242 Energy Flow and Ecosystems Although recent experiments have examined some fundamental relationships, we not have a full understanding of the effects of varied energy inputs on the richness of consumer species that naturally coexist in ecosystems For example, in most tropical forests and coral reefs there is generally a high number of species, but the cause and effect of this species diversity are open to different interpretations other than the potential importance of relatively high and continuous inputs of energy At the earth’s surface, the annual, seasonal, elevational, and latitudinal distribution of solar energy provides varied inputs of energy to deserts, grasslands, forests, wetlands, lakes, rivers, and oceans Generally, energy always flows through ecosystems but does so at different rates under different global geographical locations and local conditions of slope and aspect Depending on the latitude, ecosystems generally receive a seasonally pulsed or a continuous annual supply of solar energy for primary producers The rate of energy flow and associated biological productivity are dependent not only on the availability of energy but also on water, on combinations of different macronutrients (nitrogen and phosphorus) and micronutrients (trace elements such as iron, manganese, and silica), and on the presence of an assemblage of interactive plant and animal species Some natural food webs in extreme environments, such as hot springs, saline lakes, caves, or certain deep-sea thermal vents, have relatively simple linear food chains and have species adapted for specific habitats For example, dark, closed caves that are deep underground only receive indirect sources of detrital energy from sunlit surfaces aboveground and are typically characterized by a relatively low number of endemic species not found on the surface Deepsea thermal vents in the oceans rely solely on chemical energy derived from microbial breakdown of gases such as hydrogen sulfide and are characterized by sulfur bacteria and unique consumer species These simple ecosystems continue to provide an opportunity to test some fundamental concepts regarding food webs and energy flow relationships at Oak Ridge National Laboratory on the carbon cycle Analysis of time lags requires an understanding of how rapidly organic detritus accumulates and then breaks down to recycle carbon, nitrogen, phosphorus, and other materials These insights are critical in current discussions regarding carbon dioxide accumulation in the atmosphere as fossil fuels are burned (i.e., coal, oil, and natural gas taken from storage that accumulated over geological timescales and are now being rapidly cycled back into the atmosphere following combustion) Debates regarding global warming, the greenhouse effect, and where the ‘‘missing’’ carbon is in the present-day ecosystem require a thorough understanding of the entire biosphere and the carbon cycle as it relates to other nutrient cycles Studies of deep-sea vents and hot mineral springs illustrate another distinct class of ecosystems This food web is not solar driven but depends on chemical energy sources used by chemosynthetic microbes Geologic sources of hydrogen sulfide and other gases provide examples of chemical energy pathways that may well have been the first modes of ecosystem formation by the earliest microbial species on Earth before the evolution of photosynthetic species Various lines of evidence, such as the banded iron formations in pre-Cambrian rock strata, indicate that the earliest atmosphere lacked oxygen, suggesting that chemoautotrophs dominated the first phases of evolution Once oxygen-producing photoautotrophs evolved and dominated the oceans and lakes (and later developed terrestrial forms of green plants), their high levels of primary productivity resulted in an accumulation of oxygen in the atmosphere and a decrease in carbon dioxide (possibly through carbon uptake and storage by plants and deposition of sedimentary limestones) This early shift into a photoautotrophically based ecosystem apparently put the chemoautotrophs at a competitive disadvantage in an oxygen-rich environment These remnants of the earliest species of microbes coexist with specialized species of tube worms, shrimps, and crabs that now dominate deep-sea thermal (DDT) vents where these chemoautotrophs use inorganic chemical compounds as energy sources Multiple Energy Pathways It has been evident since Lindeman’s work that energy travels along different pathways and includes microbial species and macro-species in various interconnected relationships Experimental assemblages are now being widely used to provide some insights into which mechanisms control ecosystem dynamics There is evidence for biotic control mechanisms (interspecific competition for resources, predation, parasitism, and mutualism), abiotic controls (nutrient limitation and frequent and/or intense disturbances), and combinations of controls in different ecosystems These studies have also provided important insights regarding two main energy pathways The distinction between direct, solar-driven, photosynthetically based food webs and indirect transfers of stored energy in the form of detritus (that can be wind driven or washed into habitats) has sorted energy flows into two main classes The earliest work on ecosystems recognized this bimodal classification and it remains an important organizing framework in linking aspects of terrestrial and aquatic ecology The importance of organic detritus as a means for storage of energy was recognized by studies of Jerry S Olson in the 1960s Lake Ecosystems To illustrate the flow of energy through ecosystems it is useful to consider some examples derived from lake studies These convenient habitats have been used for comparative ecosystem studies because distinct boundaries provide clear definitions of inputs and outputs (Figure 3) The main boundaries include any inflowing and outflowing rivers as well as the lake surface-atmosphere and the sediment–water interfaces and also shorelines and topographic ridges (that delimit drainage basins) Water temperatures, nutrient inflows and outflows, and mixing and transport processes all influence species distributions and abundances in generally predictable ways Solar energy transformed into organic matter through the process of photosynthesis is the main source of energy for most ecosystems, especially in large lakes Different sizes and types of plants in lakes vary greatly in their rates of productivity For example, in shallow-water ecosystems solar energy can be used by microphotoautotrophs (attached algae or suspended phytoplankton) and macrophotoautotrophs (pond weeds such as cattails and water lilies) The ratio of the biomass of organisms