Invasion in patchy landscapes is affected by dispersal mortality and mate‐finding failure

Abstract

Range expansions are a function of population growth and dispersal, and nascent populations often must overcome demographic Allee effects (positive density dependence at low population densities) driven by factors such as mate-finding failure. Given the importance of individual movement to mate finding, links between landscape structure and movement may be critical to range expansion; however, landscape effects on other factors including mortality may be equally or more important. In one of the most comprehensive investigations of the interactions of these processes to date, we combined field experiments, simulation modeling, and analysis of empirical spread patterns to investigate how landscape structure affected the spread of the gypsy moth in Virginia and West Virginia. In experiments designed to assess how landscape attributes affect mate finding, we found adult males resisted leaving forest patches and the probability of locating a pheromone source declined more rapidly over distance in non-forest matrix than in forest. We used these findings to develop individual-based simulation models of gypsy moth population dynamics and spread in complex patch-matrix landscapes. The models produced an Allee effect that strengthened with reductions in forested area, but owing more so to dispersal mortality than to effects on mate location. Predicted maximum rates of population spread grew with increases in forest area due to increasing success of long-distance transport events. Evaluations of empirical data showed relationships between spread rates and landscape structure largely consistent with model predictions. We conclude rates of spread were largely driven by long-distance dispersal events, the success of which was influenced primarily by dispersal mortality of larvae in unsuitable matrix, and that landscape effects on mate location played a secondary role. Though influences of landscape structure on mate location appear to be unimportant to the spread of the gypsy moth, we predict they would have stronger effects on more dispersive species.

Why come back home? Breeding-site fidelity varies with group size and parasite load in a colonial bird

“…The consequences of philopatry versus dispersal can profoundly affect gene flow, average relatedness, demography, disease spread, mating success and social behaviour (Clobert, Danchin, Dhondt, & Nichols, 2001; Greenwood & Harvey, 1982; Johnson & Gaines, 1990; Walter, Firebaugh, Tobin, & Haynes, 2016). Ecological and behavioural correlates associated with site fidelity have been explored in many species (e.g.…”

Sperm storage reduces the strength of the mate‐finding Allee effect

“…have not yet been addressed among individuals of the same species (Orr & Brennan, 2015). Our results suggest that longer sperm storage durability may be especially useful for the populations that occur in patchy habitats with low population densities and show low movement ability (Walter et al, 2016). Given the potentially major importance of longer sperm storage for the biological fitness of species with limited movement ability, future studies may evaluate this trait from experiments run in captivity, in which females are isolated from males (e.g., Palmer, Rostal, Grumbles, & Mulvey, 1998).…”

“…Some studies have evaluated the biological consequences F I G U R E 4 Probability of the extinction (P ext ) of T. graeca for the different simulated scenarios for 200 years. The different sperm storage durability scenarios (0, 1, 2, 3, and 4 years) are indicated by different colors of mate-finding limitation (Carver, Wolcott, Wolcott, & Hines, 2005;Fromhage et al, 2016;Gascoigne et al, 2009) and have even implemented individual-based models to address this question (Berec & Boukal, 2004;Berec et al, 2001Berec et al, , 2018Coates & Hovel, 2014;Engen, Lande, & Saether, 2003;Gregory, Bradshaw, Brook, & Courchamp, 2010;Walter et al, 2016). However, to the best of our knowledge, this is the first study to assess the ecological role of sperm storage in relation to the mate-finding Allee effect in a spatial context.…”

“…Mate-finding Allee effects may also frequently occur for populations in expansion, where densities that border the species range are typically low (Shaw, Kokko, & Neubert, 2018;Walter et al, 2016). We expect that a long sperm storage durability would be important for species conservation and to contribute to successful range shifts in a climate change scenario (Estrada, Morales-Castilla, Caplat, & Early, 2016;Morrison, Estrada, & Early, 2018).…”

“…However, individuals may fail to find mates when population density is low (Berec, Kramer, Bernhauerová, & Drake, 2018;Gascoigne, Berec, Gregory, & Courchamp, 2009). This effect might be amplified in patchy landscapes (Walter, Firebaugh, Tobin, & Haynes, 2016), especially for species with limited movement ability (Anthonysamy, Dreslik, Douglas, Marioni, & Phillips, 2014). Mate limitation has major consequences for reproductive fitness and can cause a mate-finding Allee effect (Berec, Boukal, & Berec, 2001;Gascoigne et al, 2009).…”