From a talk given by Carol Boggs November 5 in the California Academy of Sciences BioForum series
Today there are more species than in the past, but most species once on Earth have gone extinct. Ballpark estimates: species have a lifespan of 1 to 10 million years, there are 10 million species, so extinction rate is 1 to 10 species/year. This is the background rate, but todayâ€™s rates are actually much higher: 2 â€“ 3 orders of magnitude (100 â€“ 1,000) times faster than the background rate for birds and mammals between 1850 and 1950 (and presumably even faster today).
The average species has 220 populations (members of a species in one location), or about 1.1 â€“ 6.6 billion populations worldwide. Thatâ€™s a lot of populations. But theyâ€™re being destroyed by habitat loss (paving, logging, building) alone at the rate of 1,800/hour, about 16 million/year. As Professor Boggâ€™s group discovered, the loss of populations can be bad news for nearby populations.
Why do we care?
There are the ecological goods and services, such s flood control, soil, and production of timber and (to a lesser degree) pharmaceuticals. There are the existence values â€“ many of those habitats give us pleasure. There are options values â€“ we may want to take advantage of services in the future.
Besides habitat loss, overexploitation and invasive species have been important causes of species loss.
Most data come from a few groups: pests (eg, agricultural, so thereâ€™s a monetary motivation to study), beings that affect our health, and beings that we have an emotional attachment to (charismatic megafauna or spotted owls).
Professor Boggs was part of a group following the Bay checkerspot butterfly in the hills above Stanford. The serpentine soils there discourage invasive grasses (and the insects that follow them). There were three populations fairly close to each other. (The magic marker had just been invented, and by capture, release, and recapture, they were able to determine that there was little overlap between the populations; the butterflies donâ€™t generally travel far.)
The butterflies prosper when there is good overlap between the timing of the larvae and the food. The extent of overlap depends on precipitation and temperature. While the mean precipitation changed little over the time studied, there was a noticeable increase in variability in precipitation beginning around 1971. Population size differed, as did habitat (more or less sunny, for example). One population was destroyed when its home was paved over. The area supporting the longest-lasting population was topographically heterogeneous (it was valley-shaped with both a north and south slope, so plant and insect species could shift slopes as rain changed).
The habitat where variation was largest saw the first extinction. Populations prospered in different years (one did well with tons of rain, one did better when there was less rain), so the butterfly metapopulation (def: group of populations near each other) showed more flexibility than did any population alone. One area was recolonized by another, and again went to extinction.
A metapopulation spreads the risks to populations due to environmental variations and disease, and allows repopulation in event of extinction.
Several other explanations were explored and discarded, and only the increase in rain variability appeared to matter. It wasnâ€™t known whether the climate changes observed were caused by human-caused changes to the atmosphere, though an increase in variability is compatible with predictions. Models of the butterfly populations were run with rain variability patterns from 1932-1970 and 1971+: the populations should have survived hundreds to many of hundreds of years with less variability, but only decades with the more recent climate.
Biologists make a big deal about the need to protect populations, and not just the species, and this study provides some of the explanations why. Other butterflies nearby can protect a population from going extinct, and several populations in the same area increase the chance that one or more will survive stresses such as climate change.