Limitations of Photo Pairs

Close examination of repeat photo pairs taken throughout Denali National Park and Preserve has certainly given park staff something to think about!  The photo pairs reveal a broad suite of changes occurring over recent decades that may indicate an ecosystem undergoing a fundamental transformation.

But can researchers really be sure the change they see in a photo pair has happened?  Of course!  Photographic evidence, assuming enough landmarks are present in the pair to ensure they were taken of the exact same location, is solid proof that a change has occurred.  What researchers can’t be certain about is why that change occurred, or what might happen next.

Take this photo pair for example.  One could see the tent camp in the early photo, and the trees in the recent photo, and assume that the campers must have planted the trees.  Alternatively, one could determine that the disturbance caused by the tent campers facilitated the growth of those disturbance-adapted tree species.  Examining only one photo pair, researchers have a limited frame of reference.  Looking at other photo pairs of tent camps, and photo pairs of similar landscape positions may help.  In order to gather more information, researchers sometimes investigate historical records.  The campsite pictured here was used by both the Alaska Road Commission and the Mt. McKinley Tourist and Transportation Company in the 1930s.  For decades after the camp was removed, the area was called “the soapberry patch” because the early successional shrub (soapberry) grew there in abundance, spurred by the disturbance the camp caused.  Bears were frequently seen in the area eating soapberries.  However, the trajectory of vegetation succession has continued, and soapberries have been replaced by willows and other woody vegetation.  Do you think bears still frequent this area now that the available food has changed?

Individual photo pairs provide sure proof that changes have occurred, but as you can see, there is more to understand than what you see at first glance!  The interplay between humans, animals, and vegetation can be complex, but a collection of photo pairs from across the landscape, in coordination with an understanding of site histories of particular locations, allows us to make educated guesses about why the observed changes may be occurring and how widespread they are.

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