Resource‐Based Carrying Capacity Estimates for the Florida Manatee ( Trichechus manatus latirostris ) in a Subtropical Estuary

ABSTRACT Resource‐based carrying capacity can constrain abundance and population dynamics of large herbivores, including in aquatic ecosystems. Seagrass decline in the Indian River Lagoon (IRL) has raised concern that reduced forage availability may limit foraging opportunities for large marine herbivores, particularly the Florida manatee ( Trichechus manatus latirostris ). We applied a resource‐based carrying‐capacity model to evaluate how long‐term changes in seagrass availability may have affected the number of manatees the IRL can support. Carrying capacity was estimated from mapped seagrass areal extent, seasonal mean seagrass percent cover measured along fixed transects, a standardized seagrass productivity value and mean annual food consumption per manatee. As expected, carrying capacity estimates were higher in summer than in winter, but they also declined significantly throughout the study period. Between 2003 and 2021, the estimated resource‐based carrying capacity decreased markedly in both seasons, declining by 98.3% in summer and 97.3% in winter, indicating a substantial reduction in seagrass forage available to manatees. Because the model assumed a constant rate of seagrass productivity and intake, did not differentiate among seagrass species and did not account for forage removal by other herbivores, it effectively treated all annual production as forage exclusively available to manatees. These estimates should therefore be interpreted as production‐based upper bounds rather than measures of realized carrying capacity. Nevertheless, the model provides a useful framework for linking habitat degradation to forage availability and long‐term changes in ecological carrying capacity, with important implications for seagrass restoration and manatee conservation.