Lessons from two decades of detailing an invasion front

What can we learn about population dynamics from invasive species? A recent synthesis article published in the Journal of Animal Ecology examines the contributions from one of the best-documented cases of species invasion, the spread of the gypsy moth in North America. Dr Kristine Grayson started working with gypsy moth as a USDA National Institute of Food and Agriculture Postdoctoral Fellow at Virginia Commonwealth University with Dr Derek Johnson. Since then, she has continued this work five miles down the road at the University of Richmond, where she joined the Biology Department as an Assistant Professor in 2015.

It might seem counterintuitive to pay tribute to the contributions of invasive species, yet these systems offer a unique window for studying population dynamics and species distributions. Zebra mussel, Argentine ant, spotted knapweed, cane toad, and grey squirrel are just a few examples of the multitudes of species that have offered important insights into ecological processes. We highlight the contributions from research on gypsy moth, which established in North America after escaping from the home of Étienne Trouvelot around 1869 in Medford, Massachusetts. The spread of the gypsy moth in North America particularly stands out for the historical information available for understanding spread, the long time-series of the invasion, and a large-scale monitoring program that has resulted in detailed knowledge of the invasion front.

Gypsy moth
Male gypsy moths are attracted to flightless females by mating pheromones, which have been synthesized and deployed as an attractant in traps (Photo credit: University of Richmond)

Population dynamics at range edges are fundamental to understanding the limits to species distributions. What factors determine that a species which is abundant in one area gradually declines to be absent in another? How do climate and habitat or predators and prey play roles in determining where a species is found? What happens to the dynamics of a population when a species becomes rare? These questions have increased in urgency due to shifting geographic ranges occurring under climate change.

A major challenge to answering these questions is having detailed data on where a species occurs and where it does not. Conducting surveys in low density populations is challenging, as anyone who studies rare and endangered species well knows, particularly on a large scale. This is where the US national program to manage the spread of gypsy moth is so unique. The Slow the Spread (STS) program in cooperation with the USDA Forest Service and state partners, has monitored the US invasion front for over two decades, spanning an area from Wisconsin to North Carolina with up to 100,000 pheromone baited traps per year. These traps are highly effective at detecting male moths in low density areas. Additionally, because the trapping zone spans the established area, the transition zone, and areas that are uninfested, the precise location of the range edge can be determined each year.

Slow the Spread programme in 2012
Example of the area trapped under the Slow the Spread programme in 2012 where each dot represents an individual gypsy moth trap (a) and the associated captures of male moths (shown in detail in panel b). Trap catch data is used to generate a continuous surface of gypsy moth abundance (Graphic by P. Tobin and L. Blackburn).

We synthesized how the STS program, although designed to prioritize areas for active management, has ended up as an ideal system for testing ecological theory related to spatial spread, range limits, low density population dynamics, and Allee effects. The STS data set offers one of the most extensive and detailed records of a species border ever attempted and, despite decades of prolific work, there are still a wealth of opportunities to continue to advance ecological theory using this system. Researchers are making strides in using landscape tools to test cross-scale dynamics and new statistical approaches are being designed to better understand low density populations. The work of my lab and others examines the potential for plasticity and evolutionary change in developmental traits across the invasion to facilitate future spread. Through the synergy between invasive species management and fundamental research, this system has resulted in significant insights for the field of ecology and we look forward to seeing the new avenues of research using the gypsy moth invasion front in years to come.

gypsy moth research team
Kristine Grayson and students Amber Yang, Melisa Quiroga-Herrera, and Nana Banahene conducting a gypsy moth reciprocal transplant experiment at the southern invasion front (Photo credit: University of Richmond)

More info:

Grayson KL, Johnson DM. Novel insights on population and range edge dynamics using an unparalleled spatiotemporal record of species invasion. Journal of Animal Ecology 2017;00:1–13. https://doi.org/10.1111/1365-2656.12755

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