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Light At The End Of The Tunnel For The Tasmanian Devil

Image Credit: Mattias Appel, CC BY-ND 2.0

Image Credit: Mattias Appel, CC BY-ND 2.0, Image Cropped

Quantifying 25 years of disease‐caused declines in Tasmanian devil populations: host density drives spatial pathogen spread (2021) Cunningham et al., Ecology Letters, https://doi.org/10.1111/ele.13703

The Crux

While the Tasmanian Tiger has made news this last month for all the wrong reasons, there’s still another famous species of Tasmanian mammal which deserves just as much attention (probably more given that we can still save this one from extinction). The Tasmanian devil has seen its populations declined considerably over the last three decades, largely due to the emergence of a transmissible facial tumour, the devil facial tumour disease (DFTD).

The way the devils interact mean that even at low densities, the disease can still be transmitted through a population. The aggressive nature of Tasmanian devil mating (which occurs even when there are few devils around) is a big transmission vector. This unfortunately means that extinction due to DFTD was recently thought to be a likely endpoint.

Today’s authors wanted to test to how strongly the devil density influenced the spread of DFTD, and whether the drop in population that the disease causes means that we’re likely to see the disease’s effects wear off at some point, and Tasmanian devil populations stabilise.

What They Did

Long-term data is an absolute must for a study like this. Luckily, the Tasmanian government has run ‘spotlight surveys’ along 172 road transects for the last 25 years. These involve driving slowly along a 10 kilometre stretch of road and recording mammal presence using a handheld spotlight. This was combined with further surveys designed to obtain density at smaller scales to come up with a predictive estimate of devil density in Tasmania from 1985 to 2035.

The team also used occurrence data for DFTD to figure out how quickly it initially spread through Tasmania, and modelled the spread into a new region against the density of the devils in that region.

Did You Know: Devil Reintroduction

The Tasmanian devils are an Australian icon, and a lot of money has been put into figuring out how to save their species. Suggestions have been made to reintroduce DFTD-free population back onto mainland Australia, where their presence may even help reduce the effect of cats and foxes. However it is also possible that the introduction of a new predator could instead put added pressure on mainland species already threatened by invasive predators. Studies into this are ongoing, and you can check out more on them at the articles linked below.

Read More: Releasing the Devil

What They Found

Tasmanian devil density may have played a large role in the initial spread of the disease, explaining why it spread so quickly through certain parts of Tasmania. This isn’t hugely surprising, though the precision with which the authors modelled its spread will be absolutely crucial for effective conservation.

What is really interesting is that the Tasmanian devil population back before the disease struck were probably much lower than initially thought. If this sounds depressing, the other big takeaway is that based on the predictions here, the decline in devil numbers should ease off soon, meaning the disease is unlikely to result in the extinction of Tasmania’s most iconic endemic species.

The study predicts that Tasmanian devil extinction is unlikely, but that doesn’t mean we can relax just yet (Image Credit: Mathias AppelCC0 1.0)

Problems

Normally authors will mention interesting future research which could build on the research they’ve carried out. Standard practice. Here, my ‘problem’ is that the authors mention some research so incredibly tantalising I’m angry at them for bringing it up. What will be important in the future is looking at devil genotypes. The genetic makeup of some devils will make them more resistant to the disease, and identifying and moving these individuals to areas where the disease is rampant could help fight DFTD. Having said that, it could also help produce more aggressive strains of the disease. GIVE ME ANSWERS.

So What?

This is a good news story, which often feel quite scant in the world of ecology. But it doesn’t mean the devil is out of the woods yet. Actually the woods themselves are a massive problem, seeing as Australia’s rates of deforestation are among the worst in the world. We need to constantly monitor the population to figure out where local extinctions are likely.

This study is also a fantastic example of how important long-term monitoring is for ecologists. Studies like the one used here are hard to fund (more on that here), but their value to ecologists in allowing us to figure out what drives population fluctuations is enormous.

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who has spent way too much time looking at photos of Tasmanian mammals over the last 2 weeks. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.

Polly Want A City? Population Boom Sparks Call For Cull Of London’s Invasive Parakeets

When someone imagines London, they probably visualise Big Ben, Buckingham palace, and an overly patriotic use of the Union Jack. What they probably don’t picture is flocks of bright green parrots occupying every tree branch and streetlamp in view. However, urban populations of invasive parrot species are becoming more readily observed globally, and in London, there are fears the population may be growing too fast!

Earlier this year, the UK saw headlines announcing that the government has been advised to cull the iconic birds following a recent increase in numbers. But with their bright colours making them a unique addition to the fauna of the city, and their nonchalant nature towards locals and tourists, many are opposed to the cull. So what is the right thing to do when we get attached to an invasive species? And are parrots on their way to becoming the next globally distributed ‘pest’?

Read more

Not All Datasets Are Created Equal

Image Credit: Chinmaysk, CC BY-SA 3.0, Image Cropped

Species data for understanding biodiversity dynamics: The what, where and when of species occurrence data collection (2021) Petersen et al., Ecological Solutions and Evidence, https://doi.org/10.1002/2688-8319.12048

The Crux

With the rise of the internet, GPS’ and smartphones, the amount of openly available species occurrence data has reached previously unfathomable numbers. This increase is mostly due to the engagement of the citizen scientist – regular people getting out there in nature and taking part in data collection and research. From people taking photos of flowers in their backyard to organised salamander spotting safaris, citizen scientists have opened up data that previously would have cost massive amounts to produce.

The Global Biodiversity Information Facility (GBIF) is the largest hub of such data, collating data ranging from amateur observation to museum specimens to professional surveys. It is well-known, however, that this kind of openly available data comes with a myriad of caveats: some species groups are reported much more than others (I am looking at you, bird-watchers), and “roadside bias” (see Did You Know?) haunts the records. But how are the records distributed among different land-cover types on a country-scale, does it differ between groups of conservation concern, and does it depend on who the reporters are?

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“Those Things Are Evil”: Prediction Intervals in Mixed Models

Suppose we study salamanders and want to predict body mass based on their body length. We also want to account for different access to food and differing levels of competition at each site we’ve collected our salamanders from. So we fit a linear model with a random effect for site as we only have samples from a subset of sites. (Want a refresher on random effects? We’ve got you covered.)

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From Deforestation to the Pandemic: How Destroying Ecosystems Increases Novel Infectious Diseases

This is a guest post by Professor Emma Despland

Zoonotic diseases, or diseases that jump from animals to people, are not a new phenomenon.  Many well-known human diseases first originated in animal populations. In some cases, animals are the main sources of human infection and human-to-human transmission is rare or null (e.g. rabies); other diseases persist in animal populations and occasionally jump to humans, seeding a human outbreak (e.g. plague), and yet others jumped from animals to people long ago and have been circulating in human populations ever since (e.g. measles).  However, novel zoonotics have been appearing with disturbingly increasing frequency.  

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Predators Under Nightlights

This is a guest post by Dr. Mark Ditmer.

Streetlights like these have meant that for some animals, hiding in the dark is now increasingly difficult (Image Credit: ME Stoner, CC BY 2.0, Image Cropped)

Artificial nightlight alters the predator–prey dynamics of an apex carnivore (2020) Ditmer et al., Ecography, https://doi.org/10.1111/ecog.05251

The Crux

The earth is no longer dark at night – artificial lighting has degraded the dark nighttime conditions that many species have evolved with throughout their evolutionary history. This change is only accelerating, with human expansion and intensity of radiance continuing to increase annually. We already know that elevated light levels can disrupt ecological processes like pollination or migration, as well as have a litany of negative effects on individual species, from physiological stress to predation risk. But it’s hard to get an idea of how the increase in ‘light pollution’ affects free-roaming wildlife, especially large mammals, and especially at scales relevant for making conservation policy.

In areas like the American west, the rapid growth of urban areas and the accompanying spread of light pollution create a rapidly changing ecosystem, one that sees many conflicts between humans and wildlife. One particularly species of particular interest is the mule deer (Odocoileus hemionus), which seeks out sources of forage on the edges of and within towns and cities (e.g. parks, farms), especially in arid regions. The primary predator of mule deer – the cougar (Puma concolor) – also navigates and hunts near human development where their prey congregate, but tend to avoid human presence more so than deer.

Today’s authors wanted to assess how artificial lighting, both where it occurs and its intensity, can shape the behaviors and predator-prey interactions of these species across the American West ranging from the edges of bright urban regions, such as Salt Lake City (Utah) and Reno (Nevada), to areas receiving minimal light pollution like Grand Canyon National Park.

What They Did

The authors used a massive dataset that included GPS-locations from 263 mule deer, 56 cougars, and 1,562 locations where cougars successfully killed mule deer. The resulting location data were combined with estimates of anthropogenic light pollution (more on this in Did You Know?).

Several different analyses were performed on the combined light and GPS-location data, along with other variables representing environmental (e.g., snow cover, land cover, terrain) and human factors (e.g., distance to roads, housing density). The aim was to figure out whether A) light has any influence on the behavior of each species, B) cougars avoid areas with high light pollution, allowing deer to forage freely wherever and whenever they want (the ‘predator shield hypothesis’), or C) cougars exploit the higher densities of deer seeking forage around areas with elevated light pollution (e.g., parks, golf courses, agriculture; the ‘ecological trap hypothesis’).

Did You Know: A Space Agency’s Ecological Impact

In this study we used remote sensing data to determine the amount of light pollution in a given environment. Yet the sensors only pick up the total amount of light, and can’t tell us what is a product of our activity and what is a natural source of light. To separate the two, we used light data which was recently developed by the U.S. National Aeronautics and Space Administration (NASA). This dataset removes the contributions of natural sources of light (e.g., moonlight, fire, atmospheric spray) from our data and results in values of just the human-created nighttime light emissions.

What They Found

The behaviors of both species changed greatly with levels of light pollution, as did the predation risk for deer. The behaviour changed across different scales as well. Cougars killed deer in study sites with the high amounts of light pollution, but within those sites (e.g., edge of Salt Lake City, Utah) cougars selected to hunt and kill in the relatively darkest locations. In contrast, in the darker study areas, cougars killed deer in areas with the relatively more light pollution than the surrounding area. However, even though cougars killed deer in the darkest spots within the bright urban interface, those locations generally had much higher levels of light pollution than the brightest kill sites in the low light pollution study areas.

The study found that cougars made kills in bright regions, but generally only within the darker parts of those regions (Image Credit: ME Stoner, CC BY 2.0)

Deer living in brighter urban areas tended to forage at night, potentially to avoid direct human interactions. This shift might have benefited deer by avoiding humans, but as they sought out more natural and dark locations in these areas, cougars would wait in ambush.

In the end, the authors concluded that their findings fell in a gray zone between the predator shield and ecological trap hypotheses dependent on scale. Areas with high levels of light and subsequent human activities provide excellent foraging opportunities for ungulates (as this study measured as well), but adaptable predators can follow and take advantage – at least in environments that they feel are safe enough.

Problems

This is an observational study, so it’s hard to fully tease apart what effects are driven by light and what are driven by other human factors. We did our best to account for the other more traditional sources of the human footprint, reporting effect sizes for each, but there’s always a chance we’re attributing some effects to light pollution that could be caused by some other aspect of our presence.

So What?

Work like this shines a light on (pun intended) how different species will respond to the ongoing urbanization trends humans are driving in much of the planet.

Although many wildlife ecology studies consider various human alterations to habitats and the consequent changes in animal behavior, most studies fail to consider the sensory environment and the pollutants (e.g., noise, light) that can impact wildlife populations in their analyses. How wildlife use an ecosystem can impact everything from human-wildlife interactions to pulses of nutrients to the soil based on shifting areas of kill sites/carcasses.

Dr. Mark Ditmer is a post-doctoral researcher at the Centre for Human-Carnivore Co-Existence and Wittemyer Lab at Colorado State University. You can read more about him at his profile here or follow him on Twitter @MDitmer.