FRAGMENTATION CAN CAUSE INBREEDING

by Whit Gibbons

February 10, 2008

The paths to extinction for a species are many, and they are traveled in many ways. Some routes, both natural and man-caused, are obvious. Among natural causes that could lead to extinction for a species are the appearance of a new disease that eliminates individuals more rapidly than the host species can develop a resistance. Or natural climate change might occur at rates that certain species cannot adjust to. Or major natural disasters, such as volcanoes or tsunamis, may cause the extinction of species restricted to oceanic islands.

Of the many ways that humans have been responsible for the extinction of plant and animal species, such as unconstrained removal from the wild or the introduction of diseases, parasites, or predators, one assault on the environment stands above all others--habitat destruction. The loss of natural habitat is a death knell for many native species. The fragmentation of natural habitats into smaller and smaller parcels as a result of commercial urbanization and agriculture throughout much of the country and the world will gradually spell the end for thousands if not millions of species.

A study in Finland provided documentation of a far less obvious phenomenon that can also result in extinction of natural populations--inbreeding. Research by Ilik Saccheri and colleagues at the University of Helsinki implicated inbreeding as a contributor to declines and ultimate extinction of small, isolated populations of a butterfly called the Glanville fritillary. Inbreeding, in which close relatives in a population mate and produce offspring, has long been considered a less-than-optimal situation because of the greater likelihood that deleterious genes will be expressed.

The study in Finland was able to examine the total effects inbreeding can have when an entire population inbreeds. In a region having more than 1,600 meadows suitable for small populations of the Glanville fritillary butterfly, the investigators searched meadows for larvae of the butterflies. The presence of larvae, of course, indicated that adults were breeding and producing offspring.

During a four-year study period, the researchers located more than 500 meadows with larvae, then selected 42 populations for genetic tests to determine how variable each individual and population was genetically. Although sophisticated laboratory techniques must be used to assess the genetic makeup of individuals, once the analysis is accomplished, comparing the genetic variability present is simple. An assumption is that when there is low genetic variability among offspring, the parents were closely related. In contrast, high variability in genetic makeup among offspring is considered to be a healthy sign within a population. With this in mind, the scientists compared the genetic makeup of butterfly larvae, the offspring, in the 42 different meadows and followed the fate of each population to see if it persisted in the meadow or became extinct.

The researchers found that the lower the genetic variability of a population, the higher the probability of its extinction. That is, the most-inbred butterfly populations were most likely to decline in numbers and eventually go extinct. Several components in the life cycle of the butterflies were identified as being adversely affected by inbreeding. For example, the survival of larvae was lower. Likewise, those reaching adulthood did not live as long. Also, the hatching rate of the eggs was lower in the populations with high inbreeding.

The severe consequences of inbreeding demonstrated in the Glanville fritillary butterflies in Finland could occur naturally. But such situations are anything but natural when human beings isolate animal species into small populations and create greater opportunities for inbreeding. By fragmenting habitats so that normally widespread populations are separated into small populations, we create situations in which inbreeding is more likely to occur. We now know that such inbreeding can lead to extinction.

Habitat fragmentation occurs every day throughout the United States and the world. When modification of a forest results in two or more small wooded areas instead of one large area, animal species may be isolated into small inbreeding populations. The consequences of such isolation can adversely affect the long-term welfare of some species. Knowing this, we have one more reason for curtailing the fragmentation of habitats.



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