Global tiger density linked with forest carbon stock, top-down and bottom-up

Tiger (Panthera tigris) survival, as apex predators in forest ecosystems,
largely depends on abundant prey in healthy, intact forests. Because large
herbivore prey are drivers of plant biomass, we reasoned that tiger
distribution and density are probably also closely linked with forest
carbon (C) stock, the management of which is critical for mitigating
climate change. However, whether tigers exert top-down control of forest C
stocks or are passive surrogate C indicators bottom-up is a salient
unanswered question in conservation and management, particularly in
trophic rewilding. Here, we compiled estimates of global tiger presence
and density to test the top-down effects of tigers on forest C stocks and
tiger-carbon relationships along a gradient from “empty forests” without
tigers to “target state” ecosystems with tigers living at different
abundances. Our results showed that tiger presence was associated with
higher forest vegetation C stocks, lower C emissions, and higher C inputs
globally. Top-down effects via ungulate biomass were stronger in less
established forests. Furthermore, forest vegetation or soil C stocks
increased with tiger density or reached tiger-carbon peaks in four forest
habitat types covering most of the tiger range. Our findings reveal that
tigers, represented by their presence and density, are both an indicator
and a driver of forest ecosystem C stocks, depending on underlying
ecological conditions, and could safeguard forests against future C
emissions and improve our understanding of climate-C cycle feedback.