Wētā on a Warming Mountain: Acclimation or Local Adaptation?
A rock wētā population on a New Zealand schist field occupies a continuous altitudinal gradient from 900 m to 1400 m. Field-collected adults have critical thermal maxima (CTmax, ramping rate 0.25 °C min⁻¹) of 38.2 ± 0.4 °C at 900 m and 36.9 ± 0.3 °C at 1400 m, but animals were measured within 2 h of collection. After 6 weeks in a common garden at 15 °C, CTmax is 37.8 °C and 37.1 °C respectively. First-generation offspring reared at 15 °C give 37.9 °C (lowland) and 37.3 °C (alpine). In a reciprocal transplant, 40% of alpine adults survived 12 months at the low site versus 85% at their home site; the design used two 0.25 ha enclosures per site. A separate acclimation trial showed that 25 °C for 10 days raises CTmax by about 1.5 °C in both populations. The Department of Conservation must decide whether to move alpine animals to low-elevation sites to escape a spreading fungal pathogen.
1. Using the field, common-garden and F1 CTmax values, quantify how much of the altitudinal difference is acclimatory and how much is heritable. What is the defensible conclusion?
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Field difference = 38.2 − 36.9 = 1.3 °C. After 6 weeks at a common 15 °C, the difference is 37.8 − 37.1 = 0.7 °C. So about 0.6 °C (roughly 46%) of the field difference was generated by the animals' recent thermal history, i.e. acclimation, not fixed genetic difference. The acclimation trial independently confirms large plastic capacity: +1.5 °C in both populations, and the similar slopes suggest the two populations have similar reaction-norm shapes rather than different plasticities. The F1 comparison retains 37.9 − 37.3 = 0.6 °C when both are reared identically from eggs, which adult acclimation cannot explain, so roughly half the difference is heritable (or maternally/developmentally transmitted). Conclusion: the populations differ genetically in heat tolerance, but the genetic component explains less than half of the field pattern. The correct description is a small shift in a reaction norm, not two fixed 'heat-tolerant' and 'cold-adapted' types - the two datasets are complementary, not contradictory.
2. Evaluate the reciprocal transplant as evidence for local adaptation and propose two design changes that would strengthen it.
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The 40% versus 85% survival is a genuine fitness cost of being in the wrong environment, the classic signature of local adaptation. But the inference is weak. Two enclosures per site means site is completely confounded with enclosure: with n = 2 you cannot separate 'low elevation' from 'this particular plot'. The enclosures themselves alter temperature, predators and food, so the 'low site' is only a proxy for the real low-elevation environment. Survival alone ignores reproduction, which is what determines whether a translocated population persists. Most importantly, the actual reason for translocation - the pathogen - is absent from the experiment, so the measured contrast is not the management-relevant one.
Changes: (1) replicate properly, ≥6 enclosures per site, crossing population with site and repeating across years, with thermal loggers inside enclosures to confirm the intended temperature contrast; (2) measure a temperature-relevant performance trait (CTmax, sprint speed, egg production) plus survival and reproduction across a full life cycle, and add pathogen exposure as a treatment so the experiment matches the decision being made.
3. The translocation is proposed. Argue both sides from these data and identify the single measurement that would most reduce uncertainty.
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For: there is a heritable component to heat tolerance and a measurable survival cost at low elevation, but the alpine population faces a pathogen that could extirpate it; moving some animals preserves gene pool and buys time.
Against: 40% survival at the low site is a severe cost; the lowland population is genetically distinct and could be swamped or exposed to the pathogen; moving animals may spread the pathogen to the recipient site; and the experiment never tested whether translocated animals would breed, so 'survival' may overstate success. The 0.6 °C genetic difference is also small relative to the 1.5 °C plastic capacity, so animals may cope without moving.
Key uncertainty: whether the genetic difference is large enough to matter for persistence, and whether the low site is genuinely warmer at the microhabitat scale the wētā uses. The measurement that would most reduce this is a multi-year reciprocal transplant with pathogen exposure and reproduction measured, combined with thermal mapping of microhabitat refugia at the low site. A lower-risk alternative is assisted gene flow into a mid-elevation site with thermal refugia.