Ever have one of those new neighbors who moved into town and was so successful that they just seemed to take over your turf? Well that’s the problem that a native ginger plant in Las Cruces Biological Station is having. Over the past few decades, the habitat of the native plant, Renealmia cernua, has been slowly inundated with an invasive ginger variety, Zingiber spectabile. We don’t know exactly why these invasive species are so successful, but one compelling explanation is called the Enemy Release Hypothesis or ERH.
Here’s how it works: when the invasive species is in its own home range, it has plenty of enemies that it grew up with and has to spend lots of energy fighting them off. But when it moves away to a new neighborhood, it escapes all of its enemies and gets to spend its energy growing and reproducing instead. Which makes it hard for the native species already living in that neighborhood to compete.
One way we can test this is by looking at who the plants’ enemies are. You can imagine there are two types: the personal grudges, who specifically prey on the type of plant it grew up with (“specialists”), and the meanies, who prey on just about anyone (“generalists”). Under the ERH, we expect that native plants get more grief from specialists than the invasives do (since the invasives haven’t ruffled as many feathers in their new neighborhood yet), but that both natives and invasives get the same grief from generalists.
In Las Cruces, the ginger plants’ enemies are insects, who eat the leaves (“herbivory”). To see if the ERH holds true in this system, I categorized chomp marks on 341 leaves of both ginger species into two insect groups. One type of bite mark—small holes in the middle of the leaf—indicated beetle herbivory, which is the generalist. Edges shredded by a small insect indicated caterpillar herbivory, which is the specialist.
So what did I find? Indeed, the native Renealmia cernua was eaten more by caterpillars, but there was no difference in beetle herbivory between the native and invasive Zingiber spectabile. And overall, 24% more native gingers were experiencing some sort of herbivory than invasive gingers.
It seems that the Enemy Release Hypothesis is holding up in the Las Cruces ginger neighborhood—but what happens next? Are the caterpillars going to catch on to the invasive ginger’s tastiness, and start eating it too? Will the invasives have to defend themselves and become less productive? What if new insects move into town? We might just have to return to Las Cruces to find out.