Conservation of energy and mass

Energy is the capacity to cause change, or to do work (to move matter against an opposite force). In ecology, we focus principally on solar radiation, which is used by autotrophs (primary producers such as plants and algae) to build organic compounds. These compounds can then be passed to heterotrophs (consumers such as herbivores and carnivores) in food, and dissipated as heat. As you may remember from the first law of thermodynamics, energy cannot be created or destroyed, only transferred or transformed.

Matter is anything that has mass and takes up space. In ecology, we focus on common chemical elements that organisms use—principally carbon, nitrogen, and phosphorus. Chemical elements are mostly recycled, but smaller amounts also flow in and out of an ecosystem’s boundaries over time. Autotrophic organisms assimilate inorganic chemical elements from the air, soil, and/or water, and transform them into organic form by incorporating them into their biomass. Some of these chemical elements are added to the biomass of other species when consumed via the food web. Detritivores (organisms like millipedes, burying beetles, clams, and small crabs) and decomposers (e.g., fungi, some bacteria, and some snails and insects) break down organic wastes (e.g., remains of dead organisms, excrement, fallen foliage) and return them to the environment in inorganic form. The loop is closed. Figure 1 shows how both energy and chemical elements are transferred in ecosystems. Note that, unlike chemical elements, energy cannot be recycled. Instead it enters, flows through, and exits an ecosystem (entering most frequently as solar radiation and leaving as heat during processes such as cellular respiration).

Figure 1. Energy and nutrient dynamics in an ecosystem. Energy (in orange) enters, flows through, and exits and ecosystem as heat. Chemical nutrients (in blue) cycle within an ecosystem–through the trophic levels and eventually back to the primary producers–although, nutrients can leave an ecosystem through processes like erosion and leaching (not shown). Primary producers are autotrophs and consumers are heterotrophs. More specifically, primary consumers are herbivores; secondary consumers are carnivores that eat herbivores; and tertiary consumers are carnivores that eat secondary consumers. Adapted from Campbell et al. (2008) by Nadav Gazit/AMNH

Primary productivity

An ecosystem’s primary production consists of the amount of energy that autotrophs (producers) convert to chemical bonds in organic compounds during a given time period. (The compounds are subsequently broken down to generate energy-storing molecules like ATP.) The vast majority of this energy enters an ecosystem through solar radiation and is processed via photosynthesis. (However, in some ecosystems such as in some caves and deep sea hydrothermal vents, autotrophic communities use chemical energy instead of sunlight, a process called chemosynthesis.)

Ecologists identify two types of primary productivity: gross primary productivity (GPP) and net primary productivity (NPP). GPP is the total amount of energy converted via photosynthesis in a given area and period of time. GPP is often expressed in Joules (J) or kiloJoules (kJ) per square meter per year. Not all of this energy is stored, because the autotrophs use some for metabolic processes, which are fueled by cellular respiration. The difference is the NPP: the energy captured by primary producers in a given area and period of time (GPP), minus the energy used by the primary producers for cellular respiration (Figure 2).

NPP = GPP – Respiration

Figure 2. Flow of energy from the Sun to GPP to NPP. Approximately 99 percent of the solar energy that reaches plants (primary producers) is reflected or passes through them without being absorbed. Only one percent is harnessed by photosynthesis and contributes to GPP. The producer then uses approximately 60% of that energy for cellular respiration, leaving 40% available for other organisms (NPP). This means that almost all life on the planet (excluding organisms that get their energy from chemosynthesis and all the organisms that depend on them) is supported by less than 1% of the solar energy that reaches Earth! Adapted from Ricklefs & Relyea (2014) by Nadav Gazit/AMNH

Secondary production and efficiency

NPP is the energy that moves through the food web. Its movement and distribution depends on how efficiently food energy is converted to biomass at each step in each food chain within a food web. In most ecosystems, herbivores only consume a small fraction of the NPP (available plant material). Moreover, they digest only part of the plant material (assimilation) and excrete the rest as waste. The amount of chemical energy in food that consumers convert to new biomass (through growth or reproduction) during a given period is called secondary production.

Let’s focus on a hypothetical example. Say a giraffe eats an Acacia tree leaf containing 200 J of energy: 50% of the energy is lost through its feces (100 J) and approximately 48.5% is used in cellular respiration (97 J). This leaves 1.5% (3 J) for growth (addition of new biomass). Some detritivores may eat the feces, but most of this energy flows out of the ecosystem as heat. This is why we say energy flows through ecosystems rather than cycles within them. We can use the following equation to measure how efficiently animals transform energy:

Production efficiency = Net secondary production x 100% / Assimilation of primary production

In the case of our hypothetical giraffe, net secondary production is 3 Joules and assimilation (the amount of energy not lost through feces) is 100 Joules. Thus its production efficiency is 3 x 100% / 100 = 3%. Animals’ efficiencies vary widely. For example, mammals (like giraffes) and birds typically range between 1-3% because it takes a lot of energy to maintain a high body temperature. In contrast, some insects and microorganisms can have efficiencies that average 40% or more.

How do we measure energy flow within an ecosystem? Instead of calculating the efficiencies of each organism within the food web, we can simplify the food web by calculating the trophic efficiencies across it. As mentioned in the Community Structure and Dynamics essay, the trophic structure of a community can be described in terms of its trophic levels: primary producers, primary consumers, secondary consumers and so forth. Although different ecosystems have different trophic efficiencies, the average trophic efficiency is about 10%, meaning that about 10% of the energy stored in the biomass at each trophic level is converted to biomass at the next level. This means that 100 kg of vegetation biomass can support about 10 kg of herbivore biomass, which can support about 1 kg of carnivore biomass. A useful way to visualize the difference in energy at each level is to draw a trophic pyramid where the relative energy in each trophic level is represented by different sized bars, stacked from primary producer up to the highest trophic level in that community (Figure 3).

Figure 3. An idealized terrestrial trophic pyramid of net production. In this trophic pyramid, trophic levels are visualized by the amount of net production (in Joules) for each trophic level with the assumption that trophic efficiency between each level is 10%. Note, the exact efficiency percentages vary among species and ecosystems. Adapted from Campbell et al. (2008) by Nadav Gazit/AMNH

Limits to production

Net primary productivity differs a great deal among ecosystems around the world, in part because the amounts of sunlight and precipitation that reach terrestrial ecosystems limit primary production (Figure 4). For example, little sunlight reaches tundra regions at high latitudes—little energy enters the system—so they are one of the least productive biomes. Tropical rainforests, on the other hand, which are found around the equator and receive intense sunlight year-round, are approximately 20 times more productive. On a more local scale, production depends on the relative availability of key soil nutrients. Chemical elements like nitrogen and phosphorus are the building blocks of organic molecules that organisms need in order to grow and reproduce. As we’ll see in the next section, animals can also moderate NPP.

Figure 4. Global patterns of NPP estimated from spectral data gathered by NASA’s MODIS satellite. This animation shows satellite imagery (remote sensing) data–one way to monitor the NPP of the Earth’s terrestrial vegetation–from October 2003 – October 2013. Values range from near -1.0 to 6.5 grams of carbon per square meter per day. A negative value means decomposition or respiration overpowered carbon absorption; more carbon was released to the atmosphere than the plants absorbed. Productivity in mid-latitudes is tied to seasonal change, with productivity peaking in the warmer months. Northern boreal forests, for example, experience high productivity in June and July and then a slow decline through fall and winter. Tropical forests in South America, Africa, Southeast Asia, and Indonesia have high productivity year-round, corresponding to abundant sunlight, warmth, and rainfall. ©wikimedia.

The carbon cycle

The carbon cycle is closely tied to energy flow. Why? Photosynthesis depends upon it. Plants, algae, and other photosynthetic organisms transform solar energy into chemical energy. During this process, these organisms use solar energy, water (H₂O), and CO₂ in the atmosphere (or dissolved in water) to create carbon rich carbohydrates, such as glucose (C₆H₁₂O₆), trapping the solar energy in the bonds that hold these atoms together. Thus, you might see NPP expressed in terms of carbon (grams of C per m² per year — or day in the case of Figure 4). The chemical energy stored in an organism is passed through the food chain and metabolized through cellular respiration. In the process, the waste product of CO₂ is released back into the atmosphere or water, completing the cycle.

However, not all carbon cycles through organisms and directly back to the atmosphere. A variety of processes operating at different rates and timescales contribute to the global carbon cycle. For example, carbon can be sequestered for varying lengths of time in soil, forests, the ocean, rocks and fossil fuels, and released in a short period of time through natural and human-caused events (see Box 1).

Nitrogen and phosphorus cycles

Figure 5. The relationship between wolves, moose, trees, carbon, and global warming. This causal loop diagram visually represents series of events and relationships that occur in a boreal system when wolves are not present to keep moose populations at lower densities. For the purposes of clarity, we have chosen to show only some of the components and relationships within the system. For example, moose negatively influence ecosystem carbon uptake/storage through browsing of trees and vegetation (as shown) but they also increase CO₂ emissions through priming soil microbial activity through fecal deposition, and through direct respiration (not shown because collectively, soil and moose respiration is orders of magnitude less than tree carbon uptake/storage). Although not all feedbacks within this system are represented, we have highlighted with double lined arrows example feedbacks that are expected to occur. For example, up to a certain point, the reduction of trees/vegetation will lead to increases in fire frequency or severity, which will further decrease tree/vegetation biomass–a feedback loop that reinforces the effects of reduced carbon uptake/storage and reduced canopy. What would happen to the intensity of these relationships if wolves were reintroduced into this system? ©Lizette Vernon/AMNH

As mentioned earlier, NPP differs greatly among ecosystems, in part because the amounts of sunlight and precipitation, but also because nitrogen or phosphorus availability can limit production in ecosystems. Primary producers need nitrogen and phosphorus for growth and reproduction because they are the building blocks of many critical biochemicals such as ATP, amino acids, proteins, and nucleic acids (which DNA is made of). Therefore, if there isn’t enough available nitrogen in an ecosystem (in a boreal forest, for example) or phosphorus (in a freshwater lake, for example) for all the organisms in need of these resources, growth will be limited.

Figure 6. Algal bloom. Algae blooms are a regular phenomenon in Florida’s Lake Okeechobee during the summertime, but this photo, taken in July 2016, shows a larger than usual bloom. It covered 85 square kilometers (33 square miles). Here, blue-green algae, also known as cyanobacteria, grow swiftly when the lake is warm in summer and nutrients like phosphorus and nitrogen enter the water through runoff from farms. Algal blooms occur all over the world and not only in freshwater systems (e.g., “red tide” is a bloom of red-pigment producing algae in coastal regions). High densities of algae can lead to high concentrations of toxins (a byproduct of algae normally diluted in water) which directly affect aquatic organisms by impairing their survival, growth or reproduction, and can have effects throughout the food chain. Indirectly, algae affect aquatic animals because high densities of algae require larger than normal amounts of dissolved oxygen in the water, which can cause death from oxygen deprivation in aquatic animals and create what are known as “dead zones”. ©USGS.

Some organisms have developed adaptations to living in areas with low nutrient availability (in part because of soil composition). For example, various bacteria and fungi are able to use more forms of nitrogen (in both inorganic and organic chemical compounds) than plants, which has led to the evolution of mutualistic relationships between fungi/bacteria and plants. In the case of the bacteria Rhizobium and legumes, the bacteria makes more nitrogen available to legumes than what’s available to them in the soil, and in turn the legumes provide sugars and protection for the Rhizobium.

Our understanding of nutrient limitations and chemical cycles is important for sustaining high agricultural yields. Agriculture removes nutrients from ecosystems (plants uptake nutrients from the soil and then are harvested and shipped outside of the ecosystem, and the nutrients do not cycle back into the soil where the plants grew). To counteract this loss of nutrients, large supplements (e.g., fertilizer) are added to the soil. Although these added nutrients lead to better and consistent crop growth, the nutrient runoff from the fields can pollute aquatic ecosystems (eutrophication), and stimulate excess algal growth (Figure 6).

Human activities dominate chemical cycles

As Earth’s population has grown, human activities have disrupted the basic function of ecosystems: their species composition, energy flow, and chemical cycling. Largely because of industrialized forms of agriculture and our growing reliance on fossil fuels, human activities are not influencing most chemical cycles more than they are influenced by natural processes. If we continue at this rate, we may be altering Earth systems to the point that is unsustainable for humans. How do we know when we’ve gone too far — have we passed the tipping point?

The Stockholm Resilience Center has identified nine “planetary boundaries” —thresholds that could generate unacceptable change, if crossed, (Figure 7). This framework helps us assess our risk of exceeding the capabilities of the global system. For example, the biodiversity crisis we discussed at the beginning of the course is represented by the red ‘genetic diversity’ zone in the figure under ‘biodiversity loss’. We are also approaching or already beyond the sustainable change that the nitrogen and phosphorus cycles are able to sustain.

Figure 7. Risk status for nine planetary boundaries. For each category of concern, the threshold for a “safe-operating space for humanity” is represented by the planetary boundary that lies at the intersection of the green and yellow zones. Categories depicted in yellow and red have already exceeded the planetary boundary. Those in yellow are in a zone of increasing risk and uncertainty, while those in red are in a zone of high risk and uncertainty. Processes for which planetary boundaries have yet to be quantified are represented by gray wedges; these are atmospheric aerosol loading (i.e., particulates in the atmosphere), novel entities (i.e., new substances and modified life forms that have the potential for unwanted geophysical and/or biological effects), and functional diversity (i.e., the value, range, distribution, and relative abundance of functions performed by organisms within an ecosystem). For more details on each category and the planetary boundary framework, see Steffen et al. (2015). Adapted from Steffen et al. (2015) by Lizette Vernon/AMNH

What can we do? In the next essay and next week, we explore some approaches scientists and policy makers are taking to meet the challenge of maintaining the earth’s functioning systems.

REFERENCES

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Chapin III, F. S., Matson, P. A., & Vitousek, P. (2002). Principles of terrestrial ecosystem ecology. Springer Science & Business Media. New York, NY, USA.

Pastor J, Dewey B, Naiman RJ, McInnes PF, Cohen Y. 1993. Moose browsing and soil fertility in the boreal forests of Isle Royale National Park. Ecology 74:467-80.

Ricklefs, R. E., & Relyea R. (2014). Ecology the Economy of Nature. W.H. Freeman and Company. New York, NY, USA.

Schmitz, O. J., Raymond, P. A., Estes, J. A., Kurz, W. A., Holtgrieve, G. W., Ritchie, M. E., … & Christensen, V. (2014). Animating the carbon cycle. Ecosystems, 17(2), 344-359.

Steffen, W., Richardson, K., Rockström, J., Cornell, S. E., Fetzer, I., Bennett, E. M., … & Folke, C. (2015). Planetary boundaries: Guiding human development on a changing planet. Science, 347(6223), 1259855.