Age of Extinction: Collapse

Some things are greater than the sum of their parts. We live in a period of extinction, with nearly 4,000 critically endangered animal species. These population collapses have many causes, none of which would be catastrophic individually, but are cumulatively disastrous. One of the greatest challenges these die-offs pose is disentangling their complexity. Emerging diseases, new or rapidly increasing illnesses, often play a role, and disease can be the straw that breaks the camel’s back. In this post, I want to highlight three emerging wildlife diseases that are contributing to major population declines. My hope is that this essay will illustrate the impact of wildlife diseases, their relationship to human welfare, and the need for more research and greater understanding.

Colony Collapse Disorder

Colony collapse disorder (CCD) has received a lot of press (for a wildlife disease), because it has implications for agriculture. CCD is a condition where worker bees abruptly disappear en masse. Though it can affect wild or feral bees, it has been most closely studied in the honeybee, where it wreaks havoc, causing approximately 1/3 of all honeybee hives to collapse each year. CCD results from the cumulative impact of the laundry list of issues facing honeybees, including fungal, bacterial, and viral diseases, mites, pesticides, climate change, nutritional deficiencies, and some commercial beekeeping practices3,9.

CCD is the culmination of a cascade. In monocultures bees have little diversity of food, because only one crop is being grown. This results in nutritional deficiencies that weaken their immune systems9. The truly staggering amount of pesticides in the environment (a typical honeybee colony has residue from more than 120 pesticides) further weaken the bees8,9, leaving them vulnerable to a wide array of pests and pathogens. In fact, several tests have demonstrated that it takes the one-two punch of insecticides and infection to knock down honeybee fitness3.

Human welfare is intimately connected to the wellbeing of bees; 70% of crops depend at least in part on animal pollination, usually by insects, accounting for 35% of food production3. What is bad for bees is bad for us: the same pesticides that pollute colonies leach into ground water and are highly persistent in the environment8. Not only that, what is good for bees is good for us: farmers who plant a whole field with one crop earn $27,000, while those that left 1/3 of the field unplanted for bees to forage and nest in earn $65,0009.

extinction
Not the bee’s knees.

Chytridomyosis

Amphibian populations are collapsing around the world. The current extinction crisis affects about 50% of amphibian species. The declines are related to a number of issues, including habitat loss, climate change, and chytridomyosis1. Chytridomyosis is caused by the fungus Batrachochytrium dendrobatidis, which colonizes amphibian skin and keratin10. How exactly it causes illness is still unclear, but its effect is undeniable; chytridomyosis epidemics have led to the total extinction of several amphibian species in the wild. The disease is a generalist, infecting over 350 species, the widest known host range of any pathogen, and has spread to every continent except Antarctica1.

The effect of chytridomyosis is difficult to predict, as susceptibility to the disease varies between species, and even between populations of the same species10. Life history, especially how aquatic the species is, plays a major role, as the fungus is transmitted by water4. Environmental factors, such as temperature and altitude, affect the growth of the fungus, making them critical to disease spread. Humans have also played a role, for good and ill; trade of infected amphibians has helped spread chytridomyosis around the world1, but humanity has also put conservation efforts into full swing. Myriad approaches are being explored, including captive breeding programs, such as the Amphibian Ark Program, treating habitats and amphibians in the wild, vaccination, and species reintroductions1,10.

Sudden Aspen Decline

Sudden Aspen Decline, which has the tragically appropriate acronym SAD, is a condition that kills more than 90% of trees in a stand within 3 to 6 years with little to no regeneration5. Aspens are clonal, meaning that they share an underground root network and all the trees in a stand are part of one giant organism (they are possibly the largest living things on Earth), so a disease affecting one tree could potentially afflict the whole stand. The impact of the disease varies across regions and aspen clones2, but currently affects wide swaths of aspen forests in the American west, including approximately 17% of aspen-dominated forest in Colorado7.

SAD isn’t well understood, but it is thought to be caused by a whole consortium of factors, including pollution, drought, defoliation by pests, and pathogens2. Nonetheless SAD is definitely reshaping aspen forests; sick clones experience branch dieback, and lose canopy cover and root mass, radically changing conditions on the forest floor. These shifts could mean trouble for human health; in Coloradan forests where SAD has been particularly destructive, the small mammal community is less diverse and there is an increase in the prevalence of Sin Nombre Virus (a particularly virulent hantavirus)5.

No rest for the weary

Humanity is suffering from crisis fatigue; the world seems too full of problems to face6. And yet. They won’t disappear if we give up. They will multiply. They will return, even more daunting, if we cast them aside. We can’t turn away from the complexity of the world; we have to embrace it, explore it, and, ideally, understand it. That is our greatest hope. We may be tired, but we can’t afford to rest.

References

1. Fisher, MC, TWJ Garner, & SF Walker. (2009). Global emergence of Batrachochytrium dendrobatidis and amphibian chytridomyosis in space, time, and host. Annual Review of Microbiology, 63: 291-310.

2. Frey, BR, VJ Lieffers, EH Hogg, & SM Landhäusser. (2004). Predicting landscape patterns of aspen dieback: mechanisms and knowledge gaps. Canadian Journal of Forest Research, 34:1379-1390.

3. González-Varo, JP, JC Biesmeijer, R Bommarco, SG Potts, O Schweiger, HG Smith, I Steffan-Dewenter, H Szentgyörgyi, M Woyciechowski, & M Vilà. (2013). Combined effects of global change pressures on animal-mediated pollination. Trends in Ecology & Evolution, 28(9):524-530.

4. Kilpatrick, AM, CJ Briggs, & P Daszak. (2009). The ecology and impact of chytridomyosis: an emerging disease of amphibians. Trends in Ecology and Evolution, 25(2):109-118.

5. Lehmer, EM, J Korb, S Bombaci, N McLean, J Ghachu, L Hart, A Kelly, E Jara-Molinar, C O’Brien, & K Wright. (2012). The interplay of plant and animal disease in a changing landscape: the role of sudden aspen decline in moderating sin nombre virus in natural deer mouse populations. Ecohealth, 9:205-216.

6. Redford, K, & MA Sanjayan. (2003). Retiring Cassandra. Conservation Biology, 17(6):1473-1474.

7. Sturrock, RN, SJ Frankel, AV Brown, PE Hennon, JT Kliejunas, KJ Lewis, JJ Worrall, & AJ Woods. (2011). Climate change and forest disease. Plant Pathology, 60:133-149.

8. van der Sluijs, J, N Simon-Delso, D Goulson, L Maxim, JM Bonmatin, & LP Belzunces. (2013). Neonicotinoids, bee disorders and the sustainability of pollinator services. Environmental Sustainability, 5:293-305.

9. Winston, M. “Our bees, our selves: Bees and colony collapse”. The New York Times. 15 July 2014. A25. NYTimes.com. Web. 15 July 2014.

10. Woodhams, DC, J Bosch, CJ Briggs, S Cashins, LR Davis, A Lauer, E Muths, R Puschendorf, BR Schmidt, B Sheafor, & J Voyles. (2011). Mitigating amphibian disease: strategies to maintain wild populations and control chytridomyosis. Frontiers in Zoology, 8:8.

Image source: Creative Commons, http://yourownfood.org/2013/05/most-pesticide-heavy-fruits-and-vegetables/

White Nose Syndrome: Winter is Coming

Aliases: white-nose syndrome, WNS

Unless they directly endanger humans (i.e. avian flu), wildlife diseases don’t often get much press. This is shortsighted; even if we are immune to the illness, our welfare is intimately linked to that other animals. White-nose syndrome (WNS), a fungal disease that is decimating bat populations in the northeastern US and Canada, is an excellent example of the wide-ranging effects of wildlife diseases.

Since its discovery in eastern New York State in 20072, confirmed cases of WNS have been found in 25 US states and 5 Canadian provinces3, reaching as far south as Mississippi2. The fungus that causes WNS, aptly named Pseudogymnoascus destructans, thrives cold and humid environments, such as caves and mines, where bats commonly hibernate2 (these areas are called hibernacula). Consequently, hibernating bat species are the most affected by WNS. To date, WNS has killed more than 5.7 million bats in northeastern North America1, and has caused bat population declines of approximately 80% in the northeastern US3.

But hibernacula aren’t the only things that WNS will leave barren. WNS typically affects long-lived bat species, which only produce a single offspring (aka pup) per year. This means that the hard-hit bat populations will be slow to recover and build back up to their former size. The severe drop in bat numbers will not only affect natural ecosystems, but also profoundly impact agriculture. Bats are fantastic insect predators, and they are the biggest consumers of many agricultural pests. In fact, bats provide an estimated $4-50 billion worth of insect pest suppression a year3, so their absence could affect everything from crop yield to food prices.

white nose syndrome
Killer cuddling.

Cause: WNS is caused by P. destructans infection in the muzzle, ears, tail and wings of bats2,3. Although the transmission of WNS is not well understood, it is thought to be transmitted directly from bat to bat during hibernation, and possibly by humans who carry the fungus out of infected hibernacula on clothing and equipment. There’s good, bad, and really bad news about WNS’s range. The good news is that of the more than 20 hibernating bat species inhabiting the US and Canada, only 7 species have had confirmed cases of WNS2. The bad news is that two of the affected species are endangered and three more have been detected carrying the fungus, though they had not contracted WNS2. The really bad news is that P. destructans may be a generalist fungus, so all bat species within its range may be at risk5.

Consequence: P. destructans infection causes skin erosion and may result in white fungal growth, though it doesn’t always. It may also result in abnormal behavior during hibernation, such as daytime winter flights, when temperatures are at or below freezing, movement toward and clustering near hibernacula entrances, and death2,3. These behaviors may deplete the affected bat’s fat stores and cause emaciation3. Although mortality rates vary by site and species, it can be as high as 100%2.

Cure: While research exploring possible treatments is underway4, there is currently no cure for WNS. In the meantime, the US Fish and Wildlife Service is trying to combat its spread; it has put a voluntary moratorium on caving in infected states2.

References

1. About white-nose syndrome. White-Nose Syndrome.org Web. 7 July 2014. https://www.whitenosesyndrome.org/about-white-nose-syndrome

2. Frequently Asked Questions. White-Nose Syndrome.org Web. 6 July 2014. https://www.whitenosesyndrome.org/faqs

3. White-nose syndrome. National Wildlife Health Center. 29 May 2014. Web. 6 July 2014. http://www.nwhc.usgs.gov/disease_information/white-nose_syndrome/

4. White-nose syndrome. US Fish & Wildlife Service. June 2014. Web. 7 July 2014. https://www.whitenosesyndrome.org/sites/default/files/resource/white-nose_fact_sheet_6-2014_1.pdf

5. Zukal, J, H Bandouchova, T Bartonicka, H Berkova, V Brack, J Brichta, M Dolinay, KS Jaron, V Kovacova, M Kovarik, N Martínková, K Ondracek, Z Rehak, GG Turner, & J Pikula. White-nose syndrome fungus: a generalist pathogen of hibernating bats. PLOS One, 9(5), e97224.

Image source: Wikimedia Commons, http://commons.wikimedia.org/wiki/File:Hibernating_bats_(5600306079).jpg