The Chilika Wetland: Quadrat Data, Residues and a Conversion Proposal
A 40-hectare freshwater wetland near Chilika is proposed for conversion to prawn ponds. A class samples ten 1 m² quadrats along a transect. Reed A occurs in 7 quadrats with 84 individuals in total; submerged plant B occurs in all 10 quadrats with 12 individuals in total; an invasive fern occurs in 9 quadrats with 260 individuals. Waterbird counts at the site fell from 4,200 to 1,100 over eight years. Residues measured in the same period: water 0.002 ppb, plankton 0.5 ppm, small fish 2 ppm, eggs of fish-eating birds 25 ppm. Macrophyte species richness in the area fell from 34 species in 1998 to 19 species in 2023. The developer argues that because 19 species remain and the wetland is 'not unique', conversion should be permitted. The wetland also buffers storm surges for two villages and supports 300 fishing families.
1. Calculate density and frequency for reed A and plant B, interpret the contrast between them, and state which statistic alone would mislead the class.
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Density is total individuals divided by total area sampled: for reed A, 84/10 = 8.4 individuals per m²; for plant B, 12/10 = 1.2 per m²; for the invasive fern, 260/10 = 26 per m². Frequency is the number of quadrats containing the species divided by the number of quadrats, as a percentage: 7/10 = 70% for A, 10/10 = 100% for B, 9/10 = 90% for the fern. The contrast is informative. Plant B is present in every quadrat but at very low density, so its individuals are thinly and evenly scattered. Reed A is absent from three quadrats yet reaches 8.4 per m² where it does occur, so it is clumped or patchy. Frequency alone would rank B as the most widespread species and could be read as dominance, which is wrong; density alone would miss that B is ubiquitous. The judgement is that density, frequency and the identity of the species must be read together: the invasive fern at 26 per m², more than three times the density of the reed, shows where the standing biomass has gone while native richness fell.
2. Explain the residue sequence from water to bird eggs, and why it is evidence about the food chain rather than about contaminated water alone.
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The sequence runs 0.002 ppb in water, 0.5 ppm in plankton, 2 ppm in small fish and 25 ppm in the eggs of fish-eating birds - a rise of more than ten million-fold from water to egg. The mechanism is biological magnification. The compound is lipophilic, poorly metabolised and poorly excreted, so what an organism ingests is stored in fat rather than lost. Because roughly 90% of biomass is lost at each trophic step, a predator must eat many prey to build its own body, and it concentrates all the pesticide contained in that prey into a much smaller mass, so concentration rises by about tenfold per level. Bird eggs sit at the top of this chain and are lipid-rich, so they receive a large dose, and the residue is deposited in the egg, exposing the embryo. This is why the pattern is evidence of transfer along the food chain and not merely of polluted water: the water concentration is far too low to explain the eggs by direct exposure. It also gives a mechanism for the fall in waterbirds from 4,200 to 1,100.
3. Evaluate the developer's argument and state what a conservation decision should actually weigh, and what extra data you would demand.
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'Nineteen species remain' is the wrong test. Species richness counts species, not their abundance, identity or function: losing fifteen species while gaining an invasive fern at 26 per m² leaves the count at nineteen but changes the community completely, and the reed beds that nesting birds use may be replaced by a fern mat. The waterbird fall from 4,200 to 1,100 and the residue load in eggs show the food chain is already stressed, so conversion to prawn ponds would add nutrient loading, salinisation and further habitat loss on top of an existing decline. 'Not unique' ignores the ecosystem services the developer does not pay for: the wetland buffers storm surges for two villages and supports 300 fishing families, and neither value appears in a species list. The decision should weigh irreversibility, protected status under wetland and Ramsar-type criteria, and livelihood dependence, and follow the precautionary principle where evidence is incomplete. Before any approval I would demand a full survey of all 40 hectares rather than one transect, biomass and evenness data, mapping of the invasive fern, and a residue survey of the proposed pond discharge.