“This,” said I at length, to the old man—“this can be nothing else than the great whirlpool of the Maelström.”
Edgar Allen Poe (Descent into the Maelstrom)
One of the key concepts in conservation biology is the extinction vortex, a model that shows how small populations get sucked in a maelstrom of genetic and demographic misery that culminates in their extinction. The reasoning behind is model is straightforward: small populations are more vulnerable to genetic drift and inbreeding, leading to a loss of genetic diversity. This genetic decline might prevent organisms from adapting to rapidly changing environments which translates into lower survival and reproduction rates. The result is an even smaller population that is again more vulnerable to the diversity-reducing forces of genetic drift and inbreeding. This negative cycle continues until the population disappears. An extinction event. A recent study in the Journal of Field Ornithology examined whether the Red-billed Tropicbird (Phaethon aethereus) should worry about descending into this treacherous vortex.
Low Genetic Diversity
At first glance, the Red-billed Tropicbird seems to be doing fine. This seabird species is considered “Least Concern” by the IUCN, although populations are declining due to competition with invasive species and habitat degradation. On local levels, however, Red-billed Tropicbirds are listed as threatened. In Mexico, for example, there are only 14 confirmed and six potential breeding colonies that vary in size from two to 1600 pairs. Absolute numbers only tell you so much, what about the genetic diversity of these colonies?
José Alfredo Castillo‐Guerrero and his colleagues sequenced one mitochondrial gene and 10 microsatellites from Red-billed Tropicbirds in the Mexican Pacific. The genetic data revealed surprisingly low levels of genetic diversity. The researchers found that 85 individuals all carried the same mitochondrial variant. For comparison, similar studies on other seabirds reported 47 variants for 55 Sooty Terns (Onichoprion fuscata) and 106 variants in 292 Masked Boobies (Sula dactylatra). The lack of genetic diversity in the mitochondrial DNA of Red-billed Tropicbirds was also reflected in the microsatellites: 7 out of ten markers showed no variation.
Should we worry?
The extremely low genetic diversity suggests that Mexican Red-billed Tropicbirds have been sucked into the extinction vortex and might be more vulnerable than we think. But is that really the case? From a theoretical point of view, the extinction vortex makes sense: smaller populations continue to lose genetic diversity which leads to even smaller populations that continue to lose genetic diversity, and so on. But several genomic studies have reported species with extremely low genetic diversity that seem to be doing just fine. For example, reconstructing the historical demography of the vaquita porpoise (Phocoena sinus) indicated an effective population size of less than 5000 for over 200,000 years. One potential benefit of small population sizes is genetic purging. In this process, deleterious variants appear more often due to inbreeding and get expelled from the population because individuals carrying these variants fail to survive or reproduce. This probably allowed the vaquita porpoise to maintain a healthy level of genetic diversity.
Another aspect to consider is the distribution of genetic diversity across the genome. There might be particular genomic regions with high levels of genetic diversity that can provide the raw material for rapid adaptation to changing conditions. Just because 10 microsatellites are not very diverse does not necessarily mean that the entire genome is deprived of useful genetic variants. It is therefore important to carefully quantify genetic variation across the genome and understand the functional contents of regions of high and low genetic diversity.
The extinction vortex is certainly a useful concept, but we should be careful not to follow it blindly. It is important to take into account other potentially positive processes, such as genetic purging. This does, however, not mean that we should not protect the Red-billed Tropicbirds in Mexico. Their low genetic diversity does set some alarm bells ringing and we need to take proper conservation actions.
Castillo‐Guerrero, J. A., Piña‐Ortiz, A., Enríquez‐Paredes, L., van der Heiden, A. M., Hernández‐Vázquez, S., Saavedra‐Sotelo, N. C., & Fernández, G. (2020). Low genetic structure and diversity of Red‐billed Tropicbirds in the Mexican Pacific. Journal of Field Ornithology, 91(2), 142-155.
Featured image: Red-billed Tropicbird © Dominic Sherony | Wikimedia Commons