Category: ecology

Suburban ecological traps

Suburban ecological traps

An ecological trap is a situation that may seem appealing to organisms, but that is actually detrimental to them.  The situation that made me think of this tonight is the intense mate-calling of frogs in our backyard.  We have a swimming pool in our backyard.  We put a cover over it in the winter to prevent leaves and gumballs (from the sweet gum tree) from accumulating in it over the winter.  Then in the spring, we scoop all the leaves and gumballs off the cover and remove the cover to “open” the pool.  It’s easier to scoop the leaves and gumballs off the cover than it is to try to scoop them out of the bottom of the pool.

This year, I don’t know if we were a bit late or if the frogs were a bit early, but frogs began laying eggs in the water on top of the pool cover before we had gotten a chance to remove it.  All of a sudden, it seemed, there were hundreds of tiny tadpoles in the water on the pool cover.  While we typically have no problem draining the insects from the pool cover to their deaths in the terrestrial habitat of the back yard, we couldn’t fathom killing literally hundreds of vertebrates– the tadpoles.  So, to make a long story short, over a period of a few weeks, we removed overall a grand total of a few thousand tadpoles and transported them to a nearby pond to complete their little froggie lives.  Then we removed the pool cover.  End of story, right?

Well, I had thought so.  These are grey tree frogs, and they typically lay their eggs in still water.  Our pool filter is constantly moving the water, so it doesn’t qualify as “still water”.  To me, at least.  Do insects lay their eggs in our swimming pool?  We have observed dragonfly mating in the pool area and it is possible that they lay eggs in our pool.  But if they do, the eggs would get sucked into the pool’s filter and cease to exist.  I’m certainly fine with that happening to any mosquito eggs that may be laid in the pool.  What about dragonflies?  They’re more “charismatic” than mosquitos…  And what about frogs, which are fellow vertebrates, after all– a characteristic that for some reason makes us more willing to empathize…

So, this evening, the frogs in our backyard were calling at a decibel level topping 80 at times. There were a half dozen frogs that I could easily see around the pool, and more calling that I couldn’t see.  I watched as a female selected the male calling from the raft we leave in the pool as a “life-saving device” for critters that may fall in.  I guess, actually being *in* the pool made him the king of the area.  Amplexus occurred, but then I watched at the female, with the male on her back, swam half the length of the pool, climbed out (with the male still hanging on), and disappeared into the night without releasing eggs.  There is no other body of standing water within at least a few hundred yards, so it seems that this mating encounter was fruitless.

amplexus

So, is our pool, and are other suburban pools, an ecological trap?  The presence of the swimming pool as a water body lures the frogs into thinking it’s adequate mating habitat.  The males expend a good deal of time and energy calling for mates and defending territories where their offspring will never be produced.  The females seek out mates and carry them around for a while but, if eggs are laid, they are surely sucked into the pool’s filtration system and never hatch– thus no offspring will result from their efforts.  This would be the same for dragonflies, mosquitoes, and any other organisms trying to lay their eggs in water.

I propose that backyard swimming pools are a substantial ecological trap, contributing to the recent and significant declines in populations of amphibians and insects with aquatic larval stages.  A quick search on Google Scholar revealed zero hits for papers on “swimming pool” and “ecological trap”.  Think I could get funding to study this?  😉

Low pollinator populations this summer

I have noticed unusually low populations of pollinators this summer.  Very rare is the occasion this year that I have seen any pollinators at all on specific plants that, in past years, have been swarming with bees, flies, and wasps every time I walk by them.

One possible reason for the low numbers of pollinators could be the ever-increasing use of pesticides.  For the last several years, field biologists have been warning that pollinator numbers have been getting dangerously low.  You may recall the hype about Colony Collapse Disorder affecting honeybees or the outcry from many scientists and environmentalists that the abundant use of neonicotinoids (a group of insecticides outlawed in the EU but abundantly used here in the U.S.) is having severely negatively effects on native pollinators.

But, the low numbers this year could be due to the fact that we had an exceptionally warm winter.  Many plants began flowering remarkably early this year and many insects emerged very early due to the warm ground temperatures.  Then, in mid-March, we had about a week of very cool days with nights below freezing, and even a dusting of snow.  Possibly, insects that had emerged from hibernation early due to the abnormally warm soil temperature were unable to sustain themselves through that cold snap.  I certainly noticed far fewer canker worms in our trees this year than there typically are, possibly for that same reason.

When the numbers of a species fall below a certain threshold, it becomes very difficult for them to bounce back.  The many types of insects we rely on to pollinate our crops and that are relied upon for food by so many species of birds, amphibians, and reptiles are suffering a myriad of hazards these days.  Between pesticide use, habitat destruction (clearing of land for development), and the wacky climate patterns we’re experiencing due to the melting of the Arctic ice cap, it would be surprising indeed if populations of insects were stable!

At what point will we need to begin hand-pollinating our crops?  Apparently, in some parts of China, they’re already doing so.  I don’t think that is the kind of “job creation” our next generation will appreciate our leaving them…

Leaf miners

Leaf miners

I love the little designs I see in leaves made by “mining” insects.  In the picture above, you can see trails from leaf miners on the leaves of columbine and on a species of lily.  The lines are made by larvae of beetles, flies, or moths that live inside the leaf.  They crawl around in between the upper and lower layers of the epidermis, munching on the mesophyll cells.  Being sandwiched between the layers of the leaf epidermis, the larvae are safe from predators while they’re growing.  As adults, they emerge from the leaves, and then the females will lay their own eggs on the surfaces of other leaves and the cycle will begin again.

leaf cross-section

The shade of a tree

The shade of a tree

Having the shade of a canopy tree is greatly appreciated on a sunny day.  How often do we think about how long it took that tree to get large enough to provide us with that shade?  How long will it continue to live and provide us with shade?  While many trees can live to be hundreds of years old, it is not the norm.  Think about how many acorns each oak tree produces each fall.  Consider how many of those spiny balls fall off each sweet gum tree; those each contained a few dozen seeds.  Now think about how many hundred-year-old trees there are where you see those acorns and sweet gum balls.  Not very many of those babies make it to old age.

Many of the largest, oldest trees in Charlotte were planted about a hundred years ago.  It is not unheard of for trees to live to be a few hundred years old; some trees have been alive for more than a thousand years.  But trees growing in an urban, or even suburban, environment have to put up with numerous stressors they would not have in a forest.  They are subjected to significant air pollution, light pollution, soil compaction, and reduced water availability.  The soil is nothing like what would be found in a forest.

I have two tulip poplar trees in my front yard that are probably around 70 years old.  This is relatively old for urban trees.  They provide ample and greatly appreciated shade to the house and yard.  They’re healthy now, but if I want to maintain a shady house and yard, I need to plan for their replacements now.   One might argue that there’s not room in my front yard for more than two large, mature trees.  That’s true.  But my planning now for the demise of the two current residents doesn’t mean I am going to plant two large trees now.  It means I will plant small trees now.  Those small trees will grow very slowly for the next decade or two, in the shade of the large poplars which have deep roots, drawing nutrients and water from depths unavailable to the small trees.  Then, in a decade or two or three, when the tulip poplars are no longer healthy and come down, I will have a couple of sub-canopy trees that will then be able to quickly take advantage of the extra water, nutrients, and light available to become good shade trees in a handful of years as opposed to my having to wait another 40 years for newly planted trees to grow.

IMG_8026