Ever wonder why your neighborhood has certain kinds of birds and your uncle, who lives across the country, has a different set of birds? And why aren’t there hundreds of kinds of birds in your backyard rather than just a few dozen? Well, it all comes down to succession – the development of communities. As an environment builds from raw land, plants and animals make their way into the niches that provide what they need to survive. A penguin can’t live in the desert and a roadrunner can’t live in the arctic. Well, we know that, but how does a community of birds develop? An ecosystem can hold just so many species of birds (or plants or insects), so that every new arrival has the potential to cause the extinction of an existing species. There are new birds arriving at all stages, but not all of them make it. Let’s look at an example or two.
A well-studied example of succession is a volcanic island in Indonesia called Krakatoa which lies between Sumatra and Java. Krakatoa erupted with explosive force in 1883, killing 36,000 people and destroying two-thirds of the island. It was so powerful that tsunami waves rocked ships off the coast of South Africa. Although a tragedy for the local populace, it afforded an opportunity for gathering firsthand knowledge of the evolution of an ecosystem and its avifauna. In 1889, six years after the eruption, seeds had blown or washed in and vegetation started to recover, but there were no resident birds. By 1908, plants were taking a good hold and thirteen species of birds had taken up tenancy. By 1920 tropical forest plants began to cover the island. In the period 1919-24, there were 28 breeding bird species, but two previous occupants had disappeared. In 1928-34, 171 plant species were identified along with 29 bird species, but three earlier bird species were gone. In 1951-52 there were 33 species but three former species disappeared, and in 1984-86, 36 bird species were residents but four previous occupants were absent. Today there are 38 surviving species.
More recent examples of the successional process are the Icelandic island of Surtsey that arose anew from the Atlantic Ocean over the period from 1963-1967 and the eruption of Mt St. Helen’s in 1980 that covered the surrounding countryside in ash. Today there are merely twelve species of birds surviving on the isolated Surtsey, 20 mi (32 km) from Iceland, mostly seabirds. Gulls are the most abundant and have had a significant influence on the growth of plants on the island as the birds not only bring in seeds but fertilize the soil with guano. The entire avifauna in the enormous blast area around Mt. St. Helen’s perished, but birds were seen in the area four days after the eruption. Since then over 80 bird species have colonized the mountain from the surrounding ecosystems. There are variations on the successional theme, especially when human influences come into play, but the underlying concept is that ecosystems develop in an orderly and predictable way and at maturity are dynamic but stable. And in every ecosystem each bird species occupies a particular niche.