Harlowheath metapopulation: corridor or patch restoration on a fixed budget
The Harlowheath silver-spotted skipper occupies six patches of lowland heather separated by conifer plantation and improved pasture. Survey data (area / adult butterflies / total butterfly species) are: P1 210 ha / 480 / 34; P2 95 ha / 210 / 27; P3 60 ha / 140 / 22; P4 22 ha / 0 / 14, with 6 adults recorded in 2015; P5 11 ha / 8 / 9; P6 4 ha / 0 / 5. Two marked individuals have been recorded moving 1.4 km and 2.6 km. P4 lies 700 m from P3 and 2.1 km from P2. The trust has £180,000 to spend over five years and cannot raise more. Options costed by staff are: (A) active management of P1 at £40,000 a year; (B) a 300 m by 30 m heather corridor linking P2 and P3 at £120,000; (C) restore P4 at £55,000; (D) a head-start breeding programme at £30,000 a year.
1. Show why option A cannot be delivered as written, and calculate what each remaining option leaves available for anything else.
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Option A: £40,000 × 5 years = £200,000 against a £180,000 budget. It overruns by £20,000 before any monitoring is funded, so it is not deliverable as stated. It is also management only - it protects the status quo and adds no new patch.
Option B: £120,000 leaves £60,000. That is 1.5 years of P1 management, or 5 years of management at £12,000/yr, which is unlikely to cover a 210 ha site. It also creates a corridor that must itself be maintained if scrub and conifers are not to close it.
Option C: £55,000 leaves £125,000. That funds roughly three years of P1 management (£120,000) or five years of lower-intensity management plus monitoring, and still restores a patch.
Option D: £30,000 × 5 = £150,000 leaves £30,000, and head-starting produces captive animals with nowhere better to go unless habitat is restored - it treats the symptom, not the habitat.
So the real choice is B versus C, and C releases nearly three times as much uncommitted money as B.
2. Using species–area and metapopulation reasoning, recommend B or C. Address the SLOSS argument explicitly and state the main risk in your recommendation.
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Case for the single large patch (SLOSS in favour of large): P1 at 210 ha holds 480 adults and 34 species and is almost certainly the only population comfortably above a minimum viable size. Small patches lose species: P6 at 4 ha holds 5 species, P5 at 11 ha holds 9, P4 at 22 ha holds 14 - richness tracks area. Spending on small patches may buy habitat that extinction debt has already condemned.
Case against relying on P1 alone: a single large patch is one fire, one disease outbreak or one bad season from losing the whole taxon locally. The empty P4 is direct evidence that local extinctions happen in this network.
Cost per hectare: the corridor is 300 × 30 m = 0.9 ha for £120,000, about £133,000/ha, and its benefit depends on dispersal that has been recorded only twice. P4 restoration is £55,000 for 22 ha, about £2,500/ha, and raises the number of patches above roughly 10 ha from three to four. At 700 m from P3 it is well inside the observed 1.4–2.6 km dispersal range, so colonisation is plausible.
Recommendation: C, plus spend the residual £125,000 managing P1, P2 and P3 and monitoring P4 for colonisation over five years. Risk: P4 is never recolonised and the money is spent on an empty patch. That is why the decision must carry a pre-agreed review point at year three, and why B becomes the better buy if P4 is still empty and dispersal data remain thin.
3. Identify the two pieces of new evidence that would most change your recommendation, and explain how you would collect them.
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1. Whether P4 can actually be recolonised. The recommendation assumes P3 adults cross 700 m of hostile conifer and pasture. Two sightings are not a dispersal rate. Evidence that would flip the decision: mark–recapture or a genetic study showing P4 and P3 are effectively isolated, or three seasons of no colonisation despite suitable habitat. Collection: a systematic transect between P3 and P4 with marked individuals, plus non-lethal wing samples for genetic differentiation between patches.
2. Whether P1 is genuinely viable or just large. If P1 is quietly declining (extinction debt from past fragmentation), the 'protect the large patch' logic collapses and connectivity becomes urgent. Collection: annual counts on P1 with a consistent method across at least five years, plus an estimate of habitat condition (heather age structure) inside P1.
A third, weaker candidate is fire risk, which would argue for spreading risk across patches but is harder to quantify.
The point is that the decision is a bet on dispersal and on P1 stability; both are testable, and the budget should fund the test rather than assume the answer. A judgement that cannot state what would prove it wrong is not a judgement, it is a preference.