How do the same fish species end up in different lakes hundreds of miles apart? —S. Snyder, Sebring, Fla.

Megan McPhee, an assistant research professor at the University of Montana's Flathead Lake Biological Station, offers this explanation:

There are two general explanations for how a fish species might end up in different lakes separated by great distances.

The first is termed “vicariance” by biogeographers, who study the distribution of organisms. In this case, a species originally occupies a much larger, continuous range. Over long periods, geologic, biological and climatic events cause populations to go extinct in scattered places throughout that range, leaving behind isolated present-day populations. For example, during the late Pliocene and Pleistocene epochs (between two million to 0.5 million years ago), western North America experienced a much wetter climate and giant lakes occupied many basins. When an increasingly arid climate swept into the area, large lakes dried into separate, smaller ones—leaving fish species isolated.

The second explanation is based on dispersal, or movement of individuals away from the population in which they were born. Sometimes these individuals spread into new areas previously unoccupied by members of their species. In fish, where most species require male-female sex to reproduce, a minimum of one of each sex would be needed to colonize a new lake. Whereas the chance of this event happening once is quite small, in time enough opportunities will arise for these rare dispersals to result in the colonization of new lakes.

Over longer periods, dispersal is often facilitated by headwater capture—a river tributary erodes through the divide between two rivers, thus linking them. Although this mechanism is primarily responsible for the transfer of river-dwelling fish, it can also move lake fish between basins if they spend part of their life cycle in rivers.

Finally, humans are responsible for moving fish great distances. In many cases, this dispersal occurs intentionally because people want to fish for a particular species outside its native range. In the late 19th century the U.S. Fish Commission made concerted efforts to introduce carp—as a food fish—into the waters of the western U.S., where they can now be found in lakes and reservoirs throughout the region. Unsanctioned or even accidental transfer occurs when people release aquarium fish or empty out bait buckets into wild habitats. We have learned that nonnative fish (including carp) often prey on or compete with native species, so such practices are now largely discouraged.

How does Bluetooth work?

Bluetooth Special Interest Group executive director Michael Foley transmits an answer:

Bluetooth is a short-range wireless communications technology that replaces the cables connecting electronic devices. It uses the principles of device “inquiry” and “inquiry scan.” Scanning devices listen in on known radio frequencies for devices that are actively inquiring. When the scanner receives an inquiry, it sends a response with the information needed to forge a connection.

A group of devices then forms a so-called piconet, which may contain one master and up to seven active slaves, with additional slaves that are not currently participating in the network. (A given device may be part of more than one piconet, either as a master or as a slave.) In a piconet, the devices are synchronized to a common clock and frequency-hopping pattern, and they share a radio channel. The pattern, which the master device determines algorithmically, helps to combat interference and fading. The basic hopping pattern cycles through the 79 available frequencies, but it may be adapted to exclude those that are used by interfering devices, improving Bluetooth's coexistence with static (nonhopping) systems, such as Wi-Fi networks, that may be in the vicinity of a piconet.

The wireless link is subdivided into time units known as slots. Bluetooth-enabled devices transmit data in so-called packets during these slots, with frequency hopping taking place between the transmission and reception of packets. All this complexity, of course, goes on without the user being aware of anything more than the task he or she is trying to complete, such as talking hands-free on a cell phone or listening to music on wireless headphones.

HAVE A QUESTION?... Send it to experts@SciAm.com or go to www.SciAm.com/asktheexperts