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