Competition Modeling of the Combined Impact of Pterygoplichthys spp. and Invasive Trichogaster spp. on Native Freshwater Fish: Stability, Sensitivity, and Management Scenarios

Invasive fish threaten native freshwater fish communities, yet mathematical studies have mostly examined a single invader, leaving the combined effects of co-occurring invaders and their underlying mechanisms unquantified. This study develops a three-species ordinary differential equation model of janitor fish (Pterygoplichthys spp.), an invasive gourami population (Trichogaster spp.), and a native fish, in which the invaders affect the native fish through resource competition and an additional stress-induced mortality term. Equilibria and their local stability were analyzed by Jacobian linearization, the system was solved with the fourth-order Runge–Kutta method over 15 years, and normalized sensitivity indices and management scenarios were evaluated. All parameters and initial densities are hypothetical but literature-informed. The two invaders coexist at a locally asymptotically stable equilibrium (10,177 and 7,420 ind/km), whereas no positive three-species equilibrium exists and native exclusion is the only stable outcome. Without intervention, the native population declines by about 99% within 10 years. Competition accounts for more than 96% of the pressure on native fish, with janitor fish contributing about 61%, and native density is most sensitive to the janitor fish carrying capacity and the invasive-to-native competition coefficients. Harvesting janitor fish, even to complete removal or combined with habitat restoration, cannot keep native density above 50% of its carrying capacity, because gourami is released from competition. Effective management therefore requires the joint control of both invaders. These theoretical projections require field calibration before site-specific application.