Heterogeneity exists on multiple scales. Species abundance can often be associated with abiotic factors, but you cannot assume, based on field observation alone, that highest population density occurs where the abiotic environment is most favorable. Chthamalus tolerates dessication, occurs in the upper tidal zone, but could occur lower if it wasn't excluded by Balanus More on interactions in a few weeks.
Individuals have an equal chance of living anywhere in an area.
Random distribution Only occurs if environment is relatively homogenous and if individuals aren't interacting. Most plant populations have a clumped distribution. They cannot move and space themselves out to avoid competition. Shrubs in arid environments appear to be regularly spaced. Measurements produced conflicting results and arguments over sampling techniques and spatial statistics.