Methods for calculating coexistence mechanisms: Beyond scaling factors

TL;DR

This paper compares four methods for calculating species coexistence mechanisms within Modern Coexistence Theory, highlighting the practical advantages of simple comparison and speed conversion methods over traditional scaling factors.

Contribution

The study introduces and evaluates two novel methods—speed conversion factors and invader-invader comparison—for measuring coexistence mechanisms, providing guidance for empirical applications.

Findings

Scaling factors are less suitable for empirical studies.

Simple comparison is effective for species with similar life histories.

Speed conversion factors are advantageous for species with different generation times.

Abstract

How do species coexist? A framework known as Modern Coexistence Theory measures mechanisms of coexistence by comparing a species perturbed to low density (the invader) to other species that remain at their typical densities (the residents); this invader-resident comparison measures a rare-species advantage that results from specialization. However, there are several reasonable ways (i.e., methods) to compare invaders and residents, each differing in practicality and biological interpretation. Here, using theoretical arguments and case studies, we compare four such methods for calculating coexistence mechanisms: 1) Scaling factors, the traditional approach where resident growth rates are scaled by a measure of relative sensitivity to competition, obtained by solving a system of linear equations; 2) The simple comparison, which gives equal weight to all resident species; 3) Speed conversion factors, a novel method in which resident growth rates are scaled by a ratio of generation times, and; 4) The invader-invader comparison, another novel method in which a focal species is compared to itself at high vs. low density. We conclude that the conventional scaling factors can be useful in some theoretical research, but are not recommended for empirical applications, i.e., determining the mechanisms of coexistence in real communities. Instead, we recommend the simple comparison and speed conversion factor methods. The speed conversion factors are most useful when comparing species with dissimilar generation times. However, ecologists often study coexistence in guilds of species with similar life-histories, and therefore, similar generation times. In such scenarios, the easier-to-use simple comparison method is reasonable.