|
Environment, Ecology & Earth's biodiversity
|
The dynamics of pine marten predation on red and grey squirrels
Abstract: Invasive alien species pose one of the greatest threats to global biodiversity. In parts of Europe, introduced eastern grey squirrels (Sciurus carolinensis) have caused regional extinctions of the native red squirrel (Sciurus vulgaris). However, exposure to pine martens (Martes martes) has been demonstrated to reverse the competitive outcome between red and grey squirrels. The mechanism whereby this effect occurs remains unclear. It is hypothesised that direct predation, facilitated by a lack of behavioural response, is the mechanism driving this relationship. We review the literature and reanalyse a new dataset to provide further data on the occurrence of both squirrel species in the scats of pine marten. Both squirrel species occurred in the scats of pine marten confirming its role as a predator of these species. Predation of grey squirrels was significantly higher than red squirrels and was recorded only in spring and summer. Our review provides evidence for the mechanism driving the current decline in grey squirrels in Ireland and Scotland and supports the hypothesis that in the presence of a shared predator, direct predation influences the outcome of species interactions between native red and non-native grey squirrels. 
Furry engineers: sea otters in California's estuaries surprise scientists
When Brent Hughes started studying the seagrass beds of Elkhorn Slough, an estuary in Monterey Bay on California’s central coast, he was surprised by what he found. In this highly polluted estuary, excessive nutrients from agricultural runoff spur the growth of algae on seagrass leaves, which kills the plants. Yet in 2010, Hughes noticed that the seagrass beds were thriving. It did not make sense.  *This image is copyright of its original author FacebookTwitterPinterest Records show that otters were once abundant in California’s estuaries. “This is the highest nutrient concentration that I had ever seen on the planet,” says Hughes, a biologist at Sonoma State University. “Any model would suggest there should be no seagrass there and yet it was expanding.” Hughes set out to solve the mystery. He examined every possible factor, including water quality, temperature and changes in seagrass coverage over time, going back 50 years. He was not making any progress until he was approached by a boat captain named Yohn Gideon who had been running wildlife tours in the slough since 1995. Over the years, the captain had handed clickers to his passengers, asking them to count the sea otters they saw. Hughes overlaid the captain’s sea otter counts with historical seagrass coverage data and realised the two graphs were almost perfectly in sync. When sea otter numbers went up, seagrass went up, too. “You don’t see that very often in ecology. That was a eureka moment,” he says. While Hughes had often encountered sea otters in the slough during his dives, he never paid attention to them. Now they were clearly linked to seagrass health, but how? Keystone species Sea otters may be North America’s smallest marine mammal, but they have a huge appetite. Unlike other species that rely on a layer of fat called blubber to keep them warm, otters use their dense fur and a high metabolic rate to maintain body heat in the chilly waters of the northern pacific. To achieve that, they must eat 25% of their body weight every day, munching on a variety of marine invertebrates, including sea urchins, clams and crabs.  *This image is copyright of its original author FacebookTwitterPinterest Seagrass in the Elkhorn Slough estuary. When the otters first moved into the slough in the 1980s, they put their big appetites to work eating crabs. With fewer crabs to prey on them, the California sea hares – a sea slug – grew larger and became more abundant. The slugs fed on the algae growing on the seagrass, leaving the leaves healthy and clean. Hughes had discovered a trophic cascade that had made the seagrass beds the healthiest of any estuary he had seen on the west coast. Since the otters arrived in the slough, the seagrass has recovered and increased by more than 600% in the past three decades. Sea otters had already shown that they were capable of a large influence on the ecosystem. In the 1970s, biologist James Estes was conducting research in the Aleutian Islands in Alaska and noticed some areas where the seafloor was covered with sea urchins. As herbivores, urchins feed on kelp, and when their numbers are not kept in check by predators, no kelp remains. In contrast, in places where sea otters were present, kelp forests were thriving. Estes demonstrated that by eating urchins, sea otters created the opportunity for kelp to flourish. Sea otters gained their official title as a keystone species.  *This image is copyright of its original author FacebookTwitterPinterest California sea hares feed on the algae growing on seagrass, which improves the health of the leaves. Advertisement
But the discovery that sea otters could also be important players in estuaries came as an ecological surprise. In fact, scientists had not even expected sea otters to survive in an estuary. Hunted to near extinction by fur traders in the 18th and 19th centuries, sea otters had only survived in a few small, isolated populations in the North Pacific. Then, after the Fur Seal Treaty was signed in 1911, conservation efforts led to an increase in otter numbers. In California, the population grew from 50 in 1914 to about 3,000. On the coast of British Columbia, they were reintroduced between 1969 and 1972 and there are now about 8,100 of them. Since the otters were first recovering in kelp forests along the open coast, the scientists who studied the animals assumed that this was their primary habitat. When they started turning up in Elkhorn Slough in the 1980s, they thought that was an anomaly, failing to realise that they were in fact reoccupying old habitats. Archaeological and historical records indicate that otters were once abundant in California’s estuaries, prior to being driven to local extinction through over-hunting. “Scientists are subject to perception bias,” says Tim Tinker, a wildlife biologist at the University of California, Santa Cruz. “Estuaries were traditionally very important to otters. But we did not really appreciate them as such. The ecological effects of otters on the outer coast dominated our thinking.”  *This image is copyright of its original author FacebookTwitterPinterest An otter enjoys a swim among the kelp. Advertisement
Estuaries were not even considered in the US fish and wildlife service’s plan for the recovery of the sea otter in California, listed as threatened under the US Endangered Species Act. Quote:Anywhere sea otters are going, I can guarantee change is going to happen Now, scientists and wildlife managers are turning to estuaries as potential places where sea otter numbers could grow. In the past decade, the expansion of sea otters on the California coast has been curbed unexpectedly by the presence of the great white sharks that are recovering from overfishing and by-catch, at the southern and northern edges of the otter range. Shark bites have become the primary source of otter mortality. “The population is constrained by a shark wall,” Tinker says. Estuaries could provide otters with an important refuge from sharks and other unfavourable coast conditions, such as storms and warming events. A recent paper suggests that San Francisco Bay, California’s largest estuary, could support about 6,000 otters, more than double the current population. “Once fully recovered, between a quarter and one third of the entire population in California could be accounted for by otters living in estuaries,” Tinker says.  *This image is copyright of its original author FacebookTwitterPinterest Biologist Brent Hughes working in Elkhorn Slough. Scientists also believe that sea otters could be conservation allies, with their potential to help restore other polluted estuaries in California. “It is a two-way street,” Tinker says. “To have a resilient population of sea otters we may need them in estuaries but estuaries may also need sea otters.” Seagrass ecosystems perform important ecological functions. They provide nursery habitat for young fish and invertebrates, and they protect shorelines from storms and waves. They control erosion by holding down and trapping sediment. Seagrass absorbs carbon from the atmosphere and buries it in its roots, acting as a carbon sink. Hughes’ discovery of the positive impact that sea otters had on Elkhorn Slough’s seagrass made him wonder whether they could benefit other places. Still working in the slough, he observed that sea otters moved into the salt marsh portion of the estuary in 2012 and started having a positive influence on this fragile habitat. Before the otters’ arrival, numerous striped shore crabs were burrowing into the muddy banks and fed on the marsh roots, accelerating the erosion of the marsh. By eating the crabs, the otters are now helping make the salt marsh healthier, slowing down erosion, a discovery that could be significant for salt marshes elsewhere. “Salt marshes are one of those ecosystems that globally are in a state of decline. In California, we lost 90% of them,” says Hughes.  *This image is copyright of its original author FacebookTwitterPinterest Scientists believe that sea otters could help restore other polluted estuaries in California. In south-east Alaska and British Columbia, scientists are studying other ways that otters may be beneficial to seagrass beds. “Anywhere sea otters are going, I can guarantee change is going to happen,” Hughes says. But the long-term influence of these furry ecosystem engineers is not yet fully understood. Quote:Sea otter recovery is different from the recovery of any other species because they have such disproportionally big effects on the ecosystem For example, when sea otters look for clams to eat, they dig in the seagrass beds, leaving holes behind. “When you are watching otters digging for clams, you can see a cloud of sediment come up to the surface,” says Erin Foster, a marine ecologist studying the interactions between otters and seagrass at the Hakai Institute in British Columbia. “It looks like a bombed-out landscape when you come across a fresh site where there has been recent digging,” adds Margot Hessing-Lewis, also a marine ecologist with the Hakai Institute. Foster and Hessing-Lewis are investigating the possibility that when sea otters disturb the sediment they could be helping seagrass become more resilient to environmental changes by making the seagrass flower more often, potentially increasing the genetic diversity of the plants. As scientists are still learning about the full impact of sea otters on different habitats, they wonder about assisting the growth of the population, especially in California where the shark predation is making it difficult for otters to expand into new areas. Elkhorn Slough was the first estuary in California recolonised by sea otters, a process that was greatly accelerated by the release in the slough of 37 otters that had been rescued and rehabilitated by the Monterey Bay Aquarium in the 2000s.  *This image is copyright of its original author FacebookTwitterPinterest Sea otters eat 25% of their body weight each day. Advertisement
The return of this top predator also comes at a cost to humans, since sea otters compete directly with shellfish fisheries which had developed in their absence. A recent study that analysed the costs and benefits linked to the reintroduction of sea otters to the west coast of Vancouver Island in British Columbia found the benefits – including increasing fish populations, carbon sequestration and ecotourism – outweighed the losses to invertebrate fisheries by seven times. However, the benefits are not felt equally, especially among indigenous communities who rely on shellfish harvesting for food security. “Sea otter recovery is different from the recovery of any other species because they have such disproportionally big effects on the ecosystem,” Tinker says. “For most depleted species you are just worried about the conservation of the species but with sea otters, you are thinking how the entire ecosystem is going to change when they recover.”
The distribution of biodiversity richness in the tropics
Abstract: Abstract We compare the numbers of vascular plant species in the three major tropical areas. The Afrotropical Region (Africa south of the Sahara Desert plus Madagascar), roughly equal in size to the Latin American Region (Mexico southward), has only 56,451 recorded species (about 170 being added annually), as compared with 118,308 recorded species (about 750 being added annually) in Latin America. Southeast Asia, only a quarter the size of the other two tropical areas, has approximately 50,000 recorded species, with an average of 364 being added annually. Thus, Tropical Asia is likely to be proportionately richest in plant diversity, and for biodiversity in general, for its size. In the animal groups we reviewed, the patterns of species diversity were mostly similar except for mammals and butterflies. Judged from these relationships, Latin America may be home to at least a third of global biodiversity.
How beavers became North America's best firefighter
The rodent creates fireproof refuges for many species, suggesting wildlife managers should protect beaver habitat as the U.S. West burns. The American West is ablaze with fires fueled by climate change and a century of misguided fire suppression. In California, wildfire has blackened more than three million acres; in Oregon, a once-in-a-generation crisis has forced half a million people to flee their homes. All the while, one of our most valuable firefighting allies has remained overlooked: The beaver. A new study concludes that, by building dams, forming ponds, and digging canals, beavers irrigate vast stream corridors and create fireproof refuges in which plants and animals can shelter. In some cases, the rodents’ engineering can even stop fire in its tracks. “It doesn't matter if there’s a wildfire right next door,” says study leader Emily Fairfax, an ecohydrologist at California State University Channel Islands. “Beaver-dammed areas are green and happy and healthy-looking.” For decades, scientists have recognized that the North American beaver, Castor canadensis, provides a litany of ecological benefits throughout its range from northern Mexico to Alaska. Beaver ponds and wetlands have been shown to filter out water pollution, support salmon, sequester carbon, and attenuate floods. Researchers have long suspected that these paddle-tailed architects offer yet another crucial service: slowing the spread of wildfire. This beaver-dammed wetland in Baugh Creek, Idaho, is a so-called "emerald refuge" that can serve as a firebreak and refuge for other species during wildfires. PHOTOGRAPH BY JOE WHEATON, UTAH STATE UNIVERSITY DEPARTMENT OF WATERSHED SCIENCES “It’s really not complicated: water doesn’t burn,” says Joe Wheaton, a geomorphologist at Utah State University. After the Sharps Fire charred 65,000 acres in Idaho in 2018, for instance, Wheaton stumbled upon a lush pocket of green glistening within the burn zone—a beaver wetland that had withstood the flames. Yet no scientist had ever rigorously studied the phenomenon. (See California’s record blazes through the eyes of frontline firefighters.) “Emily’s study couldn’t be more timely,” says Wheaton, who wasn’t involved in the research. “This points toward the importance of nature-based solutions and natural infrastructure, and gives us the science to back it up.” Fire refugia Inspired in part by Wheaton’s observations, Fairfax and colleague Andrew Whittle chose major wildfires that had occurred since 2000 in five U.S. states—California, Colorado, Idaho, Oregon, and Wyoming—and scoured satellite images for nearby beaver dams and ponds. (Beaver infrastructure is so impressive that it’s visible from space.) Then, using a statistical measure of plant health, they calculated the lushness of the surrounding vegetation before, during, and after the fires. Unsurprisingly, thriving, well-watered plants tended to appear vivid green in the satellite photos, while dry plants looked comparatively brown. (Read more about how wildfires get started—and how to stop them.)  *This image is copyright of its original author Surveys of beaver dams revealthe locations of green safe havens Sharps Fire (2018)
Using satellite images, scientists remotely surveyed beaver dams in different areas that have burned in wildfires, such as the Sharps fire in Idaho. ENLARGED BELOW 1 mi IMAGES FROM BEFORE SHARPS FIRE 1 km By measuring vegetation along creeks both before and after fires, the researchers found that beaver dams offer protection from fires. On average, vegetation near dams burned three times less than in areas lacking dams.) (you can find this image in the article as it cant be pasted) RILEY D. CHAMPINE, NG STAFF; SOURCES: EMILY FAIRFAX, CSUCI; GOOGLE EARTH; ESA A green, hydrated plant, of course, is also less flammable than a desiccated, crispy one. And that’s what makes beaver ecosystems so fireproof. In beaver-dammed stream sections, Fairfax and Whittle found, vegetation remained more than three times lusher as wildfire raced over the creek. Beavers had so thoroughly saturated their valleys that plants simply didn’t ignite. These lifeboats don’t merely protect beavers themselves: A broad menagerie—including amphibians, reptiles, birds, and small mammals—likely hunker down in these beaver-built fire “refugia,” Fairfax says. Although wildfire is a vital force that rejuvenates habitat for some creatures, like black-backed woodpeckers, it can devastate other animal populations. Beaver habitat also protects domestic livestock and agricultural lands, adds Fairfax, whose study was published this month in Ecological Applications. “If you have a beaver wetland, your cows can take advantage of that refuge and fare better during wildfire than if you had to pack them out on trailers.” Embracing beavers In addition, beavers may help an ecosystem recover from a wildfire. In northern Washington State, Alexa Whipple, the director of the Methow Beaver Project, found that beavers promoted the recovery of native species, like willow and aspen. Beaverless streams, by contrast, were more likely to become colonized with invasive plants after a burn. Whipple also found that beaver ponds improved water quality by capturing the phosphorus-laden sediment that runs off torched hillsides. (Learn how wildfires are increasing worldwide.) “If we have a wetter landscape, we are going to resist fire and recover from it better,” says Whipple, whose results haven’t yet been published in a peer-reviewed journal. “My hope is that wildfire can be the gateway for people to understand the whole suite of benefits that beavers offer.” Despite all the good that beavers do, thousands are killed every year for flooding roads, cutting down trees, and causing other damage to human property. Employing smarter, more humane policies—using nonlethal flood-prevention devices like “Beaver Deceivers,” for example, and relocating trouble-making individuals instead of killing them—could heal our relationships with beavers and wildfire alike, Fairfax says. “Strategically embracing beavers in local watersheds could provide reassurance that you have wet soils and wet plants around your town,” Fairfax says. In fact, as her paper’s title suggests, the U.S. Forest Service might want to consider a new animal mascot: Smokey the Beaver.
Status and Ecological Effects of the World’s Largest Carnivores
Abstract Large carnivores face serious threats and are experiencing massive declines in their populations and geographic ranges around the world. We highlight how these threats have affected the conservation status and ecological functioning of the 31 largest mammalian carnivores on Earth. Consistent with theory, empirical studies increasingly show that large carnivores have substantial effects on the structure and function of diverse ecosystems. Significant cascading trophic interactions, mediated by their prey or sympatric mesopredators, arise when some of these carnivores are extirpated from or repatriated to ecosystems. Unexpected effects of trophic cascades on various taxa and processes include changes to bird, mammal, invertebrate, and herpetofauna abundance or richness; subsidies to scavengers; altered disease dynamics; carbon sequestration; modified stream morphology; and crop damage. Promoting tolerance and coexistence with large carnivores is a crucial societal challenge that will ultimately determine the fate of Earth’s largest carnivores and all that depends upon them, including humans.
And here's an article questioning the notion that apex predators have such a large impact on their respective ecosystems, offering a different perspective
https://www.nature.com/news/rethinking-predators-legend-of-the-wolf-1.14841
A very interesting (not yet peer reviewed) paper here on the imbalance between the biomass of large herbivores in ecosystems when accounting for said ecosystems' primary productivity, barring Africa where they assume it approaches this proposed baseline. They then use this to predict the likely natural biomass of large herbivores Europe should harbour, and the implications this has for rewilding.
Exploring a natural baseline for large herbivore biomass Abstract: The massive global losses of large mammals in the Pleistocene have triggered severe ecosystem changes including changed nutrient cycles, fire regimes and climate, shifts in biomes and loss of biodiversity. Large herbivores create and diversify resources and living space for other organisms and thereby play an important role in ecosystem functioning and biodiversity conservation. However, even today large herbivores are regulated, hunted and driven to extinction to a degree where intact large-herbivore communities are largely non-existent. Consequently, natural density and biomass of large-herbivores for restoration of ecosystems are poorly known. To address this knowledge gap, we apply the scaling pattern for consumer-producer relationships and show that the biomass of large herbivores in ecosystems across the world is considerably lower than expected from primary productivity. African ecosystems have the strongest consumer-producer relationship and assuming that African ecosystems approach a natural baseline, we use this relationship to predict large herbivore biomass in Europe as an example. Our findings indicate that restoring large herbivore biomass would entail increasing large herbivore biomass by orders of magnitude in most ecosystems, which potentially changes the perspective on large herbivores in conservation and restoration projects.  *This image is copyright of its original author
A distinct ecotonal tree community exists at central African forest–savanna transitions
Abstract
Anthropogenic modification of forests means only 40% of remaining forests have high ecosystem integrity
Abstract Many global environmental agendas, including halting biodiversity loss, reversing land degradation, and limiting climate change, depend upon retaining forests with high ecological integrity, yet the scale and degree of forest modification remain poorly quantified and mapped. By integrating data on observed and inferred human pressures and an index of lost connectivity, we generate a globally consistent, continuous index of forest condition as determined by the degree of anthropogenic modification. Globally, only 17.4 million km2 of forest (40.5%) has high landscape-level integrity (mostly found in Canada, Russia, the Amazon, Central Africa, and New Guinea) and only 27% of this area is found in nationally designated protected areas. Of the forest inside protected areas, only 56% has high landscape-level integrity. Ambitious policies that prioritize the retention of forest integrity, especially in the most intact areas, are now urgently needed alongside current efforts aimed at halting deforestation and restoring the integrity of forests globally.
Postwar wildlife recovery in an African savanna: evaluating patterns and drivers of species occupancy and richness
Abstract As local and global disturbances reshape African savannas, an understanding of how animal communities recover and respond to landscape features can inform conservation and restoration. Here, we explored the spatial ecology of a wildlife community in Gorongosa National Park, Mozambique, where conservation efforts have fostered the recovery of large mammal populations after their near‐extirpation during Mozambique’s civil war. We deployed a grid of 60 camera traps and used a hierarchical, multi‐species occupancy modeling approach to examine patterns of occupancy and its environmental and anthropogenic correlates for different functional groups and species. Our survey provides strong evidence that wildlife in Gorongosa is recovering. Throughout the study area, modeled species richness was comparable to richness in less‐disturbed savanna systems in Tanzania and Botswana, and exceeded estimates of richness from a mixed‐use landscape outside the park and from postwar (1997–2002) aerial surveys. However, the mammal community in Gorongosa differs from prewar conditions and from those of more intact systems, with few large carnivores, low occupancy probabilities for large ungulate species that were dominant prior to the war, and high occupancy for other ungulates that are now ubiquitous. Associations with tree cover varied among species and guilds. Contrary to our expectation, there was no effect of lake proximity on community and group‐level occupancy, and previously dominant floodplain ungulate species now occupy more wooded areas. Mammals were more likely to occupy areas that burned frequently, as post‐fire vegetation regrowth provides high‐quality forage, highlighting the importance of Gorongosa’s fire regime. Occupancy was lower in areas with more illegal hunting, and higher closer to roads, potentially because roads were established in areas of high wildlife density and facilitate animal movement. Continued multi‐species monitoring in Gorongosa can shed light on the different recovery trajectories of ungulate species and the consequences of ongoing large carnivore restoration, guiding conservation interventions.
Herbivore Body Size Influences Grazing Behavior, Poop Quality
The paper E. le Roux et al., “Animal body size distribution influences the ratios of nutrients supplied to plants,” PNAS, doi:10.1073/pnas.2003269117, 2020. Animals eat plants. Animals poop. Poop nourishes plants. It may sound simple, but it’s not. Elizabeth le Roux, an ecologist and Newton International Fellow at the University of Oxford, studies this complex cycle in the South African savanna, particularly as it relates to larger animals and how they differ from small species in their effects on ecosystems. As a graduate student at Nelson Mandela University in South Africa, le Roux began investigating how animal dung varies across the grassy plains of Hluhluwe-iMfolozi Park and whether that variation influences the local plant life. At 15 sites, she and her colleagues assessed the density of the vegetation, used camera traps to record how many of each animal species frequented a particular area and how long they stayed, surveyed the amount of dung and noted what species it came from, and sampled dung, soil, and plants to assess nitrogen and phosphorus levels.  *This image is copyright of its original author BIG DUNG: Areas with denser vegetation and thus lower visibility (left) are more likely to be frequented by larger animals—which have less to fear from predators—than by smaller animals, according to a recent study. Compared with smaller herbivores such as impala, large herbivores such as elephants and rhinos produce dung with relatively lower phosphorus content. These differences in dung are associated with variable availability of nutrients for plants growing in the savanna, the researchers found, suggesting that the animals’ body sizes could influence ecosystem functioning. WEB | PDF © MELANIE LEE The team hypothesized that vegetation-dense, low-visibility areas would be frequented by large animals, which are less likely to be targets for lions and other predators that sneak up in the foliage. Indeed, camera traps showed that smaller grazers such as impala stick mainly to open areas, while elephants and rhinos often graze at denser sites. Read “Can Rewilding Large Predators Regenerate Ecosystems?” When it came to the animals’ dung, the team predicted that larger animals would excrete proportionally less phosphorus. That’s because larger animals have relatively larger skeletons, le Roux says, and thus may require more phosphorus from their food. This hypothesis, too, was supported by the data—phosphorus was scarcer in larger animals’ dung, both as a proportion of dung mass and relative to nitrogen content. The researchers also analyzed the grasses at their sites to see if dung nutrient differences were reflected in plant tissue. Here, the data hinted that phosphorus:nitrogen ratios may be lower in grasses in vegetation-dense areas frequented by large herbivores, but the effect was weak. That’s not surprising, le Roux says, as plant nutrient ratios are influenced by factors other than dung, such as soil microbes and fire. Ecologist Harry Olde Venterink of Vrije Universiteit Brussel praises the study for its innovative use of camera traps to collect quantitative data on animal behavior, and says it provides good evidence that larger animals are using sites differently than smaller ones and may be influencing nutrient availability via their dung. His team recently showed that herbivore dung quality in Europe influences plant community diversity (Sci Rep, 9:5675, 2019), and he says the new study is a further step toward deciphering how animals influence nutrient cycles. However, Venterink questions some of the study’s assertions—in particular, the idea that species with larger skeletons need more phosphorus, a connection le Roux agrees isn’t well established. Venterink suggests that differences in dung nutrients could simply reflect differences in animals’ diets. Le Roux says the findings could help researchers understand the effects of adding or removing larger species from ecosystems. As humans influence wild animal populations, often “we’re changing the average [body] size of the herbivore community,” she says. “We need to understand: What are the consequences?”
A great article about trophic cascades and landscapes of fear, as well as the competing theories surrounding these. Below I'll post some parts I found most interesting as it's a long article. By all means read the whole thing though, these types of things are why ecology fascinates me.
Can Rewilding Large Predators Regenerate Ecosystems? "Estes’s observations were quickly followed by a series of observations from other researchers that pointed to the existence of trophic cascades in other aquatic ecosystems, such as those in lakes and rivers. In the 1990s, Os Schmitz, an ecologist at Yale School of the Environment, discovered that a terrestrial predator, the nursery web spider Pisaurina mira, could similarly create a trophic cascade, although in this case he uncovered an entirely different mechanism. It turns out that the spiders didn’t have to kill their prey to affect the ecosystem; they just had to scare them into skipping a meal. Schmitz placed leaf-chewing grasshoppers, specifically Melanoplus femurrubrum, in a small, grass- and herb-filled enclosure, and observed that the vegetation flourished after he added nursery web hunting spiders, even without any immediate change in grasshopper numbers. Then, in a series of experiments in which Schmitz glued the spiders’ mouthparts shut so they could still instill fear but not consume their prey, he demonstrated that their mere presence was enough to allow grasses to flourish, as grasshoppers would forgo a meal to avoid becoming one. Biologists now call such fear-driven effects behaviorally mediated trophic cascades, distinguishing them from density-mediated ones that involve the predators’ consumption of their prey." "Utah State University ecologist Dan MacNulty questions whether adult elk, which are large and thus difficult for wolves to kill, would be so afraid of wolves as to miss out on a good meal, he says. Indeed, he and his colleagues have tracked wolves and elk with radio collars and found that elk often don’t avoid areas frequented by the predators, and that the ungulates seem to be more concerned with avoiding cougars (another name for pumas, Felis concolor). To Schmitz, this makes sense, given pumas’ sit-and-wait hunting strategy. As he’s learned from comparing spiders with different hunting styles, predators that tend to ambush their prey are more likely to create behaviorally mediated cascades. The wolves at Yellowstone typically hunt by chasing elk across the landscape. Because the elk can often see them coming from a distance, Schmitz explains, there’s no point avoiding certain areas. " "Even without reintroducing predators into the wild, researchers elsewhere are using experimental approaches to detect trophic cascades already in action. In 2008, in the rugged Andean terrain of San Guillermo National Park in Argentina, Donadio wanted to understand how pumas influenced their prey, llama relatives known as vicuñas (Vicugna vicugna). He noticed that, in open grasslands where they’d easily see a predator approaching, the vicuñas’ heads were usually buried in the grass eating, only occasionally popping up to look around. In meadows with taller grasses and canyon areas where pumas could lurk behind rocky outcroppings, on the other hand, the vicuñas spent less time eating and more time on watch. To test the effects of these behavioral differences on vegetation, Donadio constructed a number of 20-meter-by-20-meter exclosures—fenced areas intended to keep the vicuñas out, though the herbivores (as well as the pumas) could still frequent the general area. Sure enough, he observed that the growth of grass inside the exclosures in the grasslands shot up compared with grass in surrounding control plots, while grass growth in the canyon and meadow exclosures did not, suggesting that vicuñas were indeed sacrificing grazing opportunities there to avoid an ambush. This behaviorally mediated cascade is created by the complexity of the animals’ habitat, Donadio says, and in turn, it helps shape the environment. If the pumas weren’t there, “the vegetation in the canyons [and meadows] would look exactly like the vegetation in the plains.” By enhancing the diversity of habitats in San Guillermo, pumas may be creating new niches for other species, he explains, and in doing so, enhancing biodiversity." "In several locations within Iberá’s 1.3 million–hectare protected area, biologists with specialties in entomology, ornithology, predator ecology, and animal behavior are busy characterizing various facets of the ecosystem that they suspect the jaguars might influence. Populations of oversized rodents called capybaras (Hydrochoerus hydrochaeris) might plummet after the predators’ arrival, and their behavior may radically change, PhD student Belén Avila of Argentina’s Institute of Subtropical Biology hypothesizes. Right now, the capybaras are acting fearlessly, Donadio says, even dozing on the paths cutting through the area. But once they realize there are killers lurking about, individuals are likely to become more cautious and vigilant, which means they’ll spend less time eating, possibly affecting grass abundance. The jaguars could also reduce the number of smaller predators such as pampas and crab-eating foxes, which are abundant at the moment, and in doing so protect the endangered birds that the foxes sometimes eat. As the researchers track these and other outcomes over the coming years, Donadio says, “it’s going to provide really, really good information when it comes to the importance of large predators on landscapes and biodiversity.” "Pringle did, however, find evidence of cascading interactions between other species in the same ecosystem. In clearings where impala (Aepyceros melampus) gather to avoid being ambushed by leopards (Panthera pardus), the researchers noticed an abundance of acacia trees (Acacia etbaica), which carry thorns to defend against intense herbivory. In other areas where the bush was thicker—areas less frequented by the impala, probably due to fear of predation—“there were a bunch of plants that actually tend to be much more palatable,” Pringle says." "Similar studies have reported that underwater playbacks of mammal-eating killer whales (Orcinus orca), but not of local fish-eating killer whales, trigger harbor seals (Phoca vitulina) to dive to safer depths."
A look on grazing lawns, their nutrient cycles and how rainfall and predators can impact these and the size of herbivores who inhabit them (from: Predators and rainfall control spatial biogeochemistry in a landscape of fear)
"Rainfall could become a mitigating factor by determining the size range of herbivores that occupy a specific grazing lawn, and accordingly the ratio of N  that is released there. Rainfall increases the quantity of grass that is produced. However, that grass has poorer quality, making it harder to digest. Larger herbivores stand to disproportionately benefit from this because they require high quantities of forage, plus they have the physiological adaptations to digest poorer-quality forage (16). Smaller herbivores, alternatively, require less quantity, but their digestive adaptations necessitate that they seek high-quality forage (16). Thus, the mean body mass of herbivores present on a given lawn should increase along a gradient of increasing rainfall (Fig. 1B). That is, grazing lawns with lower rainfall (higher forage quality) should be frequented by herbivores from the smaller end of the size range, whereas lawns with higher rainfall (higher forage quantity) should be occupied by herbivores from the larger end of the size range. Now predators could complicate the matter by further determining which sizes of herbivores occupy a given grazing lawn. In savanna ecosystems, susceptibility of herbivore prey to predation decreases with herbivore body mass (Fig. 1C), with the very largest species even being completely invulnerable to predation (17, 18). Consequently, smaller herbivores should be more fearful than large herbivores, and therefore be more inclined to avoid grazing lawns that pose high risks of being captured by predators. Risky grazing lawns tend to be those surrounded by high vegetation because it hides predators that wait in ambush (19). Hence, more open grazing lawns should attract herbivores from the smaller end of the size range because the greater visibility makes them less risky places to feed (19). Taken together, the amount of rainfall and level of potential predation risk should create a joint gradient in the sizes of herbivores occupying different grazing lawns, thereby offering a way to explain spatial contingency in biogeochemical properties across landscapes. High-rainfall, high-risk sites should have the highest proportion of large herbivores. That proportion should dwindle as rainfall and risk become lower (Fig. 1D). Such patterning in herbivore size should then determine patterning in associated landscape biogeochemistry (Fig. 1E) due to the size dependence of N released in dung. By virtue of fast nutrient recycling, the size dependence of released N should be further reflected in the patterning of N in grass foliar tissue (Fig. 1E)."Edit: The emojis are supposed to be Nitrogen: Phosphorous but wildfact automatically makes colonP an emoji lol |
| Users browsing this thread: |
| 15 Guest(s) |