ON THE WRONG SIDE OF TOWN
Where we humans alter the face of the earth, it becomes more difficult for most other organisms to thrive. Quite a number, though, manage to exploit those changes and to benefit. These species often end up as human commensals, living in a state of dependence just a bit south of parasitism. During the second millenium CE, the Black Rat (Rattus rattus) and later, the Norway Rat (R. norvegicus) followed humans around the world, as the House Mouse (Mus musculus) did centuries earlier. A hundred years ago, lost African Cattle Egrets (Bubulcus ibis) established themselves in the Guianas, and, exploiting the newly created habitat of pastureland, began to spread from there. By 1940 they were breeding in Florida. Utah's first record came in 1963, and by 1980 it was a common breeder here. Today the species thrives throughout the Western Hemisphere. More often than not, a newly introduced species that achieves such a degree of success does so as a human commensal. Otherwise, it generally does it by out-competing established natives.
Currently, the most impressive case of tetrapod invasion is probably the Eurasian Ring-necked Dove (Streptopelia decaocto). Originally native to southeastern Europe, central Asia and Japan, its range began to expand in the 1930s, eventually stretching west to Ireland and Iceland, and north to Norway.
In the 1970s, introduced Eurasian Collared Doves established themselves in the Bahamas, and over the next decade they spread into Florida, from where they've been dispersing across the continent. At this point, little is known of their impact on native species. The general consensus is that they exploit a niche between that of the native Mourning Dove (Zenaida macroura) and the old-timer invasive Rock Dove (Columba livia). Some have suggested that it is taking over to some extent the ecological function of Passenger Pigeons (Ectopistes migratorius). Over the past three years, they've established healthy populations in several towns in Colorado, Idaho and Montana, and many of them have been recorded in my own state. I spent a good part of last week touring around eastern Utah, where I saw quite a few of them, always close to agricultural areas, in a band running from the town of Roosevelt, east nearly to the Green River. It's not surprising that these birds are expected to behave as ecological intermediates between Mourning and Rock Doves. In size they fall between those two species, and they seem more social than the Mourning, and less so than the Rock. The fact struck me, though, that I never saw those other species in any sort of proximity to Collareds, despite what appears to be a wide overlap in habitat preference. In fact, a sizable Collared colony exists on the west end of Jensen, ten miles east of Vernal. Locals informed me that the birds appeared two years ago. Native Mourning Doves have been relegated to the eastern part of town, and the two species appear to me to live in absolute segregation.
Erasian Collared Dove photo taken by CPBvK in Jensen, Utah July 14, 2007