Tuesday, March 01, 2016

On Diversity

Last fall I wrote a series of essays reflecting on Pope Francis's remarkable encyclical, Laudato Si' (subtitled On Care For Our Common Home). I emailed those essays over a period of months to some Catholic acquaintances. Although raised a Catholic, I am no longer a believer, much less a follower of any religion. Nevertheless, I was struck by the urgency and moral clarity of the pope's message. I was particularly eager to see how Catholics I know would reconcile the pope's strong warning about how we are destroying God's creation (and our own home) with their previous denial of climate change and its implications for life on Earth. I'm sorry to report the initial reactions are not encouraging. If even the pope can't get through to hard-core Catholics, who can? Certainly not me.

In his book-length encyclical Pope Francis takes on climate change and much more. There's a section on the horrifying loss of biodiversity we are now experiencing. (As far back as the early 1990s E.O. Wilson warned that we are entering the sixth great extinction spasm in the 4 billion year history of life on Earth, and, unlike the others, this one is caused by human activity.)

The United Nations Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services has just released a scientific assessment (based on 3,000 scientific papers) warning that around 40 percent of the world's pollinating insects are in danger of extinction. That should hit you like a gut punch. Pollinators are crucial to maintaining the web of life on Earth, and also—if self-interest is what motivates you—to the world's human food supply. The loss of such a large number of them would have staggering implications for all of life, including us.

The U.N. assessment underscores the unnatural and disturbingly high rates of extinction we are now experiencing (we are losing tens of thousands of species per year). Last fall I mused on biodiversity in the seventh in my series of email essays on the pope's encyclical. I thought it might be useful to reproduce that essay for a wider audience, so here it is.

Years ago I asked a botanist in Missouri if I should be concerned about the proliferation of a native shrub, buckbrush, on my tallgrass prairie restoration site, and what if anything I should do about it. He discussed options, and said that "I would not use herbicides if there's much diversity."

Diversity is a big deal to biologists—not just to Pope Francis—because it epitomizes the health of biological communities. I have a fat book on my bookshelf entitled Restoring Diversity, along with a number of books on prairie conservation and restoration. The theme of diversity is intimately entwined with all ecological discussion. One of the great ecology books of all times is E.O. Wilson's The Diversity of Life.

Biologists sometimes need to quantify diversity for assessment and monitoring. An important numerical measure of the botanical "quality" of a natural area includes diversity. Called the "Floristic Quality Index" (FQI), it is an arithmetical product that's proportional to the square root of the number of native plant species that area contains. (The other factor in the product is the mean "Coefficient of Conservatism" of all the area's species; the CoC designates a "quality" coefficient to each species. So the product is FQI = average quality x square root of number of species.) In general, then, an area with a greater number of native species is regarded as higher quality than one with less species. I have for years calculated the FQI of my own restoration site.

Biological communities (of all types) that are not diverse are often referred to as being "depauperate." A useful sort-of synonym is "impoverished." Restoration typically entails trying to increase diversity. The diversity of my own site has increased substantially over the past 16 years, but there's tremendous room for more improvement. A lot of native species "came with" the place, but I've added many more by both purchasing and locally collecting seed. I have also increased the restoration's health and diversity by restoring more natural conditions (such as frequent fire) than what prevailed when the site was an overgrazed pasture.

Diversity is a complicated subject operating at multiple scales. It can involve the amount of genetic variation within a single species, and also (more commonly) the variety of different species in a particular area. Habitat fragmentation decreases diversity because smaller areas can support fewer total species. There's actually a mathematical relationship between the size of an area and the ultimate diversity it can support. Thus there are limitations to what is possible on my own restoration site, which unfortunately will never be bigger than some tens of acres. But compared to the land that surrounds it, it is already amazingly diverse.

In general, habitats with high (both inter- and intra-species) diversity are healthier and more resilient. In particular, they are more able to survive what biologists call "stochastic events," which are random natural disruptions (a severe prolonged drought, for example) that cause (temporary, it is hoped) damage to the biota of the area. The typical path to extinction involves reduced numbers, reduced range, and perhaps reduced genetic diversity (leading to "inbreeding"; think of the Florida panther as an example) of a species over time. A species thus weakened ultimately succumbs to some stochastic event or another and becomes extinct. Diverse ecosystems, then, have the resilience necessary to bounce back from severe disruptions.

Diverse ecosystems are healthier and more resilient because they contain a stronger web of connections. In his encyclical Laudato Si', Pope Francis repeatedly emphasizes the connectedness of things, which is true in theological, social, and purely physical or ecological respects. (So there's even connection in the connectedness!) The amount of biological interaction and, indeed, cooperation in a healthy ecosystem is staggering; most people are completely unaware of it. The connections span both scale and taxonomic hierarchy. It's hard to give a sense of how profoundly interconnected everything is by citing just a few examples, but I will try.

Gardeners have heard of how legumes (such as peas and beans) are plants that can harbor certain bacteria in their root nodules that "fix" nitrogen, and thus increase the local fertility to the plant's benefit. You can buy powdered cultures to inoculate garden legume seed with this bacteria, which is known generically as Rhizobium.  But Rhizobium is actually a genus of bacteria containing many species, and each species of Rhizobium is adapted to a particular species or perhaps genus of leguminous plants. It turns out that tallgrass prairie has a multitude of native legumes: prairie clovers, leadplant, goat's rue, the indigos, the bush clovers, and many, many more. For the nitrogen fixation process to work, each of these disparate native legumes requires a particular cooperating species of Rhizobium. The plant might do OK without the proper Rhizobium present, but it will do better with it. So many connections.

Moreover, the root systems of many (perhaps most) prairie species are colonized by a type of fungus broadly called Mychorrhizae. The fungus's fine hyphal threads extend out from the plant's roots, and become in effect an extension of the plant's root system. It is a true symbiotic relationship: the roots exude carbohydrates that nourish the fungus, and the fungus assists with nutrient uptake from the soil which it directs to the plant's roots. The benefit to both is substantial. Some species of prairie plants don't require this association but do better if it is present; others have very coarse roots and require it for health or even survival. (Such requirement for the mycorrhizal association is referred to as "obligate"). As with Rhizobia, particular species of mycorrhizal fungi are matched to particular species of plants. If the required species of Mycorrhizae is not present, the plant may not do well, or may not be present at all. The fungal species and their host plants mutually require each other: Agricultural fields that have been tilled annually eventually lose their below-ground complement of native Mycorrhizae (becoming "depauperate"), and it can be difficult to reestablish some native plant species on those soils. It's worth noting that the most fertile and productive agricultural soils, such as in the midwestern U.S., were actually created by the complex connections and interactions occurring below-ground in tallgrass prairie over millennia.

These examples don't even scratch the surface of connections in nature, but they can give you a hint of why the connectedness of things is so important.

Some species of prairie plants won't take hold except in a community with a diverse mix of other high quality prairie plants, and a healthy compliment of below-ground processes. Even though I take liberties with the term in my own activities, it can take literally centuries for a particular area to truly be called a "prairie," because the connections that must be made, both below (tallgrass prairie can contain nearly two-thirds of its total biomass below the soil surface) and above ground, are slow and protracted.

The connections are unending. Some plants require very specific pollinators and can't survive without them. Some pollinators require very specific plant hosts and can't survive without them. Other relationships are a bit looser but still important. Monarch butterflies use milkweed species (genus Asclepias) for all phases of their life cycle. The butterflies are important pollinators of the milkweeds, but other insects pollinate them too. Hummingbirds are important pollinators of certain plants with very deep tubular calyxes, such as royal catchfly, where the hummingbird's long beak and tongue are required to access the deeply-contained nectar. The superior strength of the bumblebee (itself endangered by climate change) is required to pry open the tightly overlapping flower parts of the closed gentian; lesser insects cannot get inside to perform the pollination services. Evening primroses open at night, and so are serviced by evening- and night-flying pollinators such as sphinx moths.

Life on Earth is a true web of connections, and when those connections are weakened, other connections are in turn weakened or broken in a destructive cascade that's important even if it isn't immediately apparent to the casual observer. (If you live in a city you might be especially unaware of those connections and, in particular, of when they are broken.) You might wonder why we sometimes fixate on large "charismatic" or "keystone" species, not realizing that their presence or absence speaks to the overall health of the multitude of biological processes of an entire ecosystem that they sit atop. The northern spotted owl, for example, requires old-growth forest. Old-growth forest is itself a particular rich and complicated ecosystem with all kinds of connections at all biological scales. If owls aren't present, lots more is gone too. If we can save the owls, we're saving an entire (and important) habitat type.

As Pope Francis says, we should cherish nature because it is God's creation, and has value in His eyes. Because he saw that it is "good." All of nature, all creatures, give glory to God. But we should also understand that nature is the thing that sustains us in ways we can understand, and some that we can barely imagine. The web of life on Earth produces oxygen we breath. It filters our water. It stores our water and meters it out at a rate we can use. It prevents floods, protects our coastlines, minimizes erosion. It nourishes us. It handles the complex cycling of nutrients through the biosphere, so that life of all kinds can be continuously regenerated. Nature builds soil. It pollinates our crops. The list of so-called "ecosystem services" is practically endless, and we diminish them at our peril. We are all too good at thinking we're only pulling this thread or that, when in fact we are unraveling all kinds of natural complexity upon which we depend.

Pope Francis gets it right:
It may well disturb us to learn of the extinction of mammals or birds, since they are more visible. But the good functioning of ecosystems also requires fungi, algae, worms, insects, reptiles and an innumerable variety of microorganisms. Some less numerous species, although generally unseen, nonetheless play a critical role in maintaining the equilibrium of a particular place. (paragraph 34)

For his part, biologist Edward O. Wilson, in The Diversity of Life, said this about what would happen if there suddenly were no insects:

So important are insects and other land-dwelling arthropods that if all were to disappear, humanity could probably not last more than a few months. Most of the amphibians, reptiles, birds, and mammals would crash to extinction about the same time. Next would go the bulk of flowering plants and with them the physical structure of most forests and other terrestrial habitats of the world. The land surface would literally rot. As dead vegetation piled up and dried out, closing the channels of the nutrient cycles, other complex forms of vegetation would die off, and with them all but a few remnants of the land vertebrates. The free-living fungi, after enjoying a population explosion of stupendous proportions, would decline precipitously, and most species would perish. The land would return to approximately its condition in early Paleozoic times, covered by mats of recumbent wind-pollinated vegetation, sprinkled with clumps of small trees and bushes here and there, largely devoid of animal life. (p. 133)

So many connections.

Copyright (C) 2016 James Michael Brennan, All Rights Reserved


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