The “World Is Green” (WIG) hypothesis is a foundational idea in ecology that seeks to explain a fundamental paradox of life on Earth. Proposed by ecologists Nelson Hairston, Frederick Smith, and Lawrence Slobodkin in 1960, the hypothesis addresses the apparent contradiction between the vast number of plant-eating animals and the overwhelming global abundance of vegetation. It suggests a mechanism that allows for the flourishing of plant life despite constant herbivore pressure.
The Puzzle: Why Herbivores Don’t Strip the Planet
The base of nearly every terrestrial food web is formed by plants, which convert solar energy into biomass through photosynthesis. Since plants represent the largest energy source, and herbivores are capable of high reproductive rates, one might logically expect these consumers to eat most of the available greenery. A simple energy transfer model suggests that herbivore populations should rapidly increase until they consume almost all primary production.
If this were the case, the landscape would look largely barren, or at least heavily grazed. However, observation of most ecosystems, from forests to grasslands, reveals a world dominated by plant life. The sheer volume of uneaten plant material is vastly greater than the total mass of herbivores. This visible abundance of green biomass established the core ecological problem the WIG hypothesis set out to solve.
The Solution: Predators and Top-Down Control
The original WIG hypothesis offered a definitive answer to the puzzle by arguing that herbivores are not limited by the amount of food available to them. Instead, the hypothesis proposed that the world remains green because herbivore populations are consistently regulated by their predators. This explanation shifts the focus of population control from the availability of resources at the bottom of the food web to the pressure exerted by consumers at the top.
In this three-tiered system, the producers (plants) are limited by resources like nutrients and sunlight, but the primary consumers (herbivores) are limited by the secondary consumers (predators). Since predators keep herbivore numbers relatively low, the plants are effectively released from intense grazing pressure and can accumulate a large amount of biomass. This transfer of control from the top of the food chain downward is known as a trophic cascade.
The concept suggests that the influence of an apex predator “cascades” through the food web, indirectly benefiting the plants two levels below. For example, an increase in a carnivore population leads to a decrease in its herbivore prey, which in turn leads to an increase in the plant population. The hypothesis posits that herbivores are “green-limited” only because they are constantly being hunted.
Refining the Theory: Bottom-Up Limitations and Plant Defenses
While the WIG hypothesis provided an elegant explanation, subsequent research showed that the control of plant biomass is more complex than a simple top-down model. Modern ecology recognizes that not all green biomass is equally palatable or available, introducing the concept of plant defenses. Many plants have evolved chemical compounds, such as tannins, alkaloids, and terpenes, that are toxic or reduce nutritional value.
These chemical deterrents, along with structural defenses like thorns, spines, and tough, fibrous leaves, mean that a significant portion of the world’s green material is inedible. This defense mechanism limits herbivores, effectively reducing the actual food supply regardless of predator presence. In this view, herbivores are limited by plant quality, not just quantity.
Another major refinement is the recognition of bottom-up control, which emphasizes the role of resource availability in limiting primary production. In environments where resources like nitrogen or phosphorus are scarce, plant growth is naturally restricted. This limitation then restricts the herbivore population, as their numbers cannot exceed the capacity of the plant base.
The relative importance of top-down and bottom-up forces often depends on the specific ecosystem. For instance, top-down control is often stronger in aquatic systems, while terrestrial ecosystems frequently display a mix of both types of regulation. Modern ecological understanding synthesizes these concepts, concluding that the persistence of a green world results from a dynamic interplay between predator regulation, plant defense, and nutrient availability.