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.
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.
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/