My wife has houseplants. When she waters them, she sometimes walks behind me, dribbles a little water on my head, and says, "Grow little flower grow."

But let me squirt her with water from the hose just once and she won't help with the yard work anymore. OK, maybe it was twice. Three, max!

Automatic sprinklers have put an end to a lot of science education. I learned, as a child, the effect of putting one's thumb over the end of the open hose to create a bottleneck.

If the opening is made smaller, but the weight of all the water behind the bottleneck is pushing water through, the water must move faster — and farther. The evidence of bottlenecks, under pressure, is well known and demonstrated with garden hoses.

But what if there is a bottleneck and very little pressure from behind? Well, of course that creates a dam and restricts movement. I learned about that from playing in an irrigation ditch. In the case of water, a lake is created.

In my case, it also created a very angry grandfather. In the case of traffic, it creates a traffic jam. In the case of living organisms, it can restrict population size.

Animal and plant reproduction occur through time, like the flow of water. We often measure how long it takes for a natural process to occur and in what direction it goes.

For example, we know how much water passes a given point each minute. But we don't always recognize the bottlenecks that affect the process between points of measurement. This is especially true in the biological sciences.

When we see a decline in a population of certain species of plants or animals, we usually respond by providing more food or restricting predation. Sometimes both, or either, of these can be helpful.

But there are other requirements for a population of organisms to grow or remain healthy, and they often depend on bottlenecks in their timelines.

Take an infectious disease. To increase in a population, the organism that causes the disease must find a new, uninfected, non-immune organism to infect.

Vaccination interrupts the number of available hosts for the disease, creating a bottleneck that restricts further infections. Vaccination has a public health role, as well as an individual's protective role.

A study by Dr. Andrew Higginson at the University of Exeter in England found that, when nest sites are hard to come by, the species that suffer most are those that nest later in the year.

He analyzed population changes in more than 200 bird species and 40 bumblebee species around the world and found that nesting sites and availability of food sources can both become bottlenecks.

In other words, the early bird not only gets the worm, it gets the nesting site.

Native bees, such as bumblebees, the blue orchard bee, and many others often have specific nesting sites that are used from year to year.

However, because of the places chosen and the native bee life cycle being somewhat invisible for much of the year, these sites are often obscure from our sight.

They are then easily disturbed or destroyed by humans who don't even know they are there. The same is true of many small insects that are not well-studied or understood.

Long-term scientific studies over extended areas are infrequently done. They are difficult, tedious and expensive to conduct.

In addition, many scientists have very short attention spans. My wife suggests that might just be me.

Anyway, we may find that the decline of insect populations has more to do with loss of nesting and breeding sites than with lack of food, predation, or even pesticides.

As humans disrupt nesting sites without even knowing it, their actions become bottlenecks for many animals. It may not be enough to sprinkle a little water on their heads and encourage them to grow.

Gary McCallister, gmccallister@bresnan.net, is a professor emeritus of biological sciences at Colorado Mesa University.

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