Revisiting Srivastava & Lawton 1998

In a paper published in The American Naturalist in 1998, Diane Srivastava and John Lawton, using experiments on tree hole insect communities, tested the “more individuals hypothesis”, the idea that more productive support higher species richness because they have a greater number of individuals. Srivastava and Lawton found that more productive tree holes had higher species richness but did not have greater numbers of individuals, a finding that ran counter to the prediction of the hypothesis. However, when communities were disassembled by reducing productivity, the hypothesis was supported. Twenty-one years after the paper was published, I spoke to Diane Srivastava about her motivation to carry out this study, memories of fieldwork, and what we have learnt since about the assembly of disassembly of tree hole and other similar communities.

Citation: Srivastava, D. S., & Lawton, J. H. (1998). Why more productive sites have more species: an experimental test of theory using tree-hole communities. The American Naturalist, 152(4), 510-529.

Date of interview: 3 December 2019 (conducted in-person at idiv, Leipzig)

Hari Sridhar: What was the motivation for you to do this particular piece of work?

Diane Srivastava: Well, I wasn’t sure what I wanted to work on, with my PhD. There was a lot of work happening around biodiversity, in various ways, both as a response variable, and as an explanatory variable. John (Lawton) suggested that I go through some of his recent papers and see if there are some ideas that he was working on that I found interesting. I got from his secretary this big binder of papers, and I started going through them and I got interested in productivity-diversity theory. It seemed like nobody really had a good idea of why there were these relationships. There were all these hypotheses. I thought maybe I wanted to work on it,so I kind of wandered back into John’s office and said, this one kind of tweaked my interest. And he said, indeed, there is a system here where you can manipulate productivity. I’ve always been interested in these tree holes around Silwood Park. That’s where I was doing my PhD. It’s this campus that’s in the country, a little bit away from London. It had all these old beech trees. The way beech grows, it often creates these cavities, especially where the roots crossover. These collect water and collect detritus and I knew that they had all these aquatic invertebrates in there. Years before I did my PhD, there was another scientist who did a sabbatical there and looked at some of the insects in the tree holes. So we knew there were insects there. It also seemed like a system where I could manipulate productivity. Productivity is the input of basal resources in the system. It’s also a system that I could really control as I could use artificial tree holes. If you put some dead leaves in a container and fill it with water and make sure the containers black, insects think it is a tree hole: it smells like a tree hole and looks like a tree hole. They are pretty happy to oviposit in it. So, I could manipulate the amount of basal resources – leaf litter – to change productivity. We did this and found that with more resources there is greater diversity. We were sure that there is a pattern, but that didn’t really get us to the ideas yet. Then, I guess, one of the things that I started wondering is, if this was causal, then I should be able to, kind of, go in reverse. I should be able to have tree holes that had tons of resources, and then I should be able to pull out some of the resources and diversity should go down, right? Lo and behold, there was something different going on. It turns out that the insects weren’t really limited by the amount of dead leaves, but they’re using the brownness of the water as a cue for whether the tree hole frequently dried out or not.  The more leaves there are, the browner the water is. So that’s why you get a correlative response, but when you did the manipulation both ways, adding the detritus and then removing it, you didn’t get the same pattern, because the cue was misleading. But, actually, I think, that is not the reason that the paper had impact in the end. I think the paper had impact because, in trying to think through the mechanisms, I had to read the literature. And I found, that a lot of people had this one idea, around sampling and the number of individuals. If you have a certain amount of energy in the system, that can support a certain biomass of organisms. So, if they’re all about the same size, then a certain number of individuals. And then well, how many species can that many individuals be divided into? Naively, you might think, every individual could be a different species. Yes, but you’re not going to have a viable population like that. So, there is some relationship, just through pure sampling effects: if you have more individuals, there may be more species represented by those individuals.  I wasn’t the first person to think of this, by a long way. Decades of ecologists had thought about this, going all the way back to Darwin. But, I guess, I needed a little shorthand to refer to this idea. And I don’t know why nobody gave it a little nickname before, but I did, and I called it the “more individuals hypothesis”. It was one of about, I think, four different hypotheses that I identified by reading the literature. The other one was, as you increase resource productivity, there’s probably a spectrum of resources, there’s some very rare ones that eventually become abundant enough that they could support a specialist species. But if there aren’t a lot of resources in total, then the rare resources will not be that abundant, and probably only a generalist could use them. There were a total of four hypotheses, so I gave these things little nicknames. And, to be honest, the reason I think people cite that paper is because it laid out, fairly clearly with some easy to remember names, these hypotheses. I hope it is also remembered because it’s a nice demonstration of how you can use experiments to choose between different theoretical hypotheses.

HS: Who was the scientist who had earlier looked at insects in these tree holes?

DS: Bill Bradshaw. He is a professor at University of Oregon.

HS: At that point, were tree holes already being researched?

DS: Yes and no. Tree holes had been certainly studied but not so much in testing community ecology theory. Roger Kitching did some seminal population work in England on tree hole insects in, I think, the 1970s, and then some more food web-based work in Australia. Kitching’s book on phytotelmata came out, I think, around the time I did my PhD. There’d also been some very nice work by Bill Bradshaw, Christina Holzapfel, and Phil Lounibos which had looked at interactions between species. They were doing some of the groundbreaking work there. Close cousins of tree holes are a different type of phytotelmata: pitcher plants. These had already been used by quite a few people – Tom Miller, Nick Gotelli, Aaron Ellison and others – to test community ecology. So there was already this idea that phytotelmata, in general, could be these natural microcosms. But tree holes hadn’t really been used that much.

HS: What are phylotelmata?

DS: Plants that create water-holding containers, so things like tree holes, pitcher plants, bromeliads, which I work on now, and then there are a bunch of other things, like Heliconia bracts and bamboo internodes.

HS: Stepping back a bit, could you tell us how you got interested in ecology and ended up doing a PhD in ecology?

DS: Well, I grew up in Nova Scotia and spent a lot of my time outdoors. I was canoeing in the summer, hiking throughout the year, cross-country skiing in the winter. My parents were both scientists. My mother was a fisheries biologist and my father was a geophysicist. So, I think I had the magic combination of being outside a lot and also having science at the kitchen table all the time. The other thing that happened was I became really interested in environmental issues. When I was doing my undergrad, I started off with doing a joint degree in biochemistry and biology, because I couldn’t make up my mind which way I was going to go. And I didn’t make up my mind until sometime between my third and my fourth year, when I got a summer job sampling lake plants throughout Nova Scotia, which involved canoeing around lakes. And I contrasted that experience with the summer before, which I spent on the top floor of a very tall building, in a lab without windows that opened, playing with toxic chemicals. And I thought, well, I think it will be a lot more fun to be an ecologist. At that point everything clicked into place.

HS: How did you decide to go to Imperial College?

DS: Well, after I did my Bachelor’s, I went to University of Toronto and did a Master’s. I worked on the effects of snow geese on graminoids. That was up in near Churchill, Manitoba, in the sub-Arctic. I had two field seasons wandering around the tundra, evading polar bears, lying on my belly and painting little dots on grasses and sedges that were all like one inch tall and then watching their leaves die. That degree was through the Botany department, but it was really about the long-term effects of snow geese on plants. It turned out to be a very interesting story, and it really got me interested in the interaction between plants and animals. I asked my MSc supervisor, at the time, who was Bob Jefferies, out of the University of Toronto, for advice on where to go for a PhD. He suggested John Lawton, who was one of the best plant-animal interaction people. That’s how I ended up going to Imperial.

HS: What did the application and selection involve? Was there a qualifying exam or an interview?

DS: I guess the most important thing was that I got a scholarship. I applied for a Commonwealth scholarship, and I was lucky enough to get one. And then, I went to meet with John. I guess, it was a mutual interview, to see if he wanted me there, and if I wanted to be there.

HS: Do you remember which year this was?

DS: 1993.

HS: In the Acknowledgments for this paper, in addition to the Commonwealth scholarship, you also mention support from the Canadian Women’s Club.

DS: Yeah, I’m really thankful for this organization. The Commonwealth scholarship is exactly three years, and a PhD in England is also exactly three years. I was not quite done by three years; I needed three months more funding. I applied to this organization, I went for an interview in the centre of London – bought a shirt just half an hour before the interview – and they gave me the three months funding that just let me finish everything up. That was, actually, really important, because, in the last few months of my PhD, I developed repetitive strain injury in the backs of both my hands from basically typing in a cold, poorly heated, drafty British house. It really slowed me down. I would handwrite some of my thesis chapters, and I’d mail it off to some friends, and they’d sit there and type it up for me and email it back. Getting through the last little bit of it was challenging. Part of the manuscript of this paper was written like that, through the generosity of friends helping me out.

HS: Did the work in this paper form one chapter in your PhD?

DS: Yeah, it was one chapter out of something like four chapters. I had another chapter on tree holes that was about the role of drought disturbance. And then, I had another chapter that’s totally different, where I went off to Cameroon with a team that included John and we looked at effects of forest harvesting on butterflies, and we looked at bracken herbivores. Totally different things. And then I had a theory chapter on local versus regional richness plots. It was a very unconventional PhD. John seemed quite happy to just let me go off in all directions. I think, initially, he wanted me to try a lot of different things and see what worked. Well, I guess, a lot of things worked, so I ended up with this unusually diverse set of thesis chapters.

HS: Did you convert the other chapters into papers as well?

DS: Yes.

HS: Tell us about what it was like to work with John Lawton, and about your interactions during the making of this paper.

DS: John was at the peak of his career then. He was extremely busy. He was also the president of the Royal Society for the Protection of Birds, and he was the director of the Centre for Population Biology at Silwood Park. He was involved in a lot of stuff and always seemed super busy. But all of us grad students noticed that, we’d go in for a half hour meeting, and it was like sitting down for five hours with somebody else, in terms of the quality of the interaction. We always came out like, oh, wow, that was great. I’m good to go. I had this great conversation and now I kind of feel like I’ve got a good idea about which way I should go here. He was always very encouraging, but also super sharp. He would also do this thing where he would sit in a seminar and kind of nod off, and then towards the end he would just snap his head up, be totally awake and ask some devilish question. And you’d be left wondering, where did that come from?

HS: Tadashi Fukami has written a piece about this paper in the ESA bulletin, in which he mentions that your paper and other work from your PhD inspired John Lawton to write his famous paper titled “Are there general laws in ecology?” Did John Lawton discuss these ideas with you and other students when he was writing this paper?

DS: Yes. I think he was writing that paper right about the time where I was writing up all that chapters to my thesis. I think there were a few things that happened in there. One was that I discovered that there was perhaps a bit of a statistical artifact with the way that we were thinking about the regional control of community richness. John had published a number of papers using this method, so, power to him, because he was very supportive, didn’t blink an eye and really took it on board and said, oh, yes, yes. He was very supportive of me discovering this. That was one thing that came out then I think the other paper that influenced him was this productivity-diversity paper, and the ability that we had of uncovering the mechanisms by a careful combination of experiments of assembling and disassembling communities, and working out those two processes. That paper that John wrote is, in some ways, his despair of us continuing to look at very small spatial scales to figure out general rules. The pervasive influence of local environmental conditions and the strong contingencies by species traits were both things that he was very aware of. I think the future he saw,if you want to come up with general things, is to take a few big steps back and look at things at a much larger spatial scale. There, he was really influenced also by the advent of a lot of work that was happening around the time in macroecology. That paper also influenced a lot of us, both his graduate students at the time, but also, going forward, a lot of ecologists, thinking about where they were going in their ideas, in their careers, what scale they should be focusing on. That particular paper has had a lasting impact in my own research. There is a line in there that, to paraphrase, it says, it’s pretty unlikely that you’re going to find some general rules, but if you do find a general rule, treasure it.  And so, when I started working with bromeliads, which are basically the tropical equivalent of tree holes, I decided I wanted to not just look in one area, because I probably would not find generality there, but look at a whole bunch of different sites. Only then might I find some general rules. That’s why I created the Bromeliad Working Group. He’s right. With this large bromeliad database that we have, from 35 sites across the Neotropics, it is hard to find general rules.There are a lot of things that are contingent. But we have found things that we now can see are mechanisms that, you know, occur from Argentina, all the way up through South America, Central America, to Mexico or the Caribbean. These are geographically robust mechanisms. And we do try to treasure that, as John said.

HS: Could you give us a sense of what it was like to actually do these experiments? What was your daily routine, who were the other people involved etc.?

DS: Sure. The trees with the natural tree holes are all around Silwood Park, which is a small graduate college based around a manor house with some fields and natural woodlands. It was extraordinary to be able to do field work just a few steps away from your lab. I set up these artificial tree holes near the base of these beech trees. All my field work was in Silwood Park. In one of my thesis chapters, I was really interested in how drought disturbance affected the communities. I had 100 natural tree holes marked around Silwood Park. Every two days, I would spend the first hour of the day marking the water depth in each tree hole. I did it for a whole year. I got the weather data for that year. And then I built a model of how water changes and how it relates to the characteristics of the tree holes. We could then use that model to go back in time, over climate data, and see how prevalent drought was in the system and so on. I don’t think I ever would have been able to do that if the tree holes weren’t immediately outside the lab.

HS: You thank D Brunner and L Turnbull for assistance in the fieldwork. Could you tell us a little more about who these people were and how they helped?

DS: Yeah. Daniella Brunner was a biologist from Switzerland who was doing the equivalent of her Master’s there, and my girlfriend at the time. And Lindsay Turnbull was my housemate. I think they both helped, Daniella especially, with some of the lab and field work. Lindsay, helped with some of the conceptual work and the analysis.

HS: You also thank V. Lauck for help in typing the manuscript.

DS:. I mentioned earlier that I had repetitive strain injury. A lot of friends, like my housemates Andy Hector and Xanthe Christophers, helped type up my thesis. But when I was converting my thesis chapter into a manuscript to submit to American Naturalist, I had already moved to UBC to start my postdoc. So I hired an undergraduate work learn student to help with formatting the manuscript – that was Ms. Lauck.

HS: Can we go over the other names in the Acknowledgements too?

DS: Okay. Jan Bengtsson was a visiting prof from SLU in Uppsala, the Swedish agricultural university. Tim Blackburn is a macroecologist who was at Silwood Park and helped me understand some of the ideas behind the “more individuals hypothesis.” Mick Crawley taught me stats. He taught me a program called GLIM, which was used widely by ecologists before R became popular – and his book on GLIM for ecologists was the precursor of his popular text on R. Tony Ives and Matt Leibold were visiting researchers at Silwood Park, and both helpful in sitting down with and talking through some of the ideas. Bob Paine! We have some amazing people here, in terms of ecological theory. Phil Warren is a food web biologist who was on my examining committee, and his ideas on the role of community assembly helped me think through the possibility that productivity could affect colonization and extinction differently.

HS: Did this study involve taxonomic work to identify all the species you found in the tree holes?

DS: Um, I was pretty lucky. I’ll have to refresh my memory, but I think there were eight or 10 species. Something like that. So, there wasn’t that much. And I was fortunate that Roger Kitching had already done his PhD at Oxford in the early 70s, and had already spent some time figuring out who they are. So I had a pretty good head start there. The only thing that were really problems, as far as I remember, were the mosquitoes. Mosquito larvae are always difficult – it’s all about the scales on the comb, etc. I had some people look over my identifications, and Francis Gilbert was very helpful identifying a syrphid fly for me. Plus the British insects are some of the most thoroughly described insects in the world, so lots of published resources.

HS: After this study, did you continue to work on tree hole communities for a while?

DS: After this, I decided that tree holes were fun, but they weren’t fun enough. They didn’t have any predators. British tree holes don’t have predators, presumably because they freeze over in the winter. The likely predators are things like odonates, and their life cycles wouldn’t be compatible with that kind of disturbance. I was sitting in my office one day when a friend of mine – Raphael Didham – came by and said, “hey, what can you tell me about Vancouver?” And I said, “Oh, it’s a super nice place to live, why are you asking me?” Raphael said, “Because there’s this job ad for this postdoc at UBC, and I’m thinking of applying”. I said, “Oh, you should really apply”. And then I thought for a bit and said, Raphael, do you mind if I also apply for this job? This was towards the end of my PhD, when I still had several months. I was just starting to write up. But the problem was that the deadline was in three days, and it needed a one page research proposal. I remembered this conversation I had with Jane Memmott. Jane had done her PhD in Costa Rica, and also done some work in Peru. Jane had had told me, listen, there are these bromeliads that also contain water and they have predators. They’re going to have lots of trophic levels, unlike the British tree holes. And so, I remembered this conversation, sat down and wrote a research proposal for this system I had never seen, let alone done any work on, and UBC funded me for the postdoc. And so, I was like, oh, well, I guess I better go down and find some bromeliads and look inside them and see what’s there. That’s what I did. Jane suggested a friend of mine hers, Erik Olson, who drove me up and down in Costa Rica. We looked inside bromeliads everywhere. And from then on I shifted to bromeliads. Raphael, by the way, went on to a faculty job in Perth and has continued to do really cool work on tropical insects.

HS: You haven’t gone back to tree holes after that?

DS: Actually, after about a decade of working on bromeliads, I decided that I wanted to look at the effects of fragmentation. But it’s hard to transplant bromeliads all over the landscape, so I remembered the artificial tree holes I had used for my PhD. We’ve now run several projects looking at the effects of forest fragmentation on the food web and on ecosystem functions. [And now, because of the pandemic, I’m starting an urban tree hole project in Vancouver in 2021].

HS: Who did you do your postdoc with?

DS: I did two postdocs. First, I was a postdoc in the Biodiversity Research Centre which had just started off at UBC. I was a postdoc without a supervisor, I was supposed to be the glue for the centre, to run seminar series, and so on, that brought people together. And then, for my second postdoc, I was an NSERC postdoctoral fellow at UBC with Bill Neill, who was a lake ecologist. And then I got a faculty position at UBC.

HS: What do you remember about the writing of this paper? Did you write the paper first or the thesis chapter?

DS: This one I wrote as a thesis chapter. And then, I moved to UBC, and I wrote it up for The American Naturalist. This was already the time when I was having a lot of trouble with repetitive strain injury. So, some of the writing was actually speaking. At that point, I was using voice recognition software. I was fortunate to receive, an early version of this software. But I have a really weird accent, if you haven’t noticed. I grew up in Nova Scotia, but my mother had a strong British accent, and so I ended up with this hybrid between the two. This software had a really hard time analyzing my voice. Basically, the only way to make it work without a lot of errors was for me to speak with this really strong fake American accent. Then it worked perfectly.  So yes, I spoke in an American accent to this software, to write the paper. I submitted it. The comments came back and they were like, okay, we will consider this but we want you to reduce it by 30%. That was massive! At that point, I was actually just of leaving for Costa Rica for several months. So I asked them, can I give this back to you in three months? At that time, in Costa Rica, I had no access to email for three months straight, so there was no way to resubmit it. They said yes.

HS: Do you remember the name of the software you used?

DS: Yes. It was called Dragon NaturallySpeaking.

HS: Was The American Naturalist the first place you submitted this to?

DS: Yes.

HS: Was the revision straightforward?

DS: Yeah. I think it was one of those which today would be called a ‘reject and resubmit’. At that time, it was still called a major revision. It was definitely a major revision; there was a lot to do. It wasn’t just a little bit of tinkering. I remember working really hard on these revisions over a long time.

HS: Do you remember how this paper was received when it was published, in academia or in the popular press?

DS: I often saw it being cited as a source for the phrase “more individuals hypothesis”. I feel a bit guilty about that, because, as I said, I certainly wasn’t one of the people who first formulated this hypothesis. I was simply paraphrasing other people’s ideas. But I think that, because people liked the phrase, it meant something concrete, they put my paper as a citation. I think that is part of the reason it took off at that point. And then I ended up teaching ecology and using textbooks where, there’s my citation next to the phrase, “more individuals hypothesis”, and I have to explain to my students really, please, this wasn’t my idea!

HS: I don’t know how much the bromeliad and tree hole research communities overlap and interact, but do you still keep track of tree hole research?

DS: Absolutely. I think people who work in one phytotelm habitat know about the research in all the other ones. The whole idea is, these are natural systems that are contained and small enough that you can actually manipulate them and replicate them, so you can ask very similar sorts of questions about community ecology in one phylotelm habitat, like a tree hole, as you can in another one, like a bromeliad or pitcher plant. There is a lot of phylogenetic similarity in the types of species that occur in these habitats.

HS: Does tree hole research still continue in Silwood Park?

DS: That’s an interesting question. When I was doing bromeliad work, Tom Bell asked if he could be a volunteer with me in Costa Rica. Tom had just finished his Master’s degree with Dolph Schluter, he had some time before his PhD was going to start, and he wanted to come to the tropics and do some work. So we thought up this experiment where we looked at extinction cascades in bromeliads. And then Tom went off to do his PhD, but I guess he had been influenced by the idea of phytotelms being really nice model ecosystems in ecology. So, when he did his PhD, he decided to work in tree holes, and he looked at the bacteria in tree holes. He was able to isolate bacteria from tree holes and use them in some really lovely experiments about biodiversity and ecosystem function, amongst other things.

HS: Did he do his study in Silwood Park?

DS: He did it in Wytham Woods near Oxford, the same site where Roger Kitching studied tree holes for his PhD in the early 1970s.

HS: Have you been back to Silwood Park after your PhD?

DS: I went back a few times. But then, after a while, the people I knew had all graduated.

HS: Does ecological research continue at Silwood Park?

DS: Yes. They have lost ownership of some buildings, like the manor house, but the beech trees are still there and the tree holes are still there.

HS: Is the “more individuals hypothesis” still relevant in your work today?

DS: I don’t directly study the “more individuals hypothesis” anymore. I think that was a really good question to look at in the British tree holes, which are basically just detritus and detritivores. So, it’s quite easy to think about bottom-up effects in that system. I then moved to a system that has other trophic levels, and I became really interested in top-down effects. So, I’ve expanded my research to think not just about diversity within trophic levels but also the diversity among trophic levels. So, in some ways, the follow-up to the original American Naturalist paper on why more productive sites have more species is another American Naturalist paper on bromeliad food webs. But perhaps what had a more lasting impact from the “more individuals hypothesis” was the idea that we need to think about null models in ecology, and that we need to think about sampling effects, and that we need to first control for these sorts of sampling effects before we ascribe ecological mechanisms. That lesson that I learned in that paper, I think has been a constant in the rest of my career.

HS: I want to read out the last sentence of the abstract to see what resonance it has with the work you’re doing now. You say, In tree holes and more generally, the More Individuals Hypothesis is an insufficient explanation for increases in species richness with productivity, because it neither accounts for the different processes of local colonization and extinction, nor allows body size to correlate with extinction risk.” What are your thought, today on this take away from the paper?

DS: Well, I think I was right, in some ways. I think what I was starting to get at then was a bit more of a patch dynamic approach to community ecology, where you think about colonization and extinction, and how there is the interplay between these processes that determine the number of species you observe in a patch. Eventually, I was able to take those ideas and develop it into a more spatially explicit version of food web ecology. The original paper was published in 1998, just a couple of years before the millennium when there were a whole bunch of new ideas around metacommunity ecology. I think I still think about communities in terms of colonization and extinction, and how the environment and the traits of species affect these two processes.

HS: What’s your relationship with this paper now? Do you ever go back to it? Is it something that you use in your teaching? Have you ever read it again after it’s published?

DS: To be honest, I haven’t read it recently. When I teach about productivity-diversity relationships, I have definitely gone back to the ideas in this paper. I do teach about the “more individuals hypothesis”. I talk about the “more specialization hypothesis”. So, I think it’s still there in the background. I don’t think I often reread my papers. But the ideas are still things that I talk about and teach about.

HS: Is this work an important part of your identity in the field?

DS: I think in a few years after I published this paper, and perhaps also the Journal of Animal Ecology paper on local and regional richness, if people recognized my name that was generally what they recognized it for. But a lot of time has passed since then – more than 20 years – and, unfortunately, we tend to only think about looking at fairly recent papers. So, I think, perhaps, if you ask graduate students if they know of this paper, they may well not know about it. They probably know about my more recent work. They probably know a lot about the bromeliad work, in particular.

HS: Would you count this piece of work and this paper as one of your favorites?

DS: Yeah, I think it is. I like this paper, perhaps, because I took a fairly philosophical approach to the question. In explaining each of the hypotheses to myself, I felt that I needed to break it down into a series of premises. And now, teaching people how to write science papers is part of my job at UBC. I teach students how to think through their arguments in a series of premises. And I think I’m proud of this paper because I was able to formulate hypotheses like that,and it helped me reason through things. And I guess, given the popularity of the paper, other people appreciate it, the clarity comes with reducing an argument to its premises.

HS: What would you say to someone who’s about to read this paper today? Would you guide his or her reading in some way? Would you suggest other papers they should read along with this?Would you add any caveats they should keep in mind when they are reading?

DS: I will suggest they read one paper that is older and one paper that is more recent. I would suggest that they went back to some of the original ideas, to some of Preston’s ideas in the 1960s about how the number of individuals is related to the number of species. That had a strong influence on my thinking. I would also suggest that they go and look at some of the really cool new studies that have been done, where they’ve tried to use some really large databases to test some of these ideas. I was using experiments in a limited number of tree holes to test the underlying premises of the “more individuals hypothesis”, but structural equation models also allow the testing of on mechanisms on large observational datasets. A nice synthesis of both experimental and observational tests of the more individuals hypothesis is the 2018 Ecology Letters review by David Storch and colleagues.

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