Seasonalityr

The cues that time the living world

Preface

Many years ago, long before we were generally aware of climate change, and we certainly weren’t talking about it, I got a phone call. Out of the blue. From someone I didn’t know, whose name I don’t remember. The call was from Alaska; I’m in Hawai’i. “The whales came back two days early.” That was the message. “Why are you telling me this?” I answered, truly confused. “Because you need to know that,” was the reply. That was the end of the conversation. That exchange, to a large extent, changed my outlook.

A living calendar had moved by two days, and the news traveled stranger to stranger because it simply had to. Think about what that call assumed. It assumed that the timing of the living world is real, that it can shift, and that a shift is worth a long-distance call to a person you’ve never met. The caller was right on every count. This document is about that timing, and about making it legible enough that a shift of two days is something a person can see, and feel, and need to know.

We usually talk about seasonality as a fact of climate. A place is wet in summer, or cold in winter, or dry for half the year. That’s true, but it isn’t the part that matters most. Seasonality matters because living things use it. A plant times its flowering, a bird times its migration, an animal times its breeding, and each of them reads the season from signals in the environment. Being out of phase is expensive. Flower a week early and the pollinators aren’t out yet. Arrive a week late and the flock has already paired. So organisms key their cycles to cues, and the cues are exactly what a place’s seasonality offers them.

That’s the shift in view this document asks you to make. Seasonality is not a description of the weather. It is the set of cues a place provides for the timing of life. Some cues are dependable and some are not. Daylength is astronomically perfect but says little near the equator. Temperature is strong and a little noisy. Rainfall is the least reliable of the three, often so unreliable that an organism can only react to it, not plan by it. A place hands out some mix of these cues, with some mix of reliability, and that mix is its seasonality in the sense that biology cares about.

The work here takes an accepted picture, the climate diagram, and carries it all the way to a map. We turn monthly temperature and rainfall into a few indices, add a daylength channel that comes free from latitude, and sort places into a small set of cue regimes. Then we map the regimes across four parts of the world. The reward is that the maps reproduce the climate geography you already know, but they read it as cue structure rather than as climate boxes. The everwet equatorial core comes out as the place where none of the cues carry much signal, which is exactly where ecologists find the weakest, most staggered seasonal timing. Index and biology turn out to be the same fact.

The reading example that runs through the document is a trio of North American deserts: the Mojave, the Sonoran, and the Chihuahuan. They’re nearly identical on paper, dry most of the year in all three, and yet they’re plainly different deserts. What separates them is when the rain comes and whether it comes once or twice. That’s a question of phase and sequence, not of how much of each kind of month a place has, and it’s the reason this is not a simple counting problem. The deserts are a good test because you can check the answer two ways, against the textbooks and against direct experience, and a result you can check two ways is a result worth trusting.

A word on what this is and isn’t. What’s documented here is an adventure, true to the spirit of exploration. We could have pushed on to a destination, a fine-tuned model of seasonality with every gap closed. But much of the fun is in the voyage. We’ve built the vehicle, the R code, now gathered into the seasonalityr package, and the results are interesting enough that we both think there’s a destination just over the horizon. The honest gaps are named as you go, and the last chapter points straight at them. The invitation, by the end, is to take the vehicle and sail further.

The book was written together, by Kim Bridges and Claude. The code is real and runnable, the data is public, and every map can be rebuilt from scratch. We hope you come away seeing the year a little differently, and watching for its shifts.