Climb a mountain and the world changes around you. The trees thin out, the vegetation shifts from dense forests to open slopes. The birds are different. So are the butterflies, the beetles, the moths. This pattern, where communities of species gradually replace one another as you climb, is one of the most documented patterns in ecology. But a new study by Dr. Pritha Dey, reveals something unexpected: even as the species composition changes, their overall traits remain the same.
Geometrid moths, the family whose caterpillars are the familiar inchworms, are a favourite group for ecologists studying how life responds to environmental gradients. Pritha sampled these moths across eight sites in and around the Kedarnath Wildlife Sanctuary in Uttarakhand, spanning 1,500 metres of elevation from warm, oak-forested valleys up to cool slopes dominated by Rhododendron. Over two spring-summer seasons, she collected 697 specimens from 120 species, photographed each one, and measured their bodies: body length, wing dimensions, wing loading (how much weight each unit of wing area has to carry), and maneuverability.
Species diversity declined with elevation, as expected, and community composition shifted substantially from bottom to top. The moths found at high elevations were largely different from those at low elevations, with the subfamily Larentiinae, associated with herbaceous vegetation, becoming more dominant higher up. So far, so familiar.
But when the trait distributions of these communities were measured, the picture changed. Body size, wing loading, and maneuverability showed no significant differences across the gradient. The overlap in trait space between communities at different elevations was 0.84 out of a maximum of 1, indicating that communities at the top and bottom of the mountain were functionally almost identical despite being made up of largely different species.
This is puzzling. Environmental gradients are generally thought to act as filters, letting through only species whose traits suit local conditions. Cold temperatures, thinner air, and reduced oxygen at high elevations should, in theory, favour moths with particular body plans: perhaps smaller bodies, or proportionally longer wings to generate lift in lower air density. These kinds of trait shifts have been documented in bees, beetles, and other insects across elevational gradients. For geometrid moth assemblages in the Himalayas, these environmental filters do not affect their overall morphological traits t"One of the most surprising findings was that even though the moth species composition changed with elevation, the overall traits remained remarkably similar," says Pritha. "This suggests that different species may still be playing similar ecological roles across elevations."
One reason may be that geometrid moths are already well-equipped for a range of conditions. Their large, low-loading wings allow them to fly across a wide span of temperatures without extensive warm-up, giving them a broader thermal tolerance than many other insects. Another possibility is functional redundancy: the Larentiinae moths that dominate at high elevations have different diets, different host plants, and different life histories from the Ennominae species more common below. Different species, it seems, can arrive at the same functional solutions independently.
Pritha is already thinking about what comes next. "While this study provides an important snapshot, it also highlighted how much we still do not know. Seasonal variation and fine-scale environmental measurements could reveal very different dynamics, and that is something I am especially excited to explore next. Moving forward, I hope to combine long-term monitoring with environmental and trait data to better understand how insect communities respond to changing landscapes and climates."
The study was published in Ecology and Evolution. Specimens are deposited at the Research Collections Facility, NCBS.
Link to the full study - https://onlinelibrary.wiley.com/doi/full/10.1002/ece3.73083
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