Sinking sharks and freshwater pandas
When people think about Earth’s larger animals they usually think of the tiger burning bright in Asia’s forests, or great herds of wildebeest sweeping across the African savannah. The amazing cinematography of shows like the BBC’s Blue Planet has even brought the larger species of the world’s oceans into our homes like never before.
However, freshwater environments also hold an astounding array of large vertebrate species, 207 in total, including sharks, dolphins, manatees, turtles, rays, hippos and more.
In a new paper published in Diversity and Distributions, He et al., present their research into the status of large freshwater fauna. They identify these charismatic species as potential flagships whose effective conservation could help to conserve thousands of other freshwater species, but their plight is indicative of the state of the world’s freshwater ecosystems.
Global Distribution of Freshwater Megafauna. Global Freshwater Biodiversity Atlas (atlas.freshwaterbiodiversity.eu)
Within this category of ‘freshwater megafauna’, one amazing group epitomises the challenges faced by freshwater species; river sharks. Of the more than 400 known species of shark, almost all reside near permanently in salt water, with the Bull Shark (Carcharhinus leucas) most famed for its forays into estuaries and up rivers. However, the Bull Shark is not a ‘true’ freshwater shark. Instead that title belongs to five little known species of the genus Glyphis.
Most shark species are adapted to saline environments and would overhydrate in freshwater. If a saltwater shark were placed in freshwater, water would flow into their cells through osmosis causing intoxication as key electrolytes are flushed out of their bodies. Another problem for most sharks in freshwater is that they also sink! Sharks rely on their fatty livers for buoyancy, but because of the lower density of freshwater, saltwater sharks’ livers would need to be eight times bigger to keep them afloat. River sharks solve this problem by having denser and fattier livers with more oil which makes them more buoyant. As for adapting to freshwater, scientists are still not sure exactly which adaptations they use, whether like freshwater rays they have smaller rectal glands for secreting salt (meaning their body chemistry more closely matches that of their surrounding environment, preventing osmosis), or whether they have an altogether different adaptation which allows them to tolerate freshwater.
But what makes river sharks emblematic of the challenges freshwater species are facing?
Unlike their ocean-dwelling cousins, river sharks, and many other freshwater species occupy murky water which obscures them from plain view. This means that many freshwater species are not seen, even by people who live on the banks of their river habitats.
This makes it challenging to raise awareness about the need to conserve them – it’s hard to protect something (or to want to protect something) you can’t see.
It almost takes a leap of blind faith and the rewards are, patently, less visible. Many people do not even know that river sharks exist, so their extinction would, to them, be the status quo.
Speartooth shark (Glyphis glyphis) in Melbourne aquarium. Bill Harrison. CC BY 2.0
Living in these environments also makes it particularly challenging to conduct scientific research on these species. For example, we still understand very little about the diet, ecology and taxonomy of river shark species. Whilst data on other freshwater species is improving, aided by the work of organisations such as the IUCN FBU, these species still receive less academic attention than terrestrial species (Di Marco et al., 2017).
For example, there are more than 250 species of freshwater turtle and just 7 species of marine turtle, yet from 2013 to 2015 there were 40 times more papers on marine turtle conservation than on freshwater turtle conservation (Tershy pers. comm., 2017). Meanwhile just 0.03% of articles in leading journals Nature and Science focus on freshwater species (Synchronicity Earth, 2012).
As well as the low levels of public and academic attention, the other notable characteristic of river sharks is their critical conservation status. In fact, there isn’t a single river shark species which can be described as anything other than Endangered or Critically Endangered, and in some cases having enough data to even make an assessment is problematic (this is generally a bad sign!).
For freshwater megafauna more broadly, He et al., find that 71% of large freshwater species are in decline. They also show that the threats to freshwater species are increasing: since the early 1990s human pressure has increased throughout 63% of the global ranges of freshwater megafauna. Like many freshwater species, this decline is not driven by just one threat, but by the intersection of multiple activities including damming and habitat fragmentation, overexploitation, pollution and land use change in surrounding watersheds (IUCN Red List 2018).
But He et al., don’t draw attention to freshwater megafauna only to despair about the status of freshwater species. They do so because these species can help to safeguard freshwater habitats and species by acting as flagships for these ecosystems’ conservation. But how?
- Freshwater pandas: It’s a lot easier to get people to enthuse about the conservation of the giant panda (or a hippo) than the Liangbei toothed toad (or a minnow). In this respect freshwater megafauna have a great and as yet untapped potential “to communicate to the public, to policymakers, and to donors the immense value of freshwater ecosystems, including a unique biodiversity” (Carrizo et al., 2017 p.7). Much of the focus in conserving freshwater ecosystems has been around the value of natural infrastructure, the economic value healthy freshwater ecosystems can give to humans. But freshwater megafauna can remind us of something else too – the inherent value of conserving freshwater biodiversity.
- Sounding the alarm: Many freshwater species are also good indicators of the health of freshwater ecosystems. For example, as a result of their long gestation periods, delayed maturity, slow growth, and status as an apex predator ranging over large areas, river sharks are some of the most sensitive species to human pressures. These species can therefore act as sentinels warning us of our impact on the world’s freshwater systems. Similarly, where freshwater megafauna are thriving we can look at which management regimes are in place, and understand how such approaches might be replicated to the benefit of other species and habitats.
- A basis for site protection: If the public and policymakers are aware of these species’ existence and anxious about their endangerment, then large freshwater species can also prompt the protection of key freshwater habitats. Currently 84% of global freshwater megafauna populations are found outside of protected areas (Carrizo et al., 2017). Researchers have already found that this ‘umbrella’ approach could be particularly effective for freshwater environments as 93% of the distribution ranges of freshwater species co-occur with freshwater megafauna, and 60% of threatened species are found within the collective megafauna range (Carrizo et al., 2017).
Whilst there isn’t a Blue Planet II on the cards for freshwater megafauna, there are opportunities to increase awareness of the amazing diversity of species which live in freshwater environments. This provides a chance to inspire policymakers, donors, the public and the private sector to reconsider how freshwater environments are understood, as valuable and biodiverse environments worth protecting.
This umbrella approach could benefit thousands of other freshwater species as well as benefitting millions of people who also share these environments and rely on natural and healthy freshwater ecosystems for their livelihoods. But, as the case of the unknown river sharks underlines, it would also support the conservation of some of the most amazing and enigmatic species on the planet.
Carrizo S.F, Jähnig S.C, Bremerich V, Freyhof J, Harrison I, He F, Langhans S.D, Tockner K, Zarfl C, Darwall W, 2017m Freshwater Megafauna: Flagships for Freshwater Biodiversity under Threat, BioScience, Vol.67. Issue 10, pp.919-927
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He F, Bremerich V, Zarfl C, Geldmann J, Langhans S.D, David J.N.W, Darwall W, Tockner K, Jähnig S.C, 2018, Freshwater megafauna diversity: Patterns, status and threats, Diversity and Distributions, Diversity and Distributions, pp.1-10
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