Barred owls native to forests east of the Great Plains have been moving west for the last 150 years. They used riparian habitat along major rivers to extend their range to Montana and from Montana moved north into Canada, southwest into Washington (1965), Idaho, Oregon, and finally arrived in California (1976). Barred owl bullies compete with the Pacific Northwest’s native northern spotted owls and add to population declines. Spotted owls are having a rough time as the invaders move in and wildlife ecologists are unsure about how to handle this. A new model developed at Michigan State University that used existing data can help determine management of this owl conflict. The model also is useful for other invasive or at-risk animal populations.
As I’ve described before, models can misjudge real world interactions, but through use they are refined and become more accurate. We need fast cheap ways of understanding interactions between invasive and native species and at-risk populations as problems are arising rapidly and need attention sooner than intensive research can provide guidelines.
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Spotted owls are smaller, less aggressive, and have narrower habitat requirements than barred owls; they need dense forests with standing dead trees for nesting. Barred owls can live in suburbia if enough large trees are present, They kill spotted owls, but also will breed with them as both are in the same genus — Strix. In my part of the Sierras we see barred owls, spotted owls, and intermediate forms. Barred owls (Strix varia) have bars on their chests. Spotted owls (Strix occidentalis) have — guess what — large oval white spots and are listed as Threatened under the ESA. The hybrid owls have a mix of both owls’ physical characteristics, resulting in blotchy patterns.
Attempts to help spotted owls include killing barred owls. A four-year U.S. Fish and Wildlife Service experiment is killing 3,600 barred owls in the northwest to see if northern spotted owl populations benefit. Early results from killing 378 owls in the past three years shows that “floaters” are returning and re-establishing nests.
As soon as they're forced out [by barred owls], the Northern Spotted Owls become “floaters“ . . . They just kind of float on the landscape. They don’t try to reproduce. They’re kind of just hanging on, eating enough to survive. They stop vocalizing, making them hard to track, and they’re sometimes forced into open areas away from the forest’s protective canopy where they become easy prey to larger raptors….
Accurate and cost-effective ways of managing this conflict and characteristics that influence interactions between the two owl species are determined by the "Dynamic N-occupancy" model developed at Michigan State University.
"Our model estimates population abundance and demographic rates, such as survival and reproduction, from relatively 'cheap' data [...]
The MSU model . . . uses data on two simple factors: presence or absence of animals across space and time . . . [and] is capable of providing accurate estimates of local abundance, survival rates and population gains - including reproduction and immigration - while accounting for the fact that the presence of a species may be detected imperfectly during sampling.
Researchers tested the model on barred/spotted owl interaction comparing results using both existing data and a radio-tracking study. The barred owl evaluation using 26 years of data collected by the U.S. Forest Service, a data set easy and cheap to obtain, was consistent with that of the radio-tracking study. Usually this kind of information requires sampling areas repeatedly, capturing and marking owls, then tracking them and later recapturing them. All this is labor and time intensive and expensive. And the model can use different inexpensive or existing data sets to analyze other animals. For example, it can estimate monarch populations from tagging and citizen science data reported to Journey North, or assess bird populations using Audubon’s Christmas Bird Count data.
It can work especially well on tracking the increase of invasive species or the decrease of endangered species….
"We've developed a crucial tool in helping explain why populations are changing and at what costs. . . We are like wildlife detectives. People come to us when they can't solve their mysteries. We're able to quickly sift through extensive data sets, involve citizen scientists in data collection, and help them solve their problems."
Decades of data can offer valuable insights, but sorting through it, accomodating variations in how the data was acquired and in measurements used requires an analysis that accounts for lack of standardization. If this new model bears out its promise, old data can be compared with new citizen science data to identify what is happening on the ground and what actions might be the most effective. For the owl conflict, the model showed high survival rates for barred owls and a strong connection between their regional population size and ability to move into areas and establish dominance. They found that barred owl population growth has slowed over the last five years, suggesting that the area is saturated.
"Simply put, the model is telling us the rate at which barred owl numbers are increasing and offering clues as to why that's happening. This, in turn, can help us understand how endangered spotted owl populations in the same region may respond….”
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