Beyond species: why ecological interaction networks vary through space and time

Poisot, Stouffer, & Gravel. bioRxiv DOI: http://dx.doi.org/10.1101/001677. Beyond species: why ecological interaction networks vary through space and time

An interaction network based around populations (left) may not look at the same as one based around species (right). Species may sometimes interact with one-another, but that doesn’t mean that they always have to. Figure 1 in Poisot et al.


Lynsey McInnes

Lynsey McInnes

Welcome back to PEGE! We’ve taken a break over the summer, but we are hopefully back fully revived for the new season. We decided to pre-pick a bunch of papers so that we are not scrabbling for a suitable paper a day before the next post is due. This also means that you can follow along with us and pre-read our choices too. From the ten papers already chosen, you will see many familar themes….if nothing else PEGE is helping Will and I home in on what we care about.

And we start with a paper that falls nicely into one of my major preoccupations…population-level processes!

The paper reads like a call to arms by Poisot et al. for network, and other, researchers to get a grip and realise where the relevant processes are occurring. Namely, it is not sufficient to study interaction networks at the species level, but rather we must explicitly acknowledge that interactions differ across the extent of any network as a function of both local trait and abundance distributions (and any feedbacks among these). In short, species will not always interact where they cooccur if, for example, there is very few of one party or a species is not expressing a particular trait value. Sounds intuitive, right? But from my very vague knowledge of network literature and Poisot et al’s larger survery, it doesn’t sound like this variability is commonly addressed.

As you can imagine, I am totally behind the authors that populations are the appropriate scale at which to quantify network interactions. Perhaps because I am already behind them, I came away a little disappointed that they did not provide more discussion of how feasible their approach might be or provide real world examples.

For instance, they outline a model of when they expect interactions to occur and this, again, made intuitive sense. However, it would have been great if they had filled the model in with some data, even with simulated data. They highlight themselves that all the factors they have lumped into their error term (landscape, climate, other species, etc. etc.) will be most difficult to parameterise or indeed characterise. It would have been nicer to see a great attempt to approach this.

For instance, when might useful properties of a network emerge at the species level after all. Do plus and minus interactions often cancel out (so that the species level does kind of become the most useful level to operate at)? Are there clear clines in interaction strength (so that, for example, populations experiencing certain climates always (or often) interact)? Do populations of more than one species replace each other in certain interactions? Can a population-level model of interaction networks be used to study how perturbations (e.g. environmental change, habitat fragmentation, etc.) might affect the network?

I also wonder how genetics might help? Are there diagnostic signals of which populations are likely to interact or not? A threshold level of genetic diversity? Fixation of certain alleles? A certain age of population? A certain isolation of population? Are there useful ways to put genetics to use in characterising interaction networks? My feeling is this is already a field and I don’t know much about it….

Finally, I would have really liked the authors to provide a practical cheatsheet on what types of data to collect. How many populations to study, what traits to measure, how to measure them, how much uncertainty can we cope with, and so on. At least for one or two study systems with which they are familiar.

I recognise that the authors have probably thought about or even have answers to most of the above and not everything can be covered in one paper. Perhaps I was extra harsh because I already agreed with them and did not need convincing, so I was ready for their inevitable followup paper already. Hopefully the paper convinces waverers of the merits of population-level study, both because this is where the action takes place, but perhaps more importantly (at least if you are of the monitoring networks for their continued existence bent) because understanding at the level at which interactions occur will enable us to piece together how diversity pattern look or will probably look in the future.

As ever, exciting times!


Will Pearse

Will Pearse

Aaaaand we’re back! I’m glad Lynsey picked this paper, because I have a secret soft-spot for interaction networks. This is both an interesting suggestion for future work and a good (opinionated! hooray!) review of the state of the field. The general idea seems to be that we shouldn’t assume interaction networks are static: abundances and environmental factors determine what interacts with what, but (most interestingly to me), dependent on what other species are interacting with.

I like the idea, and the authors definitely sold me on it in their review. My only criticism (and I’m saying this to try and be helpful, since this is a pre-print) is that they really didn’t go into enough depth on the ‘what other species are interacting with’ (indirect interactions). The manuscript leads wonderfully up to the point where I’m really looking forward to the magical solution to indirect interactions and then… it stops. It’s a hard problem to solve, so I sympathise! Personally, since a lot of the interactions the authors describe as completely shifting the whole system are so major and difficult to a priori predict, I think just having the presence of certain keystone species as a factor that alters the probabilities of everything else is a fine way to handle it. Treat all the interction strengths as a statistical model (as they suggest, and as others have too), and then look for particular species that change almost everything. Maybe that’s not so mathematically pretty or flexible for smaller changes, and you’d either need some fancy machine learning or (shock! horror!) a priori hypotheses to find those keystone species, but I’d be happy with that.

The authors talk a lot about how interaction networks could ‘improve’ neutral models. A lot of (all?) neutral models are inherently about single trophic levels, so I’m not surprised that interaction networks would make big changes to them, but this did get me thinking a lot about some of the cool work that’s been done on neutral evolutionary models and their ‘non-neutral’ ecological outcomes. We absolutely need more models of interaction networks that take into account evolution; and I don’t just mean measures of phylogenetic dispersion (which are good), but integrated models where we simulate evolution under interactions. We can even fit models to phylogenies now, which is awesome, but maybe it’s time we went a little further.

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Will plant movements keep up with climate change?

Richard T. Corlett and David A. Westcott. Trends in Ecology and Evolution 28(8): 482-488. DOI:10.1016/j.tree.2013.04.003. Will plant movements keep up with climate change?

It's a plant moving. Look, do you have any idea how hard it is to find a picture every week?

This plant can move fast enough… From Wikipedia


Will Pearse

Will Pearse

I picked this paper (out of Lynsey’s selection) because I had a long chat with someone about this at ESA. We concluded then that we didn’t really know whether plants could move fast enough, and to be honest that’s pretty much the conclusion I came to at the end of this review.

Box 4 of the review lists outstanding problems that include “ignorance of the factors that currently limit species ranges”, “largely unknown to what extent plants can acclimate to climate change”, and a number of other factors. The section “can plants track climate change?” lasts only two paragraphs – we apparently have no idea whether plants can track climate change or not. The authors give a number of (for what it’s worth, quite reasonable) reasons they probably can’t, but I think they’d agree that we don’t actually know. Frankly, I’m slightly shocked that we don’t know more about this.

I’m not convinced that animal-dispersed species are necessarily going to fare much better in the face of climate change. This assumes that animals with larger territories are going to move more easily (not necessarily true), particularly given we don’t fully understand the mechanisms by which species would shift their ranges. An animal that eats fruits that moves when it’s hungry is not going to disperse that fruit polewards, because it’s hungry and hasn’t eaten that fruit!

Long-distance dispersal as a mechanism by which individuals trapped in a sea of bad habitat can save the species is an interesting idea. I think this would benefit from a simulation study, but I sense it would require species to be able to colonise in the face of a quite severe numerical disadvantage (small number of immigrants, lots of incumbents). Still, this is a nice idea I’d like to think about for longer…


Lynsey McInnes

Lynsey McInnes

This is a funny paper. On the one hand, a very useful, succinct review of the different factors involved in thinking about how plants might respond to climate change and why this is of interest to ecologists/conservation scientists/mankind and on the other hand, a frustratingly on-the-fence expose of plant movement research to date and its likely next steps.

The authors undoubtedly do a great job in summarizing many recent studies (check out the reference list, its stacked with refs from post 2010). This subject is most definitely timely and popular. And yet it seems we don’t know much. For example, conclusive answers are absent for questions such as what determines a plant species’ current range? How much more range could a species occupy with unlimited dispersal/removing other species/new climates? Is this period of climate change different to past ones (cities in the way, etc.)? The author’s box 4 neatly summarizes the extent of our lack of knowledge!

I got to the end of the paper and found myself wondering (a bit like last week), should we worry? Or should we worry in a more focused way? Does the identity of individual plant species matter as long as the ecosystem is still functioning healthily? If you are a ‘rubbish’ species, has your time come? Perversely, I do just find the outstanding questions listed in box 4 of interest in and of themselves, but firmly, firmly believe that for conservation purposes, they are not the correct ones to be focusing on. I’m not a conservation scientist so I’m allowed (have allowed myself) to ponder these questions, but if wanted to be practical, I reckon we need to think more about functional types (mentioned in box 1), more about corridors to facilitate movement, more about redundancy, more about … ?

Can we ever know – ‘Will plant movements keep up with climate change?’ Seems like this is not a yes or no question. It will depend on the specific set of traits the plant species has/the environment it is found in/the interactions with other species (plants, dispersers, pollinators) it has now and could have in the future? Different camps want an answer for different reasons. Generalities seem to be in place already (and are well-summarised in the paper). However, if we want to conserve species or functions, we need more than these generalities, it seems. If want to use this broad question to learn some fundamentals on the biology of plants, my suggestion would be we need more of everything: more field studies, more theory and perhaps most importantly more of a recognition and exploration of interacting forces: a bit of evolution, a few influential abiotic factors, one or two key biotic interactions, a whole host of more minor ones, across and within trophic levels, some anthropogenic effects, short- and long-distance dispersal and this whole shebang playing out against a shifting climate.

I think the paper left me unsatisfied as it was pitched too broad and thus felt too shallow. What are these authors interested in? What piece of the puzzle will they tackle? Ja, perhaps that’s unfair to ask of from a review, but I’m curious anyway.

Finally – I did very much appreciate this line from the paper: ‘The involvement of government agencies, nongovernmental organizations, and citizen-science networks will be essential, given the focus of academic science on novelty.’

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