Tuesday, September 15, 2015

Squirrel Depredations--the Other Side of the Coin

This is the season of seed dispersal for many plants.  Plants have a significant disadvantage compared to animals: they don't move.  But because they depend on sunlight for energy, adult plants must have a way to get their children away--out of the parents' shade.  So if the adults can't move, the children have to.*

Plants have many ways of dispersing their young.  If you've blown on a downy gray dandelion head, or pulled burs or beggars' ticks from your clothing after an autumn walk, you've seen examples of two of the most popular means of dispersal.  Many plants build their fruits so that wind can carry them away.  Dandelions and milkweeds have their silky parachutes, basswood fruits hang glide to the ground on a wing-like bract, samaras twirl to the ground slowly enough to drift a bit on the breeze.  Other plants depend on animals to move them.  Berries have a taste and appearance that attracts animals; they are typically eaten by birds and mammals, and their seeds are pooped out later in convenient fertilizer.  (The seeds inside the fruit are built to safely withstand digestive processes.)  Like a flower's offering of nectar to bees, you can think of tasty fruit as part of a business deal.  Beggars' ticks make no deal, offering animals nothing at all in return for dispersing their kids: they simply send them to hitchhike on fur or clothing.

Maples like this sugar maple are among the many fruits that are wind-dispersed.

It will be a month or more before milkweed pods split, 
each seed drifting off under its own plume. 

 Hang-gliding basswood fruits don't usually get far, but the one below landed half a block away.

Blackberry, above, and nightshade, below, attract animals with bright color and reward with sugar.

Flowering dogwood produces highly-nutritious fruit and advertises it with bright color.

Witchhazel is unusual: instead of using wind or animals, its "ballistic" seeds 
are ejected with enough force to travel up to thirty feet.

What about the acorns of oaks?  An acorn that germinates where it falls is doomed.  They are much too heavy to be wind-dispersed, and the trees make no attempt at it.  On the other hand, the oak babies are not enclosed in a tasty fruit, and are not adequately protected from digestion.

But the acorn embryo is itself a very nutritious food.

 Oaks such as the black oak above and white oak below feed many more animals--large and small--
than just the jays and squirrels principally responsible for dispersing them.

One of many dangers of urban life.  (At least the end was quick.)

In other words, the oak tree offers its children as food to animals.  A bit like Abraham offering to stab his only child!

How could oaks possibly reproduce successfully this way?

The acorn is a staple food of squirrels and jays.  Both of these animals store food for the winter by burying them in small, scattered caches to retrieve later as needed.  Both these animals have prodigious memories.  And squirrels, at least, often move their nut stores repeatedly, with the acorns getting farther and farther from the parent tree.  But their memories aren't quite perfect, and nuts are forgotten or not needed.  And sometimes a squirrel or jay does not survive to eat all its stores.  These forgotten nuts are only a few percent of all those hidden, but they are well-placed to germinate and found the next generation.

The oak may sacrifice 99% of its kids for the sake of the one percent.  And of course the babies that survive to germinate face many other risks, so their survival is still not assured.  On the other hand, of the thousands of acorns a tree produces each year, over a reproductive lifetime of many decades, only one offspring* must survive to healthy adulthood in order to continue that oak's genes in the next generation.  Only one.

Still, it's a cruel calculus for the oak.

 This squirrel scolded me for interrupting.  When I was able to imitate his raspy note,
we scolded back and forth for nearly a minute.

*Earlier in the year their sperm need to move, also, and they do this inside pollen grains.  That's another thing plants can't do the same way many animals do: sex.
**Of course, that's the rule for any living thing: if parents produce more than one surviving offspring a piece on average, the world would be overrun by that species in fairly short order.  (But "on average" covers a lot if variation in the short term!)  And of course the human species has repeatedly found ways around this rule, to the woe of most other species. 

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