In a paper published in Ecology in 1992, Donald Strong reviewed the empirical literature on “actual” and “potential” trophic cascades, based on which he argued why trophic cascades are likely to be restricted to simple, low-diversity food webs. Twenty-six years after the paper was published, I spoke to Donald Strong about his interest in food webs, memories of writing this paper, and what we have learnt since about trophic cascades.
Citation: Strong, D. R. (1992). Are trophic cascades all wet? Differentiation and donor-control in speciose ecosystems. Ecology, 73(3), 747-754.
Date of interview: Questions sent by email on 20th December 2017; responses received by email on 27th June 2018.
Hari Sridhar: I would like to start by asking you about your motivation to write this paper, as part of this special feature in the journal Ecology. How did you get involved in this and why did you choose to write on this topic?
Donald Strong: As I recall, I was asked to contribute to a Special Feature.
HS: Stepping back a bit, could you tell me how and when you got interested in the topic of food webs and trophic cascades? I notice that most of your work leading up to this paper was on insect herbivory and parasitism, although in 1988 you wrote a ‘Special Feature’ in Ecology on food webs.
DS: I was long impressed with the arguments in HSS (Hairston, Smith& Slobodkin. The American Naturalist, Vol. 94, No. 879. 1960:421-425). I also was impressed with the arguments counter to HSS, especially that by Murdoch (“Community Structure, Population Control, and Competition”-A Critique The American Naturalist 100, 912 ,1966: 219-226) and by Erhlich and Birch (The “Balance of Nature” and “Population Control” Am. Nat. 101, 97, 1967) among others. I had been working on parasitoids attacking the eggs of the rolled-leaf hispine beetles in the neotropics (summarized in Strong and Sanderson, 2006, Cenozoic insect-plant diversification in the tropics, PNAS 103, 10827-10828) and had formed a top-down view of this system.
HS: Would you remember how long it took you to write this paper and when and where you wrote it?
DS: I conceived of the idea driving through Houston in August 1991 with my family on our trek from Florida State University to the University of California, Davis, where I was to begin a position at the Bodega Marine Lab. I wrote the first drafts during my first couple of weeks in my new position.
HS: You acknowledge a number of people “who shared freely..their expertise and ideas”. Could you tell us a little more about who these people were, how you knew them and how they helped:
DS: I recall sharing drafts of the paper with many of these people. Some of them disagreed, but I cannot recall the details. I recall Mark Hay objected, but cannot recall the nature of his objections. Gary Polis was very supportive, and we went on, in a similar vein, to publish “Food web complexity and community dynamics” 1996, Am. Nat. 147 (5), 813-846. This paper received lots of citations, as you can see in Google Scholar.
HS: Do you remember what kind of attention this paper received when it was published?
DS: It was published in the very early days of the internet, so responses were not so quick or easy. I did receive a letter from Nelson Hairston objecting to the article. I considered his objection weird because I was such a fan of HSS.
HS: In retrospect, how do you think writing this paper influenced your research trajectory? Did you put to empirical test some of the ideas you outline in this paper?
DS: Food webs fascinated me at that time and led to my lab’s work with entomopathogenic nematodes, which killed root feeding caterpillars and protected a native plant from being killed by the caterpillars (eg. “Climate affects predator control of an herbivore outbreak”, Preisser& Strong, 2004 Am. Nat. 163 (5), 754-7620.)
HS: Today, 25 years after you wrote this paper, I would like you to tell us where you stand with regard to some of the things you said then: a. “My premise is that true trophic cascades in the community sense are a relatively unusual sort of food web mechanics. I argue that, over the full range of ecological communities, evidence is that these cascades are restricted to fairly low-diversity places where great influence can issue from one or a few species.”
DS: I now believe that trophic cascades and other kinds of top down influences are very important in nature. Many have been severely disrupted by humans. At the same time, simple trophic cascades are far from the whole story of top down influences. And, there exist multiple examples of food chains and food webs that are dominated by bottom-up forces. Some of these arguments are laid out in Strong & Frank, 2010. Human Involvement in Food webs. The Annual Review of Environment and Resources 35: 3.1–3.24.
HS: b. “there is insufficient experimental evidence for an understanding of speciose food webs in any general sense”
DS: Progress is being made on these questions. See Strong and Frank 2010 for a bit of this.
HS: c. “Trophic trickles rather than cascades are a more appropriate metaphor for communities on and in more diverse aquatic ecosystems. Contrast the image of a torrent raging over an austere glacial landscape with that of dissected brooks and creeks in a heavily vegetated watershed. Both conduct great volumes of water, but while in the former, flow is unified, in the latter it is well split, finely differentiated, and thus greatly buffered.” (this is lovely writing!).
DS: Thank you. I had forgotten the specifics, As my Grammy used to say, “The Lord has delivered it onto you.”
HS: In Table 1 you list what you see as “potential trophic cascades”. Since this paper was written, has there been any evidence for trophic cascades in this system?
DS: Hell yes! One can google up many examples. They grow by the month.
HS: In Table 2 you list what you see as differences between systems that are and aren’t likely to show trophic cascades. To what extent do you think these were borne out by subsequent research on food webs?
DS: If Table 2 was ever ahead of its time, time has passed Table 2 by.
HS: One of the examples you discuss in some detail is grazing by the giant tortoises on the island of Aldabra. Since the time of this paper, have you kept track of what has happened on this island to the tortoises and the vegetation? Have you visited the island anytime and seen the tortoises?
DS: I have not visited Aldabra and would love to go there. The better example is the predation by humans on Galapagos tortoises on the Galapagos and the enduring adaptations of vegetation to their grazing (these are difficult to ferret out because of grazing by feral goats, horses, & cattle introduced by humans to the islands. I anticipate visiting the Galapagos for the first time in November*. Even better example is the extinction of Moa species by humans on New Zealand and other islands in the Indian Ocean.
*Update from Don Strong on 7th July 2020: My son gifted me for my 75th birthday a trip to the Galapagos. I witness many things there including the tortoises. It was a short visit and I was not able to infer much ecology beyond seeing these wonderful places, species, and be encouraged by the Ecuadoran curation of the landscape and biota.
HS: As a thought experiment, if you were to rewrite this paper today, what would its main takeaway be?
DS: I would do lots of weasel wordsmithing to lessen the valid criticisms that have been leveled against it.
HS: Have you ever read this paper after it was published? If yes, in what context?
DS: Yes. To explain it to students.
HS: Would you count this paper as a favourite, among all the papers you have written?
DS: No. My favorites have fewer citations. The hispine beetle papers (example) are very dear to my heart because they represent a time in my career when I was overjoyed by working in the tropics.
HS: What would you say to a student who is about to read this paper today? What should he or she take away from this paper written 25 years ago? Would you add any caveats?
DS: Sure, many caveats. As I have said, if this paper were ever ahead of its time, time has passed it by.
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