Niche incumbency, dispersal limitation and climate shape geographical distributions in a species-rich island adaptive radiation

Algar, A.C. et al., 2012. Global Ecology and Biogeography 22(4): 391-402. DOI:10.1111/geb.12003. Niche incumbency, dispersal limitation and climate shape geographical distributions in a species-rich island adaptive radiation

Below, we give our first impressions of this article. Please comment below, or tweet Will or Lynsey (maybe use #pegejc). Think of this as a journal club discussion group!

Will Pearse

Will Pearse

I saw this work presented at Imperial College London, and I remember being impressed with Algar’s honesty. I can’t remember when I last saw someone actively drawing attention to the potential pitfalls in an analysis, and it  made me trust him all the more. This paper is a neat, self-contained look at what controls Anolis distributions, and concludes (quite rightly) that a number of factors play a role. I really like the approach they take, and I think it has potential to answer a lot of ecological and evolutionary questions.

The setup of niche incumbency as whether species tend to co-exist with things that are similar to them is nice and general, and I like the creation of these ‘morphological landscapes’ of how similar species are to one another. I think that’s a great way to visualise things across a landscape, and fits in quite nicely with previous posts about using phylogenetic similarity to improve the fit of species distribution models. Perhaps the only (likely unfair) criticism I’d make is the use of phylogenetically-constrained measures of species’ traits to make this landscape. We constrain according to phylogeny when we’re trying to ask questions about evolution; I think these processes of incumbency play out in ecological time, and so I don’t think it matters if species are similar because of niche conservatism. Put another way, I don’t care how or why species are similar, I care whether they are similar, and so I don’t see the need to control for phylogeny. Following on from this, I would be interested to ask what aspects of niche we can ascribe to the traits they measure, what aspects we can ascribe to phylogenetic similarity, and then what aspects are non-overlapping between the two. This would help us understand whether phylogeny is capturing something different to the traits they used – sort of the phylogenetic middleman problem turned around the other way.

I wonder what the effect of different spatial scales of action are on these results. Plotting maps like those in figure 2 implies that we can link these processess at the same spatial scale; lizards compete with one-another on smaller scales than climate variables, and this might complicate matters. Equally, small-scale environmental variation would mess around with this even further, although I’m not sure how I would model that (anyone?). If we really think blue-sky, perhaps agent-based modeling could be used to get at this sort of thing, although given current computer constraints we’d probably end up being limited to single species-pair interactions, and one of the great strengths of this paper is that it isn’t so limited.

Lynsey McInnes

Lynsey McInnes

Algar et al. generate measures of morphological similarity of co-occurring congeners and of dispersal cost to determine whether niche incumbency (i.e., a morphological (and thus ecologically) similar species is already there) or dispersal limitation (i.e, some factor prevents a species reaching ecologically suitable habitat) contribute in determining distribution patterns of Anolis lizards in Hispaniola or whether they are totally determined by climate (or by nothing in particular (in their null model)). They find some signal strength for niche incumbency and dispersal limitation, although measures of climate are still overwhelmingly the most explanatory factors.

I liked this paper a lot. It felt like the authors had spent a great deal of effort thinking about how best to quantify some notoriously tricky factors in a bid to unpick what really underlies the distribution of a diverse group in a restricted area. They also admirably do not harp on about the stacks of papers that generate climate-only SDMs with a biotic interaction/dispersal caveat stuck in at the end (I include my own papers here!).

I’ve been wondering for ages how to go about getting a better understanding of how the ranges of members of a clade are determined in a certain area (in my head I populate a 100×100 grid with a single species in a single cell and see the lineage diversify into x number of species each with abutting ranges vying for occupancy – in a heavy-handed dismissal of climate, I over-emphasise the role of biotic interactions perhaps). I imagine you can get quite far with (much better developed) simulations of range dynamics along the lines of Pigot et al. But this paper represents an impressive attempt to get at that type of question with empirical data (sure, Hispaniolan anoles are probably the system with the most comprehensive data ever – but why not start high?). I do wonder what other systems this approach could be used in (maybe the non-adaptive radiation of salamanders where expectations might be different if many species are ecologically equivalent? A bigger effect of phylogeny perhaps?). Amassing all the necessary data is always going to be a problem when you try and look at more explanatory factors, right?

This setup might also be useful to bring more clarity to the ‘ecological limits’ explanation for the prevalence of slowdowns in diversification. The idea has taken off in recent years and is extremely intuitively appealing, but the actual mechanisms by which a large clade in a large area (whatever large might mean) actually experience ecological limits remains, perhaps surprisingly, unclear. Do members of the clade prevent further diversification through their occupation of all available niches? What determines the niche breadth of the constituent species? Does niche breadth of the constituent species change as a clade becomes more diverse? Aspects of these questions have been tackled before, but, not to my knowledge, all in one go. Perhaps these questions are relevant to a broader time-scale than what is the focus of this paper, but they run along the same lines.

The result – that there is often a significant effect of niche incumbency and dispersal limitation, but climate still matters is not at all surprising, but it is nice to see it quantified. I wonder at the usefulness of their null model – all non-null models were so obviously going to be better – perhaps comparing climate-only versus climate + extras would have been sufficient? But maybe this is a more philosophical question on the relevance of null models.


About will.pearse
Ecology / evolutionary biologist

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