The Bigmouth buffalo isn’t the prettiest fish in the water, nor is it particularly appealing to eat, but it has unsung qualities that scientists say are increasingly important in a climatically challenged, ecologically homogenizing world: It gets really big and really old.
The bottom-feeding fish, which swims through much of the Mississippi River basin, can grow up to 50 pounds and live for 100 years or more. When fishermen haul it out of rivers, though, they often toss it aside.
“People catch them and just throw them on the bank,” said Keller Kopf, an ecologist at Charles Darwin University in Australia. “These are 90-year-old fish.”
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Kopf’s recent research finds that these old Bigmouth buffalo—and many other animal species—are essential to thriving ecosystems, providing buffers from the impacts of climate change-induced weather extremes. Like old, large trees, these species are now being understood as uniquely valuable assets in their ecosystems. And like old, large trees, they are in decline.
“People assume old individuals are unimportant. We have that with humans,” Kopf said. “I think older individuals, across a wide range of animals, are really quite under-appreciated.”
Kopf and eight research colleagues from around the world recently authored a first-of-its-kind analysis in Science based on a review of nearly 10,000 research studies. Most research, up to now, has focused on the negative effects of aging in animals—increased cancer rates, decreased rates of reproduction and higher mortality. “But there’s all this other evidence being published showing there are lots of benefits of older individual animals across a wide range of species,” Kopf said. “And an over-emphasis on those negative aspects provides an incomplete picture, particularly when it comes to how we manage wildlife and fisheries, and we’ve overlooked those benefits.”
The researchers found that in many animal species age is an asset, especially in environments where climate change presents greater threats or has altered habitats. These species, which accumulate knowledge over their lifetimes, tend to take a long time to grow large, but fall into different categories. Some of them—like whales or elephants—expend a lot of energy into parenting and raising a small number of individuals. Some—like tuna—expend a lot of their energy into dispersing huge numbers of eggs and individuals, but essentially none on parenting.
The animals pass on information, both environmental and social—what the authors call “cultural transmission”—to their communities and offspring that’s critical for their survival. “These older individuals are the ones that have the greater knowledge,” Kopf said. “Some of these species know the best time of year to leave, to migrate a long distance, and where to go and how to navigate conditions.” When resources are scarce, these species know how to find water and shelter, or to locate “novel foods” that they normally don’t eat.
“We’re calling it wisdom,” Kopf said. “That’s a term that, generally, in the past, was only used for people—for human beings. But by many of the most basic definitions of how human beings define wisdom, there are animals that meet that criteria, and those tend to be older individuals, just like in humans.”
While reproductive capacity generally declines in most animals, some species actually have more and stronger offspring as they age. Some species participate in the reproductive success of those younger than they are after they themselves are no longer able to bear offspring by “enhancing grandparental or community support,” including finding habitat, shelter, food or protection from predators. Humans are the best-known species that support the “grandmother hypothesis”—the idea that grandmothers take care of everything, allowing the younger females to have babies. But the hypothesis also applies to other species, including some whales, particularly orcas.
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Another group of species benefits from age in a different way. It takes a long time for some species, known as ectotherms—cold-blooded animals including most reptiles, fish, amphibians and invertebrates— to get big and they never stop growing, unlike humans or other mammals. As they get bigger and older, they produce more propagules—eggs, spores or any other material through which a species propagates.
“It’s the largest individuals that have the highest fecundity,” said Kirk Winemiller, another author of the paper and an ecologist at Texas A&M who has advanced what’s known as life history theory. “The oldest individuals are actually the most valuable for the stock if we’re talking about fish because they produce massive amounts of propagules.”
Many of these species only propagate during certain parts of the year or when certain conditions occur—a strong river flow in the spring that allows certain species to spawn in flooded areas, for example. “They have the best chance of winning the sweepstakes when the appropriate conditions are encountered and surviving potentially less advantageous environments,” Winemiller said.
“When a patch of forest opens up that allows light to penetrate for seedling survival, the tree that can get the most seeds distributed into that space is the one that’s going to have the most successful descendants,” Winemiller added. “But you’ve got to live a long time to have lots of chances.”
In some cases, those chances are getting slimmer and slimmer. Researchers have found that the Bigmouth buffalo, for example, hasn’t had optimal breeding conditions and hasn’t added to its population in half a century. “That’s because they put locks and dams on the upper Mississippi and they’ve altered the conditions of the flow,” Winemiller said. “This species is not getting those optimal conditions that come along every so often, and if you don’t conserve those old individuals that have the reproductive potential, if the conditions do come along, that population is doomed.”
Winemiller says he worries about another old, big fish called the Alligator gar that has become the target of sport fishers in the South, who have dubbed it a “river monster.”
“I’m concerned that if people are harvesting these large, old, fecund females, we’re going to lose the reproductive potential to sustain these populations,” he said. “In the wildlife arena, we take out the oldest males, with the biggest tusks or antlers—the objective is the trophy. Those are the largest animals. Those are the oldest animals. Those are the wisest animals. And in the case of fisheries, they are the most fecund animals.”
Kopf and his coauthors say they hope their research provides enough evidence of the importance of big, old animals that these species start to receive special attention in global conservation efforts. They have a term for it: Longevity conservation.
The United Nations Convention on Biological Diversity says that countries should protect the structure and function of ecosystems, but does not consider longevity in its framework. Similarly the International Union for Conservation of Nature Red List, the global inventory of species at risk of extinction established in 1964, does not consider the loss of old individuals from populations as a threat.
Fisheries management policies also don’t factor in age, so a population of fish might be considered well managed or healthy, even though it’s made up of mostly young and middle-aged fish.
“We’re starting to learn that the removal of those old individuals is actually one of the major drivers making those populations really volatile,” Kopf said. “So we need to have targeted management efforts to protect those older age classes of marine species and terrestrial species across the board.”
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