The variety of life on Earth exists as a myriad of microscopic and macroscopic forms. This biological diversity, or biodiversity, can be described at scales ranging from planetary, biome, or ecosystem to site-specific. Biodiversity includes genetic variation within species, the variety of species in an area, and the variety of habitats within a landscape. Roughly 1,750,000 species have been described and formally named, though estimates of the total biodiversity of life on Earth range from 5 to 30 million, with some estimates as high as 100,000,000 total species.
Most of terrestrial life reproduces, feeds, lives, and dies in the Critical Zone (see "Soil Biodiversity" at Wikipedia for a brief introduction to soil biodiversity topics). Soil is intimately linked to biodiversity: soil provides the substrate through which much of the terrestrial biosphere interacts with the Critical Zone and its other component "spheres," and biota exert a significant influence on soil formation and Critical Zone processes. For example, plants absorb atmospheric carbon dioxide and store it in roots—eventually root respiration adds carbon dioxide to the soil atmosphere, changing, and at times controlling, weathering rates and other chemical processes within the soil and Critical Zone. A primary function of roots is to absorb water and nutrients, activity that directly influences the hydrosphere by drawing in soil moisture and dissolved constituents. This influences recharge rates and the chemistry of the soil and groundwater system. Roots also anchor plants within the Critical Zone and physically erode rocks by penetration and wedging, thus influencing interaction with the lithosphere.
But it is yet more complicated than that! Each species or organism may have a unique role in an ecosystem and therefore may interact with the Critical Zone in a different way. Furthermore, the interactions between organisms can be unique to species or unique to a specific habitat or ecosystem. Let's think about plants again. Cellulose, a potential source of energy, is tough and insoluble to most organisms. But termites (and grazing animals) contain microorganisms in their digestive tracts that convert cellulose to sugars usable by them and their hosts. Thriving termites in turn clear the landscape of dead plant litter, recycle contained nutrients back to the soil, and enhance soil porosity and permeability through construction of their subterranean chambers. An overall decrease in soil productivity and fertility has been observed in the absence of termites; thus, if not for those gut flora, a substantially different, and perhaps less fecund, ecosystem might exist.
Most cultures have recognized the importance of biodiversity for boosting overall ecosystem productivity and resilience to disasters. Yet continued destruction of habitat and species by human society is leading to what some biologists and paleontologists call the “Sixth Extinction,” perhaps the most rapid and destructive of extinctions in Earth history. Imagine total global human destructiveness perhaps exceeding the comet impact that occurred at the end of the Cretaceous Period that finished off the dinosaurs!
In Lesson 10, we will consider the various means by which biologists and other natural scientists classify life on Earth. From there, we will launch into concepts of ecology, examining some of the interactions and processes between organisms and their environment—this examination will gradually focus in on the Critical Zone and soil processes that involve biota. We will complete Lesson 10 with an online forum to discuss the merits of placing a financial value on ecosystem processes. In Lesson 11, we will take a closer look at various expansive ecosystems of Earth and consider the relationship between them and the rest of the Critical Zone.
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Learn about some interesting examples of the role of life in the Critical Zone.