How do interactions between species influence their spatial distribution in an ecosystem? To answer this question, we introduce a spatially extended ecosystem of generalized Lotka-Volterra type, where species can diffuse and interactions are nonlocal. We compute the criterion for the loss of stability of the spatially homogeneous ecosystem, and we show that the stability of the uniform state crucially depends on the most abundant species, and on the interplay between space exploration during the timescale of reproduction and the interaction range. Focusing on the spectrum of the interaction matrix weighted by the species abundances, we identify a Baik-Ben Arous-Péché transition that translates into a transition in the final patterns of the species distribution. Finally assuming that the disorder is small, we exhibit an explicit solution of the dynamical mean-field equation for the species density, obtained as the fixed point of nonlocal Fisher-Kolmogorov-Petrovski-Piskounov equations. Our work paves the way of future combined approaches at the frontier of active matter and disordered systems, with the hope of better understanding complex ecosystems like bacterial communities.

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• Ecological pattern formation
• Disordered systems
• Dynamical mean field theory
• Random matrix theory