New paper on ecological disequilibrium

Think of a community populated by saguaros, prickly pears, and agaves – do you imagine a warm place or a cold place? These species are warm-adapted, so a guess that the community is warm is a reasonable one. But what if the community were actually cold and snowy, like below? This would reflect ecological disequilibrium, a mismatch between the niches of species and the climate observed in a community.

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Disequilibrium seems unreasonable – we do not expect to find bananas growing in Siberia – but it is not impossible. And this possibility challenges several areas of ecology that we all rely on. Computer models projecting species’ geographic distributions in the future assume that species have an instantaneous response to environmental change; similarly, paleoclimate reconstructions based on fossils or pollen also assume that communities’ past compositions reflect past climates.

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Rapid climate change or slow species change can result in disequilibrium. Species may persist in place even as the climate changes away from their physiological tolerances; alternatively, a rapidly changed site may remain un-colonized by physiologically appropriate species because they have not yet had the time to disperse there, e.g. after glacial retreat following the last Ice Age, as seen above. These are just a few of the possible scenarios that can result in more modest versions of the Siberian banana or snow-covered agave scenario. But how would we know if they were occurring?

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We just published a new paper in Ecology (PDF reprint here) that explores ecological disequilibrium. We build a mathematical framework that lets us measure the strength of disequilibrium in a community, then test the framework in hundreds of forest communities across North and South America (data from BIEN). It comes with an associated R package, comclim, that lets other investigators assess disequilibrium in their own datasets.

What did we find? That most communities are actually in equilibrium with present-day climate. This is re-assuring for the assumptions of many current ecological models. But we also found that about a quarter of the communities we examined show strong lags with respect to present-day climate. We were unable to find strong explanations for why this subset of communities were out of equilibrium, providing an important problem for future research to focus on. We also found that disequilibrium will only become stronger under future climate change scenarios, suggesting that some of our equilibrium models will require modification in order to make accurate predictions for the future. Overall, I think the paper shows that communities’ response to climate change can be lagged and idiosyncratic, a message that underscores the complexity of interpreting past ecological change or predicting future change.

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This paper did not come together in a simple way. It began as an idea of mine to better reconstruct paleoclimate from species occurrence data, an idea that was funded by the Danish National Research Foundation and that ultimately sent me to visit the Center for Macroecology, Evolution, and Climate in Copenhagen. I originally decided to go not because of the science but because I wanted to escape a difficult personal situation at home. Once I arrived, my hosts and I soon realized that our proposal was not viable, as it assumed a level of ecological equilibrium for which we had no evidence. We re-oriented the project towards measuring disequilibrium instead. It took two years of visits to Denmark to finish major work on the project, and countless rounds of revisions and peer review to get it published. Three years later we have a product that I am proud of, and that bookends several important chapters in my life. I don’t think that the me who wrote the original proposal would quite recognize the me or the final paper that came out of this adventure.

I hope you’ll give the paper a read and see if it challenges you to think about climate change in some new ways!

Traveling back in time

After six years of living in Arizona, I had never visited the Grand Canyon. That finally changed this year. It was a marvelous ecological experience. The descent into the canyon is a journey back in time that ultimately reaches basement rocks that are nearly two billion years old. And it is also a journey through climate space, from the cold forests of the rim to the warm deserts of the river.

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The top is covered by conifer forests – pictured here is the Colorado piñon (Pinus edulis).

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Five thousand feet of elevation change covers a temperature differential of at least ten centigrade degrees. Looking down from the top, it is hard to imagine such a radical climate change could be tucked into the narrow bottoms of the rock layers.

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But things change – here, about halfway down, western redbud (Cercis canadensis, blooming magnificently in purple) and other less hardy species begin to appear.

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Further down the narrow side canyons begin to transport surface water. Dry ridges host warm desert-adapted species like Agave utahensis, but the wetter areas are able to support riparian species like cottonwood (Populus fremontii).

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And then, magically, the river appears – the hidden architect of the canyon, the gathering place of water from these narrow side-canyons and also for hundreds of miles in every direction.

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Walking alongside the river from its banks, I was impressed by its force as an ecological actor – to provide life and take it from such a wide portion of the continent, to cut down five thousand feet of rock in a few scarce million years, to so provide habitats for such diverse species, and to serve as a dispersal barrier for the many animals and plants (and people) stranded on either side of the rim.

Visiting the canyon after six years of thinking about ecology was undoubtedly a richer experience than if I had gone on the first day I moved to this part of the world. It is a marvelous teacher, and one that I am sure will continue to teach me.

Should ecologists stay home?

aa Since 2011 I have logged at least 152,590 air miles. This corresponds to 98,300 kg of carbon emissions at an approximate conversion rate of 0.2 kg C / km (LIPASTO 2009) – and probably more significant when accounting for radiative forcing (IPCC). Almost all of this has been work-related travel – journeys to field sites, trips to conferences, and transitions to new jobs. Carbon emissions have been one of the major costs of my work to better understand ecology and climate change. And I know I am not alone.

Is it all worth it? It’s an important question, and one that I have been thinking about for the past few years as my travel has accumulated. A recent paper by the Tyndall Centre for Climate Change Research (Le Quéré et al., 2015 – covered by Nature) calls for scientists – especially environmental scientists – to reduce the amount of flying we do in our work. They argue that scientists need to lead by example – we fly far more than most people do, participate in the public conversation on climate issues, and so have an important opportunity (or moral responsibility) to make more environmentally friendly choices. Why should we be exempt from contributing to the emissions targets our countries have set? Is trust lost in the scientific community when it does not abide by its own proscriptions? jet Flying less is a difficult proposition for me, for many reasons that are thoughtfully outlined in the Tyndall Centre paper. I have fieldwork in other countries or remote areas that are nearly impossible to get to via other methods, or would take sufficiently long intervals that my work would no longer be feasible. For example, reaching South America from North America is nearly impossible without air travel, because there is no road or train line that crosses eastern Panama, and boat travel takes weeks. In order to share my research, build collaborations, and get new ideas, I have to attend conferences and meetings, which are rarely close enough to home to be able to reach by other means. For example, my last job offer was the lucky outcome of a chance meeting at a conference – at a drinks session I would have not been able to attend virtually, and at a destination that would have taken four days to reach by train. I lose all of these benefits by not flying.

Yet, maybe I should. Maybe I should re-orient my science to have a more local focus, choosing field sites that are nearby, and only attending meetings that I can reach by public transit. I could rely on collaborators for foreign work, and attend far-away meetings virtually. I try. I took a Greyhound bus to my last job interview. The other year I was living in Copenhagen and was invited to give a talk in Aarhus – so instead of even taking the train, I cycled there. But it took a whole day that I could otherwise have used for working. If I took this minimalist approach to all my work, I might soon have very little work left. bike I think there is an essential issue with the kind of work that ecologists and climate scientists do. Tropical forest ecology requires tropical forest ecologists. That is, knowledge and change do not come freely; rather, they come at the cost of hard work, and that work often has a large carbon cost. Our carbon emissions are hopefully a price that we pay to invest in our futures.

But I do think we can do better. I agree with nearly everything the Tyndall Centre report outlines. We can develop local capacity, so fieldwork and projects are led by local scientists rather than foreigners. We can focus our work closer to home. We can travel only for essential fieldwork or conferences, putting off opportunities where our contribution will be small. We can do better on supporting virtual meetings, and locating physical meetings in countries that minimize total emissions (e.g. the International Biogeography Society).

I am still thinking about my own choices. I have already said no to several international conferences this year, attended one virtually, and am trying to concentrate most of my fieldwork within a thousand miles of where I hope to eventually have a permanent home. But this year I am still going to cross the ocean several times for different obligations, and still will go to some tropical sites so that I can know them by the time I write about them. It’s a start. field I hope I am investing my carbon emissions wisely.

Dusk on a bombing range

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Locating a research site inside a bombing range might seem crazy, but it is what my friend Max is doing for his doctoral research. He works in a portion of the Barry M. Goldwater Range in western Arizona managed by the Marine Corps Air Station Yuma. The area is relatively pristine and well managed because of the military presence, and active bombing hasn’t been carried out nearby for years. Plus, the ‘unexploded ordnance’ signs help to deter the Border Patrol officers from driving all over his research sites. I recently had the chance to visit this place as neither scientist nor soldier, but rather as a botanical tourist.

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The vast sand dunes west of the Mohawk Mountains are an extension of the Gran Desierto de Altar, primarily located in Sonora, Mexico. The environment looks harsh and inhospitable, but it actually holds a surprising amount of life. Stone artifacts and petroglyphs indicate long-term human habitation, but the place really comes alive after a good set of winter rains.

The dunes can store a large amount of moisture, and it is this moisture that in rare years enables a beautiful winter bloom. Plants that have persisted via roots or seeds for many dry years seize the available water resources and put all their effort into flowering.

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Dusk is the time when these ephemeral blooms are most spectacular. Many species have flowers that only open at night – their pollinators are not day-active, and perhaps the heat of the day would lead too too much water loss in delicate petals. My favorite species is Oenothera deltoides, a white-petaled evening-primrose.

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This species is visited by crepuscular hawkmoths (here probably Hyles lineata). At dusk the sky darkens with these hummingbird-size insects, and a soft flapping sound fills in the air. It is utterly magical, and one feels like an intruder into a hidden ecological dance.

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The next morning a dusty sunrise came. With the heat of the day the flowers closed, and the moths were long gone. Many flowers looked bedraggled, and finished for the season. The spring bloom lasts for only a few days, and comes only in rare years. It is a marvelous secret for a bombing range to hold.

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Winter flood

Rain in the Sonoran desert feels different depending on the season. Summer monsoon rains are violent events bringing thunder and downpours. Winter rains are more peaceful events, bringing cool air, low clouds, and steady drizzle. And for the rest of the year, there is no rain at all. Seasons have a very different rhythm here.

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When a large winter storm passed through Tucson a few weeks ago, it felt like the world was changed. Dark grey skies, the smell of water and creosote on the air, and the sound of raindrops on tin roofs and hard rocks are some of the joys particular to this part of the world.

saguaro rain

When there is rain in the mountains, the runoff flows into our canyons and washes. Because many of these surfaces are relatively impermeable, often with bedrock bottoms, the result is a series of flash floods – one of my favorite parts of desert life.

bear canyon normal

I went with a friend on an exploration of Bear Canyon in the front range of the Catalina Mountains. Normally this is a quiet canyon with seasonal flows, usually smooth and clear, no deeper than a few inches. Here you can see the canyon in spring – part of a Sierra Club ICO trip.

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Here is what the canyon looked like after that winter rain. We made several crossings of the canyon in its safer and wider sections, but the water was swift and more than hip deep. Bouldery sections created long whitewater wave trains, and larger bedrock sections become fast-flowing flumes and waterfalls.

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Further down the valley, where Bear Canyon meets Sabino Canyon, the scene was similar. Waters were high above riparian mesquite and sycamore trees, flooding a wide expanse of normally sandy and quiet terrain.

The United States Geological Survey monitors many streams, including this one. Today, as I write this, the flow rate is only 2.4 cfs (cubic feet of water per second). The day I took the previous photos, the flow was 1290 cfs – almost a factor of 600 higher. This flood wasn’t even a very large one – it was produced by only approximately two inches of rain – fairly average for a winter storm.

Rarer much larger storms can raise flow rates by factors of 100,000 – these floods are the ones that transport immense quantities of sediment, scour and downcut further the canyons, and disperse animals and plants to entirely new habitats. One of these days I hope to see one of these landscape-transforming flows.

The day after this storm, sun returned to the desert; two days later, flow rates were nearly back to baseline; by midweek, the rivers were dry again. Just another day in the desert.

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Ruins of a lost world

The Sierra Ancha Wilderness is a rugged landscape of deep canyons, and seemingly a difficult place to live. Yet for two centuries this area was inhabited by the Salado people, a group potentially related to the northern Pueblo culture that populated the wider area. Floodplain agriculture and mesa hunting provided resources, while populations were concentrated in stone pueblos and cliff dwellings. These people and their culture began to fragment in the early 1300s and disappeared completely by 1450, leaving behind traces of their life only in archaeological evidence such as ceramics and charcoal and the occasional stone structure.

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Last weekend I went in search of some of these abandoned cliff dwellings. Some are well-protected and easily visited, but others keep their secrets close. Our long and dusty journey into the mountains down a dirt road brought us to an uncrossable stream, and we set off on foot to explore some of the canyons you see in the background of this photograph.

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Scrambling up these steep canyons, I began to wonder at the logistics inherent in a cliff lifestyle. Unreliable water sources, cliff-side traverses and loose slopes, and multi-mile walks to agricultural floodplains. Why live in such an inhospitable and difficult-to-access place?

We climbed up through bedrock and cliff ledges. The cliff dwellings remained hidden, well protected by the canyon’s sharp turns and sheer sides.

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And then one appeared. On the north side of the canyon, a set of stone and mud structures appeared, perched underneath a small indentation in the rock where two stratigraphic layers came together.

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Crossing the canyon directly was impossible – a deep chasm with sheer cliffs separated us. We instead traversed the ledge further up the canyon, behind a waterfall, and then made a final scramble up.

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The buildings was of stone and mud, with large wooden beams used as roof and floor supports. Here you can see a 700-year old handprint preserved in the mud, indicating the work-intensive method of construction.

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Another nearby canyon, much narrower, held an even more impressive surprise. The approach was through the bedrock of a stream and up slippery manzanita-choked slopes. A final bend in the canyon revealed a fortress-like structure, balanced carefully on a narrow ledge.

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The logistics of construction seemed nearly impossible. Long journeys would have been needed to carry rock and mud from the floodplain below, and the long pine timbers used as cross-beams and floors for these multi-level structures would have had to been carried from elevations thousands of feet higher on the upland mesas.

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Defending these places would have been a simple task. The dwellings had a wide view of the canyon and no access routes except along a single narrow ledge. Nearby seeps could have provided water, and the cliff overhang prevented access from above.

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The beams used in construction date somewhere between 1280 and 1350 A.D., established via tree-ring methods led by the archaeologist Emil Haury at the University of Arizona. His 1934 investigation into this region (The Canyon Creek Ruin and the cliff dwellings of the Sierra Ancha) remains one of the most in-depth studies of the region, involving long field expeditions under conditions far more challenging than we experienced. This Arizona State Museum publication records more recent information.

So what happened to the Salado people? No one is completely sure, but it seems likely that a series of extreme climate events (several long and prolonged droughts) in the early 1300s made their lifestyle inviable (e.g. Waters et al., Graves et al.). Perhaps the cost of acquiring resources to support a complex civilization outweighed the costs, as Joseph Tainter had earlier suggested in The Collapse of Complex Societies. In the end, all that remain to use are ruins and mysteries.

A few miles brought us back to our car, and an easy gasoline-powered trip out of the desert and back home. On the trip back out, I began thinking about what these silent places meant to me. And I think they felt like a warning to us – and to our resource-intensive lifestyle in the contemporary southwest.

In the first photograph of this post (actually taken on the way home), you may have noticed a piece of paper left on the car windshield. Someone else was apparently thinking the same thing about the ease of travel and life in the desert.

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Only in Arizona.

Water and life

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Most of the country is experiencing a snowy winter, but it is hard to imagine that cold here in the Sonoran desert. Our latitude and placement relative to the Pacific Ocean give us the climate of a typical midlatitude desert. Life thrives in the winter, especially in the canyons where the winter precipitation concentrates as runoff. Here are a few quick shots from the Aravaipa Canyon Wilderness.

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A javelina (Pecari tajacu) crossing the water.

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Cottonwood trees (Populus fremontii) caught in late afternoon light – leafless, but very much alive.

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Saguaros (Carnegiea gigantea) and sycamore (Platanus wrightii) in a canyon bottom.

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Insects (unknown species) mating in a late afternoon sunbeam.

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The last light of the day catches the canyon walls and is reflected on the water rushing through this wilderness. There is no snow, and that is just fine.

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