Species coexistence by front pinning

• We study a new pattern formation mechanism for plant-species coexistence.
• The species differ in the tradeoff they make in capturing soil water and sunlight.
• The coexistence is due to localized population states of one of the two species.
• The coexistence is affected by the tradeoff in resource capture.
• The relevance of these findings to real ecosystems is briefly discussed.

The spatial competition between two plant species that make different compromises in capturing soil water and sunlight is studied using a mathematical model. A precipitation range along the rainfall gradient is identified where two alternative stable states coexist. The first state describes a uniform distribution of a plant species that specializes in capturing soil water, whereas the second state describes a periodic pattern of a species that specializes in capturing light. We show that this bistability range generally divides into three parts according to the dynamics of the front or ecotone that separates the two plant populations: a low precipitation range where the superior competitor for water displaces the superior competitor for light, a high precipitation range where the displacement is reversed, and an intermediate range where neither species displaces the other. While in the low and high precipitation ranges one species outcompetes the other, the intermediate range allows for species coexistence in the form of a multitude of stable localized solutions consisting of fixed domains of one species in areas otherwise occupied by the other species. These localized solutions can only be realized when one of the alternative stable states is spatially patterned. We further study two factors that affect the size of the species coexistence range: the strength of the competition for light and the form of the tradeoff between the competitive abilities to capture water and light.