Marine ecologists specializing in larvae often use two similar terms seemingly interchangeably: settlement and recruitment. What do these terms mean?
We use the word “settlement” to describe the transformation between the planktonic larval phase and the sessile/benthic adult phase. A settler is any organism that is no longer floating free in the water column–it’s now either physically attached to a habitat (like barnacles attaching to a rock) or metaphorically attached (like a fish settling around a particular coral colony).
On the other hand, we use the word “recruitment” to describe the addition of newly settled organisms into the population that scientists or surveyors have measured. So, a recruit is an organism that a scientist has seen in a settled form. Hypothetically, the number of recruits equals the number of settlers. However, this is rarely, if ever, the case. Post-settlement mortality, sampling bias, and other factors practically ensure that the measured recruitment number does not equal the actual settlement number.
In this diagram, we start out with twenty barnacle settlers on this rock face (yes, they are orange in real life!). But time has passed before our surveyor comes along to count this year’s new barnacles. Some barnacles have died between settlement and recruitment recording. So, our surveyor only finds 13 barnacle recruits! The number of recruits does not equal the number of settlers.
Time is critical when measuring recruitment. The less time between settlement and recording, the better, as fewer new settlers have died. However, it can be very difficult to reduce the time between settlement and when we measure recruitment. Factors such as tides, weather conditions, and funding can reduce scientists’ ability to minimize this bias.
Plus, real life isn’t as simple as this diagram! It’s easy to miss a barnacle or two (or three, or twenty) hiding in a crevice or in the shadow of an adult barnacle while sampling. Or, the area measured may not be indicative of larger-scale settlement patterns!
In order to separate these two concepts, we use the terms “settlement” and “recruitment” to reduce confusion and more accurately describe what we’re talking about!
Do you have a lingering question about settlement and recruitment? Ask it in the comments section and I will be happy to help as best I can!