in that the seed wings are similar to the introduced
maple species. I believe you when you say that the overall effect of
introduced earthworms on North American forests is negative. What I'm
observing is in the urban ecosystem where the earthworms and maples
seem to get along pretty well.

Andrew Joslin
Jamaica Plain, MA
Re: Earthworms and maple seeds   Lee E. Frelich
  Aug 22, 2006 07:41 PDT 

Andrew:

I am sure you are right about Norway maple and European earthworms being
well adapted to each other's presence. That's probably why Norway maple is
only invasive in areas where European earthworms have invaded first. There
is some evidence that the same happens for European buckthorn and garlic
mustard--they do not invade areas without exotic earthworms, which
literally plow the way for other exotic species to enter the ecosystem.

Lee