Simone Fontana, Matty P. Berg and Marco Moretti

Oniscus asellus in their natural environment.
Oniscus asellus in their natural environment.

In the last decades ecology has considered species as the main actors of the processes observed in nature. However, we know that species are far from being homogeneous entities, as individuals of the same species often display huge differences in features that determine their role and success in ecosystems. If this was not the case, natural selection and evolution, for example, could not occur. Moreover, interactions among living organisms, such as facilitation or competition, occur by definition among individuals rather than species. For these reasons, it is reasonable to assume that differences among individuals determine interactions across the food web, and can consequently affect fundamental processes at the ecosystem-level.

The decomposition of fallen leaves is crucial for nutrient cycling in soils, and therefore sustains multiple functions that are globally relevant. Soil invertebrates contribute essentially to the decomposition of fallen leaves, and different species can act in a complementary way to increase the efficiency of this process. This is the case when invertebrate species differ in their preference for distinct leaf species present on the ground. Although it is well known that the diet of individual consumers can vary throughout their life cycle, whether and how different individuals of the same species can show complementary feeding and therefore enhance the decomposition of multiple leaf species has not been investigated.

Here we tested experimentally the effect of body size differences in the common terrestrial woodlouse species Oniscus asellus on leaf decomposition of the two common species Acer platanoides and Betula pendula. We hypothesized complementary feeding by two distinct woodlouse body size classes, leading to decreased competition and higher overall decomposition. Our results support this hypothesis and suggest that the two woodlouse body size classes have distinct dietary behavior and preferences, which allow them to enhance the decomposition of fallen leaves. We hope that our work will stimulate further studies on within-species individual differences and their effects on the functioning of ecosystems.

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