Land-use change reshaping spider communities in the Himalayas, finds research

Land-use change and elevation are jointly influencing the functional properties of spiders in the north-western Indian Himalayas, affecting community resilience and the ecological roles of these arthropods, researchers have found.

The findings, published in the journal Insect Conservation and Diversity by scientists at the Wildlife Institute of India, describe what the authors say is the first assessment of functional variations in spider assemblages in the region. The study compared functional diversity (FD) across three land-use classes — forests (FR), agricultural lands (AG) and human-dominated regions (HD) — along a common elevational gradient of 1,500 to 4,500 metres.

What is functional diversity?

Irina Das Sarkar, the study’s lead author, told Down To Earth: “Functional diversity is mostly about the roles species play in a given ecosystem and physical (morphological) or behavioural (life history) characters that allow them to play these roles — as compared to taxonomic diversity that quantifies how many species are there.”

“For example, in a forest we have several bird species — one cracks hard seeds, another eats insects from tree bark, and a third disperses fruit seeds over long distances. They each perform different ecological functions cumulatively contributing to functional diversity. This diversity and the degree of functional redundancy (role overlap) acts like an insurance mechanism,” she explained.

The scientist added that higher functional diversity makes ecosystems more stable, as if one species becomes locally extinct, another with a similar role can compensate.

The researchers focused on three ecological traits — circadian activity, hunting strata and ballooning — and two predatory traits: Hunting guild and prey range. These traits relate directly to feeding and dispersal success, shaping ecological roles and influencing how resources are partitioned within communities.

Das said spiders are crucial links in the food web because they are both predators and prey. “In fact, they are one of the most voracious predatory arthropod groups consuming more than 600 million tonnes of insects annually, thus keeping the balance in check. Additionally, they also help keep disease spreads under check by consuming a huge quantum of vectors,” she said.

Some spiders also contribute to pollination, although at a smaller scale than bees or beetles. Many are sensitive to changes in their microhabitats, making them effective bioindicators. Others are highly synanthropic — such as house spiders — and can indicate levels of human disturbance.

Elevation and thresholds

The study documented 2,936 adult and sub-adult individuals representing 126 species across 65 genera and 26 families in Himachal Pradesh. In forests, researchers recorded 85 species across 49 genera and 21 families; in agricultural lands, 43 species across 29 genera and 15 families; and in human-dominated areas, 73 species across 45 genera and 22 families.

“Taxonomic diversity, measured as observed species richness and richness estimators, generally declined with increasing elevation although the magnitude differed across LU categories,” the study noted.

The transition between 3,000 and 3,500 metres — coinciding with the Himalayan treeline — emerged as a critical functional and community threshold. This zone marks a significant shift in species’ adaptabilities and niche partitioning.

At higher elevations, researchers observed a decline in functional redundancy, suggesting that these communities are more vulnerable to collapse because fewer species are available to compensate for losses caused by environmental stress.

“When spider FD reduces, it means fewer types of spiders are fulfilling fewer roles in an ecosystem. Thus, instead of having a variety of web-builders, ground hunters, and ambushers (all controlling different insects groups from ground to aerial), we may be left with just one or two similar types. This reduces overall insect control, makes pest outbreaks more likely and also weakens the ecosystem’s ability to stay stable after disturbances like land-use change or climate stress (which may also having unseen effects of other insect populations),” Das said.

She added that if functional redundancy reduces significantly, local displacement or extinction of species could trigger wider ecosystem disruptions.

Agriculture and homogenisation

Unlike forests or human-dominated regions, agricultural sites showed minimal variation in functional diversity across elevations.

“Functional Diversities in AG sites remains largely consistent across elevation, showing no significant variation, suggesting potential trait homogenisation, consistent with the effects of agricultural intensification,” the study observed. Some studies show that FEve (functional evenness) — which means how evenly different species share ecological roles — and functional divergence (FDiv) — which shows how different species vary in their traits — are not clearly linked to changes in elevation in natural ecosystems.

Earlier research in temperate apple orchards in the western Himalaya has reported a mix of web-builders and ground dwellers. However, this study found an over-abundance of ground obligates, particularly Lycosidae species. The authors suggested this may signal an unquantified decline in arthropod diversity in heavily manipulated agro-ecosystems in the north-western Himalaya, potentially altering ecosystem functioning in neighbouring habitats.

Human-dominated regions showed higher trait richness at lower elevations, supporting the “intermediate disturbance hypothesis”, owing to the presence of resilient synanthropic species adapted to human infrastructure.

Forests maintained the strongest influence on elevational patterns, with directional shifts in traits as habitats transitioned from broad-leaved forests to alpine pastures. While most communities were dominated by cathemeral species — active both day and night — lower human-dominated sites hosted a higher proportion of nocturnal species.

Fragile mountain systems

The researchers conclude that continued agricultural expansion and other anthropogenic activities may simplify complex natural landscapes, pushing Himalayan biodiversity towards new functional regimes with lower resilience.

In the Himalaya, they warn, the consequences may be significant. Mountain ecosystems are already shaped by steep elevational gradients, short growing seasons and climatic extremes.

When functionally distinct species are lost in such constricted systems, there are often fewer species available to compensate. This may be especially severe for high-elevation given the reduced species complexes and climate mediated impacts, Das said.