Page 664 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

628 part IV Soils, ecosystems, and Biomes
Biodiversity, Evolution, and Ecosystem Stability
Far from being static, Earth’s ecosystems have been dynamic—vigourous,energetic,andever-changing—from the beginning of life on the planet. Over time, communi- ties of plants and animals have adapted and evolved to produce great diversity and, in turn, have shaped their environments. Each ecosystem is constantly adjusting to changing conditions and disturbances. Ironically, the concept of change is key to understanding ecosystem stability.
The dynamics of change in natural ecosystems can range from gradual transitions between equilibrium states to abrupt changes caused by extreme catastro- phes such as an asteroid impact or severe volcanism. For most of the last century, scientists thought that an undisturbed ecosystem—whether a forest, a grassland, or a lake—would progress to a stage of equilibrium, a stable point, with maximum chemical storage and bio- mass. Modern research has determined, however, that ecosystems do not progress to some static conclusion. In other words, there is no “balance of nature”; instead, the “balance” is best thought of as a constant interplay of physical, chemical, and living factors in a dynamic equilibrium.
Biological Evolution Delivers Biodiversity
A critical aspect of ecosystem stability and vitality is biodiversity, or variation of life (a combination of bio- logical and diversity). The concept of biodiversity en- compasses species diversity, the number and variety of different species; genetic diversity, the number of genetic variations within these species; and ecosystem diver- sity, the number and variety of ecosystems, habitats, and communities on a landscape scale.
The origin of this diversity is embodied in the the- ory of evolution. (The definition of a scientific theory is given in Chapter 1.) Evolution states that single-cell or- ganisms adapted, modified, and passed along inherited changes to multicellular organisms. The genetic makeup of successive generations was shaped by environmental factors, physiological functions, and behaviours that led to a greater rate of survival and reproduction. Thus, in this continuing process, traits that help a species sur- vive and reproduce are passed along more frequently than are those that do not. A species is a population that can reproduce sexually and produce viable offspring. By definition, this means reproductive isolation from other species.
Inherited traits are guided by an array of genes, part of an organism’s primary genetic material, DNA (deoxyribonucleic acid), which resides in the nest of chromosomes in every cell nucleus. These traits— and especially the ones that are most successful at
exploiting niches different from those of other spe- cies or that help the species adapt to environmental changes—are passed to successive generations. Such differential reproduction and adaptation pass along suc- cessful genes or groups of genes in a process of genetic favouritism known as natural selection. The process of evolution continues generation after generation and tracks the passage of inherited characteristics that were successful—the failures passed into extinction. Thus, today’s humans are the result of billions of years of af- firmative natural selection.
New genes in the gene pool, the collection of all genes possessed by individuals in a given population, result from mutations. Mutation is a process that takes place when a random occurrence, perhaps an error as the DNA reproduces, produces altered genetic material and inserts new traits into the inherited stream. Geog- raphy also comes into play, since spatial variation in physical environments affects natural selection. For ex- ample, a species may disperse through migration, such as across ice bridges or land connections at times of low sea level, to a different environment where new traits are favoured. A species may also be separated from other species by a natural vicariance (fragmentation of the environment) event. An example is continen- tal drift, which established natural barriers to species movement and resulted in the evolution of new species. The physical and chemical evolution of Earth’s systems is therefore closely linked to the biological evolution of life.
Biodiversity Fosters Ecosystem stability
In the 1990s, field experiments in the prairie ecosystems of Minnesota began to confirm an important scientific assumption: Greater biological diversity in an ecosystem leads to greater long-term stability and productivity. For instance, during a drought, some species of plants will be damaged from water stress. In a diverse ecosystem, however, other species with deeper roots and better water-obtaining ability will thrive. Ongoing experi- ments also suggest that a more diverse plant community retains and uses soil nutrients more efficiently than one with less diversity. (More about this research at the Cedar Creek Ecosystem Science Reserve is at www.cedarcreek .umn.edu/about/.)
stability and Resilience In the context of ecosystems, “stable” does not mean unchanging; stable ecosystems are constantly changing. A stable ecosystem is one that does not deviate greatly from its original state, despite changing environmental conditions (the environmental resistance, mentioned earlier). Resilience is the ecosys- tem’s ability to recover from disturbance quickly and return to its original state. However, some disturbances are too extreme for even a highly resilient ecosystem. For example, several of the dramatic asteroid-impact