We all know that human activity can influence the lives of nearby animals, especially those top predators that now have to play second fiddle to our ever-expanding interests. However, a new study has shown that not only do our actions impact them, but also our mere presence may cause majestic killers like pumas to grow so fearful that they change their hunting habits for the worse.
In the first-ever real-time tracking of leopard populations in India, researchers have determined that the big cats are surprisingly fearless when it comes to wandering near human neighborhoods. This was determined in a new GPS study, which has uncovered how these animals try to thrive in a man’s world.
That’s at least according to a new and fascinating study recently published in the Journal Proceedings of the Royal Society B, which details how, among pumas living in California, those living closest to humans were found to kill a lot more prey, but eat less of each kill, compared to pumas in more wild and secluded areas.
This was determined after a team of scientists from the University of California captured and tagged 30 wild pumas with GPS collars so they could track their movements. The territory and hunting grounds of these animals were then identified, breaking the pumas up into those that are living either near more rural or suburban human environments. The team also investigated kills, measuring just how much of each kill was eaten before a puma elected to slip away.
What they found was startling. In areas near a higher density of human housing, female pumas in particular were found to kill about 36 percent more large prey – mainly deer – than the more “rural” pumas.
Strangely, it wasn’t that the suburban pumas were hungrier. Instead, it appears that they are eating less of each kill – revisiting kill sites less frequently and spending less time taking their meals, compared to your average puma.
Many explanations have been suggested, including social cohesion, thermoregulation, predation evasion and avoidance of biting flies. Identifying the associations between phenotypic and environmental factors is essential for testing these hypotheses and substantiating existing experimental evidence.
In contrast to recent findings, we found no evidence that striping may have evolved to escape predators or avoid biting flies. Instead, we found that temperature successfully predicts a substantial amount of the stripe pattern variation observed in plains zebra.
Figure 2. Predicted levels of hind leg stripe thickness (left) and torso stripe definition (right), from a random forest model based on 16 populations. Hind legstripethickness is best predicted by BIO3 and BIO11. Torsostripe definition is best predicted by BIO3, BIO11 and BIO13.
Mark D. Bertness, an ecologist at Brown University, began studying the salt marshes of New England in 1981. Twenty-six years later, in 2007, he started to watch them die. In one marsh after another, lush stretches of cordgrass disappeared, replaced by bare ground. The die-offs were wiping out salt marshes in just a few years.
“It’s unbelievable how quickly it’s moved in,” Dr. Bertness said.
Scientists have been witnessing a similar transformation in a number of plant species along coastlines in the United States and in other countries. And in many cases, it’s been hard to pinpoint the cause of the die-off, with fungal outbreaks, pollution, choking sediments stirred up by boats, and rising sea levels proposed as killers.
There is much at stake in the hunt for the culprit, because salt marshes are hugely important. They shield coasts from flooding, pull pollutants from water and are nurseries for many fish species.
In the journal Ecology Letters, Dr. Bertness and his colleagues have nowpublished an experiment that may help solve the mystery. The evidence, they say, points to recreational fishing and crabbing. A fisherman idly dangling a line off a dock may not appear to be an agent of ecological collapse. But fishing removes the top predators from salt marshes, and the effects may be devastating.
Once New England salt marshes started dying off, Dr. Bertness and his colleagues embarked on a broad survey. Quickly they noticed a difference between healthy marshes and sick ones. The dying marshes tended to be near docks, marinas or buoys where boats could anchor, or where there were other signs of fishing.
“It wasn’t a brilliant thing we thought of sitting around the lab,” Dr. Bertness said. “By the time we got to 10 marshes, we realized there was this huge disparity.”
Dr. Bertness and his colleagues wondered how fishing and crabbing were affecting the food webs of the salt marshes. If people pull out striped bass and blue crabs and other predators from a salt marsh, the animals’ prey species — including those that feed on plants, like marsh crabs — are left to thrive. A growing population of marsh crabs might wipe out the cordgrass in a marsh. Without the roots of the cordgrass to anchor the soil, the marsh would erode, making it harder for new plants to grow.
To test this idea, Dr. Bertness and his colleagues surveyed salt marshes in Narragansett Bay in Rhode Island, comparing marshes that were healthy with ones that were almost entirely dead. The scientists found that in dying marshes, the plants had more signs of being fed on by crabs. And when they looked for other proposed causes of marsh die-off, such as pollution, they didn’t find a correlation. They published their results in March in the journal PLOS One.
Next, the scientists took a step beyond simply observing the die-offs: They tried to cause them. If the predator hypothesis was right, then creating a predator-free salt marsh habitat should lead to the disappearance of cordgrass.
In May 2013, the scientists installed cages in a healthy salt marsh on Cape Cod. Each cage was three feet on a side, with mesh walls and an open bottom. Marsh crabs could feed on the cordgrass inside the cages by burrowing up through the mud, and the wire mesh walls protected them from predators like fish and blue crabs.
The experiment quickly yielded results. In a matter of weeks, the cages were crowded with marsh crabs, and much of the cordgrass inside the cages was dying off. “We were planning on it being a two- or three-year experiment,” Dr. Bertness said. “But by the beginning of July, I thought, ‘My God, this is really going fast.’”
William J. Ripple, an ecologist at Oregon State University who was not involved in the research, said, “This is an important new scientific discovery for salt-marsh systems, and more generally for ecology.” Scientists like Dr. Ripple have argued that predators are important to the ecological health of other ecosystems. But it’s been difficult to test the hypothesis directly the way Dr. Bertness has.
Merryl Alber of the University of Georgia agreed that the experiment showed that removing predators could decrease salt marsh grass. But she was reluctant to draw big lessons from the study. “It is still a leap to connect dieback to recreational overfishing,” she said.
Wade Elmer, a plant pathologist at the Connecticut Agricultural Experiment Station in New Haven, thinks that the full story of the salt marshes’ decline is more complex than just fishing. Dr. Elmer has identified a new species of fungus that attacks cordgrass in New England salt marshes. He has suggested that the fungus may weaken the plants in a way that prevents them from making chemical defenses to ward off the marsh crabs.
“I think we all have our pet theories that explain what we see in our backyard,” said Dr. Elmer, “but these theories often fail as soon as we look elsewhere.”
Dr. Bertness doesn’t rule out the possibility that other factors are at play in the die-off of marshes. But he argues that fishing is having an enormous impact.
“The implications of these findings for the conservation of salt marshes are huge,” he said. “We need to maintain healthy predator populations.”
In the Middle Ages, fleas carried by rats were responsible for spreading the Black Plague. Today in East Africa, they remain important vectors of plague and many other diseases, including Bartonellosis, a potentially dangerous human pathogen. The researchers concluded that the “spike in disease risk results from explosions in the number of rodents that benefit from the removal of the larger animals.”
In the Middle Ages, fleas carried by rats were responsible for spreading the Black Plague. Today in East Africa, they remain important vectors of plague and many other diseases, including Bartonellosis, a potentially dangerous human pathogen.
Research by Hillary Young, assistant professor in UC Santa Barbara’s Department of Ecology, Evolution and Marine Biology, directly links large wildlife decline to an increased risk of human disease via changes in rodent populations. The findings appear today in theProceedings of the National Academy of SciencesEarly Online Edition.
With an East African savanna ecosystem as their research site, Young and her colleagues examined the relationship between the loss of large wildlife — defaunation — and the risk of human disease. In this case, they analyzed Bartonellosis, a group of bacterial pathogens which can cause endocarditis, spleen and liver damage and memory loss.
“We were able to demonstrate that declines in large wildlife can cause an increase in the risk for diseases that are spread between animals and humans,” said Young. “This spike in disease risk results from explosions in the number of rodents that benefit from the removal of the larger animals.”
The researchers discovered this effect by using powerful electric fences to experimentally exclude large species like elephants, giraffe and zebra from study plots in Kenya. Inside these plots, rodents doubled in number. More rodents meant more fleas, and genetic screens of these fleas revealed that they carried significantly numbers of disease-causing pathogens.
The study was concentrated in an area where rodent-borne disease is common and sometimes fatal. According to Young, these rodent outbreaks and associated increases in disease risk may be exacerbating health problems in parts of Africa where diminishing wildlife populations are rife.
“This same effect, however, can occur almost anywhere there are large wildlife declines,” Young said. “This phenomena that we call rodentation — the proliferation of rodents triggered by large wildlife loss — has been observed in sites around the world.”
Downturns in wildlife numbers can cause rodent increases in a variety of ways, including by providing more access to food and better shelter. “The result is that we expect that the loss of large animals may lead to a general increase in human risk of rodent borne disease in a wide range of landscapes,” Young said.
“In this study, we show the causal relationship between disturbance and disease is alarmingly straightforward,” she added. “We knock out the large members of ecosystems, and the small species, which generally interact more closely with humans, dramatically increase in number, ultimately brewing up more disease among their ranks.
The study provides ecosystem managers with yet another reason to protect large and at-risk wildlife species. “Elephants are an irreplaceable part of our global biodiversity portfolio,” Young said, “but they also appear to be circuitously protecting us from disease.”
H. S. Young, R. Dirzo, K. M. Helgen, D. J. McCauley, S. A. Billeter, M. Y. Kosoy, L. M. Osikowicz, D. J. Salkeld, T. P. Young, K. Dittmar. Declines in large wildlife increase landscape-level prevalence of rodent-borne disease in Africa. Proceedings of the National Academy of Sciences, 2014; DOI:10.1073/pnas.1404958111
The wolves’ return to Yellowstone and the subsequent recovery of plants that elk had been eating to death in their absence has become one the most popularized and beloved ecological tales. By the 1920s humans had misguidedly wiped out most of the wolves in North America, thinking that the only good wolf was a dead one. Without wolves preying on them, elk and deer (also calledungulates) exploded in number. Burgeoning ungulate populations ravaged plant communities, including aspen forests. Decades later, the wolves we reintroduced in Yellowstone hit the ground running, rapidly sending their ecological effects rippling throughout the region, restoring this ecosystem from top to bottom. Yet today some scientists caution that this story is more myth than fact because nature isn’t so simple.
For decades scientists have been investigating the ecological role of wolves. In his 1940s game surveys, Aldo Leopold found ungulates wiping out vegetation wherever wolves had been removed. He concluded that by controlling ungulates, wolves could restore plant communities and create healthier habitat for other species, such as birds.
Since Leopold’s time, many scientists have studied food web relationships between top predators and their prey—called trophic cascades. In the 1960s and 1970s Robert Paine, working with sea stars, and James Estes, working with sea otters, showed that ecosystems without top predators begin to unravel. John Terborgh called the ensuing rampant species extinctions an “ecological meltdown.” Paine created the metaphorical termkeystone species to refer to top predators and noted that when you remove the keystone, arches and ecosystems collapse. Over the years ecologists found trophic cascades—also called top-down effects—ubiquitous from coral reefs to prairies to polar regions. However, William Murdoch and others have maintained that sunlight and moisture, which make plants grow, drive ecosystem processes from the bottom-up, making predators relatively unimportant. The Yellowstone wolf reintroduction provided the perfect setting to test these contrasting perspectives.
In the mid-1800s in his book The Origin of Species, Charles Darwin presciently described nature as a “tangled bank.” Nature’s complexity results from myriad species and their relationships with other species and all the things that can possibly affect them individually and collectively, such as disease, disturbance, and competition for food. Science works incrementally, taking us ever deeper into nature’s tangled bank as we investigate ecological questions. Each study answers some questions and begets new ones. Sometimes we find contradictory results. Learning how nature works requires what Leopold called “deep-digging research” in which we keep searching for answers amid the clues nature gives us, such as the bitten-off stem of an aspen next to a stream where there are no wolves.
Trophic cascades science that focuses on wolf effects is still in its infancy, with huge knowledge gaps. For example, we’ve linked wolves to strong effects that cascade down through multiple food web levels. However, we’re just starting to parse how context can influence these effects. Some Yellowstone studies have found that wolves have powerful indirect effects on the plants that elk eat, such as aspens, due to fear of predation. With wolves around, elk have to keep moving to stay alive, which reduces browsing pressure. Conversely, a growing body of studies are finding no wolf effect—that aspens in places with wolves aren’t growing differently than those where predation risk is low. Other studies have found that wolf predation risk doesn’t affect elk feeding behavior. In my own research I’ve found that wolves need another keystone force—fire—to most effectively drive trophic cascades. With wolves and fire present, elk herbivory drops, aspens thrive, and biodiversity soars due to the healthy habitat created by young, vigorously growing aspen.
It’s human nature to try to find simple solutions. Today we are grappling with monumental environmental problems such as climate change and habitat fragmentation. Due to the wolf’s iconic status and our need to fix broken ecosystems, the environmental community and the media have run with the science that shows a strong wolf effect. This has inspired other scientists to prove that ecosystems are more complex than that. These dissenting studies demonstrate that the wolf dwells in a tangled bank, working alongside many other ecological forces.
Tangled banks seldom yield simple answers. However, arguing about what exactly carnivores do ecologically and why we need them is fiddling while Rome burns. Large, meat-eating animals improve the health of plant communities and provide food subsidies for the many species that scavenge on their kills. A system with wolves in it is far richer than one without and can support many more grizzly bears, coyotes, wolverines, and eagles. There are things we don’t know and disagreements about what we do know. But given the accelerated human-caused extinctions we are experiencing today, a precautionary approach to creating healthier ecosystems means conserving large carnivores.
Beyond empiricism, scientists often operate based on instinct. Instinct led Darwin to dig more deeply into species adaptation and Leopold to doggedly delve into the effects of predator removal. For many of us who conduct trophic cascades science, our instincts are telling us that wolves should be conserved in as high a number in as many places as possible, due to the invaluable benefits they can bring to ecosystems. To do anything other than conserve wolves would be foolish, given all we’ve learned thus far.
Previous studies have shown that carnivores can have indirect positive effects on each other, which means that when one species is lost, others could soon follow. A team from the University of Exeter and the University of Bern has now found that reducing the numbers of one species of carnivore can lead to the extinction of others.
Published online February 28, 2013 in the journal Ecology Letters, the study shows that simply reducing the population size of one carnivore can indirectly cause another similar species to become extinct. The research shows that changes in population size, as well as extinction, can create ripple effects across sensitive food webs with far-reaching consequences for many other animals.
The research shows that species could suffer just as much from harm to another species as from being under direct threat themselves. This adds weight to growing evidence that a ‘single species’ approach to conservation, for example in fisheries management, is misguided. Instead the focus needs to be holistic, encompassing species across an entire ecosystem.
The researchers assembled experimental ecosystems with three species of parasitic wasps, along with the three types of aphids on which each wasp exclusively feeds. They set up four sets of tanks each containing the three aphid and three wasp species and allowed the populations to establish for eight weeks. Over the next 14 weeks (seven insect generations) the researchers removed a proportion of the wasps from three of the sets of tanks every day — one species from each set. The fourth set had no wasps removed.
The team found that the partial removal of one wasp species led indirectly to the extinction of other wasp species. In the absence of one wasp species, the aphid it preyed upon grew in numbers. All three species of aphid feed on the same plant so increased competition for food led to changes in sizes of the aphid populations. However no aphid species went extinct and so the indirect extinctions of the wasps were not the result of extinction of their prey. Rather, it is likely that the wasps that went extinct had difficulty searching for suitable prey among large numbers of unsuitable ones.
Lead researcher Dr Frank van Veen of the University of Exeter’s Centre for Ecology and Conservation said: “We have shown that the complex ripple effect of a change in population size across food webs is more sensitive than previously thought and that a reduction in the numbers of one carnivore can lead to the extinction of another carnivore species. We also found evidence that the initial indirect extinction can itself trigger further ones, potentially leading to a cascade of extinctions, like dominoes toppling over.”
“The insect system is handy for experimentation but the same principles apply to any ecosystem, from mammals in the Serengeti to the fish in our seas. It clearly shows that we should have an ecosystem-based approach to conservation and to the management of fish stocks and other natural resources.”
The research team has recently been awarded a £470K grant by the Natural Environment Research Council (NERC) to extend this research at a larger scale.
Dirk Sanders, Louis Sutter and F. J. Frank van Veen. The loss of indirect interactions leads to cascading extinctions of carnivores. Ecology Letters, 28 FEB 2013 DOI: 10.1111/ele.12096
University of Exeter. “Reducing numbers of one carnivore species indirectly leads to extinction of others.” ScienceDaily. ScienceDaily, 28 February 2013. <www.sciencedaily.com/releases/2013/02/130228124144.htm>