The question of whether there are more insects or fish in the world has a clear answer: insects overwhelmingly dominate the planet. This disparity is reflected in the number of distinct species, the total count of individuals, and the collective weight, or biomass, of these organisms. The success of insects results from millions of years of evolutionary adaptations that allowed them to colonize nearly every terrestrial niche.
The Numerical Reality: Species Counts and Biomass
The current count of described insect species stands at approximately one million, yet many scientists estimate the actual number of insect species to be several times higher, potentially reaching ten million or more. In comparison, the diverse group of fish, which includes ray-finned fishes (Actinopterygii), sharks and rays (Chondrichthyes), and jawless fish, comprises about 34,000 described species globally. This disparity highlights the numerical advantage held by insects.
Beyond species count, the difference in total population and biomass is significant. Estimates suggest that the collective biomass of insects can exceed that of all other animal life on Earth combined, including humans, birds, mammals, and fish. For every pound of fish in the oceans, there are likely thousands of pounds of insects distributed across the terrestrial and freshwater environments. This large population size is supported by their ability to reproduce rapidly and utilize resources efficiently.
The Evolutionary Success of Insects
The numerical dominance of insects stems from a combination of successful evolutionary traits that allowed for diversification across the globe. One advantage is their small body size, which allows different insect species to specialize and occupy distinct micro-niches within the same environment. This specialization facilitates resource partitioning, enabling many species to coexist by utilizing different parts of the same plant or soil layer.
Insects possess a chitinous exoskeleton, which provides structural support and protection against desiccation, a useful trait for colonizing dry terrestrial habitats. They also exhibit high reproductive rates, characterized by rapid generation times and the production of numerous offspring, a strategy known as r-selection. This reproductive speed allows their populations to quickly rebound and adapt to local environmental changes.
The development of complete metamorphosis, or holometabolism, in many insect orders further contributed to their success. Holometabolism involves a transformation from a larval stage to a pupal stage and then to the adult form, with each stage often occupying different ecological roles. This separation of the feeding stage (larva) and the reproductive/dispersal stage (adult) eliminates competition between life stages, maximizing resource utilization and survival.
Aquatic Constraints and Fish Diversity
The success of fish in aquatic environments is undeniable, yet they operate under biological constraints that limit their total numbers compared to insects. Fish generally have a much larger average body size than insects, which necessitates greater resource consumption per individual and supports lower population densities. Larger organisms require more space and energy, leading to fewer individuals occupying a given area.
Most fish species exhibit lower reproductive rates and longer generation times than insects. While many fish lay thousands of eggs, their reproductive strategy is less geared toward the exponential population growth seen in many insect groups. The three-dimensional aquatic environment, while vast, offers fewer opportunities for the micro-niche specialization that occurs on the complex surfaces of terrestrial plants and soil.
Fish are constrained by their status as vertebrates, requiring more complex physiological systems and greater energy investment in individual development and maintenance. Reliance on gills and water also limits their ability to exploit the terrestrial niches that insects have mastered. These factors contribute to fish diversity and abundance being significantly lower than that of the arthropod world.
Challenges in Estimating Global Population Totals
The numbers discussed are estimates, not exact counts, underscoring the challenges scientists face when trying to quantify global populations and species totals. A major hurdle is the sheer inaccessibility of many habitats where both insects and fish reside. Vast numbers of insect species are thought to remain undiscovered in tropical forest canopies, deep within the soil, and in remote freshwater systems.
Similarly, much of the ocean, particularly the deep-sea environment, remains unexplored, making it difficult to accurately census fish populations that live far below the surface. To arrive at the current figures, scientists rely on sampling techniques, such as sweep netting or specific types of trawling, and then use mathematical extrapolation and modeling. These methods provide robust figures but acknowledge the uncertainty in counting organisms that number in the trillions.