The Invasive Species That Couldn’t Invade

Wikimedia Commons

The red shiner is just a few inches long, and has no big scary fangs, no claws, no stinger, and no poisonous spines. The little minnow probably isn’t an animal that would ever strike fear in anyone’s heart, but it’s a fierce conqueror.

In fact, on paper, they seem like the perfect invasive species. They can live and reproduce almost anywhere, tolerate extreme conditions like high-temperature and low-oxygen water, eat almost anything, grow rapidly, and produce large numbers of young. And sure enough, after accidental releases from bait farms, they’re now found in a dozen states outside their native range. In their conquered territory, shiners are considered a serious threat to native species because they displace and outcompete them, prey on their young, introduce tapeworms and other parasites, and dilute gene pools through hybridization.

All this suggests that red shiners should be able to go wherever they please. But for some reason, they can’t go back home.

The shiner is native to a broad swath of the Mississippi River basin and, for almost half a century, was abundant in the creeks that fed into Lake Texoma in southern Oklahoma. Over the last few decades, though, they’ve all but disappeared there. Populations that used to number in the hundreds in the 1980s dropped to just single digits in the late '90s and early 2000s. 

This doesn’t seem to be for lack of effort on the minnows' part, though, and surveys from recent years showed a mysterious pattern of reappearance and disappearance. After severe flooding in the area in the summer of 2007, previously impassable stretches of dry land were re-watered and the minnows re-colonized their old stomping grounds. In June 2009, researchers found 81 shiners in one of the creeks. Two months later, there were only four. By November, there was just one. Then that was gone, too. Just as quickly as they settled in, the fish disappeared again.

The minnows were still abundant in other nearby streams, though, so the problem didn’t appear to be a total loss of the species in the area. It was just this handful of creeks that they couldn’t get a foothold in.

They say you can never go home again, and it looked like the shiners would agree. The little minnow that couldn’t presented scientists with a natural paradox, one that turned the standard invasive species narrative on its head. Why, biologists wondered, despite their abundance, tolerance of harsh conditions and invasiveness, couldn’t the fish re-invade the creeks they’d come from?

To find out what had locked the shiners out, University of Oklahoma zoologists Edie Marsh-Matthews, William Matthews, and Nathan Franssen decided to watch a shiner homecoming unfold. They built an artificial stream that mimicked the conditions and native fish populations of Brier Creek, where the shiners were losing the most ground after re-invasion. 

After the other fish—which included stonerollers, bigeye shiners, blackstripe topminnows and green sunfish—had time to establish themselves, the shiners were thrown into the mix in a mock invasion. At first, they seemed right at home. They were healthy, they ate well and the males chased and circled the females in the shiner equivalent of courting. By the end of the experiments, though, only 20 percent of the invaders survived. Even in a fake stream, they’d failed again.

The three scientists searched for reasons for the die-off, but couldn’t find anything. They’d stocked enough shiners at the start of the experiment. The water chemistry looked fine. The filters were clean. The algae cover was ideal. Shiners had successfully reproduced and raised young and thrived in similar experiments when housed alone, so maybe the problem was one of the other fish.

The researchers found that the more adult sunfish there were in the stream during the experiments, the fewer shiners they wound up with in the end. The sunfish didn’t seem like likely suspects at first glance. They’d been stocked at the start of the experiment as small juveniles, hardly anything to worry about, and no one directly saw them preying on the shiners. But sunfish grow quickly, and by the end of the study, they were significantly larger and could pose a serious threat to the invaders.

Death-by-sunfish fit the timeline for what was happening in the wild, too. A group of invading red shiners, swimming into the creeks during spring and summer floods, would encounter juvenile sunfish that primarily eat insects. Given a summer to grow, the larger sunfish would start mixing small fish into their diet right around the time when the survey noted the shiners disappearing.

The shiners’ failure to reinvade looks like just a matter of being in the wrong place at the wrong time. They’d come home again just before they would get added to their neighbors’ menu. Their behavior doesn’t help, either. In the experiments, the shiners tended to swim at midwater where sunfish hunt, and engaged in less defensive behavior in the face of danger. The small native fish like the bigeye shiners, on the other hand, mostly swam closer to the surface and stayed in parts of the stream that were too shallow for the larger predators.

As destructive as invasive species can be, the shiners show that even an invader can sometimes be an underdog.