The sentinel approach to quantify ecosystem function intensities

Biodiversity, through species interactions, underpins numerous ecosystem functions that can lead to ecosystem services and disservices. Quantifying these functions is crucial for evaluating the effectiveness of conservation and management strategies, as well as the impacts of land use change. However, several ecosystem functions remain underexplored or are monitored indirectly by tracking changes in the abundance of function providers rather than directly measuring the processes themselves. Here, we propose a broadened and consistent use of the sentinel approach, which relies on experimentally placed “sentinels” to measure ecosystem functions. This approach offers several advantages: it allows direct and comparable measurements across multiple functions, is suitable for monitoring both ecosystem services and disservices, facilitates the monitoring of understudied ecosystem functions, and avoids harming the providers of important services as well as the disadvantages of using proxies. While sentinel-based methods are well-established for some functions (e.g. predation, pollination), they remain underused for others (e.g. herbivory or scavenging), particularly for functions associated with ecosystem disservices (e.g. intraguild predation, pollination of weeds). Moreover, these processes are often studied in isolation, even though management interventions may generate trade-offs or synergies among them. Our suggested toolkit enables the quantification of ten ecosystem functions using sentinels: predation, parasitism, scavenging, detritivory, coprophagy, fruit consumption, herbivory, plant infection, seed predation, and pollination. In addition, six other ecosystem functions (aboveground and belowground primary production, soil fertility, water availability, aboveground and belowground secondary production) can be assessed using direct (non-sentinel-based) methods. Although focusing on ecosystem functions comes at the cost of taxonomic resolution compared to species-level monitoring, this approach provides sufficient information when the primary goal is to assess the intensity and continuity of ecological functions. These methods yield complementary, non-overlapping information and can be used alongside traditional species-level monitoring.