Article: He, F. et al. The global decline of freshwater megafauna. Global Change Biology (2019). 25:3883-3892. DOI: 10.1111/gcb.14753
Featured Image: There are 5 distinct populations of Atlantic sturgeon in the U.S., and due to overfishing and pollution, each population is listed as endangered under the Endangered Species Act. These fish have been around for a while- they are considered living fossils
Photo source: Sarah Shainker
Popular kids in the conservation world
A quick Google Image search for “endangered species” brings up a leopard, panda and polar bears, a bald eagle, and a blue whale.
Easily recognizable animals like these are called “megafauna”, and they tend to act as mascots for endangered species and the movement to conserve biodiversity. This can be a positive thing because it is easier to engage the general public with an animal that is large, cute, or majestic rather than, say, an endangered species of algae. On the other hand, the focus on charismatic megafauna can sometimes take attention away from smaller, shyer, and generally less charismatic endangered species. The Ugly Animal Preservation Society advocates for these outcasts of the animal kingdom!
Aside from their popularity, megafauna do have a few characteristics that can make them especially vulnerable to extinction in some cases. They often need complex habitats, which are more likely to be threatened by fragmentation or degradation. They usually take longer than smaller organisms to reach sexual maturity, and produce fewer offspring. This means that populations will take longer to recover from a stressful event, and will take longer to adapt to changing environmental conditions.
The case for freshwater
Even within the charismatic endangered megafauna category, not all have reached “mascot” status. This means that we’re leaving out a significant category of megafauna: those that call the freshwater environment their home. My google image search animals were photographed against the backgrounds of forests, oceans, and savannahs…without a lake, river, or stream in sight. This is due to a lack of awareness and attention, not a true lack of freshwater megafauna. Freshwater environments are home to a range of diverse gentle and not-so-gentle giants: from beavers to giant turtles, catfish and sturgeon, even stingrays!
Freshwater habitats, including rivers, lakes, streams, ponds, and wetlands, cover only 1% of the earth’s surface but harbor a disproportionately high amount of diversity, including a third of the world’s vertebrates and half of its fish species. Freshwater ecosystems also provide services that are central to human society: drinking water, hydropower, shipping routes, irrigation…the list goes on. This means that freshwater ecosystems and the biodiversity they harbor are under an especially high amount of stress from human activities.
A study of studies
A team of scientists from several European countries and the USA came together with the goal of quantifying global changes in freshwater megafauna populations over time, by looking at both population abundance (the number of groups, within a species, that are geographically separate) and distributions (where the populations are located now compared to where they were located in the past). The team also aimed to compare these changes in Europe vs. the United States. The group hypothesized that freshwater megafauna would have experienced greater declines than freshwater fauna overall due to factors including their complex habitat requirements, low numbers of offspring, and slow growth rate. They also predicted that populations of large fish would be even more vulnerable than other megafauna groups because of a history of overfishing. Finally, it was predicted that species ranges would decrease at a greater rate in Europe compared to the US because of its denser human populations, longer history of exploiting freshwater habitats, and complex political boundaries that render the enforcement of conservation policies difficult.
These hypotheses address some large-scale issues that require answering some big questions! It wouldn’t be feasible for any one scientist or team of scientists to go out in the field and measure all current populations of megafauna, not to mention somehow time-travelling to measure all past populations for a comparison.
So, the authors conducted a meta-analysis to answer their questions. That is, they chose a group of studies on various species of freshwater megafauna conducted between 1970-2012, then compiled and analyzed the data together. They chose a group of studies that covered a range of taxonomic groups (fish, reptiles, mammals) and a range of geographic regions (Europe, North America, Southeast Asia, South America, and Africa).
Freshwater megafauna under threat and under-studied
The meta-analysis showed that freshwater megafauna populations have decreased by an astounding 88% since 1970. Compared with the previously established decline of 81% for all freshwater vertebrates, these data supported the team’s hypothesis that megafauna would be shown to experience greater declines than vertebrates as a whole. The analysis also showed that megafauna have undergone intense range contractions, meaning that the range (the total area inhabited by a given species) is much smaller today than it was in the past.
The researchers also uncovered gaps in the data they analyzed due to biases in which animals people choose to study, and where they choose to study them.
The highest amount of data was found for sturgeons and salmon, while much less was found on reptiles and on fish that were not the economically important varieties of sturgeon or salmon. Additionally, even though Africa, Asia, and South America harbor over three quarters of the world’s freshwater biodiversity, just over a third of data taken successively over time comes from these areas.
The study confirmed that freshwater megafauna populations are indeed suffering due to overexploitation, dams (which break up habitats), collisions with boats, and pollution. Megafauna species are often keystone species, meaning that they maintain food web complexity, and that their presence often reflects the presence of overall high freshwater biodiversity.
The study authors pointed out the need to continue long-term biodiversity monitoring, and to fill in data gaps, so that we know where animals live and where we are losing them.
Do we need some new mascots?
The authors point out that in some cases, using megafauna species as “mascots” can help increase awareness and advocacy for biodiversity conservation as a whole. Freshwater conservation, in particular, faces a lack of focus and resources. Perhaps publicizing some alligators, sturgeons, beavers, or river dolphins to increase public awareness could help increase society’s willpower to conserve animals and plants, charismatic and non-charismatic alike, in freshwater systems.