Competitive Exclusion: The Abingdon Tortoise

Gause's Competitive Exclusion Principle

The Competitive Exclusion Principle, also known as Gause's Law, states that two species competing for the exact same limiting resources cannot coexist. One species will always have a slight advantage, eventually outcompeting and eliminating the other.

In the case of the Abingdon tortoise and the goats, they occupied the same ecological niche—that of a herbivore—and competed for the same food source. The goats' more efficient feeding habits gave them a competitive edge that the tortoises could not overcome.

Grazing vs. Browsing

The key to understanding this competition lies in the difference between grazing and browsing.

• Grazing: This is the act of feeding on low-growing vegetation, primarily grasses. Grazers, like the Abingdon tortoise, typically clip the tops of plants, allowing them to regrow.
• Browsing: This is the act of feeding on the leaves, soft shoots, and fruits of higher-growing woody plants, such as shrubs and trees. Goats, as classic browsers, are known for their ability to eat a wide variety of plants, and their feeding style often involves uprooting plants entirely, preventing them from regenerating.

While goats are primarily browsers, they are also opportunistic eaters. On Pinta Island, they not only consumed the shrubs but also aggressively ate the grasses that the tortoises relied on. Their destructive feeding habits, which include pulling up plants by the roots, meant that the food source for the tortoises was not just consumed, but completely eliminated. The grazing land, which may have seemed abundant at first, quickly became a **limiting resource** as a direct result of the goats' presence and feeding behavior.

Exploitative vs. Interference Competition

The competition between the goats and the tortoises was a prime example of exploitative competition.

• Exploitative Competition: This is an indirect form of competition where organisms compete by consuming a shared resource. The presence of one species reduces the amount of resource available for the other, without any direct interaction. The goats, with their "greater browsing efficiency" and destructive uprooting of plants, simply ate the vegetation faster and more thoroughly, leaving nothing for the tortoises.
• Interference Competition: This is a direct form of competition where organisms actively interfere with another's ability to forage, survive, or reproduce. This might involve aggression or physically preventing access to a resource. While the goats' presence was inhibitory, the specific ecological mechanism was **exploitative**, as the tortoises died out due to a lack of food, not because they were directly prevented from eating it.