Ecosystems

The niche of a species is its role within its ecosystem — how it obtains its energy and nutrients, the conditions it can tolerate, and how it interacts with other organisms and the environment.

Two species cannot occupy exactly the same niche indefinitely because they would compete for identical resources, and one species would always out-compete the other, which would then be eliminated from that area (competitive exclusion).

Biotic factors are the influences of other living organisms on a species — for example competition (for food, light or space) and predation (or disease/parasitism).

Abiotic factors are the non-living, physical and chemical features of the environment — for example temperature and light intensity (or pH, water availability, oxygen concentration).

Both act together to determine the distribution and abundance of a species: abiotic factors set the conditions an organism can tolerate, while biotic factors determine the outcomes of interactions such as competition and predation within those conditions.

Energy is lost between trophic levels in several ways:

• Respiration releases energy as heat to the surroundings (the largest loss in animals);
• energy is used in movement and other life processes;
• some material is not eaten (e.g. roots, bones) or not digested and is lost in faeces (egestion), and energy is lost in excretion of nitrogenous waste.

Because only about 10% of the energy in one trophic level is passed on to the next, after only a few transfers there is too little energy left to support another trophic level. This is why food chains usually have only four or five trophic levels and why top predators are relatively rare.

(a) Calculation. The energy fixed (38 000 kJ m⁻² year⁻¹) is the gross primary productivity (GPP), and the respiratory loss (23 000 kJ m⁻² year⁻¹) is R:

$\text{NPP} = \text{GPP} - R = 38\,000 - 23\,000 = 15\,000\ \text{kJ m}^{-2}\,\text{year}^{-1}$

(b) Meaning. NPP is the chemical energy that the producers store as new biomass after meeting their own respiratory needs. It is the energy actually available to the next trophic level (the primary consumers that eat the producers).

NPP is lower than GPP because GPP is the total energy fixed by photosynthesis, but the producers must respire to release energy for their own life processes (active transport, growth, synthesis of molecules), and this respired energy is lost as heat rather than stored. The amount left over (NPP) is therefore always less than the total fixed (GPP).

The percentage efficiency of energy transfer is:

$\text{efficiency} = \frac{\text{energy in level after}}{\text{energy in level before}} \times 100$

(i) Producers → primary consumers:

$\frac{6500}{65\,000} \times 100 = 10\%$

(ii) Primary → secondary consumers:

$\frac{520}{6500} \times 100 = 8.0\%\ \text{(2 s.f.)}$

Comment. Both transfers are inefficient, transferring only about 8–10% of the available energy, because most energy at each level is lost in respiration (as heat), movement, material not eaten, faeces and excretion. The transfers are close to the typical ~10% figure; this large loss at every step is exactly why food chains are short.

A pyramid of numbers simply counts the number of organisms at each trophic level, regardless of their size. It can therefore be inverted when a small number of very large producers support a much larger number of consumers — for example, a single oak tree (one producer) can support thousands of caterpillars (many primary consumers), giving a narrow base and a wide second level. Similarly, one host can carry many parasites. Because numbers ignore the mass and energy content of each organism, the shape can be distorted.

A pyramid of energy shows the energy stored at each trophic level (in kJ m⁻² year⁻¹) over a period of time. At every transfer between levels, around 90% of the energy is lost — as heat from respiration, in movement, in material not eaten, and in faeces and excretion — so each level always contains less energy than the one below it. This loss is unavoidable and happens at every step, so a pyramid of energy can never be inverted: it is always pyramid-shaped, with the producers at the wide base.

This is why a pyramid of energy is considered the most accurate and reliable representation of a feeding relationship — it accounts for the energy actually flowing through the ecosystem, not just the number or momentary mass of organisms.

The student should sample the moss using quadrats and measure soil moisture at the same points so the two can be compared.

Placing the quadrats. Lay out two long tapes at right angles across the woodland floor to form x and y axes, and use a random number generator to produce coordinates. Place the quadrat at each random coordinate. Random placement avoids bias, so the sample is representative of the whole area and the results are valid.

Recording abundance. At each quadrat, estimate the percentage cover of the moss (using a gridded quadrat) or count the number of individuals to give a measure of abundance. Take a large number of quadrats (e.g. 15–20) so that anomalies have less effect and the mean is reliable.

Measuring the abiotic factor. At each quadrat, measure the soil moisture with a moisture meter (or by taking soil samples and finding water content), keeping the method the same each time (same depth, same instrument) so the comparison is fair.

Analysing. Calculate the mean percentage cover and the mean soil moisture, then look for a correlation between moss abundance and soil moisture. To improve reliability the student should repeat the investigation and remember that a correlation does not by itself prove that moisture causes the difference — other abiotic factors (e.g. light) could be involved.

(a) Light fixed as GPP.

$\frac{24\,000}{1\,700\,000} \times 100 = 1.4\%\ \text{(2 s.f.)}$

Only about 1.4% of the light reaching the producers is actually fixed by photosynthesis.

(b) NPP.

$\text{NPP} = \text{GPP} - R = 24\,000 - 9000 = 15\,000\ \text{kJ m}^{-2}\,\text{year}^{-1}$

(c) Efficiency NPP → primary consumers.

$\frac{1500}{15\,000} \times 100 = 10\%$

(d) Explanation. Very little of the Sun's energy reaches the primary consumers because energy is lost at two stages. First, the producers fix only about 1.4% of the incoming light — most light misses the chloroplasts, is reflected, is the wrong wavelength, or passes through the leaf. Second, of the energy that is fixed (GPP), a large part is used in the producers' own respiration and lost as heat, leaving only the NPP (15 000 kJ m⁻² year⁻¹) as stored biomass. Finally, only about 10% of that NPP is transferred into the primary consumers, the rest being lost in respiration, movement, material not eaten, faeces and excretion. Each successive loss multiplies, so only a tiny fraction of the original solar energy ends up in the herbivores.

The key observation is that each species can survive across the whole shore when grown alone, but in the wild each is restricted to part of it. This shows that the distribution is set by both abiotic and biotic factors acting together, not by abiotic tolerance alone.

Abiotic factors. The rocky shore is a strong environmental gradient. The upper shore is exposed to air for longer, so it is subject to desiccation (drying out), wider temperature changes and higher salinity, while the lower shore is wetter and more stable but spends more time submerged. Because each species can survive the full range of these conditions when alone, abiotic factors set only the outer limits of where each could live (the fundamental niche), not the sharp boundary actually seen.

Biotic factors. The restriction must therefore come from interspecific competition. The lower-shore species is likely a stronger competitor for space (a key limiting resource on a crowded shore), so it out-competes the upper-shore species lower down and excludes it, pushing it up onto the harsher upper shore. The upper-shore species survives there because it is more tolerant of desiccation and the lower-shore competitor cannot dominate in those stressful conditions.

Putting it together. The lower limit of the upper-shore species is set by a biotic factor (competition), while the upper limit of the lower-shore species is set by an abiotic factor (its lower tolerance of desiccation/exposure). This is a classic case showing that the realised niche (where a species actually lives) is narrower than its fundamental niche (where it could live), because competition and physical stress combine.

Conclusion. Neither factor alone explains the pattern: abiotic stress limits the lower-shore species at the top of the shore, and competition limits the upper-shore species at the bottom. Distribution on the shore is therefore best explained by the interaction of abiotic and biotic factors.

Describe (with direction and quoted values). As distance from the water increases from 0 to 12 m, the percentage cover of the reed falls from 80% at the water's edge to 0% at 12 m. Over the same transect, soil moisture also falls, from 95% near the water to 18% on the dry land. There is therefore a positive correlation between soil moisture and reed cover — both decrease together moving inland.

Explain. The reed is adapted to wet, waterlogged soil and needs a high water availability to survive. Near the water, the high soil moisture (95%) provides the water the reed needs, so it grows well and dominates (80% cover). As soil moisture falls inland, there is too little water for the reed to survive and grow, so its cover decreases, reaching 0% where the soil is too dry (18%). Soil moisture, an abiotic factor, therefore limits the distribution of the reed along the gradient.

It is worth noting that the data show a correlation, not proof of causation: another abiotic factor that changes along the transect (e.g. light or soil oxygen) could also contribute, so a controlled investigation would be needed to confirm that moisture is the cause.