Читать книгу Ecosystem Crises Interactions - Merrill Singer - Страница 36

The biotic and abiotic components of an ecosystem are synergistic and reinforcing. Acreman et al. (2011) provide a good illustration of these kinds of interactions based on research on Somerset Levels and Moors (SLM), a coastal plain and wetlands located in the county of Somerset in southwest England. The SLM contains a rich biodiversity of invertebrates, plants, migratory and local birds, fish, amphibians, reptiles, and several mammalian species. Archeological remains indicate human presence since the Paleolithic. The area has been mined for its rich peat soils of decomposing organic material—used for fuel and fertilizer—at least since Roman times, but intensely so since the industrialization of extraction in the 1950s. The result is considerable ecosystem damage and peat loss.

Freshwater is ever‐present in the SLM because of its location near the outlet of a river basin, low‐lying topography, permeable peat soils, underlying aquafer, and rainfall. The resulting wet environment “supports bird [and other animal] life that maintains biological diversity, attracts tourists, protects archaeological artefacts and reduces CO₂ emissions; raising water levels to or above the ground leads to net greenhouse gas uptake by the wetland” (Acreman et al. 2011, p. 1543). In light of climate change, it is notable that peat is the largest and most efficient terrestrial store of CO₂. On average, peat wetlands sequester 10 times more CO₂ per acre than any other ecosystem. When peat is mined, however, it is exposed to air. Carbon contained within it (from decomposed biota) combines with oxygen in the air to produce CO₂, which is emitted into the atmosphere (Dunn & Freeman 2011). In this way, peatlands can be transformed by human actions from a CO₂ sink (or storage site) to a CO₂ source; that is, from a resource that restricts global warming to a source that drives it.

Another example of ecosystem synergy is provided by black bears (Ursus americana). These omnivores eat various animals (including insects), though most of their diet is based on diverse plant foods. Their lives revolve around food acquisition, reproduction and offspring care, and hibernation. They have a keen sense of smell that enables them to locate various food sources. Pregnant black bears give birth to their young in dens, and mother bears lactate for about 3 months while living there (Alt 1989). During this period, females do not eat or drink, yet they produce more than 850 ounces of high‐fat milk. After emergence from the den—a part of the abiotic environment critical to bear survival—the composition of their milk changes as new milk production is based on the nutrients available in the diet rather than on fat stores (Oftedal et al. 1993; Iverson et al. 2001). Thus, black bears must consume large quantities of fat‐rich food before denning but may have more varied diets at other times of the year, depending on the season.

Research on black bears in New Mexico (Costello et al. 2001) found that key food items taken from the environment and used for energy and cell construction, as well as to produce and feed cubs, were flowering plants, grasses, juniper berries, squawroot, acorns, beechnuts, gooseberries, huckleberries, and blueberries. Blueberries, for example, provide energy in the form of sugar (15 grams for each cup consumed), as well vitamins and dietary fiber, while acorns are high in fats and carbohydrates. Notably, black bears are able to extract protein from flowering plants and grasses (Eagle & Pelton 1980). They are highly efficient berry‐eaters, capable of consuming up to 30 000 a day if they are available in their range; they can double their body weight in years when fruits are plentiful. They harvest berries rapidly, using their sensitive and flexible lips, and swallow them whole (Landriault et al. 2018). Research in Massachusetts (McDonald & Fuller 2005) found that postdenning milk produced for cubs during the spring was higher in fat (26.7% versus 18.2%) when acorns were abundant as compared with lean acorn‐production years. These plants acquire CO₂ from the atmosphere and nutrients like nitrogen and phosphorous from the soil to build cells, grow, and produce leaves, nuts, and fruit. Bears were found to also consume a significant number of ants during part of the year. Both the bears and the primarily plant foods they consumed, as well as the ants, release CO₂ back into the atmosphere as their bodies carry out cellular respiration. The waste excreted by bears provides energy and building material for bacteria and fungi living in the soil. These decomposers, in turn, release molecules like nitrogen back into the soil and atmosphere. The same occurs when bears and plants die. These resources are then available for plants and the cycling of energy and matter continues. Bears also disperse the seeds of the plants they consume. Some berry seeds pass through their digestive tracts unbroken and are able to germinate where they land. Each summer, black bears spread the seeds of their favorite berries throughout their home ranges, enabling berry movement to new patches and future bear food supplies.

These two examples, of course, are simplified illustrations intended to show some of the interconnections and synergies that make up an ecosystem. They are useful because, in the case of black bears, the arrival of European colonialists in North America led to a drastic drop in their numbers and range. Habitat loss (e.g., deforestation) and extensive hunting and persecution resulted in local extinctions and the disappearance of bears across significant sectors of their traditional range by the early 1900s. Subsequently, organized recolonization by conservation officers using bears from growing neighboring populations has been successful in many areas. The lessons of this example suggest the potential for species loss and habitat destruction, as well as the possibility of ecosystem restoration. Restoration of damaged peatlands is a more daunting challenge because of the time it requires: peat only forms at the rate of about 0.04 inches per year, so a 30‐foot‐deep peat bed takes 9000 years to form (Chalker‐Scott 2014). While degraded peatland ecosystems have been restored using sphagnum moss and mulch, CO₂ continues to be released for a number of years through bacterial respiration during decomposition of the new organic matter.