Ecologists have for years argued the concept of diversity, what it means and what its importance is. The argument continues. A workable definition is that diversity is a combination of species richness and equitability. Stated simply, diversity is a combination of the number of bird species and the numbers of individuals in each species. It’s obvious that a community of three species with ten individuals in each is more diverse than a three species community with 26 individuals in one species and two in each of the other. But intuition goes just so far.

A classic paper by R. and J. MacArthur proffers an elegant solution. If we know the number of bird species and individuals of each in a habitat, the MacArthurs’ formula provides a measure of bird species diversity and gives us a specific number that we can use to compare communities. We know that birds segregate themselves at least partially by their foraging locations, so let’s also measure the density of the vegetation at different heights (a tall tree is different than a short shrub, and a dead tree different than a live one, for example). We plug these measures into the same formula and come up with a number for foliage height diversity. The MacArthurs’ formula demonstrates, perhaps surprisingly, that there is no relationship between bird and plant species diversities. It doesn’t matter how many different kinds of plants there are but there is a close relationship between the vegetation structure and the diversity of birds.

The more complex and varied the plant structure, the more niches are available to allow the survival of various bird species. This foliage height diversity measure is reliable enough that a competent birdwatcher can predict with some accuracy what birds and in what abundance they will occur in a specified habitat. So now we have bird species diversity down to a simple number that we can use to compare year to year in the same habitat. If the numbers change, something is happening to the bird population that should be investigated. It can also be used to compare different habitats, eliciting interesting questions as to why the differences exist.

            Why should we care about diversity? Ecologists and conservationists have argued for years that diversity means stability, and that complex ecosystems are more likely to resist and recover from perturbations than simple ones. There has been considerable discussion and  disagreement about that idea, but it does make some sense. Consider a mechanical pocket watch. The watch has a bunch of parts, some more essential than others. We can remove a few parts like the crystal, second hand, numbers on the watch face, and the watch face itself, and the instrument will still work. But keep taking parts out and at some point the watch will malfunction. Do the same with an ecosystem. Remove one of the frugivores and other frugivores will likely fill in and provide seed dispersal. Eliminate a nectar-supping pollinator and another one will probably take its place. Kill off one species of hawk and the forest will probably continue to function more or less as before. But continue to simplify the system by removing more birds and the ecosystem will suffer deleterious effects.

            Consider what would happen if a guild of birds disappeared? There would be major changes in both the plant and animal communities, or even geological effects. The Bay of Fundy, a bay of the Atlantic touching Newfoundland, Nova Scotia, and the state of Maine, is frequented by several species of shorebirds during migration. The shorebirds feed on amphipods (small shrimp-like creatures); one sandpiper might eat as many as 10,000 per day. The amphipods feed on diatoms, tiny, mostly one-celled algae; although miniscule, they are extremely abundant and play a significant role in the ocean ecosystem as they provide 45 percent of the seas’ productivity. In addition, they produce adhesive chemicals that help to stabilize shoreline sediments. So if shorebirds were to disappear, amphipods would increase, diatom numbers would be reduced, and the shoreline would erode. The Eurasian Jay is a significant disperser of many species of oak throughout Europe. The jays pick acorns off the trees and bury them in open areas – abandoned croplands, pastures, openings in the forest-and retrieve them in the fall when food is scarce. By that time the acorns have begun to germinate, but the birds can still retrieve the nut meat and leave the seedling viable. Considering that a pair of jays can scatter and hoard thousands of acorns in a season, any decline in the jay’s population would be detrimental to many oak species.