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Animals I: birds, bats and bumblebees

I should probably send Rob Hopkins a thank you note. I’m not at all sure he meant to draw attention to the Green Wizards project just as the forum at went live, but that’s the way it turned out, and the results have gone past my most improbable hopes. Measures of forum activity I’d hoped to pass in six months have been shouldered aside in six days, and the forum staff are scrambling to deal with a far more lively online community than any of us expected this soon.

To all those who have participated in the Green Wizards forum so far, I certainly owe a hearty thanks, and to judge from comments fielded there and elsewhere, the best way to express it is to plunge onward into the next phase of green wizardry and start handing out more practical information. That’s the agenda for this week, certainly, and the subject under discussion ought to be dear to the hearts of prospective green wizards. By the time you’ve finished with this week’s work, you may not be able to call spirits from the vasty deep as Glendower claimed to do, but you’ll be able to call helpful critters from the surrounding ecosystems to help maintain the balance of your garden – and yes, to forestall Hotspur’s gibe, they will indeed come when you do call them, if you do it in the right way.

Let’s start with basic concepts. A garden is an ecosystem managed in such a way that human beings get to eat a significant fraction of the net primary production of the plants that grow there. Net primary production? That’s the amount of energy each year that the plants in a given ecosystem take in from the Sun and store in the form of sugars and other compounds that can be eaten by some other living thing. Everything other than plants in any ecosystem gets its fuel from the net primary production of that ecosystem, or of another ecosystem that feeds energy into it.

You’re not going to get anything close to a majority of the net primary production of your garden onto your dinner table, by the way, and it’s a mistake to try; if you do, you’ll starve other living things that depend on a share of net primary production to keep their own dinner tables stocked, and you need these other living things in order to have a healthy and productive garden. (Ignoring this latter point is one of the critical errors of today’s industrial agriculture.) Your goal instead is to make sure that as much of the net primary production diverted from your table as possible goes to living things that earn their keep by doing something for your benefit.

Here’s an example. A certain amount of each year’s net primary production from your garden goes to feed earthworms. Any gardener with the brains the gods gave geese won’t grudge them their share, because earthworms break down organic matter into forms plants can use, and they improve the texture and drainage of soil as they do it. Charles Darwin – yes, that Charles Darwin – wrote a brilliant and too often neglected book on the role of earthworms in the creation of topsoil; what he found, to drastically simplify a classic piece of ecological research, is that earthworms are topsoil-making machines, and the more you’ve got, the better your soil and the higher your crop yields will tend to be.

Now the logical conclusion to all this, at least according to the logic of modern industrial society, is that gardeners ought to run out and buy earthworms by the carload. As it happens, this is rarely a good idea. There are bound to be some earthworms in your soil, and since earthworms are hermaphroditic and fertile most of the time, there’s generally no shortage of baby earthworms starting out on their slimy and subterranean lives. The question, if you’ve got a worm shortage, is why so few of them grow up to become the big pink nightcrawlers that haunt fishermen’s dreams.

This is where another of the fundamental principles of ecology comes into play. Liebig’s law, named after the 19th century German agricultural botanist Justus von Liebig, has the interesting distinction of being at one and the same time one of the most consistently valid principles of ecology and one of the most consistently rejected concepts in modern economics. The short form of the law is that for any organism, whatever necessary resource is in shortest supply puts an upper bound on the carrying capacity of the environment for that organism.

To understand how this works, imagine a plant growing in your garden. That plant has a variety of needs – water, nitrogen, phosphorus, potassium, an assortment of trace elements, and so on. If the soil is short of any one of them, it doesn’t matter if all the others are abundant; the nutrient in short supply will determine how well that plant can grow in that garden. Readers familiar with the rhetoric of today’s economists will recall the claim that if humanity runs out of one resource, we can always replace it with another; this claim amounts to insisting that Liebig’s law doesn’t apply to human beings – though it’s a rare economist who knows enough about nature to recognize that.

There are good reasons to think that the economists who make this claim are dead wrong, but that’s a topic for another time. The point that needs making here is that all the living things in your garden are subject to Liebig’s law, and if you want more of some particular organism in your garden, the way to get it is to find out what the resource in shortest supply is, and provide it. With earthworms, most often, it’s the sheer amount of organic matter in the soil that’s the limiting factor, and the more organic matter you put into the soil – by hoeing in compost, using mulches, planting green manures, or what have you – the more infant earthworms will mature to massive pink nightcrawlerhood and get to work improving your garden soil.

The same rule governs all the other useful critters you might want to attract to your garden. Bats are an example too rarely considered by organic gardeners. Why so many people fear and dislike bats is beyond me; any animal that can eat its body weight in mosquitoes in a single night, after all, should be a welcome guest wherever it goes. Still, the benefits bats bring to the garden outweigh the simple pleasure of not being eaten alive by the insect world’s answer to Count Dracula. Many of the grubs that cause serious damage to food crops – the corn borer, the apple maggot, and more – are the larva of night-flying moths, and night-flying moths are prime bat food.

The limiting resource for bats, nearly always, is daytime shelter during the non-hibernating months, and so one very easy way to bring bats to your garden is to build or buy a bat house and set it in an appropriate place. Both the house and its placement require a certain degree of care – bats, like nearly all other animals, are particular about their homes – but their preferences are well known and the resources given at the end of this post will provide you with the information you need.

Get a proper bat house in place, and in most cases you can count on a crew of bats finding it and taking up residence in a fairly short time, and thereafter any problems you may be having with moth larvae will become a good deal less severe. You’ll also be doing a good turn for the bats themselves; recently, a fungal disease called white nose syndrome has caused high death rates in many North American bats, and ensuring plenty of housing and habitat for the survivors will help bat populations survive the epidemic and recover quickly once it begins to pass off.

Birds are the day shift to bats’ night shift, and some varieties of birds are well worth attracting to your garden as well. Swallows, swifts, and martins – a closely related group of birds with tapering, pointed wings and a prodigious appetite for insects – are a classic example. Until the advent of chemical agriculture, farmers across North America went out of their way to encourage barn swallows to set up housekeeping in and around their farms, because swallows do exactly what their name suggests to a great many daytime insects that make life difficult for crops. Like bats and most species of birds, swallows and their relatives are particular about their homes; here, though, this is a double advantage, because homes well suited to swallows are uninviting to starlings and other birds that damage crops.

Another set of living things your garden needs is pollinators. The collapse of honeybee populations over much of the industrial world has been all over the news over the last few years, and for good reason. Without pollination by insects, many food crops don’t produce or reproduce, and honeybees have long been the primary pollinators of most commercially grown fruits and vegetables, with hives being trucked from farm to farm over hundreds of miles in season.

Exactly what is causing Colony Collapse Disorder is uncertain as yet, though some evidence points to a class of recently introduced pesticides – neonicotinoids such as imidacloprid – which are highly toxic to bees and can build up in a hive’s honey supply to lethal levels. Until the issue gets sorted out, making sure that your garden has backup pollinators in place is crucial. Domesticated honeybees are one option, but beekeeping is not a project for everyone; another, far less demanding option is to increase the population of local species of wild bees.

Spend some time outdoors watching flowering plants and you’ll quickly discover just how diverse a range of insects can play the pollination game. Many of them are bees of one kind or another, for there are thousands of kinds of bees, each with its own lifestyle and preferences. Very few of them have the complex social structure and hive life of the honeybee, and even fewer of them have a sting painful to human beings. Most are solitary, harmless, and short-lived, hatching in the spring and mating almost immediately, after which the males die and the females spend the rest of their lives laying eggs in burrows of one kind or another; each egg will hatch out the next spring as a bee of the next generation.

The orchard mason bee is one variety of solitary bee that has become a popular pollinator in some areas, especially where fruit trees are grown. The limiting factor for orchard mason bees is nesting sites for the females to lay their eggs, and this can be provided with a simple wooden block drilled with a lot of 5/16" holes. Nesting blocks for several other species with similar life patterns can also be made or bought; again, the resource section at the end of the post gives details.

Another common wild bee with a somewhat more complex life pattern is the familiar bumblebee, large and furry, that can be found visiting flowers through the summer months in most of North America. There are many species of bumblebees; all of them dwell in small underground hives which they build in abandoned burrows, and they have queens who live for several years and workers who live only one. The limiting factor in their case is not homes, but homes safe from predators such as field mice, who like to dig down into hives and eat the larvae.

The way to make Liebig’s law work in bumblebees’ favor is to take a small wooden box full of cotton wool, and with a short piece of old garden hose extending from a hole in the side maybe six inches. Bury the box in the ground in a secure, fairly dry place, so that the end of the hose just pokes out of the ground. Once a newly hatched queen finds it – which rarely takes more than a single spring – you’ll have a bumblebee hive full of pollinators who will do their duty for your garden and the wild plants around it as well.

All pollinators need something in flower to feed on for the entire period they are active, which for orchard mason bees extends from March to the end of May in most areas, and for bumblebees, depending on the species, can run from sometime in the late spring well into autumn. The absence of flowering plants can be a limiting factor for all kinds of bees, and if the area around your garden is short on flowers at some point in the season, a flowering shrub or two to fill in the gaps is a good investment. We have a buddleia in our front yard that serves as lunch counter for a dizzying array of daytime insects, including nearly a dozen species of wild bees; your local ecosystem will have appropriate shrubs that will fill the same role.

The same principle can be applied in many other ways. Just as you can encourage a species by figuring out what resource it needs is in shortest supply and providing that resource, in other words, you can limit an unwelcome species by figuring out its resource needs, and doing your best to make sure that one of those needs is as scarce as possible. As you work with your garden, and learn more about the complex ecosystem that an organic garden develops around it, pay attention to places where a little careful tinkering with variables can increase the population of something you want, and decrease the population of something you don’t want. It’s not so clumsy or random as a pesticide, as Obi-Wan might have put it: an elegant method of the more ecologically sane age toward which, willy-nilly, the pressures of the present are forcing us.


Net primary production and Liebig’s law are covered in most college textbooks of ecology, and if you’ve got one of those, it may be worth your while to read back through the sections discussing these two concepts.

For earthworms, Charles Darwin’s The Formation of Vegetable Mould Through the Action of Worms remains the classic study, and readers who can handle the leisurely pace and extensive vocabulary of an earlier age of scientific writing shouldn’t miss it.

For bats, Merlin Tuttle’s America’s Neighborhood Bats is a good introduction, and Bat Conservation International provides extensive resources for bat house construction and other details of living around bats. For useful birds such as swallows, swifts, and martins, your local chapter of the Audubon Society can get you information about the species that are local to your area and their nesting requirements.

For bees, two books by Brian L. Griffin, The Orchard Mason Bee and Humblebee Bumblebee, are good primers with plenty of detailed information; they are written from a Pacific Northwest perspective, however, and some of their advice may need adjustment in different climates.

What do you think? Leave a comment below.

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