Never underestimate a squirrel. Growing up near the Atlantic coast of North America, I was familiar with the native eastern gray squirrel (Sciurus carolinensis) as a familiar feature of forest and urban environments. It seemed to coexist in both habitats well enough, and I never heard any stories of it causing ecological damage beyond the occasional occupation of a house.
A few weeks ago, I was on a tour of the Forest of Dean in England, led by managers from the government’s Forestry Commission. I knew the eastern gray squirrel was invasive in Europe, displacing the native red squirrel, but was surprised by the level of anger directed at this species by the land managers.
I learned that it causes dieback of many native species as well. Apparently it strips bark from trees like oak and beech. Because the bark conducts water and sugars to different parts of the trunk, bark loss can easily lead to the death of the whole tree. The downstream consequence is shifts in the dynamics of forest succession and the loss of trees that would otherwise be sold for timber. And there was evidence of these losses throughout the forest.
For a North American, this was an incredible surprise – to see a species shift so dramatically in its impact in what appeared to be very similar habitat. I don’t know what exactly causes this behavioral shift, and no one else among this group of ecologists seemed to either. Maybe they do actually strip bark in their native range but no one notices, or minds. Regardless, the land managers were certain about the damages caused by these squirrels and the need to control them, and equally surprised to hear that they caused no such problems in North America.
This experience made me reflect on the difficulty of predicting how species interactions will change in novel climates and biotic contexts. And it gave me a new respect for this species. The eastern gray squirrel is full of surprises.
Spring is yet to come in parts of Europe. I spent most of April in Norway working with collaborators in Trondheim, and was impressed by how much snow still remains in the mountains.
Lakes are still frozen, trees shrouded, and the high peaks buried under white cover. It is a very different landscape than the England I returned to.
But the snow isn’t everywhere. The variation in snowpack tells a fascinating story about where spring will come first. The first day of spring really comes on the date the ground becomes bare and life can return. And when the snow melts can vary by weeks even over a few meters of distance.
Wind can compact or drift the snow in some places, and blow it away from others. On ridge lines and mountaintops there are often complex patterns of bare ground brought on by these flows.
Darker surfaces absorb more heat, and melt snow faster. The dark tree bark of these trees has produced rings around each trunk, so that the ground will eventually open up near the trees sooner than away from them.
And landscapes’ orientation relative to the sun can lead to dramatic patterns of melt-out. This south-facing slope is bare and already supporting new growth while a meter of snow still sits on the north-facing slope of the same hill.
All of the ecology and growth of the organisms that need the summer warmth must follow these later-winter rhythms. When we look at a spring landscape, we are really seeing the history and the shadow of the snow that was once there. Winter does not end all at once, not anywhere.
Spring in England means every day is longer than the one that precedes it by more than a few minutes. With the lengthening of the days comes the re-emergence of the forest. Earlier this week I went out with our research group to the canopy walkway at Wytham Woods.
It is a short climb up several ladders to reach the top of the forest.
The world changes on the journey up to the top. Leaves that look like small green dots from the ground begin to take their proper form, and branches that seemed impossibly far away come within an arm’s reach. The perspective changes from upward to sideways, and one gets a sense of how trees might see themselves and each other. As neighbors in a crowded environment, fighting amidst the sway of the wind and the bright light of the sun.
Looking out at all these trees, it becomes clear that not every tree is responding to the longer days in the same way. Each individual is following its own particular strategy, taking its own counsel on how best to respond to the environment.
This beech tree (Fagus sylvatica (Fagaceae)) has elongated its buds, but has no leaves deployed yet.
in contrast, this sycamore (an unfortunate British common name for Acer psuedoplatanus (Sapindaceae) – the same name elsewhere refers to Platanus spp. (Platanaceae)) has leafed out several days earlier, and has flowers deployed too.
The congeneric species field maple (Acer campestre) is still in bud, and has no leaves or flowers deployed at all. These buds have a beautiful waxy-red color that may assist in protected buds from ultraviolet light damage.
This English oak (Quercus robur (Fagaceae)) tree has begun to leaf out, but there seems to be wide variation among individual trees.
For example, this other English oak tree still has only buds.
Back on the ground, the understorey is also still beginning to develop. Some species look to be near their flowering peak, as for this cream-colored primrose (Primula vulgaris (Primulaceae). Others, like this purple-colored bluebell (Hyacinthoides non-scripta (Asparagaceae)) are just beginning to flower.
Why is there such divergence in the spring strategies of these different plants? Proximately, each species is likely responding to or receiving different cues. Plants have marvelous sensory abilities. Spring leaf-out and flowering is thought to be triggered by a combination of day length (via light color detection) and temperature, with the thresholds for each varying among species. Forests also support a range of different temperature and light microenvironments. The oak tree with leaves may be experiencing very different conditions than the oak tree with buds. Ultimately, however, each species has evolved to follow a different set of rules for beginning spring growth. Grow too early and delicate leaves may freeze; grow too late and those leaves may not gain as much carbon as others deployed sooner. The physiology of each species means there are different tradeoffs each species must consider. These rules work well enough in the evolutionary and statistical average for each species to successfully grow and reproduce. What happens in any given year is a different question.
Looking in the canopy, I saw these trees making gambles. Each of these individuals has chosen to either begin or delay its investment in new leaves and flowers. Some days it is sunny; some days it snows. It is too early to know whether each developmental program will fare well or badly this year. But the air is full of hope for growth and new life.
Early spring is when the elevation contrasts of Britain’s mountains are clearest. These weeks I have been wandering through England and in Wales, climbing where possible, seeing what could be found among the valleys and hilltops.
The transition between winter and spring means that the valleys are warm, and the hilltops are still cold, so the zonation between snow-free and snow-covered areas is sharp. Mud and fields transition to occasional bunchgrass tussocks covered by snow, and then areas wholly blanketed under.
These quiet landscapes are beautiful reminders of colder and longer winters in other parts of the world. But unlike these other regions, these hills are largely bereft of animals, who can easily walk downslope to more suitable conditions. I saw no tracks in the snow on any of my explorations.
The snow is going. The winter has been a warm one, and the ice is melting.
So the streams begin to fill, clearing the hills of snow and bringing life to the valleys again.
A few species have begun their spring blooms lower down, bringing some color back to the land.
This willow (Salix sp.) has male flowers with stamens covered in bright yellow pollen.
And this witch-hazel (Hammamelis sp.) has beautiful dark-purple flowers.
Soon both these species will flush their leaves and begin to grow in earnest, relying in part on the snowmelt water coming off the hills. Soon the high places will lose their white cover, and the green will return. Soon the balance will tip, and the contrast between high and low will fade away.
The red flowers of the invasive African tulip tree (Spathodea campanulata; Bignoniaceae) stand out in a Puerto Rican rain forest. This species may occupy a mathematical hole, having an ecological strategy unoccupied by other species in the community. Recent mathematical advances now allow detection of holes and better understandings of species invasions and extinctions.
Imagine an island populated by small and large animals. A reasonable assumption would be that the island might also have medium-sized species—but what if none are found? These empty spaces, or ‘holes,’ may represent missing components of ecological communities. Identifying these holes may help researchers predict the outcomes of invasions, extinctions, and long-term evolution. What we don’t see may matter as much as what we do. My new paper in The American Naturalist (link, PDF) now provides a new way to answer the previously unanswerable question—does a community have holes, or not?
The trouble is that holes are difficult to detect. Imagine a volleyball. This three-dimensional shape has an empty space inside of it, but from a two-dimensional photograph you would never be able to tell that it isn’t solid. Ecological strategies work the same way. Along any individual strategy axis (like body size, or temperature tolerance), species may be able to take on small, medium, and large values—but when considering all axes together, some combinations may never occur, just like the hidden interior of the volleyball. Yet these holes may be exactly where a new species could invade a community, or where another species may have recently gone extinct.
Computers have not been able to solve this problem before now. As the number of dimensions in the ecological strategy space increases, the number of possible ecological strategies grows exponentially. That means there are too many places to find a hole for a computer to easily find. But there is now a new set of free software tools that will enable researchers to efficiently discover such holes (CRAN link to hypervolume package). These holes may represent forbidden ecological strategies or available but unrealized opportunities that have never yet been identified. We don’t know how common holes are in real ecological communities, but we won’t know until we look. Now we can.
Winter in the United Kingdom is quiet. Organisms retreat underground or migrate to less harsh climates. Only simple winter scenes remain, painted in broad figures with a restricted palette of colors. The complexity of summer is temporarily washed away.
Fields are seas of black soil, prepared for spring planting.
Coastal landscapes are brown, dominated by the decomposing fronds of the bracken fern, Pteridium aquilinum.
Harvested fields are golden, covered by the stems and chaff of last year’s growth.
Pastures are under frost with only few species of grass still green at the surface.
These simple scenes do not endure. As the earth moves along its orbit, the skies brighten, and the air warms. The frost melts, and the ice retreats.
Little by little, the memories of the previous year return. And a few species begin to flower, ushering in the spring.
The gorse (Ulex europaeus) puts out a few tentative flowers, transforming coastal landscapes from uniform dark grays to mosaics of yellow and green.
And the first spring Crocus blooms begin to appear. I am glad for the return of a little life to these landscapes, and for the lengthening of the days.
I think every biologist has a favorite kind of organism. For some people it is a charismatic animal, like an elephant, or a whale. For me it is silverswords. I first learned about these plants in my first year of graduate school. Someone was giving a lecture on adaptive radiations, and showed photographs of a strange-looking plant on a Hawaiian volcano. It had a silvery-gray rosette with long straight leaves, and was growing out of the bare lava (here, Argyroxiphium sandwicense ssp. macrocephalum).
Then he showed another photograph of a different species, this one more shrub-like and growing in a bog, but with something of the same spirit in it (Argyroxiphium kauense). I learned that these two species were part of a group of more than thirty, and were all descended from a single ancestor that colonized the Hawaiian islands a few million years ago. From an unremarkable California tarweed came the entire Hawaiian silversword alliance – the genera Argyroxiphium, Dubautia, and Wilkesia (Asteraceae). The man giving the lecture was Rob Robichaux, a silversword expert. That very day I decided I was going to see them myself, and study them.
That was 2009. I got funding from National Geographic’s Young Explorers program in 2010 with the help of my doctoral supervisor Brian Enquist, and went to Hawaii in 2011 for fieldwork under the guidance of Rob Robichaux (below) and Bruce Baldwin (two below), with extra help from Emma Wollman.
The silversword alliance is commonly thought of as one of the most spectacular and diverse adaptive radiations anywhere on the planet, but direct measurements of functional variation among species were somewhat limited. I was intrigued by the apparent diversity of their leaves’ venation networks, as passingly described by Sherwin Carlquist in 1959. I focused the project on understanding how functionally diverse this set of thirty-odd species really was, given that they occupy all the major habitats and islands of the Hawaiian archipelago.
Five years later, the work is all done and the paper is finally out. You can read it, “Variation and macroevolution in leaf functional traits in the Hawaiian silversword alliance (Asteraceae)”, in the Journal of Ecology (link, PDF).
The short message from the paper is that the clade really is a world-class example of adaptive radiation. The variation in traits among the clade is closely matched to global ranges of trait variation across all species – and this variation seems to evolve very quickly with few evolutionary constraints. These findings provide a quantitative perspective on a charismatic group of plants, and hopefully support arguments for conservation of this endemic and often threatened flora.
It has been a dream to work on this group of plants alongside some of the botanists who love them best. A few species in the clade are now extinct, and several others are either threatened or endangered – for example, this Argyroxiphium sandwicense ssp. sandwicense individual high on a cliff face, protected from non-native sheep browsing – one of fewer than 40 individuals known to exist when Rob became involved with the ongoing state conservation efforts.
Or this Dubautia latifolia individual that I was only able to sample from a herbarium collection – no more than a few dozen individuals are known to be alive in the wild.
Or this Wilkesia hobdyi, in cultivation at the National Tropical Botanical Garden on Kaua`i.
It was humbling getting to work with such rare species, touching individuals of something shaped by millions of years of evolution and now so close to being gone. I had never held an USFWS endangered species recovery permit before this project, and felt very much the obligation to take good care of the plants I was studying, but also the challenges faced in conserving and restoring populations of these native species on an archipelago now run over by non-native species.
Yet among these challenges, the beauty of the habitats and forms of these species remains incredible. Here are a few examples of other species in the clade:
Dubautia platyphylla, growing on the dry slopes of Haleakalā volcano.
Dubautia menziesii, on cinder cones at the summit of Haleakalā.
Dubautia reticulata, a tall tree-like species growing at The Nature Conservancy’s Waikamoi Preserve.
Wilkesia gymnoxiphium, a spindly rosette growing near Waimea Canyon.
Dubautia linearis, a tall shrub in the high plateau between Mauna Loa, Mauna Kea and Hualālai.
I’m not sure what resonates with me so deeply about these plants. Maybe it is the tenacity and hope their continued existence represents. Maybe it is how their underlying form becomes multiplied and reflected on different island habitats. Maybe it is the feeling of touching a soft and silvery leaf. But it is a deep sort of affinity that persists across thousands of miles of ocean, and I am sure our paths will cross again soon.