The coupon collector urn model with unequal probabilities in ecology and evolution

TL;DR

This paper applies the coupon collector urn model with unequal probabilities to ecological sampling, analyzing how probability differences influence collection speed and parasitization, with theoretical proofs and numerical examples.

Contribution

It introduces a comprehensive application of the coupon collector model to ecological parasitization, including new proofs of stochastic dominance and insights into sampling probability effects.

Findings

Sampling speed depends on the entire probability distribution shape.

Variance comparison alone is insufficient to assess collection speed.

Numerical examples illustrate the influence of unequal probabilities.

Abstract

The sequential sampling of populations with unequal probabilities and with replacement in a closed population is a recurrent problem in ecology and evolution. Many of these questions can be reformulated as urn problems, often as special cases of the coupon collector problem, most simply expressed as the number of coupons that must be collected to have a complete set. We aimed to apply the coupon collector model in a comprehensive manner to one example -hosts (balls) being searched (draws) and parasitized (ball color change) by parasitic wasps- to evaluate the influence of differences in sampling probabilities between items on collection speed. Based on the model of a complete multinomial process over time, we define the distribution, distribution function, expectation and variance of the number of hosts parasitized after a given time, as well as the inverse problem, estimating the sampling effort. We develop the relationship between the risk distribution on the set of hosts and the speed of parasitization and propose a more elegant proof of the weak stochastic dominance among speed of parasitization, using the concept of Schur convexity and the "Robin Hood transfer" numerical operation. Numerical examples are provided and a conjecture about strong dominance is proposed. The speed at which new items are discovered is a function of the entire shape of the sampling probability distribution. The sole comparison of values of variances is not sufficient to compare speeds associated to two different distributions, as generally assumed in ecological studies.