Species undergo range shifts in response to changing climate or following an introduction to a new environment. Invasions often incur significant economic cost and threaten biodiversity. Ecological theory predicts two distinct types of expansion waves, pulled and pushed, depending on the degree of cooperativity in the population. Although pulled and pushed invasions differ dramatically in how population-level properties such as the expansion rate depend on the organism-level properties such as rates of growth and dispersal, these theoretical predictions have not been tested empirically. Here, we use a microbial model system to perform these tests and demonstrate that pulled and pushed waves can be distinguished based on their dynamics.

Abstract

Range expansions are becoming more frequent due to environmental changes and rare long-distance dispersal, often facilitated by anthropogenic activities. Simple models in theoretical ecology explain many emergent properties of range expansions, such as a constant expansion velocity, in terms of organism-level properties such as growth and dispersal rates. Testing these quantitative predictions in natural populations is difficult because of large environmental variability. Here, we used a controlled microbial model system to study range expansions of populations with and without intraspecific cooperativity. For noncooperative growth, the expansion dynamics were dominated by population growth at the low-density front, which pulled the expansion forward. We found these expansions to be in close quantitative agreement with the classical theory of pulled waves by Fisher [Fisher RA (1937) Ann Eugen 7(4):355–369] and Skellam [Skellam JG (1951) Biometrika 38(1-2):196–218], suitably adapted to our experimental system. However, as cooperativity increased, the expansions transitioned to being pushed, that is, controlled by growth and dispersal in the bulk as well as in the front. Given the prevalence of cooperative growth in nature, understanding the effects of cooperativity is essential to managing invading species and understanding their evolution.