Boreal and tropical forests underpin a wide range of critical Earth system functions, including long-term carbon storage, regional and global climate regulation, hydrological stability, and the maintenance of habitat for countless species. Because these ecosystems contribute so fundamentally to planetary resilience, their preservation is essential for maintaining a safe operating space for humanity in the face of accelerating environmental change. Biodiversity plays a central role in sustaining these functions, yet ongoing climate warming, increasing climate variability, and rapidly shifting land-use patterns are driving widespread losses of species and ecological interactions. Current global assessments suggest that tropical forests, in particular, are experiencing persistent biodiversity declines due to deforestation, fragmentation, and climatic stressors. However, scenarios that assume stronger sustainability policies, such as the Shared Socioeconomic Pathway SSP1 paired with the low-emissions RCP2.6 trajectory, indicate that improved land-use management and reduced pressures may help alleviate some of these losses. Under this same scenario, boreal regions are projected to experience a more moderate decline in species richness, though responses remain spatially variable and highly dependent on regional climate impacts. Despite these insights, many biodiversity assessments still rely on modeling approaches that overlook fundamental ecological processes, especially demographic dynamics and dispersal capacities that determine long-term metapopulation viability and ecosystem functionality. To address this gap, we apply a novel multispecies modeling framework, MetaRange, which integrates demographic rates and dispersal processes into traditional habitat-suitability-based projections. Using several key functional groups of mammals as focal taxa, we simulate potential biodiversity trajectories under the SSP1–RCP2.6 scenario to evaluate the feasibility of achieving a nature-positive future, defined here as maintaining or increasing population abundance relative to a 2015 baseline by the year 2100. Our results reveal substantial differences among functional groups, with spatially heterogeneous patterns of gains and losses driven by complex metapopulation dynamics. Ultimately, our approach demonstrates the feasibility and value of large-scale mechanistic simulations for advancing global change research and informing conservation strategies.