Size Matters for Survival: Largest and Smallest Species at Greatest Risk of Extinction

animal extinction

Story at-a-glance -

  • Body mass is associated with extinction risk, with animals at both ends of the size spectrum at greatest risk
  • The break-even point was 0.035 kilograms; the further in either direction from this body mass, the greater the risk of extinction became
  • The greatest threat to large species is hunting, poaching and unintentional bycatch, while small species are threatened most by habitat loss and degradation

By Dr. Becker

Size matters when it comes to survival among animals, according to researchers from the U.S., Australia and Switzerland, with those in the “Goldilocks zone” — not too big, not too small but rather just right — faring the best.

Analyzing more than 27,600 species (those assessed for the International Union for the Conservation of Nature’s, or IUCN, Red List), “the probability of being threatened was positively and significantly related to body mass for birds, cartilaginous fishes [like sharks and rays], and mammals,” the researchers said.1

Animals at both ends of the size spectrum were most at risk of extinction, with the break-even point being 0.035 kilograms. The further in either direction from this body mass, the greater the risk of extinction became, though for different reasons.

In a news release, Oregon State University (OSU) said that direct killing by humans — via regulated and unregulated fishing, hunting meat for consumption, using body parts for medicine or due to unintentional bycatch — was the greatest threat to large animals. Lead author William Ripple, Ph.D., a distinguished professor of ecology at OSU, said:2

“Many of the larger species are being killed and consumed by humans, and about 90 percent of all threatened species larger than 2.2 pounds (1 kilogram) in size are being threatened by harvesting.”

Among small animals, those less than 1.2 ounces (35 grams), such as the sapphire-bellied hummingbird, gray gecko, hog-nosed bat, were most at risk, in this case typically due to loss or modification of habitat — especially that related to pollution, agriculture and logging.

Large Herbivores Are at Risk of Extinction

In line with the featured study findings, in 2015 an IUCN study revealed that 44 of the 74 largest terrestrial herbivores (that’s about 60 percent) were threatened with extinction (including 12 that were critically endangered or extinct in the wild). Further, about 58 percent of such animals have decreasing populations.3

It’s estimated that more than 70 percent of large herbivore species are hunted for meat, while close to 30 percent are hunted for body parts. Many aren’t aware that 1 billion people worldwide depend on wild meat for food, and the researchers suggested that availability of this food source will decline by up to 80 percent by 2050.4 Thomas Newsome, Ph.D., of Deakin University and the University of Sydney in Australia and a co-author of the featured study, explained:5

“For the large animals, there is an urgent need to reduce the consumption by humans of harvest sensitive species to lessen the negative impacts of hunting, fishing and trapping … But it’s ultimately slowing the human population growth rate that will be the crucial long-term factor in limiting extinction risks to many species.”

Giant Fish, Amphibians, Birds and Reptiles Are Also at Risk

The featured study highlighted a need to protect not only iconic animals like elephants and rhinos but also large fish, amphibians and reptiles, including whale shark, Atlantic sturgeon, Somali ostrich, Chinese giant salamander and the Komodo dragon.6 The Somali ostrich, for instance, is a huge flightless bird native to northeast Africa. It’s classified as a vulnerable species and its population is declining due to hunting for feathers and food, egg collection and habitat loss and degradation.7

The Chinese giant salamander is critically endangered (the last step before extinction), as its population has declined 80 percent over the last three generations. Measuring nearly 4 feet long and weighing up to 66 pounds as adults, this creature is the largest living species of amphibian8 and it lives in streams and mountain lakes. According to IUCN:9

“Commercial over-exploitation for human consumption is the main threat to this species. It has also suffered from habitat destruction (e.g., from the construction of dams) and habitat degradation (e.g., water pollution from mines). Although there is commercial farming of this species, the vast majority of Chinese Giant Salamanders being traded are believed to originate from the wild.”

Atlantic sturgeon, which can grow to be 14 feet long, are also critically endangered. This is especially tragic as this species is one of the longest surviving animals, predating pterodactyls, flowering plants and tyrannosaurs.10 They depend on both salt water to live in and fresh water to reproduce in and can live to be 60 years old. “There has been more than a 90 percent population decline in the past 75 years,” IUCN noted, “based mainly on loss of habitat, along with pollution and exploitation [including ending up as bycatch].”11

The Tiniest Animals at Greatest Risk

Larger animals tend to get more attention (and protections) than tiny creatures, but the latter are at similarly grave risk of extinction, especially those that live in freshwater habitats. Although they’re generally too small to be exploited by humans via hunting, habitat loss, including for use as human housing areas or coffee plantations, is a major problem.

Clarke’s banana frogs, for instance, measure just 20 to 23 millimeters (mm) and are native to Ethiopia. They live in just two known locations, both of which are threatened, and the species is endangered, according to IUCN. The frogs live in tropical forests and forest edges and require freshwater, marshy pools for breeding. “The major threat is ongoing habitat loss due to selective logging, human settlement, and small-scale and large-scale agricultural encroachment (including coffee plantations),” IUCN reported.12

The waterfall climbing cave fish, native to Thailand, is also a vulnerable species, measuring just 2.8 centimeters (cm) long. The fish are blind with no visible eyes and are capable of climbing up slippery rocks because of their unique pelvis, which is similar to that found in land-dwelling animals.13 It’s estimated that less than 2,000 individuals remain, and the species is very sensitive to pollution, changes in water quality and other disturbances, including that from tourism activity.14

Also mentioned in the study is the sapphire-bellied hummingbird, which is critically endangered. Measuring just 9 cm, this tiny bird is native to Colombia, but its population has been on the decline since the mid-1970s due to habitat loss. It’s thought there are only 50 to 249 mature individuals left in the wild, with numbers “declining rapidly owing to pollution and mangrove cutting/die-back.”15

With such a wide variance in species at risk, it’s clear that different conservation strategies will be necessary depending on the unique threats to each species — and the strategies need to be employed sooner rather than later. If trends continue, ecosystems as we know them may be forever changed as both the largest and smallest species among us continue to die off. The researchers noted:16

“[H]uman activity seems poised to chop off both the head and tail of the size distribution of life … This compression of the size distribution of vertebrate life not only represents a radical shift in the living architecture of our planet, but is likely to precipitate consequential shifts in ecological functioning."

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