The question of what percentage of animals are herbivores is complex because the sheer number of animal species remains an estimate, and dietary categories are not always rigid. Calculating a global percentage requires a consistent definition of “animal” and a method for classifying the diet of species, many of which are tiny and poorly studied. The accepted range for this percentage varies significantly depending on whether scientists are counting species, individuals, or biomass.
The Global Estimate and Calculation Challenges
Determining the exact proportion of herbivore species is an exercise fraught with major statistical challenges, which is why a single consensus number does not exist. The best estimates suggest that the percentage of animal species that are herbivores falls somewhere in the range of 20% to over 50%. This wide margin exists because the vast majority of described animal species are invertebrates, particularly insects, and many more are yet to be discovered.
Insects alone account for over half of all described animal species, and a significant portion of this group are plant-feeders. For example, the order Coleoptera (beetles), which constitutes about 25% of all known animal species, includes many herbivorous species. Approximately 75% of all plant-feeding insects are considered specialists, meaning they feed on a narrow range of plant species. The challenge in estimating the global percentage is compounded because the diets of marine invertebrates and microscopic fauna are often not fully characterized, requiring researchers to rely on modeling and extrapolation.
The percentage also shifts dramatically depending on the metric used. While species-level estimates are uncertain, the proportion of herbivore biomass is much clearer, especially for large animals. The largest contributors to the biomass of wild land mammals are large herbivores, such as deer and elephants. The sheer mass of these primary consumers suggests their biomass is overwhelmingly dominant compared to carnivores, even if the species count is lower.
Defining Dietary Niche
The variability in the global estimate is also a direct result of how scientists categorize an animal’s diet, which is rarely a simple, binary choice. Traditional ecology defines three main feeding categories: herbivores, which primarily consume plants; carnivores, which consume animal matter; and omnivores, which consume both. However, the line between these groups is often blurry, leading to more precise ecological classifications.
A stricter definition separates herbivores into obligate and facultative types. An obligate herbivore, such as a koala, must consume plant matter to survive, as its specialized digestive system is adapted for that sole purpose. In contrast, a facultative herbivore’s diet is overwhelmingly plant-based but may opportunistically consume small amounts of animal matter for specific nutrients. This opportunistic feeding complicates strict categorization, sometimes leading to species being classified as omnivores despite a diet that is over 90% plant material.
Within the herbivore category, specialists are further defined by the specific plant part they consume, illustrating a high degree of niche partitioning. This detailed classification helps ecologists understand how multiple herbivore species can coexist in the same habitat by targeting different parts of the plant, thus avoiding direct competition. Specialized feeding niches include:
- Frugivores (fruit eaters)
- Granivores (seed eaters)
- Nectivores (nectar feeders)
- Folivores (leaf eaters)
Position in the Ecosystem
Herbivores occupy a foundational position in nearly all food webs, functioning as primary consumers at the second trophic level. They are the essential link that converts the energy stored by primary producers—plants, algae, and cyanobacteria—into a form available for higher trophic levels. This conversion process is characterized by a significant energy loss, with only about 10% of the energy typically transferred to the next level. This energy loss dictates that a substantial amount of plant biomass is necessary to support herbivores, making primary consumers the most numerous group in terms of individuals and biomass, following only the producers themselves.
The consumption of plant matter by herbivores is a powerful ecological force that shapes landscapes and maintains biodiversity. By grazing and browsing, herbivores prevent the dominance of any single plant species, allowing a greater variety of plants to flourish. They also contribute directly to the dispersal of plant life through the consumption of fruits and seeds, which are then deposited elsewhere. The loss or decline of large herbivores can have cascading effects on the ecosystem, altering vegetation structure, nutrient cycling, and the natural fire regime.