The value of a species: the whooping crane conservation paradox
This post belongs to a series written by students in the Conservation Biology course BSC4052 at the University of South Florida. This course provides an overview of major themes in conservation practice and related applied problems in biology, including: population ecology in the context of conservation, patterns of diversity, valuing diversity, threats to diversity, management actions and strategies for preserving diversity.
Author: Lorna May Brown is pursuing a Bachelor of Science in Integrative Animal Biology from the University of South Florida. Lorna was born in Spain and raised in England and is an avid traveler and a huge animal enthusiast. She looks forward to completing a Masters in the future, and hopes to pursue a fruitful career in wildlife conservation.
Original Article: Adler PH, Barzen J, Gray E, Lacy A, Urbanek RP, Converse SJ. 2018. The dilemma of pest suppression in the conservation of endangered species. Conservation Biology 33: 788-796. doi: https://doi.org/10.1111/cobi.13262
The endangered whooping crane is one of the world’s rarest species, with only around 600 individuals, including one wild self-sustaining population (French et al., 2018). Reintroduced populations have had limited success, largely due to low hatching success. One multi-year study demonstrates the role that some endemic species of black flies play in whooping crane nest desertion. This work also illustrates how conservation approaches should evaluate trade-offs and utilize a decision-analysis framework to construct management strategies that incorporate the needs of all endemic species, rather than pinning the value of one species above the other.
Hurdle to successful reintroduction
Originally occupying much of North America, the whooping crane (Grus americana) was listed as endangered in the 1970s in both the US and Canada. Over the past few decades, biologists have attempted to reintroduce self-sustaining populations with limited success. Among the reintroduced populations, the Eastern Migratory Population is one of the most successful. This population mostly inhabits areas east of the Necedah National Wildlife Refuge. However, this population has had low hatching success; with over 86% of the Eastern Migratory Population’s whooping crane nests failing (i.e. leading to egg mortality) between 2005-2010 (Urbanek et al., 2011). The reasons for this are unclear, but a team of biologists recently proposed that endemic populations of black flies in Wisconsin, such as S. annulus and S. johannseni, could swarm crane nests, which would drive the birds off nests during their incubation time.
Do flies reduce whooping crane hatching success?
Peter Alder and colleagues (2018) compared the abundance of black flies in Necedah National Wildlife Refuge to elsewhere in Wisconsin. They found a remarkably high number of adult S. annulus and S. johannseni black flies in Necedah National Wildlife Refuge, where the abundance of the whooping crane is also highest.
To test if flies indeed affected nest desertion, the research team introduced a naturally-occurring bacterium, Bacillus thuringiensis israelensis (Bti), which attacks larval black flies but has no adverse effect on other species in the ecosystem. The introduction of Bti during 2011 and 2012 led to >94% mortality of the black flies. The decrease in fly abundance was associated with a doubling of nest survival rates during these years (Barzen et al., 2018). Following the experiment, fly abundance increased and whopping crane hatchling survival decreased again.
Pests have their own ecological importance
While these endemic black fly species seem to have a drastic impact on the continued success of the whooping crane Eastern Migratory Population, they also play an important role in the ecological functioning of their ecosystems. For instance, the fecal pellets of black flies serve as food for primary consumers, including many aquatic organisms. The flies also serve as food for many predators including many amphibians and reptiles. Black flies also have an ecological function as blood-feeders, swarmers, and even disease transmitters, which are important population regulators.
Pitting the value of one species against the other
There are many options one could take to address the concerns of whooping crane hatching success. This could include: 1) “doing nothing”; 2) using Bti treatments to suppress black fly populations; 3) forced re-nesting (whereby the first clutch is removed and reared in captivity, thereby encouraging cranes to replace the clutch after the flight period of black flies); 4) release and relocation (whereby cranes are removed from their original habitat to a new, more favorable habitat); or 5) a combination of strategies. These strategies vary in cost and in which species they predominantly benefit (cranes or flies). Therefore, which strategy is best, depends on objectives and budget. For instance, a combination of forced re-nesting & release relocation/Bti treatment favored crane hatching success since new nests would hatch within Necedah National Wildlife Refuge, in captivity, and at relocated areas. However, crane welfare was highest with Bti treatment followed by release relocation since these methods decreased fly harassment and increased incubation time. On the other hand, “do nothing” was the most cost effective, but would favor black flies at the expense of cranes.
Statistical models may pave future conservation strategies
Ultimately, choosing what strategy to implement is a complex and often philosophical question. However, Alder and colleagues (2018) argue that before selecting conservation objectives and management strategies, one must have a good understanding of how different strategies would affect an ecosystem. For this reason, a multi-criteria decision analysis approach was proposed, where the trade-offs of each strategy are mathematically evaluated case-by-case to frame a conservation strategy which best satisfies the chosen objectives. This way, there is now an existing transferrable framework, so management actions are selected in a way that maximizes effectiveness for specific objectives. Such a complex model requires a lot of data which must be obtained through continued monitoring and research. However, through these statistical models, better management actions could be engineered which take into consideration a multitude of complex species interactions in ecosystems such as the one seen between whooping cranes and black flies in the Eastern Migratory Population.
Adaptive management strategies through statistical models are incredibly promising, but arguably have a long way to go before being widely implemented in the conservation of target species. However, this type of study highlights how convoluted interspecies relationships can be in ecosystems and serve as important examples of why conservation is perhaps more complicated than we imagine. For this reason, it is important to learn about ecosystem dynamics and how even small, seemingly unrelated actions can have domino effects. This is important now more than ever with human interference, as evidenced in many studies including by Gian-Reto Walther and his colleagues in their 2002 study ‘Ecological Responses to Recent Climate Change’, and even in marine ecosystems as discussed by Ove Hoegh-Guldberg and John F. Bruno in their 2010 article ‘The Impact of Climate Change on the World’s Marine Ecosystems’.
Feature image Female (Left) and male (Right) whooping cranes. Image credit: Ronald Diel – Own Work. CC-BY-SA-3.0, from Wikipedia Commons. URL: https://commons.wikimedia.org/wiki/File:Whooping_Crane_Nest.jpg
Adler PH, Barzen J, Gray E, Lacy A, Urbanek RP, Converse SJ. 2018. The dilemma of pest suppression in the conservation of endangered species. Conservation Biology 33: 788-796. https://doi.org/10.1111/cobi.13262
Barzen JA, Converse SJ, Adler PH, Lacy A, Gray E, Gossens A. 2018. Examination of multiple working hypotheses to address reproductive failure in reintroduced Whooping Cranes. The Condor 120:632–649. https://doi.org/10.1650/CONDOR-17-263.1
Hoegh-Guldberg O & Bruno JF. 2010. The impact of climate change on the world’s marine ecosystems. Science, 328(5985), 1523-1528. URL: https://science.sciencemag.org/content/328/5985/1523
The IUCN Red List of Threatened Species. (n.d.). Available from https://www.iucnredlist.org/species/22692156/110390029#population (accessed February 20, 2019). URL: https://www.iucnredlist.org/species/22692156/110390029
Urbanek RP, Zimorski SE, Szyszkoski EK, Wellington MM. 2011. Ten-year status of the eastern migratory whooping crane reintroduction. Proceedings of the North American Crane Workshop 12:33–42. http://www.nacwg.org/Proceedings%2013.pdf
Walther GR, Post E, Convey P, Menzel A, Parmesan C, Beebee TJ, Fromentin JM, Hoegh-Guldberg O, and Bairlein F. 2002. Ecological responses to recent climate change. Nature, 416(6879), p.389. doi:10.1038/416389a