Do You Manage Your Invasives With That Model?
- Once a species has invaded, it’s hard to make it disappear again. Therefore, researchers and managers are always looking for ways to manage without micromanaging: they look for patterns in the growth or spread of species that might indicate a threshold of manageability. A well-known case here in Hawaii is the introduction of decorator urchins to fields of invasive algae that have been removed mostly by hand (Stimson et al. 2007). The cost of removing that much algae every month is astronomically high. But urchins may be able to nibble away the rest of the invasive limu once big stands have been manually removed (Smith et al. 2008, response to Goreau). This method, introducing a predator that might manage an invasive species, is termed “Biological Control,” and it is very popular (it’s also highly controversial, though that is a story for another day).
Eradicating or even managing invasives requires some idea of how and why the population got so big in the first place, how it might die out, and whether it will come back. Should we isolate small populations from breeding with each other, thus letting each little population die out? Or will that merely encourage them to grow, since now each subpopulation will have more resources to exploit? In the second case, we might even want to corral them all into the same location, letting one big group compete for resources until their high density causes crowding, competition, and eventual population decline. The first method has been performed experimentally (Kramer & Drake 2010). Both methods have been employed to manage invasive species, but successes are dependent on the budget for the removal, and which species are being removed (Taylor & Hastings 2004).
This management logic is often informed by experience. It’s also, however, somewhat determined by ecological theory, which can be a powerful tool for managers.
1. Divide and extinguish. The first idea above, where small populations die out, exploits what we call Allee effects. Allee effects describe a phenomenon where small populations actually decrease in density, because there is some critical threshold to their growth. These thresholds occur in many species: populations may need a critical number of individuals to breed, to cooperatively hunt, or to protect themselves in a group against predators. If the population is below this critical density, it declines, and hopefully (in the case of invasives) eventually dies. Managers can reduce whole populations to critical size, or divide them into isolated subpopulations, which then die on their own.
2. Condense For a Fight To The Death. Oh No! The small groups from (1) started growing! If this happens, the species might not have an Allee effect. In fact, this species, when divided into parcels, actually grows faster at those low densities because there are more resources to exploit. In this case, the best management option might be to contain them and allow competition and/or crowding to reduce their numbers.
In order to decide what strategy is best for a certain invasive, it helps to know how they grow. Do small populations grow faster? Or do they need a certain number of individuals before they really get going? Once we have information on growth, we can exploit Allee effects to develop a cheaper and more well-informed management strategy. Patrick Tobin and colleagues came out with a review this month that discusses the various ways that an Allee effect could be used to manage invasive species. They also suggest some new methods that haven’t been employed yet, such as combining two Allee effects to combat the spread of one population (e.g., fragmenting the population and introducing more of a predator, or releasing sterile members of the same species into the population while also reducing pollinator visits). The idea of exploiting these kinds of population dynamics to manage invasive exotics is not new, but the theory opens up new avenues of possibility, especially for species that are costly to remove.
Goreau TJ, Smith JE, Conklin EJ, Smith CM, & Hunter CL (2008). Fighting algae in Kaneohe Bay. Science (New York, N.Y.), 319 (5860) PMID: 18187638
Kramer AM, & Drake JM (2010). Experimental demonstration of population extinction due to a predator-driven Allee effect. The Journal of animal ecology, 79 (3), 633-9 PMID: 20102421
Stimson, J (Stimson, John); Cunha, T (Cunha, Tamar); Philippoff, J (Philippoff, Joanna) (2007). Food preferences and related behavior of the browsing sea urchin Tripneustes gratilla (Linnaeus) and its potential for use as a biological control agent Marine Biology, 151 (5), 1761-1772 DOI: 10.1007/s00227-007-0628-x
Taylor, Caz M., & Hastings, A. (2004). Finding optimal control strategies for invasive species: a density-structured model for Spartina alterniflora Journal of Applied Ecology, 41, 1049-1057 DOI: 10.1111/j.0021-8901.2004.00979.x
Tobin PC, Berec L, & Liebhold AM (2011). Exploiting Allee effects for managing biological invasions. Ecology letters, 14 (6), 615-24 PMID: 21418493