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To Save Slow-Breeding Giants, Biologists Recommend New Method for Defining Endangered Species

Animals
To Save Slow-Breeding Giants, Biologists Recommend New Method for Defining Endangered Species
Asian elephants frolic in Kaudulla Wewa at Kaudulla National Park in central Sri Lanka. David Stanley / CC BY 2.0

When it comes to saving some of the planet's largest animals, a group of researchers says that old methods of conservation just won't cut it anymore.


Typically, conservationists use a methodology called population viability analysis (PVA), a species-specific risk assessment that considers when a particular population will go extinct given certain perimeters. A new study published in the Journal of Frontiers in Ecology and Evolution suggests that basing conservation decisions simply on low population counts in slow-breeding animals could cause irrevocable decline.

"Critical thresholds in so-called vital rates — such as mortality and fertility rates among males and females of various ages — can signal an approaching population collapse long before numbers drop below a point of no return," said lead author Dr. Shermin de Silva, adding that conservation efforts for megafauna should be aimed at maintaining their "demographic safe space." That is, a combination of key vital rates, such as offspring survival rate and habitat loss, that support population increases rather than short-term trends in population size and distribution

To come to this conclusion, de Silva and her team analyzed endangered Asian elephants as a case study, whose populations are thought to have been halved in less than a century. Today there are fewer than 50,000 wild Asian elephants, in part because they breed extremely slowly — oftentimes just one calf every six years or more. Mathematical modeling of near-optimal reproduction and high survival rates for calves determined that Asian elephant populations cannot tolerate losing more than 7.5 percent of females annually.

It leads to something de Silva calls a "demographic tipping point," the combination of vital rates that helps us understand how populations are impacted by a variety of environmental and human influences.

"Even large populations, if in decline, will eventually pass a point of no return that is difficult to predict in advance. Rather than wait for this eventuality, we suggest that demographic tipping points, and the corresponding safe spaces they define, can offer impetus to act early even when data are scarce," write the authors. Basing conservation approaches off of current trends in population and range is misleading for these types of animals because "short-term fluctuations may mask long-term trends" and even decisions made with the best of intentions may be ill-informed.

Their large body size makes the Asian elephant (Elephas maximus) an influential and vulnerable member of the ecosystem whose threats include overharvest, habitat loss and their time-consuming lifecycle. These megafauna need "proactive management" well before their numbers become critically low.

"But while the attention of the world has been focused on the ivory trade, for critically endangered Asian elephant populations the greatest threat is habitat loss — followed by illegal trade in live animals and parts," said de Silva, adding that habitat loss creates an "extinction debt" that slows down birth rates and increases mortalities. For these long-living species that are slow-to-breed, even small changes can make a profound difference.

This application of conservation could also be applied to other keystone species like giraffes, rhinos and eastern gorillas.

Correction: An earlier version of this story inaccurately referred to de Silva as a "him" instead of "her." The story has been corrected

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A net-casting ogre-faced spider. CBG Photography Group, Centre for Biodiversity Genomics / CC BY-SA 3.0

Just in time for Halloween, scientists at Cornell University have published some frightening research, especially if you're an insect!

The ghoulishly named ogre-faced spider can "hear" with its legs and use that ability to catch insects flying behind it, the study published in Current Biology Thursday concluded.

"Spiders are sensitive to airborne sound," Cornell professor emeritus Dr. Charles Walcott, who was not involved with the study, told the Cornell Chronicle. "That's the big message really."

The net-casting, ogre-faced spider (Deinopis spinosa) has a unique hunting strategy, as study coauthor Cornell University postdoctoral researcher Jay Stafstrom explained in a video.

They hunt only at night using a special kind of web: an A-shaped frame made from non-sticky silk that supports a fuzzy rectangle that they hold with their front forelegs and use to trap prey.

They do this in two ways. In a maneuver called a "forward strike," they pounce down on prey moving beneath them on the ground. This is enabled by their large eyes — the biggest of any spider. These eyes give them 2,000 times the night vision that we have, Science explained.

But the spiders can also perform a move called the "backward strike," Stafstrom explained, in which they reach their legs behind them and catch insects flying through the air.

"So here comes a flying bug and somehow the spider gets information on the sound direction and its distance. The spiders time the 200-millisecond leap if the fly is within its capture zone – much like an over-the-shoulder catch. The spider gets its prey. They're accurate," coauthor Ronald Hoy, the D & D Joslovitz Merksamer Professor in the Department of Neurobiology and Behavior in the College of Arts and Sciences, told the Cornell Chronicle.

What the researchers wanted to understand was how the spiders could tell what was moving behind them when they have no ears.

It isn't a question of peripheral vision. In a 2016 study, the same team blindfolded the spiders and sent them out to hunt, Science explained. This prevented the spiders from making their forward strikes, but they were still able to catch prey using the backwards strike. The researchers thought the spiders were "hearing" their prey with the sensors on the tips of their legs. All spiders have these sensors, but scientists had previously thought they were only able to detect vibrations through surfaces, not sounds in the air.

To test how well the ogre-faced spiders could actually hear, the researchers conducted a two-part experiment.

First, they inserted electrodes into removed spider legs and into the brains of intact spiders. They put the spiders and the legs into a vibration-proof booth and played sounds from two meters (approximately 6.5 feet) away. The spiders and the legs responded to sounds from 100 hertz to 10,000 hertz.

Next, they played the five sounds that had triggered the biggest response to 25 spiders in the wild and 51 spiders in the lab. More than half the spiders did the "backward strike" move when they heard sounds that have a lower frequency similar to insect wing beats. When the higher frequency sounds were played, the spiders did not move. This suggests the higher frequencies may mimic the sounds of predators like birds.

University of Cincinnati spider behavioral ecologist George Uetz told Science that the results were a "surprise" that indicated science has much to learn about spiders as a whole. Because all spiders have these receptors on their legs, it is possible that all spiders can hear. This theory was first put forward by Walcott 60 years ago, but was dismissed at the time, according to the Cornell Chronicle. But studies of other spiders have turned up further evidence since. A 2016 study found that a kind of jumping spider can pick up sonic vibrations in the air.

"We don't know diddly about spiders," Uetz told Science. "They are much more complex than people ever thought they were."

Learning more provides scientists with an opportunity to study their sensory abilities in order to improve technology like bio-sensors, directional microphones and visual processing algorithms, Stafstrom told CNN.

Hoy agreed.

"The point is any understudied, underappreciated group has fascinating lives, even a yucky spider, and we can learn something from it," he told CNN.

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