# Impact of Ants on the Order Composition of Canopy Arthropod Communities in Temperate and Tropical Forests

## TL;DR

Ants shape arthropod communities in tropical and temperate forests, with different mechanisms in each, and secondary forests struggle to recover these dynamics.

## Contribution

Demonstrates that ant activity maintains arthropod diversity in primary forests and that this function is lost in secondary forests.

## Key findings

- Trees with high ant density host more arthropods than those with low ant density in both tropical and temperate forests.
- In temperate forests, aphid honeydew likely drives high arthropod abundance, but this mechanism does not apply to tropical forests.
- Secondary forests have altered ant communities and reduced predation, leading to increased pest species abundance.

## Abstract

Canopy ants influence the diversity and composition of arthropod communities in tropical primary rainforests and temperate forests. In the rainforests, ants are active year-round, suppressing the dominance of any single species and preventing pest outbreaks. In contrast, seasonality restricts ant diversity and their impact on arthropods in temperate forests. However, this changes when ground-dwelling ants enter the canopy to prey on arthropods. Nevertheless, significantly more arthropods were collected from trees with high ant density compared to trees with low or no ant abundance in tropical and temperate forests. This may be related to the consistently high availability of aphid honeydew in temperate forests, but honeydew is less abundant in rainforests and cannot explain the high arthropod abundance observed here. In secondary forests, the functional influence of ants is lost, resulting in fundamental community changes. Our results highlight that forest regeneration requires recolonization from a primary forest and takes over 50 years to fulfill primary forest functions.

Ants are key drivers of biodiversity in both tropical and temperate forests, though the underlying mechanisms of this remain debated. In tropical lowland rainforests, ants dominate the canopy as opportunistic predators, shaping arthropod abundance and community structure. By contrast, few arboreal ant species exist in temperate forests due to climatic constraints, and predation pressure is generally low. This changes when ground-nesting Formica species enter the canopy to forage. Using insecticidal knockdown, we collected arthropod communities from trees with high and low ant abundance in both tropical and temperate forests and in different seasons. We found consistently higher arthropod abundances on trees with strong ant dominance, including preferred prey taxa such as Diptera, Psocoptera, and Lepidoptera. In temperate forests, high arthropod densities may be driven by aphid-produced honeydew, whereas in tropical rainforests, the absence of large hemipteran aggregations suggests that other mechanisms are involved. Consequently, this mechanism fails to explain high arthropod abundance in tropical primary forests. In contrast, secondary tropical forests host structurally and compositionally altered ant communities, resulting in reduced predation pressure and a marked increase in the abundance of individual species, including potential pest species. These findings suggest that biodiversity maintenance in the canopy depends on intact, diverse ant communities. Recolonization from nearby primary forests is essential for recovery, yet even after five decades, secondary forests remain ecologically distinct, rendering full restoration to primary forest conditions unlikely within a management-relevant timeframe.