The Forest Is Still Standing. So Where Have Its Animals Gone?

Along a road through Ecuador's Yasuní National Park, researchers have spent twenty years counting wildlife. What they have found is troubling: large monkey populations are dramatically less abundant, even where the rainforest canopy remains largely intact.

Compared with areas without hunting pressure, yellow-bellied spider monkeys were estimated to be 87% less abundant, while Poeppig's woolly monkeys were 70% less abundant.

The forest may still look healthy from above, but much of its animal life has changed.

Twenty years beneath the canopy

Wildlife Conservation Society (WCS) Ecuador monitored mammals and large forest birds between 2005 and 2025 in an area of approximately 100 square kilometres along the Maxus road.

Researchers repeatedly surveyed six permanent four-kilometre transects, working alongside Waorani field assistants.

In 2025 alone, the teams walked 253 kilometres and recorded 5,450 observations.

Over the study period, the combined biomass of the primate community declined by approximately 68%. Biomass measures the estimated total weight of animals in an area, revealing changes that a simple count of species may miss.

Smaller monkeys remained more common. Humboldt's squirrel monkey accounted for 44.7% of primate detections in 2025.

White-lipped peccaries also declined and were not detected during the 2025 surveys, although that does not establish their disappearance from the area.

Why missing monkeys matter

Large primates eat fruit and disperse seeds as they move through the forest. Their decline can affect the reproduction and distribution of trees and other plants.

Smaller species do not necessarily replace those ecological functions.

Roads can make remote forest areas more accessible, increasing hunting pressure even without extensive deforestation.

WCS identifies control of illegal hunting, management of access routes and connections with healthier wildlife populations as important conservation measures.

The study reveals a problem that satellite images alone cannot show: preserving a forest's trees does not guarantee the survival of the animals living beneath them.