An introduction to the ecology of populations
Last month we began a discussion on pest insects and biodiversity. An image that was introduced was that of the seasonal pulse of plant growth over the face of the globe — the shift in green, when looking at the Earth from space, from northern to southern hemisphere and back again. As long as there have been green plants, this heartbeat of green has been happening.
Each spring across the temperate zones, the explosion of fresh, tender foliage is closely followed by an explosion of the insects that eat that foliage. The population burst of insects is a resource bonanza for insectivorous creatures: amphibians, reptiles and especially birds.
In order for eco-farmers to better manage our crops and livestock (and even crops in storage), it is in our best interest to have a basic understanding of insect population dynamics. To illustrate this principle of nature, let’s turn our attention to the bane of summer picnics and farm kitchens: the common housefly (Musca domestica).
In a foundational study on houseflies done years ago, entomologist L.O. Howard learned that a female housefly produces an average of 120 eggs at a time. Statistically, half of these eggs develop into females, and seven generations are hatched every year. I.e., half of the 120 eggs (60) would be females, and each of those 60 would in turn lay 120 eggs. Do the simple math (60 X 120) and you see that an initial generation of one male and one female produces 7,200 offspring in the second generation. If we do this a second time, 3,600 females (half of the population) each produce 120 offspring, becoming 432,000 houseflies … and the summer isn’t even half over yet! By the time seven generations go by there is a staggering 5,598,720,000,000 houseflies — over five trillion!
The reproductive capacity of a population is called its biotic potential. As a rule, the smaller the organism, the larger the biotic potential, and vice versa. Elephants and blue whales increase their populations much more slowly than houseflies or bacteria.
If all of the houseflies ever produced in a generation survived, it probably wouldn’t take more than two or three years for the entire surface of the planet to become covered with houseflies several feet deep. Why (mercifully) doesn’t this happen? Because the biotic potential of the fly population (and of all populations of all organisms) is balanced by what ecologists refer to as environmental resistance: not all housefly eggs hatch, not all larvae successfully pupate, not all pupa emerge as adults, not all adults are able to mate, and many are limited by competition for resources, predation and parasitism.
An understanding of these concepts, and how to use them as tools, will help us to reduce “pest” populations and to enhance the populations of our desired target species (our crops).
First and foremost, with an understanding of population dynamics, we come to realize that the earlier in a pest’s life cycle we intervene, the greater the leverage we have over that pest’s population. If we are able to prevent the initial male and female from ever mating in the first place, we would prevent the “downstream” population boom, and we could enjoy a Labor Day picnic with no flies on the watermelon. It is only when we have allowed several generations to pass that we begin to see economically significant damage.
This is not a “pest problem” or a “weed problem” but a management issue. We allowed the population to expand past the easy stage to control. By the time we notice the “problem,” the pest population is so much greater and the size and scale of our intervention needs to be even greater still — and will meet with less success. Organic farmers and chemical farmers who practice Integrated Pest Management (IPM) know that this is the key to reducing pest control (and disease and weed control) with maximum effectiveness and minimal expense.
This article is obviously just scratching the surface of the importance of understanding population dynamics. In forthcoming articles we will be delving into ways that we can directly intervene to increase our system’s environmental resistance to pests and disease. We’ll cover topics like nutrient management, mating disruption, life cycle disruption (egg laying, pupation, emergence), pest diseases, competition for resources, parasitism and predation.
Between now and then, here’s some homework. What are your biggest weed or pest challenges? Not just generally, but specifically — “worms in the apples” is not good enough. Specifically identify the pests, weeds or diseases that are causing challenges. Learn as much as you can about its life cycle, from birth to death, and the conditions it requires to complete that cycle. From there we’ll help to outline a general strategy, and you can specifically tailor those principles to fit your operation.
Mark Shepard is a land designer and consultant and is the author of Restoration Agriculture, Water for Any Farm and the Water for Any Farm Technical Manual.
