The Clumping Transition in Niche Competition: a Robust Critical Phenomenon

We show analytically and numerically that the appearance of lumps and gaps in the distribution of n competing species along a niche axis is a robust phenomenon whenever the finiteness of the niche space is taken into account. In this case depending if the niche width of the species $σ$ is above or below a threshold $σ_c$, which for large n coincides with 2/n, there are two different regimes. For $σ> sigma_c$ the lumpy pattern emerges directly from the dominant eigenvector of the competition matrix because its corresponding eigenvalue becomes negative. For $σ</- sigma_c$ the lumpy pattern disappears. Furthermore, this clumping transition exhibits critical slowing down as $σ$ is approached from above. We also find that the number of lumps of species vs. $σ$ displays a stair-step structure. The positions of these steps are distributed according to a power-law. It is thus straightforward to predict the number of groups that can be packed along a niche axis and it coincides with field measurements for a wide range of the model parameters.

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