Questions about extinction cascades

Extinction cascades, also known as secondary extinctions or coextinctions, are extinctions that occur as a result of another, “primary”, extinction. They’ve been a popular topic for many years in the ecological “network” literature. For instance, if you have a food web (a “network” showing who eats whom), or a plant-pollinator web (a “network” showing who pollinates whom), you can remove species from the network and see whether extinction cascades result, under some assumptions about how the ecology of the system works.

I used to be up on the modeling literature on extinction cascades, but gradually stopped following it closely as my interests shifted away from food webby topics. But recently I read the new review of the theoretical and empirical literature on extinction cascades by Colwell et al. (2012). I was curious to come back to this topic after some time “away”, to see how things had changed. Extinction cascades are both fundamentally interesting, and potentially of considerable applied importance, so I wanted to see how the literature had developed. Reading Colwell et al.’s review was useful, but left me with some questions. That’s not a criticism of the review, I just have questions because I think about what I read. So I decided to turn those questions into a post, to see if readers who know more than I do can address them. I know lots of folks read this blog to learn something from me, but sometimes (as with my post on SEMs) I use it as a way to learn from readers. And even if you can’t help me out with my questions, hopefully what follows will be an interesting example of someone “thinking out loud” about the current literature.

1. Reading Colwell et al., I was struck by just how limited the empirical evidence on extinction cascades is. According to Colwell et al., there aren’t many well-established empirical examples of extinction cascades at all, and the few that we have mostly involve highly-specialized ecoparasites going extinct when their one and only host goes extinct. And what indirect evidence there is seems like fairly weak sauce to me. For instance, introduced species often lack some of the parasites, predators, and pollinators from their native ranges–a pattern that could well arise from the absent species just so happening not to have been introduced along with the focal species. Extinction of obligate specialist parasites when their only host goes extinct is the minimum possible extinction cascade–a host goes extinct, and so do its obligate specialist parasites, with no further knock-on effects. While that may be an important phenomenon, it seems rather different than the sort of thing often considered in the modeling work with which I’m familiar. The modeling literature that I used to read focused on more exotic possibilities like numerous primary extinctions reducing a previously-generalized food web to a bunch of linked specialists, which then collapses entirely after one more extinction, like a Jenga tower from which one too many blocks have been removed. Is this just me? Should I be surprised that the empirical evidence on extinction cascades is limited? If we don’t have empirical examples of the more exotic sorts of extinction cascades, why is that? And if those more exotic sorts of extinction cascades truly don’t happen, does that mean a lot of our modeling work on extinction cascades is rather detached from empirical reality?

2. Then again, maybe there are lots of good examples of extinction cascades–just not the sort of extinction cascades contemplated by the existing literature. Bob Paine (1966) found perhaps the classic example of an extinction cascade, which he termed keystone predation. He removed Pisaster seastars from a patch of rocky intertidal, and the patch was eventually taken over by blue mussels that, in the absence of selective predation on them by seastars, were able to outcompete everything else. But I have the impression that the extinction cascade literature mostly (not entirely, but mostly) ignores this sort of possibility. The theoretical and empirical literature reviewed by Colwell et al. seems very focused on secondary extinctions that occur when some species loses all of its prey, or all of its hosts, or all of the mutualists on which it depends. The possibility of secondary extinctions due to dynamical processes like competition, apparent competition, etc., appears not to be much considered in the extinction cascade literature. Indeed, examples like Paine (1966) apparently aren’t even regarded as extinction cascades at all: in reviewing the empirical literature on extinction cascades, Colwell et al. (2012) don’t cite Paine (1966), or any other removal experiment from the community ecology literature on diversity and coexistence. That strikes me as quite a narrow focus–a focus on only one particular sort of extinction cascade–but I suspect I’m missing something. I guess there must be some motivation for this narrow focus that I’m not getting–can anyone help me out?

3. Back when I was following the extinction cascades literature, the models I knew about were “network” models. (Colwell et al. also discuss what they call “statistical” models of extinction cascades, but I won’t get into those) Back when I was up on the literature, network models of extinction cascades mostly were based on simple but unrealistic assumptions about how primary and secondary extinctions work. I have the sense from the Colwell et al. review that some of those assumptions are getting relaxed, but only some of them:

• It used to be that network models of extinction cascades all assumed that “network rewiring” is impossible. That is, they assumed that the observed predator-prey or plant-pollinator interactions are the only possible ones. While that’s a natural starting point, it is of course unrealistic in most cases (and not just because observed networks invariably are undersampled and so omit rare interactions, although they are and do). Further, it’s especially unrealistic for species that have recently lost some of their prey, or plants, or pollinators. That is, it’s especially unrealistic in the very situations that we’re most interested in if we’re trying to model secondary extinctions. Here’s a beautiful illustration of the lengths a predator will go to, and the changes it will make to its usual diet, to avoid starving when its usual prey are no longer available. So I was glad to see that more recent network models of extinction cascades are getting away from this assumption and allowing network rewiring (i.e. adaptive changes in who interacts with whom; e.g., see Valdovino et al. 2010 for a review of adaptive foraging models in complex food webs). Although even there, it looks like many models still assume that the observed interactions are the only possible ones? If so, that still seems like an unrealistic assumption, and it seems like it would be interesting to try to relax it. The work of my friends Owen Petchey, Andy Beckerman, and Phil Warren is a nice example of work that avoids this unrealistic assumption (see Petchey et al. 2008 PNAS), though it’s still early days and it’s unclear to me how far the Petchey et al. approach can be pushed. Is the modeling literature indeed moving in the direction of “network rewiring” models? Particularly ones that allow for the possibility of species responding to extinctions by starting to eat, or pollinate, species they’ve never previously been observed to eat or pollinate?
• Another common assumption of the older modeling literature is that primary extinctions are assumed to occur one by one, and to occur via species simply “vanishing” for unspecified reasons. Am I right in my impression that that’s still a common assumption these days? That approach to modeling primary extinctions is undoubtedly the simplest starting point. It has a venerable history in food web modeling, going back at least as far as Stuart Pimm’s classic modeling studies of “species deletion stability” in the 1980s. (Aside: “species deletion stability” is a small but relevant body of modeling work that doesn’t seem to be much discussed in the recent extinction cascade literature; it’s not cited by Colwell et al…) Assuming that primary extinctions occur via species just “vanishing” one by one (either at random, or in some pre-specified order) is probably ok if your sole focus is trying to understand the conditions under which extinction cascades will occur. But it’s surely an unrealistic assumption. Extinction is a dynamical process–species don’t just vanish (well, unless they’re experimentally removed). They first decline in some fashion, and that decline itself has dynamical consequences for the other species in the community. Declines can even lead to secondary extinctions without the declining species themselves actually going extinct. It’s not at all clear to me that the dynamical response of a community to a dynamical extinction will mimic its dynamical response to species simply vanishing. Has anyone looked at that? Further, when I think about systems in which there are lots of primary extinctions, I can’t help but think about the major causes of those extinctions–things like habitat loss and modification. Causes that affect all species simultaneously rather than one at a time, and that seem likely to obscure or even overwhelm extinction cascades arising solely as a response to primary extinction. I don’t question the value of extinction cascade modeling as a starting point, or as a purely theoretical exercise (theory does have a life of its own, independent of data). But I do wonder if existing work isn’t missing some really interesting and important possibilities in not considering realistic causes and dynamics of primary extinctions.

Again, I emphasize that this post is about me asking questions. I’m not critical, I’m curious. Looking forward to comments.