PT  - JOURNAL ARTICLE
AU  - Lauren L. Sullivan
AU  - Bingtuan Li
AU  - Tom E. X. Miller
AU  - Michael G. Neubert
AU  - Allison K. Shaw
TI  - Density dependence in demography and dispersal generates fluctuating invasion speeds
AID  - 10.1101/075002
DP  - 2016 Jan 01
TA  - bioRxiv
PG  - 075002
4099  - http://biorxiv.org/content/early/2016/09/13/075002.short
4100  - http://biorxiv.org/content/early/2016/09/13/075002.full
AB  - Density dependence plays an important role in population regulation, and has a long history in ecology as a mechanism that can induce local density fluctuations. Yet much less is known about how these endogenous processes affect spatial population dynamics. Biological invasions occur through the combined action of population growth (demography), and movement (dispersal), making them relevant for understanding how density dependence regulates spatial spread. While classical ecological theory suggests that many invasions move at a constant speed, empirical work is illuminating the highly variable nature of biological invasions, which can lead to non-constant spreading speeds. Here, we explore endogenous density dependence as a mechanism for inducing variability in biological invasions. We constructed a set of integrodifference population models that incorporate classic population fluctuation mechanisms to determine how density dependence in demography, including Allee effects, and in dispersal affects the speed of biological invasions. We show that density dependence is a key factor in producing fluctuations in spreading speed when Allee effects are acting on population densities that fluctuate locally. We show that the necessary density fluctuations can arise from either a nonmonotone population growth function where densities fluctuate locally (e.g., overcompensatory population growth), or from density-dependent dispersal when the population growth function results in constant local densities. As density dependence in both demography and dispersal are common, this mechanism of variability may influence many invading organisms.Significance Statement Controlling the spread of biological invasions reduces the cost of mitigating invasive species. However, predicting empirical invasive population spread is difficult as evidence shows the speed of this movement can be highly variable. Here, we provide a novel mechanism for this variability, showing that internal population dynamics can lead to fluctuations in the speed of biological invasions through the combined action of density dependence in demography and dispersal. Speed fluctuations occur through the creation of a variable pushed invasion wave, that moves forward not from small populations at the invasion front, but instead from larger, more established populations that “jump” forward past the previous invasion edge. Variability in the strength of the push generates fluctuating invasion speeds.