Friday, 3 January 2014

A new year - time to catch-up on last year's F1000 reviews!

I've had a very busy few months trying to get more of my own work out there in the scientific literature that I've had little time for much else.  However, I have continued to write reviews for F1000, but haven't found time until now to update this blog.  So, please read on for the reviews we did in the second half of 2013!

Back in May(!) we wrote a review on a paper that caught my eye, mainly because I'm quite a fan of trees and I really like the idea of trying to prove 'The relationship between trees and human health' by Donovan GH, Butry DT, Michael YL, Prestemon JP, Liebhold AM, Gatziolis D and Mao M (2013) American Journal of Preventative Medicine 44(2) 139-145.  The authors use the case study of the spread of Emerald Ash borer, a major invasive pest in the USA, to try to establish the importance of the loss of trees due to the insect damage in terms of human health impacts.

Our review: 

A unique combination of ecological, geographical and public health data from across the USA is analysed in order to reach a sobering conclusion: the loss of trees due to the invasive insect pest Emerald Ash Borer (Agrilus planipennis) has had a measurable impact on human mortality rates due to cardiovascular and lower respiratory tract illness in 15 US states from 1990-2007. Given that the pest has effectively only just begun its onslaught (although 100 million ash trees have already been killed, there are 7.5 billion potential victims), this should be a major cause for concern. The mechanism is unclear because this was an observational study, but could be related to factors such as reduced air quality with declining tree canopy or reduced recreational activity in parks with dead trees.

Understanding impacts of biological invasions is a current challenge, as impacts are often difficult to evaluate, uncertain and delayed {1}. This study shows that, when the environmental impact and loss of species due to a biological invasion is extreme and widespread, then such loss may indirectly have serious consequences for human health.

1. Impacts of biological invasions: what's what and the way forward. Simberloff D, Martin JL, Genovesi P, Maris V, Wardle DA, Aronson J, Courchamp F, Galil B, García-Berthou E, Pascal M, Pyšek P, Sousa R, Tabacchi E, Vilà M. Trends Ecol Evol 2013 Jan; 28(1):58-66 PMID: 22889499 DOI: 10.1016/j.tree.2012.07.013

In August we came across a paper in Ecology, that used a meta-analysis approach to answer a big question: 'Do invasive species perform better in their new ranges?' JD Parker et al. 2013 Ecology 94 985–994.

Our review:

This is a major study, using a rigorous meta-analysis to provide a very rare test of one of the assumptions of invasion biology - that invasive species perform better in their introduced range than the native range. The study is very valuable because it is surprising how rarely this assumption has been tested – indeed, the authors point out that they were not able to find comparative data for 60% of species labelled as the world’s worst invaders. They found that while half of the species tested were indeed performing better in their introduced ranges, for the other half, there was no obvious difference.

In truth, the idea that the impact of invasive species is felt through their size, greater fecundity, or greater abundance is just part of the picture: some species have an impact by simply introducing a novel life-form into a system in sufficient abundance (not necessarily greater than that in their native range) to make a difference to ecosystem function in the new range (e.g. Gamba grass in northern Australia, which has modified the fire regime to one of later, hotter fires {1}). It is also worth noting that the graphical presentation in the paper (as natural logarithms of the introduced range performance over the native range performance) visually underplays some of the substantial differences in performance shown by some of the species here.

The authors rightly call for more international research networks and partnerships to build data on invasive species from the native range. This is long overdue.

1.Testing the grass-fire cycle: alien grass invasion in the tropical savannas of northern Australia Rossiter N, Setterfield S, Douglas M, Hutley L. Divers Distrib 2003 May; 9(3):169-176 DOI: 10.1046/j.1472-4642.2003.00020.x

Our latest review, submitted in November, is a modelling paper: we don't often review modelling papers (which is surprising, given my role as an Ecological Modeller)!  I soon realised why, its much harder to distill the complexity of the methods!! Hopefully we succeeded with this one, certainly its a very interesting approach: Agent-based Bayesian approach to monitoring the progress of invasive species eradication programs Keith, JM and Spring, D (2013) PNAS 110(3) doi: 10.1073/pnas.1216146110 

Our review:
This paper combines agent-based simulation with Bayesian inference to develop a model that not only would allow control efforts to be targeted in space but also show how the management strategy should adapt over time, as control measures take effect on the invasion trajectory in space and time.

Developing a simulation model of an invasive species incursion is a difficult task: dispersal processes act at multiple temporal and spatial scales and we are often faced with a multitude of constraints including a short timeframe, limited data availability or knowledge, high uncertainty and of course, budget (for a recent overview see Parry et al. {1}). The use of an agent-based approach allows for the model to capture more detail than a commonly used grid-based approach, and this proves critical to understanding the persistence of the outbreak of imported fire ants in Brisbane. These details include capturing the effects of long-distance founding events and simulating explicitly the formation of individual nests.

The use of Bayesian inference combined with a Markov chain Monte Carlo method allows the authors to construct a likelihood model that incorporates a range of unknown parameters including the number and location of undetected nests during the study period and the phylogeny of the nests, and the authors test the importance of prior knowledge of these, which overall they find not to be very important to the trend of the estimated invasion trajectory besides knowledge of the recent decline in fire ant nest numbers.

The application of this method enabled the authors to determine which aspect of the eradication strategy had a strong influence on the success of eradication (killing immature nests before they reach maturity) and also whether the current management strategy is likely to succeed. However, although the authors are confident that their spatial model shows the range of the invasion has never declined even under management, they advise some caution: uncertainty increases over time and so spatial projections over a long time frame may show an expanding invasion front where there may be none (it is simply that uncertainty increases). Thus, density 'heat maps' should be used with caution beyond inferring current distributions. Overall, the results of the model show that a moderate increase in early treatment during the initial stages of the invasion would have contained it to a much smaller area with a much greater chance of eradication, and the model can now be used to inform ongoing eradication decisions.

1. Practical guidelines for modelling post-entry spread in invasion ecology Parry H, Sadler R, Kriticos D. NEOBIOTA18:41-66 DOI: 10.3897/neobiota.18.4305

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