National Geographic - Strange Days on Planet Earth - Part 3 of 4 - Predator

Summary

The documentary examines how removing large predators can trigger cascading changes throughout ecosystems—and whether restoring predators can help reverse them.

Artificial Islands in Venezuela

A hydroelectric dam flooded a continuous forest, leaving animals isolated on hundreds of islands. Researchers studying this “natural experiment” found unusually dense populations of some animals. Iguanas, howler monkeys, and leaf-cutter ants were damaging vegetation, while monkeys showed signs of social stress.

Researchers suspect predators disappeared from many islands, allowing prey and other consumers to multiply and disrupt the ecosystem. A biologist and his team rescued more than 9,000 animals from the rising reservoir, though many could not be saved.

Yellowstone and Wolves

Research links the loss of wolves to changes in elk behavior and browsing. Aspen tree cores indicate that regeneration largely stopped in the 1930s, around the time wolves were eliminated from the park. Heavy elk browsing may also have contributed to declining willows, eroding stream banks, and fewer beavers.

After wolves were reintroduced in 1995, researchers observed areas where willows were growing well, and beavers began recolonizing. Wolf kills also provide food for scavengers and other species. The documentary presents these findings as evidence of ecological connections while noting that other factors may also be involved.

Predators and Coral Reefs

Coral cores from Jamaica suggest the reef was relatively stable for centuries before a marked change in the 1980s, when algae began replacing coral. Researchers point to overfishing as a major cause: large predatory fish and the herbivorous fish that graze on algae have been depleted.

After a disease killed many sea urchins—another important grazer—the reef had fewer defenses against algal overgrowth. Fisheries researcher Daniel Pauly says that roughly 90% of the world’s large fish were lost over the preceding 40–50 years.

Marine Reserves in St. Lucia

Fishing-free zones have been established to help fish populations recover. Early monitoring found that some fish populations had tripled within the reserves, and researchers spotted a grouper. Fishers reported catching larger fish outside reserve boundaries.

These early signs suggest reserves may help, though continued monitoring is needed to evaluate ecosystem recovery.

Human–Predator Conflict

Restoring predators can benefit ecosystems but also create risks and costs for people. In Yellowstone, officials tracked wolves and removed one associated with livestock attacks, illustrating the difficult compromises involved in living alongside predators.

Methods and Evidence

The documentary describes several methods used to study ecosystem change:

• Survey animals and vegetation across islands and compare their densities with those on the mainland.
• Use tortoise shell rings to estimate age and identify changes in reproduction.
• Compare historical photographs and aerial images to document changes in rivers and forests.
• Count annual growth rings in tree cores to determine when aspen regeneration declined.
• Track wolves and observe their effects on prey, vegetation, scavengers, and other wildlife.
• Extract coral cores and examine the fossil record to reconstruct past reef conditions.
• Monitor marine reserves by surveying fish populations and ecosystem interactions.

Researchers and Sources Featured

• Luis Bas — Venezuelan biologist involved in rescuing animals threatened by flooding; the surname spelling is unclear in the subtitles.
• John Terborgh — Biologist studying the Venezuelan islands.
• César Aponte — Biologist studying tortoises.
• Bob Beschta — Hydrologist studying Yellowstone rivers; the surname is uncertain in the autogenerated subtitles.
• Bill Ripple and Eric Larson — Researchers investigating Yellowstone’s aspen decline.
• Doug Smith — Biologist involved in Yellowstone wolf research and reintroduction.
• Joe Fontaine — Fish and Wildlife biologist working on wolf management.
• Marina Smith — Rancher discussing livestock losses from wolves.
• Rich Aronson and Bill Precht — Marine biologists studying Caribbean coral reefs.
• Daniel Pauly — Fisheries biologist studying fishing pressure and declining fish populations.
• Kai Wolf — Manager involved in St. Lucia’s marine-reserve effort.
• Sati — Visiting biologist monitoring the reserves; the surname is unclear in the subtitles.
• Edward Mong — St. Lucian fisher discussing declining catches.