Notes: 1  When s is a string or Unicode string object the in and not in operations act like a substring test. In Python versions before 2.3, x had to be a string of length 1. 2  Values of n less than 0 are treated as 0 (which yields an empty sequence of the same type as s).
A food web describes the flow of energy and nutrients through an ecosystem, while a food chain is a linear path through a food web. Distinguish between food chains and food webs as models of energy flow in ecosystems. Organisms can be organized into trophic levels: primary producer, primary consumer, secondary consumer, and tertiary or higher-order consumer.
Heat flux and radiative flux are specific cases of energy flux that involve the rate of transfer of heat and radiation (photons) respectively. Energy flux is a function of time as well as area, and has units of J/m 2 /s or W/m 2. It is sometimes also referred to as flux density. In more technical terms, flux density is defined as the energy travelling over an area perpendicular to the flow of energy divided by time and by area. Four types of energy fluxes for ideal (flat and smooth) surfaces have been identified so far.