Friday, October 4, 2013

Melting Ice = Missing Species?

A recent feature in National Geographic magazine posed a provocative question: What would today’s world look like without ice? Earth was last ice-free during the Eocene (56 million years ago) and didn’t start cooling down until 40 million years ago. If all of that frozen water was melted into Earth’s oceans now, sea level would be about 216 feet higher than it is now, flooding Florida, the Eastern seaboard and California’s Central valley.

The global meltdown isn’t anticipated for millennia, but within the next century we can reasonably expect to see the waters rise about 1-2 meters. For the residents of low-lying oceanic islands, this could be a bit of a problem. Islands often harbor plants and animals found nowhere else in the world and these ‘endemic’ species are prime candidates for the big so-long if their homes become submerged. Just how many species are at risk of extinction simply because their island paradises are soon to be flooded?

This summer, researchers at the University of Paris-Sud published an analysis of how the loss of area to 4447 islands in biodiversity hotspots would affect endemic species. If the sea-level rises 1m, 267 islands will be completely sunk and 489 islands will lose more than half of their area, with islands in the Caribbean being the most endangered. Most islands (2,579), will make out without any area lost. However, the researchers also ran the numbers for more extreme estimates of sea-level rise (up to 6m) and found that such inundation could lead to the sinking of 826 islands with 1,423 losing more than half of their area.

How do these areal losses convert to species extinctions? One of biogeography’s few ‘laws’ is that islands with larger areas have more species following a fairly precise mathematical relationship (SAR). Calculations based on this relationship can be used to find the area that needs to be lost in order to remove the last individual of a species and, with a little more calculation, how many extinctions are probable when a given amount of area disappears.

Using these rules, the researchers estimated that 26 plant species, 1 bird and 1 reptile species are likely to go extinct. Under more extreme conditions (i.e. 6m sea level rise) these numbers rise to 300 plants, 8 birds, 18 reptiles, and an additional 8 amphibians, 3 fish, and 2 mammals. Considering that this is out of 58,195 endemic species, this is good news, at least in the compounding drama that is Earth’s climate change woes. Unfortunately, a shrinking island may be the least of most of these species concerns. Human activities, like farming and city-ing, can shrink island habitat much faster than a melting ice-cap can raise the waters.

You can find this article at:

Bellard C., Leclerc C. & Courchamp F. (2013). Impact of sea level rise on the 10 insular biodiversity hotspots, Global Ecology and Biogeography, n/a-n/a. DOI:

Monday, May 6, 2013

The Aspens that were left behind

When climates change, species move. It’s a fact of life on Earth and probably has been for the past 542 million years, even when species don’t have legs or wings or fins to get them from place to place.

Quaking aspen is one example of a seemingly stationary species that has managed in just the past 20,000 years to expand into the largest range of any native North American tree. These distinctive trees grow across Alaska and Canada all the way south along the Rocky Mountains to Mexico. Since most of their current northern distribution was buried under up to 2 miles of ice just 22,000 years ago, aspens provide a great opportunity to study how species move during climate change.

Researchers at Utah State University were curious whether the aspens that currently live in the mountains of the western United States are stragglers that have occupied this region since the species moved north after the last ice age and have subsequently climbed into the mountains to avoid the warming lowlands, or whether the southern aspens are wayward offspring of the main northern population that found the mountains to be a distant, but suitable, habitat.

In order to find the answer the researchers analyzed short sections of DNA from almost 800 individual trees in 30 different forests spanning the species’ entire range. The genetic variation among these trees showed that forests in the northern part of the aspen’s range are all reproducing with each other across thousands of kilometers, or are at least very recent relatives of one another. In contrast, forests in the south are more isolated and probably represent the remnants of two populations that took refuge on opposite sides of the Great Basin during the last ice age.

This geographic history has interesting consequences for aspen genetic diversity (how much variation there is among members of the same species in their DNA). Southern aspens have lower genetic diversity within a forest compared to aspens in northern forests, mainly because of their isolation and propensity to reproduce by sprouting clones rather than by seeds.  However, the amount of variation between forests in the south is much higher than in the north, so that, taken all together, northern and southern aspens embody about the same level of genetic diversity. This means that losing a single forest in the south will eliminate much more of the genetic diversity within the entire species than losing a single northern forest.

Unfortunately, the aspens with the most unique genes are also the most likely to disappear; climate change in the next century is expected to decrease southern aspen’s habitat by as much as 94%. Genetic diversity holds a species’ potential ability to adapt to new conditions. What incipient adaptability to current climate change is stored in the DNA of the endangered southern aspens? It is a bit ironic that the most vulnerable forests house trees whose genetic material could hold the adaptive key to outrunning the cause of this vulnerability. The question is, will this potential manifest before its basis is lost?

You can find this article at:

ResearchBlogging.orgCallahan, C., Rowe, C., Ryel, R., Shaw, J., Madritch, M., & Mock, K. (2013). Continental-scale assessment of genetic diversity and population structure in quaking aspen (Populus tremuloides) Journal of Biogeography DOI: 10.1111/jbi.12115

Wednesday, May 1, 2013

Sing a song of Sphagnum

For a plant, there is one good thing about being small; it’s a lot easier to get everywhere- for your seeds, that is. Nowhere is this more evident than where all plants are really small- the Arctic tundra.

The northern latitudes are covered by vast expanses of tree-less terrain covered by mosses and lichens that, for the most part, are the same around the world. Biogeographers believe that this similarity is facilitated by the small size of the spores of these organisms. The spores are 20-40 micrometers long (it would take about 500 spores to stretch across a dime) and can easily get picked up by the nearest breeze and transported thousands of kilometers. Upon landing, they germinate and grow into a new baby mosses and lichens. With frequent enough inter-continental exchange of spores, tundra looks like tundra looks like tundra.

Although spores are small and scientists think they travel great distances, testing this is actually quite difficult. How many spores actually do make the trip thousands of kilometers across oceans to new continents?

A scientist at Uppsala University in Sweden went out and measured how many spores of Sphagnum moss could be found at different distances from peat bogs (where Sphagnum and bog mummies live). Although 20 million spores were produced in every m2 of bog, only 6 million made it up into the air and just 4% of these managed to travel 40 meters away. Despite this rapid decline with distance from the bog, some spores are able to travel great distances. On Svalbard, the cloth spore traps showed that 1000 spores are deposited per m2 every growing season. And this is on a barren island 820 km north of the nearest Sphagnum bog (in Norway).

Oddly enough, the spores that made it to Svalbard and other islands were larger than the spores found at sites closer to bogs. Evidently, for Sphagnum, getting smaller is not a recipe for getting farther, and past a certain point, size really doesn’t matter.

You can find this article at:

ResearchBlogging.orgSundberg, S. (2013). Spore rain in relation to regional sources and beyond Ecography, 36 (3), 364-373 DOI: 10.1111/j.1600-0587.2012.07664.x

Sunday, March 17, 2013

Where have all the dry forests gone?

The environmentally conscientious citizen is well aware of the plight of the world’s tropical rainforests and our moral obligation to protect these biodiverse shelters of the next new cancer drug. But how many know of the troubles facing the tropical dry forests? (Or could even find them on a map?1) Just over 40% of tropical and subtropical forests are ‘dry’ forests where the trees lose their leaves each year, not because of a cold winter, but because of a rainless dry season.

Mopane woodland in Zambia,
a unique type of dry forest.
A study out this month in the Journal of Biogeography finds that dry forest loss in tropical Africa rivals that of deforestation in the rainforest. By comparing satellite photos from 1990-2000 across a grid of 784 locations spanning the continent, the researchers determined that 0.34% of dry forest was lost annually during the nineties compared to 0.16% in the central African rainforest (found by a previous study). This loss amounts to an area the size of Maine over the ten-year period studied and doesn’t include an equal amount of dense forest that is degraded to open forest each year.

These satellite-based estimates of deforestation are lower than estimates based on reports from individual countries and are lower than estimates of forest loss from local hotspots of forest degradation. However they are a relatively accurate summary of continent-wide forest loss. Whether local or broad scale estimates are more useful managing forest resources is a question for a conservation biologist. Regardless, 350 square-miles per year is no small amount for an ecosystem that supports more than half of Africa’s population.

So where have all the trees been going? Fields and firewood are the chief culprits. Everyone needs to eat and this requires land for growing and fuel for cooking.

What is a post about deforestation doing on a blog about biogeography? Two reasons. Images taken from satellites (‘remote sensing’) have enormous potential to inform our understanding of the geography of biological systems, which I hope to highlight this and other posts. Second, there are interesting geographic differences in deforestation rates across Africa. West African dry forests had much lower rates of tree loss than southern African dry forests, but this turns out to be because West Africa lost most of its dry forest prior to 1990 and there wasn’t much left to lose. Instead, in this part of the continent the researchers saw numerous switches between wooded shrub-land and non-wooded land that reflected agricultural practices.

Even though this research just came out this month, it’s actually rather old news. Over ten years have passed since the last satellite photo used in this study was taken. What has happened to forests in Africa since 2000? What story might recent photos tell about the plight of the dry forests?

You can find this article at:

Bodart, C., Brink, A.B., Donnay, F., Lupi, A., Mayaux, P., & Achard, F. (2013). Continental estimates of forest cover and forest cover changes in the dry ecosystems of Africa between 1990 and 2000 Journal of Biogeography. DOI: 10.1111/jbi.12084

1 Here’s a fun Google Earth map of the Holdrege life zones.

Footnote: I spent 2009 living in Malawi, Africa where I observed first-hand the impacts of rural agricultural life on the tropical dry forest ecosystem. The dependence of humans on wood has never been more apparent to me than when I flew across the border from Malawi (90 people / km2) to Zambia (13 people / km2) in a small aircraft. The political border could have practically been delineated by trees. 

Left: Zambia, Right: Malawi

Sunday, February 3, 2013

Asian Artemisia makes touchdown on Hawaiian soil and heads for the hills


Above the surf, sand, and mai-tai toting tourists, the Hawaiian Islands soar to heights that would be... uncomfortable for bikini and board short-clad beach-goers. Yet, these cold and windy high elevation environments are home to some of Hawaii’s greatest treasures; botanical absurdities reside on top of the Haleakala, Mauna Kea and Mauna Loa volcanoes that grow nowhere else in the world. But, just 6 million years ago the southeastern Hawaiian islands were nothing more than an expanse of open ocean. Where did the high-elevation flora of Hawaii come from? Are the species related to plants from boreal and temperate regions that came to the islands already adapted to a cold climate? Or, are they relatives of plants living in the lower elevation tropical climates that managed to dramatically adapt to new conditions? For the famous silverswords and woody violets, most evidence supports the former explanation, but a new study in the Journal of Biogeography has found a group of plants that seem to buck this trend.

Artemisia mauiensis grows only on the slopes of the Haleakala volcano on the island of Maui.  There are a couple other species of Artemisia that also grow only in Hawaii, but they live in the warmer lower elevations. Have these endemic Artemisias all evolved from one ancestor that originally colonized Hawaii? Given that there are 350-500 species of Artemisia distributed in a wide variety of habitats all around the world, it’s possible that the high-elevation species is a descendent of a colonizer from northern latitudes, while the lowland species came to the islands by way of a more tropical ancestor.

Using DNA from 114 species of Artemisia from around the world and an estimate of the date when the first Artemisia evolved based on 31 million-year-old fossil pollen, two researchers at the University of California Berkeley created a phylogeny of species relationships that included Artemisia mauiensis and two other Hawaiian endemics. This ‘family tree’ showed that all three of the Hawaiian species were each others’ closest relatives and descendants of an ancestor that probably lived 1½ million years ago. That ancestor likely came from Asia, since the Hawaiian species’ next closest relative, Artemisia chinensis, lives on islands stretching across the Pacific from Taiwan to the Bonin Islands. Since chinensis is a tropical plant that can’t handle the cold, it is likely that the high-elevation mauiensis evolved in a relatively short amount of time from a tropical ancestor to a plant well-adapted to cold temperatures and harsh conditions.

But how did the tropical Asian ancestor get to Hawaii? Birds, most likely. The researchers discovered that two of the Artemisia species, chinensis and the oldest of the hawaiian endemics, Artemisia kauaiensis, both have long curved hooks on the ends of their fruits that could easily get caught in a migratory bird’s feathers and accidentally carried half-way across the Pacific. The Hawaiian species that evolved more recently don’t have these hooks- which parallels other instances where plants living on islands lose characteristics that make dispersal easier.

This isn’t the first time that a warmer-weather Artemisia has changed tactics and headed for colder climes. In their paper the researchers bring up several other examples, such as how Artemisia species living in the Arctic evolved several different times from ancestors who were originally more southerly. The worldwide success of Artemisia raises the intriguing idea that species whose genes allow them to easily adapt to different environments diversify more rapidly. Such evolutionary flexibility may just be the key to life’s diversity.

You can find this article at:
Christopher R. Hobbs, & Bruce G. Baldwin (2013). Asian origin and upslope migration of Hawaiian Artemisia (Compositae-Anthemidea) Journal of Biogeography : 10.1111/jbi.12046

Tuesday, January 22, 2013

Cell phones track human migration

Here’s a question every scientist at some point asks themselves: does this data that I can easily and (relatively) inexpensively collect reasonably approximate the data that I would collect in an ideal world where I had bucket loads of money and an infinite amount of time? It may not be apparent from science news coverage, but a lot of science involves routinely checking that the methods we are using to investigate a question will actually result in an answer. So here’s a story about the unsung iceberg hiding beneath any good scientist’s crowning achievement (of which only a few ever make it into the press, anyway).

Like any other animal, humans move around. They commute from home to work, travel to see their grandparents, move to new cities, vacation in Tahiti... you get the idea. A lot of epidemiologists, sociologists, and airline companies would love to know why they do so.  But, before you can predict why people go where they do, there has to be good data that show where people actually go. Unlike wolves, humans are somewhat averse to GPS collars that track their every move, so the easiest data for researchers to access is national censuses that ask whether a person has changed residences within the last year.

But, do all of the permanent relocations that happen within any given year represent the day-to-day, or even week-to-week movements of people who don’t necessarily change homes? Census data might not be fine-scale enough to capture the kinds of movements that are important, for example, in the spread of disease. Luckily, people are willing carry little transmitters with them everywhere they go, periodically sending their locations throughout the day to the companies that provide these devices. (I’m talking about cellphones, all 6 billion of them.) Every time someone makes a call or sends a text, the nearest cellphone tower records their location and the location of the tower closest to the person they are calling. And cell phone companies have access to these records- the only thing standing in the way of this data and science is a little thing called privacy.

Cell records being difficult to access, graduate student and researcher Amy Wesolowski and colleagues decided to test whether freely available census data is able to approximate the movements shown by the fine-scale cell phone data. They were able to obtain cell phone data from Kenya during the year than matched the most recent national census and used this to calculate how many trips cell phone users made between different counties in Kenya. They then compared these trips to the relocations recorded in the census data. In the end, the movements recorded by the census were surprisingly similar to trips made by cell phone users, which suggests that, after some more validation, censuses could be useful for modeling movement over shorter time periods.

The yearly movements of humans in a moderately small African nation may not fall under the traditional realm of biogeography, but I present this research because all too often, the natural sciences categorize humans as an external force on whatever system is being studied. As a scientist, sometimes it is important to recognize that the natural laws we study also apply to us. Dispersal is the foundation of all biogeographic patterns and humans do it more than any other species on the globe.

You can find this article at:

ResearchBlogging.orgWesolowski, A., Buckee, C., Pindolia, D., Eagle, N., Smith, D., Garcia, A., & Tatem, A. (2013). The Use of Census Migration Data to Approximate Human Movement Patterns across Temporal Scales PLoS ONE, 8 (1) DOI: 10.1371/journal.pone.0052971

Wednesday, January 16, 2013

Eric and the traveling plants

A little over 1000 years ago Eric the Red sailed around the southern tip of Greenland to set up the first successful European settlement in Greenland.  But it wasn’t just people and farm animals that joined Eric in his exile- seeds of several weedy plants likely stowed away on sheep fur and hay to become their species’ first representatives on the western coast of Greenland.  For decades botanists have suspected that a suite of “Old Norse” plants first rounded the Cape by way of Viking ships. A forthcoming study in the Journal of Biogeography confirms that at least three of these species were indeed introductions, rather than simply pre-existing residents whose life-style was favored by human activities.

Researchers in the U.K. and Iceland looked for pollen from three “Old Norse” plants (Sheep’s Sorrel, Common Knotgrass, and Common Yarrow) in layers of lake sediments near farm sites in the historical Eastern Settlement of Norse Greenland. When viewed under a microscope, plant pollen takes the form of small granules whose shape depends on which species the pollen comes from. When pollen lands on the surface of small lakes, it sinks to the bottom along with other leaves and detritus, which eventually builds up a layered mud pie recording which plants lived near the lake at different points in time.

According to historical texts, the settlement was active from 985-1400 CE so the scientists looked for whether pollen was present in mud deposited before, during and after this time period to determine how the plants arrived. This record showed that the three species first appeared around the time that the settlement began, increased in abundance throughout the settlement period, and declined during the century after the settlers abandoned their farms. What’s more- the first records of each species show up close to Qassiarsuk, believed to be the location of Erik’s original farm.

It’s not that surprising that the Vikings unintentionally transported foreign plants- the introduction of exotic species is an ancient tradition among humans that continues to this day. To me, the interesting part of the story was that once the Little Ice Age kicked the settlers out of Greenland, the introduced plants also began to disappear. There is sometimes a misconception that all introduced species become invasive, causing harm to the native ecosystem. In this case that didn’t happen, likely because these plants are ruderals (i.e. weeds) that prefer to live in disturbed areas like the trampled ground around homes and farms. Once their favorite habitat was gone, the introduced plants just didn’t do as well when they had to live like the natives. At least until the Return of the Europeans.

You can find this article at:

ResearchBlogging.orgJ. Edward Schofield, Kevin J. Edwards, Egill Erlendsson, & Paul M. Ledger (2012). Palynology supports 'Old Norse' introductions to the flora of Greenland Journal of Biogeography : 10.1111/jbi.12067