Assertion: tree frog's population | /r/CapableOf | fluctuate over time
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Source sentences:
text: Herpetologists first started noting that amphibian populations were falling at many of their study sites in the 1980s; however, those early reports were anecdotal, and many experts doubted that the observed declines were cause for concern (the argument was that populations of amphibians fluctuate over time and the declines could be attributed to natural variation).