Differences in nutrient mineralisation between native and invasive grazing catfish during the invasion process.
In the highly diverse Rio Ubatiba in Brazil, the native and historically abundant armoured catfish Hypostomus punctatus (Loricariidae) has been declining since the 1990s, concomitantly with the introduction of the non-native loricariid, Parotocinclus maculicauda. Here, we assess over an 18-year period the potential impact of the establishment of invasive armoured catfish by examining dietary, tissue and excretion nutrients of these two species and the consequences of species substitution on nutrient remineralisation. To scale these differences up to the ecosystem, we also estimated the total areal excretion of each species over 18 years, pre- and post-introduction of P. maculicauda. Parotocinclus maculicauda had greater tissue %P, while H. punctatus had greater %N and %C. P. maculicauda showed a diet richer in nutrients. It also excreted significantly less mass-specific NH4-N than the native species, but the two species did not differ in their mass-specific PO4-P excretion rates. H. punctatus had higher per capita excretion rates for both nutrients, which was expected due to its greater body size. After the introduction of P. maculicauda, areal nutrient excretion dropped, due to the sharp decrease in overall loricariid biomass and the substitution of P. maculicauda, with its lower excretion rate than H. punctatus. These results indicate the two species have different stoichiometric niches and that the invasive P. maculicauda have not replaced the functional role of the native H. punctatus in nutrient mineralisation. This lack of functional replacement was mostly due to the lower biomass of P. maculicauda after the invasion compared to H. punctatus before the invasion. This species substitution can have important consequences at the ecosystem level, potentially altering nutrient cycling in the entire food web. Future studies should further investigate the causes that have led to P. maculicauda invasion and H. punctatus population decreases.