(1) We test the assumptions of the `aggregation model' of coexistence on a divided and ephemeral resource: (i) that the competing stage is aggregated; (ii) that the distributions of individuals can be represented by a negative binomial distribution; and (iii) that the degree of aggregation can be represented by a constant parameter. The assumption of no association between species is tested in a following paper. (2) Tests are made on 360 datasets representing distributions of Diptera, particularly drosophilids, on a total of 7638 resource patches. Results indicate that for this data: (i) competing stages are strongly aggregated; (ii) the negative binomial is not an exact description but may be adequate; and (iii) aggregation varies with density. (3) We relax the assumption of constant aggregation and develop a model of two-species competition where aggregation varies with density. We show that, with observed estimates of parameters for drosophilids, coexistence between pairs of species with no resource partitioning remains likely despite strong local competition.