Features of an Ecosystem (Practical)

Explain means giving reasons. The second clause asks what each data type misses, so the answer must be built around two specific gaps rather than around a general statement that both are useful.

5 marks. Define the two data types, state what each alone establishes, name the gap each leaves, then show what only the combination provides.

Abiotic data are measurements of the non-living components of the ecosystem, such as temperature, light intensity, soil or water pH, soil moisture, salinity, dissolved oxygen and wind exposure. Biotic data are measurements of the living components, such as which species are present, how abundant each is, how they are distributed, and the number of different species present.

Abiotic data alone describe the physical conditions and so establish what the environment demands of an organism: a site that is hot, dry and strongly lit demands drought and heat tolerance. What they cannot reveal is whether anything actually lives there, what does, or how much of it. Two sites with identical abiotic readings may hold very different communities, because history, dispersal, competition and predation also determine who is present. Abiotic data describe the stage and not the cast.

Biotic data alone describe the community and so establish what is there and in what numbers. What they cannot reveal is why. A count showing ferns abundant in one zone and absent from another records a pattern without any explanation of it, and offers no basis for predicting what would happen if conditions changed. Biotic data describe the cast and not the stage.

Collected together and at the same locations, the two allow a relationship to be established rather than two separate descriptions. If light intensity, soil moisture and the abundance of ferns are all recorded at the same sampling points, the pattern in the biotic data can be related to the gradient in the abiotic data, supporting a statement of the form that this species is abundant where moisture is high and light is low. That relationship is what allows the features of the ecosystem to be discussed, and it is also what makes a prediction possible.

Even together the data show correlation rather than proven cause. Ferns being abundant where it is moist and shaded is consistent with moisture and shade permitting them, but it is also consistent with a third factor, such as soil type or the shade cast by the trees, driving both. Establishing cause needs either a controlled experiment or the same correlation demonstrated repeatedly across independent sites, so a conclusion from a single site should be stated as a relationship supported by the data rather than as a mechanism proven by it.

Analyse a dataset means drawing out the relationships and saying what they mean. Every claim you make should quote a figure, and the figures should be processed rather than simply repeated.

6 marks. Identify the gradient, quantify the abiotic difference, quantify the biotic difference, link the two, name the general feature this illustrates, then state the limits.

The two zones differ systematically in one underlying variable: time exposed to air, about 8 hours per tidal cycle in the upper zone against about 1 hour in the lower. Exposure is not itself a biological demand but it drives the others, because an organism out of water heats, cools, dries and experiences changing salinity as pools evaporate. The dataset is therefore best read as a gradient of exposure, with the other abiotic differences as its consequences.

The temperature range is 32 minus 14, or 18 degrees Celsius, in the upper zone, against 22 minus 18, or 4 degrees Celsius, in the lower, so the upper zone varies over a range about four and a half times as wide. Salinity varies widely in the upper zone but is stable at about 35 parts per thousand, full-strength sea water, in the lower. The upper zone is therefore not simply harsher on average but far more variable, which is the more demanding property.

Species richness, the number of different species recorded, is 6 in the upper zone against 19 in the lower, a difference of 13 species, with the lower zone holding about three times as many. Richness is a simple count and makes no allowance for how abundant each species is, but it is sufficient to establish that the two communities differ substantially.

The two patterns correspond: richness is lowest where abiotic variability is greatest. This is explained by tolerance ranges. Every species can survive only within a range of conditions, so a zone that swings through 18 degrees Celsius and a wide span of salinity can only be occupied by species whose tolerance ranges span all of it, and few species have tolerances that broad. The lower zone, varying by only 4 degrees Celsius at stable salinity, falls inside the tolerance range of many more species. The upper zone is therefore not a poorer habitat in every sense but a more restrictive one, and the abiotic variability acts as a limiting factor on which species can be present.

The data reveal three features of this ecosystem. It is zoned, showing distinct communities at different heights rather than one uniform community. It is structured by an abiotic gradient, since the biotic pattern follows the exposure gradient. And it demonstrates that abiotic variability, not just abiotic average, limits distribution and diversity. These are features of the ecosystem rather than facts about the species, which is what the dot point asks for.

Three limits should be stated. The data are a correlation from two zones on one shore on one occasion, so they cannot establish that exposure causes the richness difference; predation, competition, wave force and substrate could also differ between the zones. Species richness from a single sampling session will under-record cryptic and mobile species and will differ with the effort spent searching, so the counts of 6 and 19 depend partly on how hard the class looked, and the two zones must have been searched with equal effort for the comparison to be fair. And two zones give no indication of how much richness varies by chance, which would need replicate sampling at each height to establish.