Sunday, October 09, 2011

Tennessee BBS: Inflection Points

As I scrolled through all those graphs for the individual species, it became clear that 45 years was not only long only enough to show trends, it was long enough to show trends in trends -- the second derivative, not just the first. Few of the graphs show a simple up or down trend over the whole period, even when allowance is made for the year-to-year wiggles. American Goldfinch provides a stark example of this; Chipping Sparrow shows another. There appear to be three periods around which these inflection points cluster. I have had trouble making sense out of some of these; perhaps readers with more knowledge of the changes in land use, legislation, and other possible impacts in the mid-south during this era might shed some light.

Turning Number 1: Late 1970s-1980

Something clearly happened around this time. I discussed this earlier as the Great Sparrow Shift; it involves other species as well, from Goldfinches to Orioles to Herons. This is the time at which several of the woodpeckers and raptors began their impressive increases. Overall, for the species that seem to show a notable shift in the direction of their trend lines at this time, the large majority of these shifts are towards the positive. Many of these are weedy and edgy species, though a marked exception is the Gray Catbird, which began a sharp drop around 1980.

Turning Number 2: Around 1990

About a decade later, there was another shifting in the trend lines for a variety of species. This is a very mixed bag, with some increasing trends flattening while other flatlines started increasing and some species entirely changed directions. As with the first turning, the majority of the shifts were positive, but less dominantly so in this case. Wood Ducks, Chimney Swifts, Great Horned Owls, and Red-winged Blackbirds are among the species that began declines at about this time.

Turning Number 3: Early 2000s?

As it is close to the end of the current dataset, this third turning is less definite. Most of these suggested trends remain hard to distinguish from ordinary short-term wobbles. Still, there are indications of another shift in the trend lines for many species in the last decade. Unfortunately, most of these possible shifts appear to be in the wrong direction, or at least the flattening out of a long-term increase. There are some positives, including the surges in Tree Swallows and Mississippi Kites. The coming decade will tell whether most of these hinted-at changes are sustained.

I would expect these turnings to primarily be associated with landscape and habitat shifts. These could come about from official programs, economics, or many other factors. I would be curious to hear suggestions as to what these factors might have been.

Tennessee BBS index:
1: Waterfowl to Herons
2: Vultures to Doves
3: Cuckoos to Woodpeckers
4: Flycatchers to Corvids
5: Larks to Wrens
6: Gnatcatchers to Waxwings
7: Wood Warblers
8: Towhees to Buntings
9: Icterids to House Sparrow
Ups and Downs
Inflection Points
Next: Summary


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