Biodiversity can be appreciated at multiple scales, from within a species and within populations (at a genetic level), to scales that encompass communities of a variety of species within a habitat or across landscapes of many habitats comprised of interacting communities of organisms. For most of us there is an understanding that higher biodiversity at each of these scales is a positive attribute, but why?
In part the answer is that with greater biodiversity there comes higher levels of redundancy. Communities with lower biodiversity are more fragile than those with higher biodiversity. Imagine a habitat with a single species of plant-eating insect and a single species of an insect-eating lizard. As long as there are just enough insects to sustain a healthy population of lizards, the there is a level of equilibrium. But, if a severe drought, or if a pandemic kills the insect, the lizard population starves. Or if a lizard-eating bird enters the community and reduces the lizard population, the insect population could increase to a level where the plant community is damaged by the insects and their voracious appetites. Either way the community collapses. However, if that community included multiple species of insects, and multiple species of lizards, that redundancy can buffer the community. The role of any one species can be filled by another and the dynamic equilibrium between predators, prey, and vegetation can be sustained.
James Estes studied a marine environment in the Aleutian archipelago that lacked biodiversity. There was a single predator species (sea otters), very few prey species (mostly sea urchins), and a single plant species (giant kelp). Sea otters ate the urchins, and the urchins ate the kelp. As long as the numbers of each were in balance (equilibrium) a dense kelp forest existed that acted as a nursery for a multitude of fish species. But then the local Orcas developed a taste for sea otter, decimating local otter populations. Without otters the urchin population exploded, and they ate all the kelp. There was no redundancy to compensate for the decline in otters; the community collapsed, and the critical fish nursery was lost.
Kevin Crooks studied coastal sage communities near San Diego. Coastal sage is generally a diverse community of plants, insects, lizards, songbirds, small seed and plant eating mammals (rodents), medium-sized omnivorous mammals and a few large predatory mammals (mountain lions, bobcats and coyotes). However, San Diego is a popular place for people to live, and the coastal sage community has been sacrificed for thousands of new homes to meet the needs of a burgeoning population of humans. Soon the coastal sage community was sliced and diced until there were just a few isolated natural habitat fragments left. Kevin's question was whether those habitat islands still retained the biodiversity of what once characterized this community. The first to go were the large predators; the big cats and the coyotes could not maintain populations in such small habitats. Then something curious happened. Without the larger predators around, the medium sized (meso-predators) mammal populations (skunks, raccoons, weasels, and opossums) exploded, and preyed upon the lizards, songbirds and small mammals to the point where the smaller creatures were no longer able to maintain populations. Excluding the top predators resulted in a “trophic cascade” and a loss of biodiversity.
Then there is genetic biodiversity at a species or population level. Darwin worried about this for his own family, even before there was a modern understanding of how genetics works. At that time, and for centuries before, European culture dictated that marriages occur within social classes and typically within a finite group of families with social and economic ties. Royal families throughout Europe intermarried to solidify strategic alliances. The result was an inordinate propensity of hemophilia and insanity. The Darwin and Wedgewood (famous for their fine china) families had similar ties of intermarriage. Darwin married his first cousin Emma Wedgewood. Darwin himself suffered in his middle and older ages from undiagnosed debilitating gastrointestinal distress that was shared by several of his cousins. Emma and Charles had ten children, seven of which survived to adulthood. Just three of his adult children had children of their own. Darwin and Emma had a long happy marriage full of love and respect, but he was guilt-ridden that their lack of genetic diversity had doomed their children, despite the fact that three of his sons were Knighted for their respective advancements in botany, astronomy and engineering. Had the Darwin-Wedgwood intermarriages continued Darwin's guilt would have likely been well-founded and his lineage may have had a short family tree. Perhaps because of his concerns, his children and grandchildren and great-great grandchildren found spouses outside of that close family circle, and there are now some 100 descendants of Charles Darwin. Today one of those great-great grandchildren, Sarah Darwin is a professor and botanist who has studied rare plants in the Galapagos Islands. Another, Christopher Darwin lives in Australia and works on his goal of halting the global mass extinction of species, and a third, Jos Barlow is a noted ecologist. Charles would be pleased.
I saw another example of the effects of genetic diversity on our community science climate change-effects survey yesterday. We were on the Boo Hoff trail, at the driest end of our survey gradient. What struck us all as curious was that of the ocotillo that dotted the hillsides along the trail, a few were leafing out, while most were still dormant. Ocotillo have adapted to dry desert conditions by leafing out after significant rainfall events, and if there is additional sufficient rain, flowering, fruiting and then dropping all their leaves and going dormant until the next rain happens. Under the right sequence of rain events they can repeat this sequence up to three times in a single year. While we did have a brief and scant rain shower about two weeks prior to our survey, most of the ocotillo were unconvinced that it was enough to risk putting precious resources into forming new leaves. But a few were convinced. Those were the risk takers, “betting” that more rain would come, and by getting a head start they would stand a better chance of completing their flowering and fruiting cycle before drought once again pushes all the ocotillo back into dormancy. If they are right, they will produce more seeds and have more potential to continue their genetic lineage. If they are wrong, they will have wasted those precious resources, and if the ensuing drought is particularly long, and hot they may not survive to reproduce again. Genetic diversity producing risk takers and conservative wait and see-ers. In an unpredictable desert climate one or the other, or both will win the survive and reproduce lottery.
Biodiversity at all scales is good.