Intraguild predation mediates co-existence between native and non-native fish

Background/Question/Methods

Understanding mechanisms that facilitate co-existence of native species with invasive species is critical for implementing effective conservation strategies. Intraguild predation is one potential mechanism which may facilitate co-existence of interacting species through reciprocal predation. Such is the case with endangered Mohave tui chub (Siphateles bicolor mohavensis), which co-occurs with invasive western mosquitofish (Gambusia affinis) in two out of four established Mohave tui chub populations. We ran a field mesocosm experiment to assess reciprocal trophic interactions between tui chubs and mosquitofish with both species maintained in allopatry or sympatry.  Data were analyzed for tui chub recruitment, mosquitofish survival and sex-specific survival rates for the sexually dimorphic mosquitofish.  Additional laboratory predation trials were conducted to assess the role of predator gape-limitation between these two species.  A single potential predator (adult tui chub or adult mosquitofish) was paired with a single potential prey (adult mosquitofish or a larval tui chub, respectively) and the survival functions were estimated using time to death data, as a function of the relative gape-size to body size ratio for each pair.  

Results/Conclusions

Tui chub larval production was significantly lower for sympatric populations ( 5.4 ± SE 3.4 larvae/mesocosm), compared to allopatric tui chub populations (33.8 SE ± 7.6 larvae/mesocosm). Reciprocally, mosquitofish population size was significantly lower (W = 155.0; P < 0.001) in sympatric tanks (22.1 ± 4.0 /mesocosm) compared to allopatric mosquitofish  populations (157.2 ± 26.9 /mesocosm). Notably, male mosquitofish, which are notably smaller than female mosquitofish, were virtually eliminated in all sympatric mesocosms.  For the laboratory predation trials, larval tui chubs had significantly lower (χ² = 74.74; P < 0.001) survival in the presence of female mosquitofish (10.0%) than in the presence of male mosquitofish (73.3%). Reciprocally, adult tui chubs preyed upon adult mosquitofish, causing a significantly lower (χ²= 11.33; P < 0.001) survival for male mosquitofish (60%) compared to female mosquitofish survival (96.7%). Analysis of predator gape size to prey depth ratio provided evidence that intra-guild predation is limited by relative gape-size of the predator.   Our experiments showed that intraguild predation between endangered Mohave tui chub and invasive western mosquitofish is structured by predator gape limitation, and may explain the long-term co-existence of these two fish species.