Introduced animals can indirectly affect decomposers through trophic cascades and habitat modifications, but whether their effects are strong enough to influence both the structure and function of decomposer communities remains unclear. We conducted an experiment on rat-invaded and rat-free islands off the coast of New Zealand to determine whether introduced rats affected the structure and function of wood-decomposing fungi. Gamma-irradiated branch segments from a single tree were placed on the forest floor on nine rat-invaded and nine rat-free islands, and fungal community structure and wood decomposition rates measured after two years of in situ decomposition. We found significant differences in fungal community structure in the wood between rat-invaded and rat-free islands. Furthermore, there was a significant correlation between fungal community structure and wood decomposition rate on rat-free islands but not on rat-invaded islands, because of decreased variability in decomposition rates on invaded islands. Despite these differences between rat-free and rat-invaded islands, mean decomposition rates were indistinguishable between the two sets of islands. These results suggest that there may be a great deal of functional redundancy in fungal communities and that removing rats from islands could reverse the rat-induced changes that we observed in the relationship between the structure and function of decomposer communities.