The effects of inter-plant interactions and density-dependent disturbances on vegetation pattern formation

Ecological interactions among individuals and disturbances are two important agents of pattern formation. In this study we investigated the interrelationships between interactions among individuals and large scale disturbances, and the resulting patterns. We categorized disturbances into three general classes, (1) those whose probability of occurrence increases with increased densities of vegetation, such as fire and disease, (2) those with a decreasing probability of occurrence with increasing vegetation densities, such as sand movement, and (3) disturbances that occur independently of vegetation densities, such as flooding. The ecological interactions among individuals were also divided to three classes: competition, facilitation and neutrality. We systematically investigated how these two types of processes interact to generate spatial patterns, using simulation models that were partially based on data collected from a shrub community in the Nizzana sand dune ecosystem. The results indicated that the different types of disturbances have fundamentally different effects on spatial patterns. Positive density-dependent disturbances, regardless of the type of interactions among individuals with which they were simulated, generated uniform spatial patterns. Patterns formed by interactions between decreasing or density independent disturbances with the different class interactions among individuals were more variable. These differences are attributed to the manner in which the difference disturbance types propagate in space.