1. Many aquatic ecosystems sustain multiple invasive species and interactions among them have
important implications for ecosystem structure and functioning. Here, we examine interactions
among two pairs of invasive crayfish species because of their close proximity and thus chance of
sympatric populations in the near future within the Thames catchment, U.K. (signal, Pacifastacus
leniusculus and virile crayfish, Orconectes virilis within a river system; red swamp, Procambarus clarkii
and Turkish crayfish, Astacus leptodactylus found within a suite of ponds). We address two questions:
do sympatric invasive crayfish occupy a smaller niche than their allopatric counterparts due to
potential resource competition? and do interactions among invasive species amplify or mitigate one
another’s impacts on the ecosystem?
2. Two fully factorial mesocosm experiments (one for each crayfish pair) were used to investigate
crayfish diet and their impact on benthic invertebrate community structure, benthic algal standing
stock and leaf litter decomposition rates in allopatric and sympatric populations, compared with a
crayfish-free control. We used stable isotope analysis to examine crayfish diet in the mesocosms and
in allopatric populations of each species in the Thames catchment.
3. Isotopic niche width did not vary significantly between allopatric and sympatric populations of
crayfish in the mesocosm experiments, and isotopic niche partitioning in all the wild populations
suggests the invaders can coexist.
4. All four species altered benthic invertebrate community structure but with differing functional effects,
often mediated via trophic cascades. Red swamp crayfish predation upon snails evidently promoted
benthic algal standing stock via reduction in grazing pressure. However, a trophic cascade whereby the
crayfish consumed native invertebrate shredders, causing a reduction in net leaf litter decomposition,
was decoupled by red swamp and signal crayfish since they consumed leaf litter directly and thus
moderated the cascade to a trickle when in sympatry with Turkish or virile crayfish, respectively.
5. Benthic invertebrate predator abundance was significantly reduced by sympatric red swamp and
Turkish crayfish but not independently when in allopatry, indicating an amplified effect overall
when in sympatry.
6. Our results suggest that the combined effect of multiple invasions on the ecosystem can reflect
either an additive effect of their independent impacts or an amplified effect, which is greater than
the sum of their independent impacts. A lack of general pattern in their effects makes any potential
management strategy more complex.