Is host tolerance to pathogens and herbivores is more beneficial than resistance in reintroduced populations?

In just about every set of reintroduction guidelines I’ve ever read, one of the primary recommendations is always to eradicate the threats that caused the extirpation of populations of the target species. However, dealing with threats that have an extensive impact are often impossible to eradicate or limit to a specific location.  In the paper summarised below, Matthew Venesky and his co-authors (Venesky et al. 2012) examine several lines of evidence to look at the incidence of pathogens and herbivores and how reintroductions can be optimised to cope with a threat that can’t easily be controlled.

The paper relies on three concepts to make their argument that particular traits are key to successful translocation of species threatened by non-native pathogens or herbivores: virulence, tolerance and resistance. Virulence is defined "as the per capita effects of a pathogen or herbivore". Host tolerance is expressed as the ability to withstand an attack with little loss of fitness. Resistance refers to the reduction of pathogen or herbivore impact through deterring infection or herbivory, or attacking the pests directly. Tolerance is thought to have a neutral or positive consequence for pest abundance whereas resistance has a negative impact on pathogen or herbivore abundance.

The main thesis is that the generation time of pathogens and herbivores (especially invertebrates) is shorter than that of their hosts and can therefore be subject to selection pressures that are exerted as a result of the negative influence of host resistance. This creates pest populations that evolve countermeasures against resistance mechanisms; the lag in the host response means that reintroduced populations suffer high mortality before developing adequate resistance. Hosts which exhibit tolerance rather than resistance do not place strong selection pressures causing increased virulence, and in some cases may even select for decreased impacts. As a result, captive breeding that selects for tolerant (rather than resistant) individuals for translocation, may maximise the chance that a reintroduction attempt will survive long enough to produce progeny.

The two case studies use examples of non-native organisms to explore how captive breeding might select for tolerance rather than resistance to cope with pathogens and herbivory. The first is the fungal pathogen Batrachochytrium dendrobatidis (Bd) that causes chytridiomycosis and has decimated amphibian populations across the world. Selection for resistance involves several suggested approaches including identifying indicators of infection but minimal loss in fitness.  The second example is the cactus moth (Cactoblastis cactorum) which was succesfully introduced to Australia to reduce the prevalence of non-native prickly pear (Opuntia spp.). Unfortunately, the cactus moth has exhibited similar voracity against two narrow endemic species of cactus in Florida. Selection for tolerant genotypes might involve identifying individuals that drop pads from the main plant when stressed by moth herbivory.

The final section of the paper adds some important caveats to the discussion that shifts in host tolerance may have unexpected consequences such as a trade-off for competitive abilities of the host and the existence of low levels of pathogens and herbivores that could act as a reservoir for invading non-tolerant communities. As a result, Venesky et al. (2012) recommend adaptive managment strategies and using an experimental approach to compare the survival of resistant and tolerant genotypes post translocation.

Venesky, M. D., Mendelson III, J. R., Sears, B. F., Stiling, P., & Rohr, J. R. (2012). Selecting for Tolerance against Pathogens and Herbivores to Enhance Success of Reintroduction and Translocation. Conservation biology, 26(4), 586–592. doi:10.1111/j.1523-1739.2012.01854.x

Decision tools for reintroduction but who actually uses them?

My post today was going to be a summary of a paper by Adam Schapaugh and Andrew Tyre of the University of Nebraska-Lincoln on Markov decision processes (MDPs). These enable conservation decision-making by dictating what action should be taken based on the state a system is in, and incorporates a reward for having taken the action. Schapaugh and Tyre have used as one of their examples, an hypothetical reintroduction to demonstrate this. MDPs require the user to describe the state variables, set what actions are associated with all the combinations of state variables, and construct a reward system that means the actions can be optimised to create the best outcome. In the paper this means that a set of state variables (e.g. source population size) affect which actions (e.g. capture and release) are undertaken to produce a target population of a given size through reintroduction.


Schapaugh and Tyre present a way of selecting the most relevant variables to include in order to make the best informed choices using MDPs. Their algorithm chooses the actions that maximises rewards where the rewards are set by the user to match their targets (e.g. creating a population of the target species). The actions which reap largest rewards are selected first; the actions which provide negligible or no reward are removed from the process. There is obvious application to species recovery programmes as a decision tool for when and how to act and this paper seems to present an approach for streamlining this potentially complex process.

I wish I was in a position to comment on the utility of their approach and critique it in a way that would be useful to conservation translocations. However, I am new to MDPs and instead would like to pose the question: who uses MDPs in the real world? This then leads me to ask: how do they (the practitioners) make the jump from algorithms in a paper to policy and implementation? And finally, how long does it take for advances such as the one presented in this paper to be absorbed into practice?

Feel free to comment - the questions above are not rhetorical and are a genuine attempt to understand the use of MDPs in relation to translocations.

Schapaugh, A. W., & Tyre, A. J. (2012). A simple method for dealing with large state spaces. Methods in Ecology and Evolution. doi:10.1111/j.2041-210X.2012.00242.x

Translocation implications for the song repertoire of the kōkako

The North Island Kōkako, Callaeas cinerea, use song in year-round territory defence and strengthening monogamous pair bonds.  Sandra Valderrama and co-workers described song repertoire in six natural populations, and two translocated populations on New Zealand's North Island to describe how population size affected this important behaviour.

They found that pairs in smaller populations have lower song diversity and higher shared song phrases than larger populations.  In many cases translocated populations are very small relative to the size of natural populations and this study was no exception – the two translocated populations consisted of only 18 and 20 individuals. Higher numbers of founding members may be helpful in accelerating population growth through more efficient pair formation and territory establishment and defence.

Their findings also have implications for selecting individuals from natural populations for translocation.  Translocations using individuals from multiple donor populations may result in individuals from smaller populations being at a disadvantage due to a smaller song repertoire and therefore, reduced ability to find mates. This may have knock-on effects for the genetic mixing of individuals from different donor populations – if the birds from smaller populations cannot find a mate due to a lack of the right ‘vocabulary’, their genes will not be represented in the newly created population.

Valderrama, S. V., Molles, L. E., & Waas, J. R. (2012). Effects of Population Size on Singing Behavior of a Rare Duetting Songbird. Conservation Biology, no–no. doi:10.1111/j.1523-1739.2012.01917.x

First ever 'Reintroduction Biologist' - heralding a new era for the sub-discipline?

Today I read an article on the Chicago Tribune website that named a member of staff at Lincoln Park Zoo as the first ever person to have 'reintroduction biologist' as their job title (http://articles.chicagotribune.com/2012-09-05/entertainment/ct-ent-0906-green-snakes-20120905_1_snakes-lincoln-park-zoo-herpetology).  Given that Phil Seddon, Doug Armstrong and Richard Maloney wrote "there is therefore now a recognizable field of reintroduction biology" only five years ago (Seddon et al. 2007), this represents quite a development and a milestone the translocation community should be aware of.

Hopefully we'll see more people join Allison Sacerdote, the Lincoln Park Zoo employee, as the profession develops.  Perhaps more importantly, we'll see increasing numbers of employees of zoos, botanic gardens and statutory agencies who adopt an 'experimental' approach called for by Seddon et al. (2007) and adopted by Sacerdote. In the featured reintroduction project, she will be comparing 'hard' and 'soft' release techniques in order to develop effective protocols for smooth green snakes in Illinois.

Seddon, P. J., Armstrong, D. P., & Maloney, R. F. (2007). Developing the science of reintroduction biology. Conservation biology : the journal of the Society for Conservation Biology, 21(2), 303–12. doi:10.1111/j.1523-1739.2006.00627.x

An introduction of the non-conservation kind

An introduction of the non-conservation kind

My name is Sarah Dalrymple and I'm a conservation ecologist working in Northern England, UK.  I have a background in plant ecology and after attempting a reintroduction of small cow-wheat (Melampyrum sylvaticum) I became interested in conservation translocations and whether they really could be a viable tool for reversing the decline of threatened species.

I have recently been part of the International Union for the Conservation of Nature (IUCN) Task Force charged with revising their Guidelines for Reintroduction and Other Conservation Translocations.  The membership reflected the double edge of deliberate movements of plants and animals being drawn from both the IUCN Re-introductions Specialist Group and the Invasive Species Specialist Group.

My current research is trying to identify examples of translocations both within and beyond a species' indigenous range in order to assess the effectiveness of interventions such as assisted colonisation when compared to within-range translocations such as reintroduction.  (If I've already confused you with the terminology, please see the definitions page.) I am building a database of conservation translocations and offering to assess the habitat suitability of translocations for practitioners who are willing to share their data - please see the project page for more details.

Finally, if you would like me to blog about any aspect of your translocation work, feel free to contact me:
s.e.dalrymple@gmail.com.

Looking forward to working with you,
Sarah