The devil is in the details: managing small populations to combat disease-led decline

Following the recent post by Billie Lazenby, Ellery McNaughton & Associate Editor Margaret Stanley share their thoughts on the article, Density trends and demographic signals uncover the long-term impact of transmissible cancer in Tasmanian devils and the importance of focusing on external threats to Tasmanian devils.

New research by Lazenby et al. highlights the important interplay between disease ecology and population dynamics in informing conservation management. Wild populations of the Tasmanian devil have managed to persist in the face of a fatal and contagious disease that puts them at risk of local extinction. Given the current absence of treatment for the disease in wild populations, Lazenby et al. suggest that managing the wild populations themselves, rather than the disease per se, may be the only way to ensure their continued survival.

Tasmanian_Devil_Facial_Tumour_Disease - Margaret Stanley
Tasmanian devil facial tumour disease (Photo: Menna Jones)

The Tasmanian devil is a medium-sized marsupial endemic to the Australian state of Tasmania. At present, the devil is endangered by the prevalence of Devil Facial Tumour Disease (DFTD), a transmissible and fatal cancer. Since its discovery in the 1990s the disease has ravaged wild devil populations, to the point where local extinctions were predicted to occur within a few decades. Such dire predictions were partially tempered by evidence of precocial breeding and potential development of genetic resistance in devils, but the extent to which this offset long-term population decline was unknown. Lazenby et al. endeavoured to provide a much-needed clearer picture of devil population dynamics over time through utilising a long-term data set of spotlight counts and spatial capture-recapture (SCR). What they found is concerning, and yet also reveals some cause for encouragement.

The bad news

The bad news first – Lazenby et al. estimate that DFTD is now present in over 80% of mainland Tasmania. Additionally, they have recorded drastic declines of 77% in devil populations since the emergence of DFTD in these populations. Despite these declines, the prevalence of DFTD within infected populations was not found to decrease with density, which confirms what has previously been put forth; that DFTD transmission is independent of the number of individual devil hosts. Small populations of devils are therefore not guaranteed relief from disease transmission simply by having low densities of individuals. Yet despite this, Lazenby et al. have recorded devil populations persisting, not succumbing to local extinction. Density-dependent disease transmission is not propping up small devil populations, so what is?

Zoo devil smaller - Margaret Stanley
Tasmanian devil in zoo as part of captive breeding programme (Photo: Ellery McNaughton)

The good news

This brings us to the good news – precocial breeding (females breeding at an earlier age) does indeed seem to be providing an offset to DFTD driven population decline. Lazenby et al. also found an additional method of population compensation, with females having more pouch young on average in sites affected by DFTD. Encouraging though this is, it seems the offset that this increased fecundity provides is only enough for persistence in the face of the disease, not for population recovery. To use a metaphor, if the devils’ clock is counting down to local extinction, then rather than stopping the clock altogether, increased fecundity just keeps adding extra seconds. Devil populations are therefore fragile, and any outside threat may well be the straw that breaks the devil’s back, leading to a decline that cannot be offset enough to avoid local extinction.

It is these external threats that Lazenby et al. suggest focusing conservation management on. Despite recent promising advances in treatment of DFTD, at present there is no way to effectively manage the disease in the wild. In the absence of a treatment, the most effective way of managing the devils is to aid in the persistence of small populations by focusing on non-disease related threats. Reducing secondary causes of mortality, such as road deaths, gives the increased fecundity of diseased devil populations the best chance at success in offsetting population decline. Managing the genetic diversity of meta-populations through translocations may increase the likelihood of a population’s persistence, and their resilience to stochastic events. These threats aren’t the most pressing or dangerous to devil populations, but by addressing them managers buy themselves (and the devils) time to deal with the larger threat of DFTD.

The plight of the Tasmanian devils as laid out in Lazenby et al.’s paper is an interesting one for disease ecology and conservation management. It highlights a situation where the best management of wild populations may actually be to ignore the disease until the tools are developed to address it directly, and instead focus on strengthening and supporting small populations; in so doing, it is hoped that population compensation will be as effective as possible in ensuring the persistence of the population. As with most things, this is probably only a temporary answer, but it may be the best stop-gap measure we have in the face of such a virulent disease.

Read the full article, Density trends and demographic signals uncover the long-term impact of transmissible cancer in Tasmanian devils.

Read more from the authors of the paper here.

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