Population Connectivity

We're all connected in some way... think of 6-degrees of separation. However, the actual distance between us varies. Some of us are more dispersive and nomadic - moving away from our hometown to explore the world; while some of us stay close to home, maybe even living on the same street as our family. The more movement of people that occurs, the more connected we are likely to become. The movement (or dispersal) of people has important implications for our country's security, population census, the spread of disease, and the spread of ideas.  Similarly, the dispersal of other organisms has important implications for population maintenance, dynamics and survival.

For many marine invertebrates, the adult stage is spent as a benthic existence, on the bottom of the seafloor, with limited dispersal.  It is their offspring during their larval stage that are released into the water column to go forth and 'discover' new areas.  Larvae of different species have different dispersal abilities - based on how long they stay in the water column, the movement of the water, their position in the water column, and so forth.  The small size of the larvae (from the thickness of a piece of paper to the size of a pencil tip) and the large size of the ocean makes them difficult to track.  Scientists have come up with numerous cleaver ways to try to figure out where individuals go - from their parents' home to their new home.  We use genetics, models, chemistry, and old fashioned collection of larvae.

Why should we put forth so much effort to track these little guys?  It is becoming increasingly important that we know how populations are connected through dispersal (of larvae).  Humans have greatly impacted the ocean.  Habitats have been destroyed and populations of sea creatures wiped out.  Our efforts to try to counter act our multifaceted damage rely on knowing animals' movements.

Lets consider a side of a seamount (underwater mountain) that has been damaged by trawling.  The trawl essentially bulldozes all of the animals in its path.  How fast will this damaged area recover?  Will the same species come back?  The answer to these questions depend on the distance to the nearest healthy populations and the ability of those individuals or their offspring to make it to the damaged site - dispersal!

Our marine protected areas are supposed to serve two functions - provide a healthy habitat for species to live in and provide a source of animals to outside locations.  First, we give the animals a nice place to live so we can have a large healthy population. Then, we want them to leave this oasis.  This is actually very tricky to accomplish.  If the organisms disperse too much, the protected area won't have a large healthy population because all of the animals would have left in a couple of generations.  Conversely, if the organisms don't disperse at all, the protected area won't act as a source of animals to outside areas that can then be fished or help reestablish ailing populations.  Understanding the scales of dispersal is thus essential to designing effective marine protected areas.