The number carrying the argument is not the 22 percent. It is the null result underneath it. The team tested how much grassland surrounded each site, how fragmented that grassland was, and whether the breeding season ran hotter, cooler, wetter or drier than normal [8]. Occupancy fell anyway: in large patches and small ones, at sites with deep interiors and sites pressed up against their edges [5]. Those are precisely the two things a land agency can buy with money, more acres and better-shaped acres.
Put the decline on an annual footing and it stops looking parochial. A 22 percent drop over 16 years is about 1.5 percent a year compounded [1]. The Cornell Lab's 2019 estimate that grassland bird populations halved since the 1970s works out to roughly 1.4 percent a year [2]. So the reserves are losing ground about as fast as the continent they were meant to be a refuge from [3]. The two metrics are not the same animal: Minnesota measured the probability of detecting a species [4], Cornell estimated numbers of birds [7]. A site keeps counting as occupied until the last pair goes, which makes the occupancy figure the more forgiving of the two, not the more dramatic.
The authors' preferred explanation is extinction debt. Less than 2 percent of the roughly 18 million prairie acres that once covered a third of Minnesota survives [11][12], which is under 360,000 acres [4], and the birds still present may be working off losses banked generations ago [13]. It fits, and this design cannot falsify it. A second reading fits the same counts: the upland sandpiper whose whistle has faded from these sites migrates from South America [10], and no Minnesota breeding survey can observe what happens to it in between. Sixteen years of visits to 40 sites tell you where birds are not. They do not tell you where they died.
Neil Gilbert, the lead author, puts the operational version plainly: a postage stamp of a preserve in a sea of cropland may not be enough, which is not the same as protection being pointless [14]. Chris Jennelle of the Minnesota Department of Natural Resources adds that these are among the best prairies left in the country [15]. Both statements point outward, past the fence line. The harder sentence to sign is the senior author's, that new reserves or more grassland inside existing ones is "probably not going to cut it" as climate conditions keep changing [16], because a state wildlife agency co-authored the paper it appears in [1]. Acreage still buys the option to have birds later. On this evidence it does not, by itself, buy the birds.
