Monday, February 05, 2018

Minimum Viable Population

Distribution of Prairie fringed orchid (Platanthera praeclara) in a portion of Ransom County, ND. 
Map from a 2008 report. 
This is a federally threatened species. In a given year, there may be 10-20,000 specimens in existence, distributed across 45 counties. This is a drastic decline from pre-settlement populations, which enjoyed vast expanses of tallgrass prairie habitat. Today, that habitat is one of the rarest in North America.

While 10-20,000 plants in 45 counties may sound like a lot, it is not. Here is why: Species have what are called Minimum Viable Populations (MVP), or extinction thresholds. These are population numbers below which they spiral into extinction. Below this threshold, the effects of inbreeding, loss of genetic variability, or loss of mating opportunities are fatal to the population. They go extinct.

There have been attempts to establish a universal MVP; in the 1980's it was suggested that at least 50 individuals were needed to prevent inbreeding and 500 individuals were needed to prevent genetic drift. Genetic drift is the change in frequency of a particular gene variant, which can be devastating if that frequency is a decline or loss of frequency. Which is to say, a loss of genetic diversity, a capacity to express traits that may be needed in a given environment. This is called a shrinking gene pool. 

But those threshold numbers, while somewhat useful, are persistently impractical because reproductive rates, existing genetic diversity, and habitat requirements vary from species to species. Consider: Those of us who have been ushered into the Space Age are aware that we are now able to look back at ourselves from a distant vantage point. A nighttime view from 223 miles above the earth shows the arrangement of human populations, bright, lighted clusters of humans with dark, somewhat vacant gaps in between. This satellite view is of the middle of North America. 

As you may have noticed, the populations of Platanthera praeclara, and in fact, most other species, are arranged in a similar way. Rather than being evenly or regularly distributed across the landscape, populations are usually clustered in nodes or centers with gaps of relatively unpopulated land in between. The entire network of lights or population clusters in a given region may be likened to a metapopulation. The arrangement of clusters is largely driven by habitat suitability. For humans, that is rivers, fertile valleys, groundwater, wind protection, coffee shops, trails. 

Removing a population cluster removes their genetic material from the metapopulation. This may be insignificant if there are enough individuals remaining in the metapopulation that contain the same genetic material. But removing many more populations will increase the risk that vital genetic material is removed from the entire metapopulation. Thus, most people understand that the total number of species must be maintained at a high level. 

However, here is where the total numbers can mislead, as in 10-20,000 orchids or 370 black-footed ferrets or 25,000 Red Knots. Back to the satellite photo. Suppose the metapopulation in the photo is 20,000 people. What would happen if all of those remaining populations were cut off from one another, if you removed the corridors between populations, the roads, rivers, railroads, and air routes? The breeding population would be reduced to the population of the city, in this scenario, maybe no more than 1000 people. The population would become like the inhabitants of a walled city surrounded by siege engines. This forces consanguineous marriage, prohibits the exchange of genetic material with outside populations, creating a community much like the royal families of ancient times. Gigantic jaws, cartoon character hands, flowerpot-shaped heads. In this scenario, the effective population is not the metapopulation, it is the individual population cluster. They are restricted to their own shallow, stagnant gene pool, sluggish, in a torpor, each generation less fit than the last. As the catapults fling blighted cattle over the walls, the inhabitants of the besieged city succumb to plague, infertility, reproductive failure, and loss of mates. Extinction approaches on fused feet. 

The lesson? This teaches us that it is not enough to consider total population numbers, it is necessary to consider the access one population has to another. This means corridors - suitable habitat patches connecting populations - are essential in maintaining an MVP.

In the end, the niches are filled, the city is reinhabited, and soon there is a bustling metropolis teeming with cane toads, mongoose, brown tree snakes, Norway rats, Asian carp, feral pigs, water hyacinth, and a broad spectrum of strip malls. At the moment, humans appear to have the genetic diversity needed to adapt to these threats. In the meantime, however, the orchids, ferrets, and Knots cannot interpret aerial photography; they have no idea what approaches their city walls.

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