Eighty-one percent of tropical trees depend on animals for seed dispersal. The relationship seems ancillary — animals carry seeds, trees grow where seeds land. But quantifying the carbon consequences reveals it is foundational.
Naturally regrowing forests with the least seed-dispersal disruption accumulate aboveground carbon at four times the rate of those with the most severe disruption. Across areas globally identified as suitable for reforestation, current levels of animal biodiversity loss yield a 57% average reduction in local carbon accumulation potential.
The mechanism is spatial: trees that rely on animal dispersal cannot distribute themselves into the configurations that maximize canopy coverage, light interception, and competitive spacing without the animals that move their seeds. When dispersers disappear, seeds fall near the parent tree, creating dense clusters of related individuals competing for the same resources. The resulting forest has lower species diversity, lower structural complexity, and lower carbon density.
This means planting trees without restoring animal populations is dramatically less effective as a climate strategy than the headline numbers suggest. The “plant a billion trees” framing treats forests as collections of trees. They are not. They are coupled animal-plant systems, and the animals determine where the trees go. Without the postal service, the letters pile up at the post office. The words are all there — but they never reach the addresses that need them.