Indicator species such as pikas or frogs can signal a biological change in an ecosystem and can be used to diagnose its overall health, helping conservation efforts. Yet, picking the right indicator is not easy.
Polar bears straddling melting ice may be the most visible global symbol of climate change. But another harbinger, say researchers, can be found skittering across the rocky slopes in the United States’ Columbia River Gorge.
The pika, resembling a small fluffy rabbit with large, mouse-like ears, dwells at cold high-altitude slopes that are not usually encroached on by humans. But, warming temperatures mean the cute critter with its thick furry coat, which is sensitive to temperature fluctuations, is quickly dwindling in population. The animals are also moving higher up mountain slopes in search of the cold climes they thrive in.
"Climate is by far the strongest factor that is apparently driving pika loss," Erik Beever, an ecologist at the US Geological Survey who has been researching the pika for over 20 years, says. "Even in places where habitat is not changing, even in places which are very remote, we’re still seeing changes."
This heat intolerance makes the pika the perfect specimen for studying the impacts of climate change on ecosystems - making them key "indicator species." The term refers to biological organisms whose presence, absence or abundance reflects a specific environmental condition - be it pollution, a disease outbreak, or climate change. So, the general health of the indicator species, or lack thereof, is a signal of the overall health of the ecosystem.
"Pikas are an early-warning indicator in the sense that they’re telling us these are the changes we can expect to see in other species," says Beever of the petite mammal, which is also found in colder climates of Eastern Europe and parts of Asia.
Picking the right indicator
Indicator species like the pika or the famous Kemp Ridley’s turtles in the Gulf of Mexico can give researchers a clue about the kinds of changes that are taking place in an ecosystem, and foreshadow future shifts. That makes them a useful research and monitoring tool.
But picking the right indicator isn’t always easy. It’s difficult for any one species to serve as an indicator as each responds to changes in their environment at different rates, and through different mechanisms, says Beever.
"From research of paleohistory, as well as what’s going on now, every time we look we see a species respond differently to climate fluctuations," he adds.
It is easy for scientists to choose the "wrong" species as an indicator, says Richard A. Snyder, a University of West Florida ecology professor, especially when they falsely believe that causation equates to correlation. For example, it was widely believed, and publicized, that the worldwide decline of frogs was an indicator of climate change.
But upon closer inspection, scientists found that their dip in population was attributed to a slew of factors that vary locally.
"You have to be careful," says Snyder. "It’s always better not just to pick one indicator but to have multiple lines of evidence for something."
Experts agree that indicator species must also be accompanied by a thorough study of what is being indicated, what is really correlated, and how this one species fits into the rest of ecosystem.
Indicating change through absence
Some species are ideal to study as indicator species due to their abundance. For most mammals, the rate of detection in their geographical area is between 16 and 40 percent. But those who visit a pika habitat are most likely to spot them scurrying over rocks: their detection rate is 90 percent.
"Because they’re so detectable, they’re a lot easier and cheaper to work with than some species are," says Beever.
But some organisms act as indicators through their absence. Just look at lichens - , a composite organism that emerges from algae or cyanobacteria. It’s often spotted on soil, bark and even roof tiles around the world. As lichens derive their nutrients from air, a shortage of them is sometimes a red flag for air pollution.
"Indicator species are a way of getting information about the environment and for any conservation effort, for any species, you have to have information to make decisions," says Snyder. "And for any species, the more information you have, the better decisions you make."
A critical part of the food chain
In addition to serving as an important sign of environmental change, indicator species often play critical roles in the food chain, from primary predators to top producers.
"A typical example would involve a predatory species occurring in comparatively low numbers, but through predation keeping herbivore numbers moderate, thereby also moderating their impact on the local plant community," writes Trevor McIntyre of the Mammal Research Institute at the University of Pretoria in South Africa in an email.
The snow leopard, one of the top predators of the high mountain food web of Central Asia, is a good example. It helps to keep the ecosystem in balance by preying on marmot populations who can degrade the mountain meadows and grass if their population explodes.
Not all indicator species have critical roles, says Beever, but those that do are also considered to have a disproportionately large effect on its environment relative to its abundance. For instance, pikas can act as 'ecosystem engineers' by changing both the composition and abundance of plant species as well as distribution of nutrients in their local surroundings.
Helping clean up pollution
And, indicator species can at times also spur environmental clean-up or conservation efforts. Take the colorful Coquina Clam, which scientists spotted scattered across the azure shores of Pensacola, Florida after the Deepwater Horizon oil spill in 2010. The clams, they discovered, retained the toxic polycyclic aromatic hydrocarbon (PAH) at higher concentrations and levels than the surrounding sand particles did.
As a result, the small seashore clams can help monitor pollution along shorelines, the researchers concluded in a paper published in Marine Pollution Bulletin this June. Adapted to harsh chemicals, the clams are sometimes even more abundant after oil spills, as they feed on bacteria growing in contaminated areas.
"If all we see is the pollution-tolerant species, then we know the situation is pretty bad," says Snyder, who is also one of the authors of the study. "Then we know we’re looking at an impact and not just some stress that’s natural to the environment."