However, the continual loss of energy due to metabolic activity puts limits on how much energy is available to higher trophic levels this is explained by the Second Law of Thermodynamics. Today we will look at how and where this energy moves through an ecosystem once it is incorporated into organic matter. Most of you are now familiar with the concept of the trophic level see Figure 1. It is simply a feeding level, as often represented in a food chain or food web.
Primary producers comprise the bottom trophic level, followed by primary consumers herbivores , then secondary consumers carnivores feeding on herbivores , and so on. When we talk of moving "up" the food chain, we are speaking figuratively and mean that we move from plants to herbivores to carnivores.
This does not take into account decomposers and detritivores organisms that feed on dead organic matter , which make up their own, highly important trophic pathways.
Figure 1: Trophic levels. What happens to the NPP that is produced and then stored as plant biomass at the lowest trophic level? On average, it is consumed or decomposed.
If NPP was not consumed, it would pile up somewhere. Usually this doesn't happen, but during periods of Earth's history such as the Carboniferous and Pennsylvanian, enormous amounts of NPP in excess of the degradation of organic matter accumulated in swamps.
It was buried and compressed to form the coal and oil deposits that we mine today. When we burn these deposits same chemical reaction as above except that there is greater energy produced we release the energy to drive the machines of industry, and of course the CO 2 goes into the atmosphere as a greenhouse gas. This is the situation that we have today, where the excess CO 2 from burning these deposits past excess NPP is going into the atmosphere and building up over time, dramatically changing our climate.
But let's get back to an ecosystem that is balanced, or in "steady state" "equilibrium" where annual total respiration balances annual total GPP.
As energy passes from trophic level to trophic level, the following rules apply: Only a fraction of the energy available at one trophic level is transferred to the next trophic level.
Typically the numbers and biomass of organisms decrease as one ascends the food chain. An Example: The Fox and the Hare To understand these rules, we must examine what happens to energy within a food chain.
Suppose we have some amount of plant matter consumed by hares, and the hares are in turn consumed by foxes. The following diagram Figure 2 illustrates how this works in terms of the energy losses at each level. A hare or a population of hares ingests plant matter; we'll call this ingestion.
Part of this material is processed by the digestive system and used to make new cells or tissues, and this part is called assimilation. What cannot be assimilated, for example maybe some parts of the plant stems or roots, exits the hare's body and this is called excretion.
The hare uses a significant fraction of the assimilated energy just being a hare -- maintaining a high, constant body temperature, synthesizing proteins, and hopping about. This energy used lost is attributed to cellular respiration.
The remainder goes into making more hare biomass by growth and reproduction that is, increasing the overall biomass of hares by creating offspring.
The conversion of assimilated energy into new tissue is termed secondary production in consumers, and it is conceptually the same as the primary production or NPP of plants.
In our example, the secondary production of the hare is the energy available to foxes who eat the hares for their needs. Clearly, because of all of the energy costs of hares engaged in normal metabolic activities, the energy available to foxes is much less than the energy available to hares.
Just as we calculated the assimilation efficiency above, we can also calculate the net production efficiency for any organism. This efficiency is equal to the production divided by the assimilation for animals, or the NPP divided by the GPP for plants. The "production" here refers to growth plus reproduction. These ratios measure the efficiency with which an organism converts assimilated energy into primary or secondary production.
These efficiencies vary among organisms, largely due to widely differing metabolic requirements. The reason that some organisms have such low net production efficiencies is that they are homeotherms , or animals that maintain a constant internal body temperature mammals and birds.
This requires much more energy than is used by poikilotherms , which are also known as "cold-blooded" organisms all invertebrates, some vertebrates, and all plants, even though plants don't have "blood" that do not regulate their temperatures internally. Just as we can build our understanding of a system from the individual to the population to the community, we can now examine whole trophic levels by calculating ecological efficiencies.
You might think of it as the efficiency of hares at converting plants into fox food. Note that the ecological efficiency is a "combined" measure that takes into account both the assimilation and net production efficiencies. You can also combine different species of plants and animals into a single trophic level, and then examine the ecological efficiency of for example all of the plants in a field being fed on my all of the different grazers from insects to cows.
Thinking about the overall ecological efficiency in a system brings us back to our first rule for the transfer of energy through trophic levels and up the food chain.
For example, If hares consumed kcal of plant energy, they might only be able to form kcal of new hare tissue. For the hare population to be in steady state neither increasing nor decreasing , each year's consumption of hares by foxes should roughly equal each year's production of new hare biomass. So the foxes consume about kcal of hare biomass, and convert perhaps 10 kcal into new fox biomass. The overall loss of energy from lower to higher trophic levels is important in setting the absolute number of trophic levels that any ecosystem can contain.
From this understanding, it should be obvious that the mass of foxes should be less than the mass of hares, and the mass of hares less than the mass of plants. Generally this is true, and we can represent this concept visually by constructing a pyramid of biomass for any ecosystem see Figure 3.
A pyramid of biomass showing producers and consumers in a marine ecosystem. Pyramids of Biomass, Energy, and Numbers A pyramid of biomass is a representation of the amount of energy contained in biomass, at different trophic levels for a given point in time Figure 3, above, Figure 4-middle below. The amount of energy available to one trophic level is limited by the amount stored by the level below.
Because energy is lost in the transfer from one level to the next, there is successively less total energy as you move up trophic levels. In general, we would expect that higher trophic levels would have less total biomass than those below, because less energy is available to them. We could also construct a pyramid of numbers , which as its name implies represents the number of organisms in each trophic level see Figure 4-top.
For the grassland shown in Figure 4-top, the bottom level would be quite large, due to the enormous number of small plants grasses. For other ecosystems such as the temperate forest, the pyramid of numbers might be inverted: for instance, if a forest's plant community was composed of only a handful of very large trees, and yet there were many millions of insect grazers which ate the plant material.
Just as with the inverted pyramid of numbers, in some rare exceptions, there could be an inverted pyramid of biomass, where the biomass of the lower trophic level is less than the biomass of the next higher trophic level.
The oceans are such an exception because at any point in time the total amount of biomass in microscopic algae is small. Thus a pyramid of biomass for the oceans can appear inverted see Figure 4b-middle. You should now ask "how can that be? This is a good question, and can be answered by considering, as we discussed above, the all important aspect of "time".
Even though the biomass may be small, the RATE at which new biomass is produced may be very large. Thus over time it is the amount of new biomass that is produced, from whatever the standing stock of biomass might be, that is important for the next trophic level. We can examine this further by constructing a pyramid of energy , which shows rates of production rather than standing crop. Once done, the figure for the ocean would have the characteristic pyramid shape see Figure 4-bottom.
Algal populations can double in a few days, whereas the zooplankton that feed on them reproduce more slowly and might double in numbers in a few months, and the fish feeding on zooplankton might only reproduce once a year.
Thus, a pyramid of energy takes into account the turnover rate of the organisms, and can never be inverted. Note that this dependence of one trophic level on a lower trophic level for energy is why, as you learned in the lectures on predation, the prey and predator population numbers are linked and why they vary together through time with an offset. Figure 4: Pyramids of numbers, biomass, and energy for various ecosystems. The Residence Time of Energy.
We see that thinking about pyramids of energy and turnover time is similar to our discussions of residence time of elements. But here we are talking about the residence time of "energy". This difference in residence time between aquatic and terrestrial ecosystems is reflected in the pyramids of biomass, as discussed above, and is also very important to consider in analyzing how these different ecosystems would respond to a disturbance, or what scheme might best be used to manage the resources of the ecosystem, or how you might best restore an ecosystem that has been degraded e.
Humans and Energy Consumption All of the animal species on Earth are consumers, and they depend upon producer organisms for their food. For all practical purposes, it is the products of terrestrial plant productivity and some marine plant productivity that sustain humans. What fraction of the terrestrial NPP do humans use, or, "appropriate"? It turns out to be a surprisingly large fraction, which launches us immediately into the question of whether this appropriation of NPP by humans is sustainable.
Let's use our knowledge of ecological energetics to examine this very important issue. Why NPP? Which biome would have the lowest net primary production? A bog is a wetland that. In aquatic ecosystems, net primary productivity is related to. In which ocean zones do crabs, mussels, anemone, barnacles, and sea stars live?
Marshes, swamps, and bogs are examples of. What is the difference between swamps and freshwater marshes? Swamps have deep water, but marshes have shallow water. Swamps have mostly woody shrubs and trees, but marshes have mostly grasses.
Swamps usually have saltwater, but marshes have fresh water. Swamps have lots of wildlife, but marshes do not. Water with a salinity that is greater than fresh water but less than saltwater is classified as.
Which ecosystems are considered standing freshwater ecosystems? Remnant herds in North America suggest that disturbances due to grazers increased local biodiversity by creating openings that rare species could colonize.
Large grazers also accelerated plant decomposition through their droppings, creating nutrient hotspots that altered species composition. Temperature deciduous forests occur in mid-latitudes Figure 4 where cool winters, warm summers, and high year round precipitation occurs Figure 9. Net primary productivity ranges from — g m -2 yr -1 with high litter production. Litter serves as a major pathway for nutrient recycling. This biome is named for the dominant trees that drop their leaves during the winter months.
These forests may have an overstory of 20—30 m tall trees, an understory of 5—10 m trees and shrubs, a shrub layer around 1—2 m in height, and a ground layer of herbaceous plants. Biodiversity is relatively high in this biome due to the niche partitioning allowed by the multiple forest layers. More complex forests are associated with a greater number of animal species; for example, bird species diversity shows a positive correlation with forest height and number of layers.
Figure 9: Temperate deciduous forest climate diagram Temperature deciduous forests occur in mid-latitudes and are characterized by cool winters, warm summers, and high year round precipitation occurs. This small biome about 1. Unrelated evergreen, sclerophyllous shrubs and trees have evolved independently in each of these areas, representing a striking example of convergent evolution.
Net primary productivity varies from — g m -2 yr -1 , dependent upon water availability, soil depth, and age of the stand. Stand productivity decreases after 10—20 years as litter and woody biomass accumulates. Recurring fires aid in nutrient cycling and many plants show fire-induced or fire-promoted flowering. Some species are able to resprout from buds protected by the soil, while others germinate from decay-resistant seeds that lie dormant in the soil until a fire promotes their germination.
Therophytes make up a large component of the flora, and their appearance is associated with openings created by fires. Figure Mediterranean biome climate diagram There are five separate regions between degrees N and S latitude with hot, dry summers, and cool, moist winters.
At latitudes beyond the boreal forest tree line lies a marshy area Figure 4 where growing seasons are very short and temperatures are below zero degrees Celsius for much of the year Figure Because of these low temperatures and short growing seasons, net primary productivity is very low in the tundra, between — g m -2 yr Productivity varies with snowfall depth and local drainage.
Rocky fields and dry meadows will have lower productivity than moist, low-lying areas and wet meadows. Figure Tundra biome climate diagram Very short growing seasons and temperatures that are below zero degrees Celsius for much of the year characterize tundras. Biodiversity in the tundra is low and dominated by mosses, lichens, and low-growing perennial shrubs. Windy conditions and low temperatures select for low growing shrubs, often with tightly-packed, rounded canopies with closely spaced leaves and branches.
Wind and ice damage help form this shape by pruning branches. Soils are low in nutrients due to slow decomposition rates and plants retain nutrients in long-lived evergreen tissues. Nitrogen fixation by lichens with cyanobacterial components is a major source of soil nitrogen. Animals have extended hibernation periods or migrate seasonally. Prentice, L. A global biome model based on plant physiology and dominance, soil properties and climate.
Journal of Biogeography 19 , Raunkiaer, C. Oxford, UK: Clarendon Press, Introduction to the Basic Drivers of Climate. Terrestrial Biomes. Coral Reefs. Energy Economics in Ecosystems. Biodiversity and Ecosystem Stability. Biological Nitrogen Fixation. Ecosystems Ecology Introduction. Factors Affecting Global Climate.
Rivers and Streams: Life in Flowing Water. The Conservation of Mass. The Ecology of Carrion Decomposition. Causes and Consequences of Biodiversity Declines. Earth's Ferrous Wheel. Alternative Stable States. Boreal forests lack the layered forest structure seen in tropical rainforests or, to a lesser degree, temperate forests. The structure of a boreal forest is often only a tree layer and a ground layer. When conifer needles are dropped, they decompose more slowly than broad leaves; therefore, fewer nutrients are returned to the soil to fuel plant growth [Figure 8].
The Arctic tundra lies north of the subarctic boreal forests and is located throughout the Arctic regions of the Northern Hemisphere [Figure 1]. Tundra also exists at elevations above the tree line on mountains. Plants in the Arctic tundra have a short growing season of approximately 50—60 days.
However, during this time, there are almost 24 hours of daylight and plant growth is rapid. The annual precipitation of the Arctic tundra is low 15—25 cm or 6—10 in with little annual variation in precipitation. And, as in the boreal forests, there is little evaporation because of the cold temperatures. Plants in the Arctic tundra are generally low to the ground and include low shrubs, grasses, lichens, and small flowering plants [Figure 9].
There is little species diversity, low net primary productivity, and low aboveground biomass. The soils of the Arctic tundra may remain in a perennially frozen state referred to as permafrost.
The permafrost makes it impossible for roots to penetrate far into the soil and slows the decay of organic matter, which inhibits the release of nutrients from organic matter. The melting of the permafrost in the brief summer provides water for a burst of productivity while temperatures and long days permit it. During the growing season, the ground of the Arctic tundra can be completely covered with plants or lichens.
Earth has terrestrial and aquatic biomes. Aquatic biomes include both freshwater and marine environments. There are eight major terrestrial biomes: tropical rainforests, savannas, subtropical deserts, chaparral, temperate grasslands, temperate forests, boreal forests, and Arctic tundra. The same biome can occur in different geographic locations with similar climates.
Temperature and precipitation, and variations in both, are key abiotic factors that shape the composition of animal and plant communities in terrestrial biomes.
Some biomes, such as temperate grasslands and temperate forests, have distinct seasons with cold and hot weather alternating throughout the year. In warm, moist biomes, such as the tropical rainforest, net primary productivity is high as warm temperatures, abundant water, and a year-round growing season fuel plant growth. Other biomes, such as deserts and tundra, have low primary productivity due to extreme temperatures and a shortage of water.
The extremely low precipitation of subtropical desert biomes might lead one to expect fire to be a major disturbance factor; however, fire is more common in the temperate grassland biome than in the subtropical desert biome.
Why is this? Fire is less common in desert biomes than in temperate grasslands because deserts have low net primary productivity, thus very little plant biomass to fuel a fire. Both the subtropical desert and the Arctic tundra have a low supply of water. In the desert, this is due to extremely low precipitation, and in the Arctic tundra, much of the water is unavailable to plants because it is frozen.
Both the subtropical desert and the Arctic tundra have low net primary productivity. Skip to content Chapter Ecosystems and the Biosphere. Learning Objectives By the end of this section, you will be able to: Identify the two major abiotic factors that determine the type of terrestrial biome in an area Recognize distinguishing characteristics of each of the eight major terrestrial biomes.
Watch this Assignment Discovery: Biomes video for an overview of biomes. To explore further, select one of the biomes on the extended playlist: desert, savanna, temperate forest, temperate grassland, tropic, tundra. Which of the following biomes is characterized by abundant water resources? Which of the following biomes is characterized by short growing seasons? Why is the tundra treeless?
0コメント