Niche construction for desert plants in individual and population scales: theoretical analysis and evidences from saksaul (Haloxylon ammodendron) forests.

摘要

Niche construction, also called "ecosystem engineering", means that organisms can modify their environments, and this phenomenon has been observed in a wide variety of ecosystems. In selecting the desert plant Haloxylon ammodendron (saksaul) found in the Badain Jaran desert in the west of China, as our research object, we reveal the mechanism of niche construction in the relationship between saksaul and soil resource content, and discuss the patterns at individual and population scales. Theoretical analyses show that the dynamics of these desert vegetation systems are governed by a positive feedback between focal ecological index and resource content at different scales. Feedback at the individual scale generates a positive correlation between biomass and individual niche fitness, which forms a biomass threshold for seedlings. Feedback at population scales generates a positive correlation between vegetation cover rate and habitat niche fitness, which forms a density threshold for planting. Mechanisms of niche construction at gene, individual and population scale are compared. The results imply that similar mechanisms of niche construction produce different patterns with altering scales, which also means that the selection unit may be altered with scales. Our study indicates that ecological or evolutionary patterns under a similar mechanism may be diverse, and the evolutionary directions induced by niche construction may be opposite with different scales, which can generate alternative stable states in desert vegetation systems. One state is characterized by low density and high local fitness but low regional fitness, with large individuals; and the other, by high density and relatively low local fitness but high regional fitness, with small individuals.