Shifting baselines in ecology highlight the gradual, often unnoticed changes in ecosystems over time, leading to a redefinition of what is considered normal or baseline conditions. Identifying historical ecological baselines, representing the original state before significant human impact, is challenging and generally requires careful analysis of long-term retrospective data.
The loss of top terrestrial predators is a prime example of shifting baselines. Historically, human actions have profoundly affected the behavior, distribution, and density of native animal species, altering food webs and often replacing wild animals with domesticated ones. Large predators, in particular, have been hunted and persecuted, leading to local annihilation or displacement and reduced numbers and distribution on regional and global scales.
The removal or displacement of large predators is associated with increases in both large herbivore prey and smaller or mesopredator populations. These increases can result in population overshoot, intensified foraging pressure, and damage to native plants and other resources. Revitalized mesopredator populations can reach high densities, leading to the decline or destruction of even smaller predators and prey populations, potentially affecting the stability and structure of animal communities.
Gray wolves (Canis lupus) in North America have experienced a substantial contraction of their historical range, almost disappearing from the contiguous 48 United States at one point. Their conservation is crucial due to wolves’ potential cascading effects on lower trophic levels. The extermination or displacement of wolves can lead to significant changes in vegetation structure, succession, productivity, species composition, and diversity, ultimately affecting other wildlife’s overall biodiversity and habitat quality.
Research in US Northwest national parks from the 1950s to 2021 reveals substantial ecological impacts of hooved animals following the removal of gray wolves and other predators. This has led to declines in long-term tree recruitment, influencing plant communities and environmental processes. Observed impacts in these parks suggest broader changes to ecosystem processes and lower trophic levels in areas where gray wolves population has declined.
Coyotes (Canis latrans) have been identified as significant predators of various threatened or endangered smaller vertebrates. Wolves can reduce coyote populations, mediating their predation of prey and smaller predator populations. Recognizing the cascading effects of large predator removal is crucial when investigating ecological questions within these landscapes.
Studying an altered ecosystem without recognizing how or why the system has changed over time due to the absence of a large predator could have profound implications for wildlife management, biodiversity conservation, and ecosystem restoration. As a starting point for future ecological studies in national parks, researchers should discuss how the presence or absence of large predators may have influenced their results and conclusions. Restoration decisions made without consideration of past conditions may continue to alter ecosystems in novel ways; fire suppression, invasion by exotic species, past overgrazing by livestock, and climate change, should be considered. Researchers should investigate park archives to exploit historical data and information to understand the effects of the loss of predators and other potential legacy factors. Various national parks in the western United States, considered the crown jewels of American wilderness, lack their apex predators, resulting in diminished ecological integrity.
Although this analysis focuses on national parks in the northwestern United States, the same issues likely apply to research conducted in other parts of the United States and globally, given the widespread and long-term impacts of the decline of large predators and the release of herbivore prey. Large predator populations are missing or depleted in many marine ecosystems as well. For example, the absence of sharks can lead to an overabundance of green turtle populations that jeopardize seagrass meadows.
Given the plethora of other anthropogenic impacts after European colonization, researchers should consider conditions before European contact to contextualize most ecosystems’ current state properly. Even then, Indigenous peoples may have had profound effects on some environments, and these effects are poorly understood. The loss of large predators is only one of many significant changes to our environment, and researchers should also focus on other factors.
Ongoing debates in ecological restoration include how to handle cases where former keystone species are now extinct and the potential risks and benefits of non-native species in ecosystems. Protecting Earth’s remaining natural areas can aid future researchers studying altered ecosystems. Understanding past conditions and how human actions have modified them is crucial for attaining ecology-based restoration goals. The desired state for restoration of a given system likely varies depending on many factors and will require careful consideration and involvement of many stakeholders and interest groups.
