NEIGHBOR COMPETITION MODIFIES DEMOGRAPHIC RESPONSES OF DESERT DUNE ANNUALS TO GRADIENTS IN SAND STABILITY

A field experiment was conducted to test the hypothesis that competiti ve interactions affect demographic responses of desert dune annuals to gradients in sand stability. The experiment was based on a three-way factorial design with two types of neighbor treatments (complete neigh bor removal vs. control), two types of habitats (stable vs. unstable s ands) and two species which characterize sandy areas of the western Ne gev Desert (Neurada procumbens and Bromus fasciculatus). Each of the e ight treatment/habitat/species combinations was characterized by three demographic variables: seedling survival to reproduction, fruit numbe r per reproductive plant and fruit number per seedling. All of these v ariables exhibited significant interactions between the effects of nei ghbor removal and habitat type, indicating that neighbor conditions we re important in determining the observed patterns of demographic varia tion. Moreover, while plants of Neurada procumbens growing without nei ghbors suffered more mortality on unstable sands, conspecific plants g rowing at natural density conditions suffered more mortality on stable sands. In the case of Bromus fasciculatus, individuals growing in the absence of competitive effects produced more fruits on stable sands, but conspecific plants growing at natural density conditions exhibited higher fruit production on unstable sands. These findings indicate th at competitive effects not only modify, but may even reverse demograph ic responses of desert dune annuals to gradients in sand stability. Ba sed on these results and evidence from two other studies it is suggest ed that seed dispersal from abiotically favorable into less favorable habitats may increase fitness of desert annuals by reducing the averag e effect of competition. (C) 1997 Academic Press Limited.