The dynamics of abundance and incidence of annual plant species during colonization in a desert

We studied colonization of annual plants in small-scale disturbancesand undisturbed soil for four years in the northern Negev desert. The experiment consisted of 24 patches of 1 m2: eight undisturbed patches, eight 30 cm deep pits and eight 20 cm high mounds. Disturbance removed the seed bank from the pits and mounds. Rainfall wasaverage during 1992 and 1993 (163.5 and 157.0 mm), very low in 1994 (97.5 mm) and very high in 1995 (283.0 mm). For all ca 100 species inthe assemblage, we measured abundance, as the average local density in patches occupied by the species, and incidence, the proportion of patches occupied. Abundance and incidence were positively correlated in the four years of the study (R2 = 0.71, 0.56, 0.56 and 0.64). The relationship became steeper during the course of colonization. A minority of species colonized all patches rapidly and increased exponentially in abundance. These high incidence and abundance species responded little or not at all to variations in rainfall. Most ofthe species experienced frequent local extinctions and colonizationsin few patches. These low abundance and low to medium incidence species responded strongly to rainfall by fluctuations in abundance. Differences per species between abundance and/or incidence in pits, mounds and matrix were evident in many species along the entire relationship. These were mostly in favor of pits. We propose that the positionsand trajectories of the individual species in the abundance incidence phase plane signify constraints on population growth along the abundance axis, and on colonization and population persistence along the incidence axis. Since species with lower incidence and abundance respond strongly to rainfall variation, we conclude that their local populations and spatial distribution are to a large extent site-limited. High-abundance and high-incidence species have few or weak constraints and experience density-independent population growth, which is onlyseed-l [ABSTRACT FROM AUTHOR]