To control the COVID-19 pandemic, governments worldwide deployed unprecedented policies. Mandatory lockdowns, mobility restrictions, and calls for social distancing and voluntary individual constraints were among the key interventions to reduce exposure risks. The resulting global reductions in human mobility constitute a massive natural experiment on whether and how changes in human activities affect natural ecosystems and sustainability. This analysis focuses on the relationship between human activities and tropical forest fires. Human activities contribute to forest fires, both directly and indirectly, threatening ecological, environmental, and climatic stability, as well as human health. Prior research of the effects of lockdowns on forest fires uses proxies for changes in human activities, e.g., covid cases and official lockdown dates. Our analysis relies, instead, on a direct indicator of human activity levels- the human mobility rate- to model the relationship between fires and reduced mobility and examine how fires patterns changed in Amazon basin countries during the initial COVID-19 lockdown in March 2020 and thereafter. Our analysis reveals that constraining human mobility significantly reduced forest fires in the Amazon regions of Bolivia, Brazil, Colombia, Ecuador, and Brazil during a brief initial period, but the effects dissipated after 30 days. After 60 days, the number of fires was higher than previous years’ average, controlling for environmental conditions. The unintended sustainability effects of behavioral changes after COVID-19 were short-lived and we document a compensatory effect that resulted in higher fire incidence. By highlighting how rapidly societies returned to business-as-usual practices, our analysis points towards the need for durable institutional and policy changes for effective shifts in human behaviors towards longer-term sustainability outcomes.