Today we had an interesting seminar with Julie Lockwood from Rutgers University. Her talk title was Killing the cuddly: Tactics for managing exotic predators to protect their native prey. Her talk was divided into two sections the first dealt with the importance of model choice in providing guidance to managers and the second part dealt with attempts to understand population growth in invasive species.
Importance of modeling
She motivated the first part of her talk with an empirical example from Rayner et al 2007. This manuscript looks at the breeding success of Cook’s petrels under three conditions first prior to any work being done to remove predators so that both cats and rats were present (success=.25-.50), then after cats had been removed but rats were still present (success=0.0-.25), and then finally once both had been removed (success=.50-.75). Here is the graph that she showed from that paper:
She then explained that most model informed management decisions that are currently made use models that only examine the predator population and look for the life stage that can be targeted to most quickly reduce its population. She suggests instead we need to take a more nuanced approach with a more complex model and use the target of keeping the prey alive rather than dropping the population of predator as quickly as possible.
She then described work that developed a model that allowed the predator to effect the prey population size. This model can accommodate biological realities such as the fact that often juvenile predators can’t eat adult prey or that only certain stages of prey are targeted at all. Once this model was developed they did a series of full block experiments with short, medium, or long lived predators and prey for a total of 9 pairings. The general results of this were that particularly in the case of long lived predators traditional approaches and her recommended approach found different optimal solutions.
- my biggest question from this part of the talk was: Can we wrap in two predators and get the "right" solution to the motivating example of Cook's petrels?
The next part of her talk was focused on population lag. This is a well documented characteristic particularly in invasive plants. It is characterized by a species being present at low density for an extended period of time and then at some point suddenly exhibiting a great increase in population size. In plants it appears that population lag can range from as little as a decade to as long as a century or more. The data available for vertebrates though was less clear. Primarily because there are not large numbers of vertebrates that have been introduced with abundant data on population size over time. However, Hawaiian birds offer the opportunity to study this phenomenon. Her team used the Audubon Christmas bird count data to look at population dynamics of 54 invasive species in Hawaii.
I’ll be honest this part of the talk got a little fuzzy for me. The data are very noisy and so apparently some method was used to define a maximum population size and data after this point were discarded (I believe). The remaining data were fit with a number of models 1) simple linear, 2) log 3) two piece linear. For the two piece linear basically every point between the 6th year and the 5th year prior to peak population was tried as the break and the break that resulted in the highest likelihood was kept. If the single break model was better than the alternatives then that was evidence for population lag in that species. Her results indicated that most species did exhibit a population lag.
- my biggest question/comment from this part of the talk was: Seems like one of those places where we need to develop or assess model adequacy, I could picture the best model here being too poor to reveal biologically important characteristics.
All in all this was a really interesting talk. My final take home from the talk was that most invasive species provide us with a long period of time when population grows slowly and during this time careful modeling can provide us with the best chance of having a successful attempt in controlling an invasive species. In addition even if we miss this low population window populations are stochastic and so perhaps we should be prepared to attack an invasive when it experiences a natural fluctuation.
Here are links to the Lockwood lab and the pages of her two former students who did much of this work. My explanations above are no doubt over simplifications and contain errors which are purely my own. If this stuff really interests you check out the publications below which contain the research I described above.
Aagaard, K. and J. L. Lockwood. 2014. Exotic birdsshow lags in population growth. Diversity and Distributions. 20, 547-554.
Robinson, O.J., N.H. Fefferman, and J.L. Lockwood. (2013). How to effectively manage invasive predators to protect their native prey. Biological Conservation. 165: 142-156.