Collective species dynamics may lead to emergent properties that cannot be deduced from species properties alone. In this paper we consider water limited systems and study emergent properties related to habitat creation and species richness. The results presented here are based on a mathematical model that captures two modes of collective dynamics. The first is associated with key species termed "ecosystem engineers", such as shrubs, which redistribute the soil-water resource and create habitats for other species, e.g. annuals. The second mode is associated with symmetry breaking vegetation patterns of the ecosystem engineers and the associated soil-water patterns. Among the findings of this work is a novel mechanism for species loss or gain events as a result of environmental changes such as drought or human disturbances. If the affected species, e.g. the annuals, owe their very existence to the habitats created by the ecosystem engineers, their local extinction (or introduction) may result from environmental changes that induce pattern transitions of the ecosystem engineers involving loss (or gain) of habitats. We illustrate this emergent mechanism with transitions between bands and spots on a slope.

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Model simulation showing the destabilization of a uniform vegetation state, as a precipitation parameter is decreased below a critical value, and the symmetry breaking vegetation pattern that evolves.