Page 655 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 19 ecosystem essentials 619
Tertiary consumers
Secondary consumers Primary consumers Producers
Seagull Worm
Cricket
1 kcal · m−2 10 kcal · m−2
100 kcal · m−2 1000 kcal · m−2
Sea near northern Europe have some of the highest lev- els of persistent organic pollutants (POPs) in any animal in the world, despite their remoteness from civilization. Many species are threatened in this manner (see the orcas in The Human Denominator, Figure HD 19d), and, of course, humans are at the top of many food chains and therefore at risk of ingesting chemicals concen- trated in this way.
Communities and Species Distributions
The levels of organization within ecology and biogeog- raphy range from the biosphere, at the top, encompass- ing all life on Earth, down to single living organisms at the bottom. The biosphere can be broadly grouped into ecosystems (including biomes, discussed in Chapter 20), each of which can then be grouped into communities, made up of interacting populations of living plants and animals in a particular place. A community may be iden- tified in several ways—by its physical appearance, by the species present and the abundance of each, or by the complex patterns of their interdependence, such as the trophic (feeding) structure.
For example, in a forest ecosystem, a specific com- munity may exist on the forest floor, while another community may function in the canopy of leaves high above. Similarly, within a lake ecosystem, the plants and animals that flourish in the bottom sediments form one community, whereas those near the surface form another.
Whether viewed in terms of its ecosystem or in terms of its community within an ecosystem, each species has a habitat, defined as the environment in which an organ- ism resides or is biologically adapted to live. A habitat includes both biotic and abiotic elements of the environ- ment, and habitat size and character vary with each spe- cies’ needs. For example, Black-legged Kittiwakes (Rissa tridactyla) are a type of gull that prefers small nesting habitats on the sheer cliff faces of offshore islands or sea stacks where their young are safe from predators (Figure 19.11). When breeding season is over, these gulls return to the open ocean for the rest of the year.
The Niche Concept
An ecological niche (from the French word nicher, meaning “to nest”) is the function, or occupation, of a life form within a given community. A niche is deter- mined by the physical, chemical, and biological needs of the organism. This is not the same concept as habitat. Niche and habitat are different in that habitat is an en- vironment that can be shared by many species, whereas niche is the specific, unique role that a species performs within that habitat.
For example, the White-breasted Nuthatch (Sitta carolinensis) is a small bird that occurs throughout the United States and in parts of Canada and Mexico in
(a) A pyramid shape illustrates the decrease in energy between lower and higher trophic levels. Kilocalorie amounts are idealized to show the general trend of the energy decrease.
Potential human biomass
81 kg beef
810 kg wheat
Consumption of grain-fed beef
(b) Biomass pyramids illustrate the difference in efficiency between direct and indirect consumption of grain.
▲Figure 19.10 Energy pyramids and biomass pyramids.
livestock feed rather than to human consumption. In some areas of the world, forests are being cleared and converted to pasture for beef production—in most cases for export to developed countries. Thus, dietary patterns in North America and Europe are perpetuating ineffi- ciency, since consumption of animal products requires much more energy for each calorie produced than con- sumption of plant products.
Biological amplification When chemical pesticides are applied to an ecosystem of producers and consum- ers, the food web concentrates some of these chemi- cals. Many chemicals are degraded or diluted in air and water and thus are rendered relatively harmless. Other chemicals, however, are long-lived, stable, and soluble in the fatty tissues of consumers. They become increas- ingly concentrated at each higher trophic level. This is called biological amplification, or biomagnification. In the 1970s, scientists determined that the pesticide DDT was biomagnifying, especially in birds, building up in their fat tissues and causing a thinning of eggshells that caused hatchling mortality. The subsequent ban on DDT for agricultural use is now credited by many experts as saving the Brown Pelican and Peregrine Falcon from extinction.
Thus, pollution in a food web can efficiently poison the organism at the top. The polar bears of the Barents
810 kg wheat
Direct consumption of grain