Black surfperch. Photo: Dan Schwartz (seawindows.org)
By Reef Check California Director Dr. Jan Freiwald
In a recent paper published in the Journal of Ecology, authors Darren Johnson, Reef Check's Jan Freiwald and Giacomo Bernardi used data on genetic diversity and Reef Check California's survey data from southern and central California to investigate the strength of population regulation in black surfperch (Embiotoca jacksoni) and how it is affected by their genetic diversity. Population regulation is the mechanism by which natural populations grow or shrink in size depending on their current population size (i.e. number of individuals). This is often related to the resources, predators and other habitat features present at a site. This type of regulation is called density-dependent population regulation because it is dependent on how many individuals are present in a given habitat and ultimately limits the number of individuals that can be supported by an environment. This is an essential process in ecology because it determines how many individuals of a species can be present in an area or habitat. Differences in the ways animals utilize available resources might determine how tightly regulated their populations are. For example, if individuals of the same species can utilize a wide variety of food resources, their population might be less regulated (i.e. their populations can grow larger) than when all individuals compete for the same resource. The way in which individuals use different available resources is linked to their variability, meaning that populations in which individuals differ more in their morphology or physiology will be able to use more resources than populations in which all individuals are similar. As much of the morphological characteristics of individuals are determined by its DNA, it is likely that genetically diverse populations are less tightly regulated and therefore can grow larger than populations with low genetic diversity.
In this study, they evaluated how much of the variation in density-dependent regulation between local black surfperch populations can be explained by their genetic diversity. Black surfperch are a common species on nearshore reefs in southern and central California. Reef Check has monitored this species since 2006 and the authors used monitoring data from 45 sites ranging from San Diego to Monterey.
Genetic diversity is an important source of biodiversity and in this study they showed that it can have a strong influence on how populations grow or shrink. Because genetically diverse populations are spread out more uniformly on the reef, they are able to use more of the available resources and grow faster when resources are available than low diversity populations where individuals are staying closer to each other and therefore are competing for resources more intensely. The results of this study suggest that the size of populations with higher genetic diversity can be up to twice as large compared to that of low-diversity populations if they experience similar environments. This study demonstrates that loss of genetic diversity can have real consequences for the size of fish populations on local reefs. In light of the potential loss of genetic diversity in wild fish populations, for example through overfishing or environmental change, this finding highlights how important it is to not only protect species, but also work to maintain their natural diversity through management and conservation.