Terrestrial isopods, commonly known as woodlice, pill bugs, or roly-polies, are often mistakenly classified as typical insects or carnivores. These small, segmented organisms are crustaceans, making them more closely related to shrimp and crabs. Isopods are not carnivores, whose diets consist primarily of live prey, but rather they belong to a different functional feeding group. Their diet is overwhelmingly plant-based and focused on materials that are already dead or decaying. This dietary preference places them firmly in the category of decomposers, which perform an important ecological function.
Classification as Detritivores
The definitive classification for terrestrial isopods is detritivore, meaning they feed on detritus, which is dead organic matter. This includes fallen leaves, decaying wood, and other plant-based waste. Isopods are primarily herbivorous, consuming leaf litter, especially hardwood varieties like oak and maple, and rotting wood softened by fungi.
Detritivores play a major role in the ecosystem by fragmenting larger pieces of detritus into smaller particles. This mechanical breakdown increases the surface area, allowing bacteria and fungi to complete decomposition much faster. Isopods benefit from this fungal growth, as fungi often provide necessary nitrogen that they cannot easily acquire from plant matter alone. Their feeding action is a foundational step in recycling nutrients back into the soil, which supports new plant growth.
Their diet also frequently includes consuming the microorganisms that live on the detritus. Isopods seek out specific ratios of nutrients, including proteins, sugars, and calcium, which they gain from different sources. They often ingest soil crust or other materials containing live microbes, which assist in the digestion of fibrous plant matter. This symbiotic relationship highlights the complexity of their detritivorous feeding strategy.
Scavenging and Protein Consumption
The misconception that isopods are carnivores often stems from their opportunistic scavenging behavior and occasional consumption of animal matter. While their diet is mainly plant detritus, isopods readily consume dead insects, molted exoskeletons, and other protein-rich materials they find. This protein intake is not driven by predatory hunting, but by the need for specific nutrients lacking in leaf litter.
The most significant required nutrient is calcium, which is necessary for their crustacean exoskeleton. They often consume their own shed exoskeletons (molting) to reclaim this calcium. When protein or calcium is scarce, colonies may exhibit cannibalism, consuming weaker or recently deceased individuals. This behavior signals a nutritional deficiency in their environment, rather than an inherent predatory instinct.
Some isopod species show a higher preference for protein and actively seek out sources like dried shrimp or insect remains. This supplemental consumption ensures they receive enough nitrogen and other micronutrients for healthy growth and reproduction. While they are not carnivores, their diet is omnivorous, meaning they consume any dead organic matter to meet their complex nutritional requirements.
Isopods in Ecosystems and Captivity
In natural environments, the detritivore feeding habit of isopods makes them indispensable for the health of soil and forest floors. By accelerating the decomposition cycle, they play a large part in nutrient cycling and making those nutrients available for plants. Their constant activity also helps to aerate the soil, which improves its structure and drainage.
This beneficial function is why isopods are frequently utilized by hobbyists in enclosed environments like terrariums and vivariums. When housed with reptiles or amphibians, they are known as a “cleanup crew” (CUC). In this captive role, they consume uneaten food, decaying plant material, and animal waste, contributing to a self-sustaining ecosystem.
Their presence in these enclosures helps prevent the buildup of mold and harmful bacteria by rapidly processing organic waste. Isopods contribute to a stable environment by recycling nutrients back into the substrate, supporting the growth of live plants. Their scavenging nature is entirely functional, demonstrating their role as ecological recyclers both in the wild and in captivity.