Question

1: What do trophic relationships illustrate about an ecosystem?

2: About how much of the sun's energy is converted, captured, or fixed by primary producers?

3: Net Primary Production is always lower than Gross Primary Production. What happens to the difference?

4: Does energy flow through an ecosystem or is it recycled? Why?

5: How does climate determine the distribution of ecosystems?

Answer 1

1)

The feeding positions in a food chain or web are called trophic levels. The different trophic levels are defined in the Table below. Examples are also given in the table. All food chains and webs have at least two or three trophic levels. Generally, there are a maximum of four trophic levels.

Trophic Level	Where It Gets Food	Example
1st Trophic Level: Producer	Makes its own food	Plants make food
2nd Trophic Level: Primary Consumer	Consumes producers	Mice eat plant seeds
3rd Trophic Level: Secondary Consumer	Consumes primary consumers	Snakes eat mice
4th Trophic Level: Tertiary Consumer	Consumes secondary consumers	Hawks eat snakes

• Many consumers feed at more than one trophic level. Humans, for example, are primary consumers when they eat plants such as vegetables. They are secondary consumers when they eat cows. They are tertiary consumers when they eat salmon.
• Energy is passed up a food chain or web from lower to higher trophic levels. However, generally only about 10 percent of the energy at one level is available to the next level. This is represented by the ecological pyramid.
• With less energy at higher trophic levels, there are usually fewer organisms as well. Organisms tend to be larger in size at higher trophic levels, but their smaller numbers result in less biomass. Biomass is the total mass of organisms at a trophic level.

2)

• The total solar radiation striking the Earth's outer atmosphere, about half of it is reflected back to space by ice, snow, oceans, or deserts, or absorbed by gases in the atmosphere - for example, the atmosphere's ozone gas layer absorbs nearly all ultraviolet light, which makes up about 9% of the sun's radiation.
• Overall, of the light that reaches Earth's surface, only about half of it is in the wavelength range that can be used by plants in photosynthesis (~400-700 nm wavelength) - this is called the Photosynthetically Active Radiation, or PAR.
• Plants strongly absorb light of blue and red wavelengths (hence their green color, the result of reflection of green wavelengths), as well as light in the far infrared region, and they reflect light in the near infrared region. Even if the wavelength is correct, the light energy is not all converted into carbon by photosynthesis.
• Some of the light misses the leaf chloroplast, where the photsynthetic reactions occur, and much of the energy from light that is converted by photosynthesis to carbon compounds is used up in keeping the plant biochemical "machinery" operating properly - this loss is generally termed "respiration", although it also includes thermodynamic losses. Plants do not, then, use all of the light energy theoretically available to them.
• On average, plant gross primary production on earth is about 5.83 x 10⁶ cal m^-2 yr^-1. This is about 0.06% of the amount of solar energy falling per square meter on the outer edge of the earth's atmosphere per year (defined as the solar constant and equal to 1.05 x 10¹⁰ cal m^-2 yr^-1).
• After the costs of respiration, plant net primary production is reduced to 4.95 x 10⁶ cal m^-2 yr^{- 1}, or about 0.05% of the solar constant. Note that this is the "average" efficiency, and in land plants this value can reach ~2-3% and in aquatic systems this value can reach ~1%. This relatively low efficiency of conversion of solar energy into energy in carbon compounds sets the overall amount of energy available to heterotrophs at all other trophic levels.

3)

• Gross primary productivity, or GPP, is the rate at which solar energy is captured in sugar molecules during photosynthesis (energy captured per unit area per unit time). Producers such as plants use some of this energy for metabolism/cellular respiration and some for growth (building tissues).

• Net primary productivity, or NPP, is gross primary productivity minus the rate of energy loss to metabolism and maintenance. In other words, it's the rate at which energy is stored as biomass by plants or other primary producers and made available to the consumers in the ecosystem.

Plants typically capture and convert about 1.31.31, point, 3 - 1.6% of the solar energy that reaches Earth's surface and use about a quarter of the captured energy for metabolism and maintenance. So, around 1% of the solar energy reaching Earth's surface (per unit area and time) ends up as net primary productivity.

Net primary productivity varies among ecosystems and depends on many factors. These include solar energy input, temperature and moisture levels, carbon dioxide levels, nutrient availability, and community interactions (e.g., grazing by herbivores). These factors affect how many photosynthesizers are present to capture light energy and how efficiently they can perform their role.

In terrestrial ecosystems, primary productivity ranges from about 2 %in highly productive tropical forests and salt marshes to less than 1001g/m​2​​/y.net primary productivity changes on shorter timescales , which shows seasonal and year-to-year variations in net primary productivity of terrestrial ecosystems across the globe.

4)  

• Energy can pass from one trophic level to the next when organic molecules from an organism's body are eaten by another organism. However, the transfer of energy between trophic levels is not usually very efficient.
• On average, only about 10%of the energy stored as biomass in one trophic level (e.g., primary producers) gets stored as biomass in the next trophic level (e.g., primary consumers). net productivity usually drops by a factor of ten from one trophic level to the next.
• For example, in one aquatic ecosystem in Silver Springs, Florida, the net productivities (rates of energy storage as biomass) for trophic levels were
• Primary producers, such as plants and algae: 7618 kcal/m​2​​.Primary consumers, such as snails and insect larvae: 1103kcal/m​2​​.
• Secondary consumers, such as fish and large insects: 1111kcal/m​2.
• Tertiary consumers, such as large fish and snakes: 555 kcal/m​2​​.
• Transfer efficiency varies between levels and is not exactly 10%, but we can see that it's in the ballpark by doing a few calculations. For instance, the efficiency of transfer between primary producers and primary consumers is:
• Transfer efficiency=14.
• energy transfer is inefficient and there are several reasons. One is that not all the organisms at a lower trophic level get eaten by those at a higher trophic level.
• Another is that some molecules in the bodies of organisms that do get eaten are not digestible by predators and are lost in the predators' feces (poop). The dead organisms and feces become dinner for decomposers.
• Finally, of the energy-carrying molecules that do get absorbed by predators, some are used in cellular respiration (instead of being stored as biomass)​.

5)

• Climate is a key factor that determines where different species can live. This principle holds true across the many branches of the tree of life, from animals (like our friend the polar bear) to plants to microbes. Each species needs its own specific set of conditions for survival, many of which are directly or indirectly related to climate.
• If climate conditions in an area change, the species that can live there may also change. For instance, a drop in rainfall may mean that a region can no longer support the plant species it previously did, becoming more desert-like instead. Such changes can have cascading effects on ecological networks, with shifts in plant communities affecting all the animals that depend on them.
• This principle holds true for any change in climate, whether it affects a tiny area or a large one. However, it's especially important in light of the global climate change that is now taking place. As a result of human activities, scientists predict a rise in average temperatures . For species sensitive to small differences in temperature, this could be a devastating change.
• Climate is the average weather in an area over a long period of time. Weather refers to the conditions of the atmosphere from day to day. Climate is generally described in terms of temperature and moisture.
• Temperature falls from the equator to the poles. Therefore, major temperature zones are based on latitude. They include tropical, temperate, and arctic zones . However, other factors besides latitude may also influence temperature.
• For example, land near the ocean may have cooler summers and warmer winters than land farther inland. This is because water gains and loses heat more slowly than does land, and the water temperature influences the temperature on the coast.
• Temperature also falls from lower to higher altitudes. That’s why tropical zone mountain tops may be capped with snow.
• Climate and Plant Growth
• Plants are the major producers in terrestrial biomes. They have five basic needs: air, warmth, sunlight, water, and nutrients. How well these needs are met in a given locationdepends on the growing season and soil quality, both of which are determined mainly by climate.
• The growing season is the period of time each year when it is warm and wet enough for plants to grow. The growing season may last all year in a hot, wet climate but just a few months in a cooler or drier climate.
• Plants grow best in soil that contains plenty of nutrients and organic matter. Both are added to soil when plant litter and dead organisms decompose. Decomposition occurs too slowly in cold climates and too quickly in hot, wet climates for nutrients and organic matter to accumulate. Temperate climates usually have the best soil for plant growth.

Climate and Biodiversity

• Because climate determines plant growth, it also influences the number and variety of other organisms in a terrestrial biome. Biodiversity generally increases from the poles to the equator. It is also usually greater in more humid climates.

Climate and Adaptations

• Organisms evolve adaptations that help them survive in the climate of the biome where they live. For example, in biomes with arid climates, plants may have special tissues for storing water . The desert animals also have adaptations for a dry climate.
• In biomes with cold climates, plants may adapt by becoming dormant during the coldest part of the year. Dormancy is a state in which a plant slows down cellular activities and may shed its leaves. Animals also adapt to cold temperatures. One way is with insulation in the form of fur and fat.