82 Natural systems and greenspaces
ocean currents, pollutant formation and transport, and long-distance migratory
birds. Rather than including a separate section here, these principles are occa-
sionally integrated into the upcoming section on ecosystem, community, and
population ecology, as well as the following sections on water, air, and soil
ecology.Ecosystem, community, and population ecology
The main focus is terrestrial, i.e., the ecological patterns and processes
on land. Three broadly overlapping topics, progressively narrowing in scale, are
introduced: (1) ecosystems; (2) natural communities; and (3) natural populations.Ecosystems
Local ecosystems,from natural to degraded, appear in aerial photos or
satellite images as the basic spatial units of landscapes in an urban region
(though the ecosystem concept can apply, e.g., from a tiny acorn to the globe).
Energy f lows one waythrough an ecosystem, that is, from sunlight to producer,
through the food chain, and ends up as heat dissipated in the atmosphere. In
contrast, materials and mineral nutrientseither f low one way through or cycle within
an ecosystem. Ecologists consider ecosystems to be basic units of ecology and of
theEarth’s surface.
Anecosystemis a space where species interact with the physical environment
(Ricklefs and Miller2000,Odum and Barrett2005). A city pond, a meadow, and
apatch of forest are ecosystems.Ecosystem structurerefers to the distribution of
energy, materials, and organisms, whileecosystem functioningrefers to the flows
of energy and materials in food chains and cycles. The termmaterials(some-
what analogous to matter, elements, or biogeochemicals) is used as a contrast
to energy, and primarily refers to water,mineral nutrients(chemical elements
including nitrogen, phosphorus, potassium, sulfur, calcium, zinc, etc. required
by organisms), and chemical pollutants.
Biomassis the amount of living tissue present, and is typically in five forms
relatedbyfeedingortrophic levels: (1)producers,the green photosynthesizing
plants; (2)herbivores, which eat plants; (3)predators,which consume herbivores;
(4)top predators, which eat predators; and (5)decomposers,which break down
and gain nutriment from dead tissue of all five groups. In an area of land
the total biomass of the first four feeding levels progressively decreases and is
appropriately called apyramid ofbiomass.For example in a large suburban park,
thetotal biomass of producers (e.g., trees and shrubs) is typically huge, herbivore
biomass (e.g., caterpillars, mice, and deer) moderate to small, predator biomass
verysmall, and the total biomass of top predators (e.g., coyotes, large wildcats,
eagles) is tiny (Figure4.1).