Photo: Lupines below Öræfajökull, and sheep grazing at Sandfell, Iceland (S. Heard)
Last summer, we were driving around southern Iceland, admiring the fields of lupines (beautiful, even though they’re invasive) and the gamboling sheep (also invasive, at least to the extent they’re allowed to graze free). Before long, we noticed an interesting pattern: we saw dense fields of lupines, without sheep; and we saw thousands upon thousands of sheep, in fields without lupines – but we drove for days without ever seeing sheep and lupines together.
Being a nerd scientist, I came up with a hypothesis to explain this pattern: lupines are toxic, and sheep avoid them. That hypothesis was lonely, so I came up with another one to keep it company: lupines are delectable, and sheep graze them down wherever they can.
Perhaps you noticed that my two hypotheses assert completely opposite processes. And yet they predict exactly the same pattern: the checkerboard* of sheep and lupines observed separately, never together. At least in ecology, it’s often easy enough to come up with hypothesis sets like this. The converse is doable too: one process, but predictions of totally opposite patterns (in phylogenetic community ecology, for example, it’s been claimed that competition can generate both phylogenetic clustering and phylogenetic overdispersion).
It’s also why a good number of senior ecologists** crankily proclaim that “you can’t infer process from pattern”. I have been guilty of crankily proclaiming this myself (for example, in this paper). But of course, this proclamation is nonsense. If it were really true that you can’t infer process from pattern, we’d have no science of astronomy, no deep-earth geology, no particle physics, and no phylogenetics (just for starters).
My inclusion of particle physics on that list, by the way, should make it clear that the process/pattern issue is not the same thing as the contrast between experimental and observational data. It’s easy to slip and think it is – to think that observational data are pattern, but experiments measure process. This misapprehension is one reason that many biologists fetishize experiments over observation. Actually, it’s very common for us to run experiments in which we still observe and interpret a resulting pattern, rather than the process we’re interested in. When high-energy protons collide in accelerators, for example, we don’t observe the collisions themselves, or even the short-lived exotic particles formed in those collisions, but rather the spray patterns of slightly-less-exotic particles arising from cascades of particle decay. In fact, it’s an interesting philosophical question whether we ever observe process directly (but not one I’ll tackle today).
So how do we infer process from pattern, if the world is full of logical booby-traps like my sheep and lupines? It’s both simple and complex: it involves inferring process from pattern with care, identifying patterns that are predicted by one hypothesized process, but by no other. This is far from trivial, because the set of processes we could hypothesize may be very large, and thus it’s hard to know whether a pattern is uniquely predicted by a particular hypothesized process, or whether we just haven’t thought hard enough yet. Such uniquely predicted patterns may rarely be accessed by a single study; instead, they may be compounded outcomes of many different observations and experiments. This is one reason that conclusions in science are tentative, and that confidence in a hypothesis tends to come after the consilience of evidence from many studies and many perspectives (as, for example, with natural selection as the major mechanism of evolution).
So “you can’t infer process from pattern” is just one of those things people like to say because they think it makes them sound rigorous and clever. It’s a slogan. Politicians like to bandy these about, and sometimes, we scientists do too. Real rigour and cleverness don’t lie in slogans; they lie in careful thought that recognizes the complexity of nature.
Oh, and about the sheep. Later we saw a lupine field with a remarkably straight edge – which turned out to be one sheep’s-neck length beyond a fence. Now that’s a pattern predicted by one process (sheep eat lupines) but not the other (lupines poison sheep). So until further evidence accumulates, I’ll infer that sheep love lupines.
© Stephen Heard (firstname.lastname@example.org) October 26, 2015
- Thoughts on time and change from Iceland: Where the Earth shows its bones
- What do we do when invasives are beautiful, and supply ecosystem services?
- On tentative inference: our literature is not a big pile of facts
*Checkerboard patterns were much discussed in community ecology in the 1970s and 1980s, as evidence (to some) of competition, and as evidence (to others) of sloppy thinking about competition.
**I name no names, but it won’t take much literature searching to find some. Or you can buy me a beer and ask me about my PhD supervisory committee.