In addition to Darwin’s and Wallace’s foundational theories of evolution by natural selection and Ernst Haeckel’s recognition of ecology as a discipline in the mid-19th century (cf. Loxdale, 2010 and references therein), later refined by figures such as George Evelyn Hutchinson and Charles Elton in the early to mid-20th century (Slobodkin and Slack, 1999; Richardson and Pyšek, 2008), the next major advance in understanding how nature functions was John Thompson’s treatise on co-evolutionary processes (Thompson, 1994). These developments have shaped how ecologists and evolutionary biologists conceptualise interactions among species and their environments.

From this perspective, no organism exists as a free ecological agent; rather, all species are defined by their associations with others. Even animals regarded as dietary “generalists” are constrained by factors governing what they can exploit. These include morphological traits (e.g. teeth, jaws, mandibles), behavioural specialisations (e.g. solitary vs. cooperative hunting), chemical and physiological capacities (e.g. pursuit ability, detoxification), spatial overlap with prey, and cost–benefit trade-offs influencing feeding efficiency. These interactions are dynamic, shaped by coevolutionary feedback that continually refines ecological relationships.

In assembling this Research Topic of Frontiers in Ecology & Evolution, we sought contributions illustrating these co-evolved species relationships and underscoring that all organisms—living or extinct—are, in essence, ecological specialists. As the poet John Donne (1572-1631) wrote, “No man is an island, entire of itself; every man is a piece of the continent, a part of the main” (Partington, 1992). Similarly, no animal is ecologically independent of its nutritional associations. Each species occupies a unique ecological niche defined by its evolutionary history and resource dependencies.

Habitats are rarely homogeneous but rather heterogeneous across altitudinal, geographical, and environmental gradients. Consequently, a species’ diet may vary across locations and habitats, whether terrestrial or aquatic. Apparent generalism should therefore be interpreted cautiously. Indeed, generalism runs counter to the direction of evolutionary processes—selection, adaptation, and specialisation. If true generalism persisted, evolution itself would stagnate. In reality, most animals are specialists, their niches fine-tuned by natural selection to local conditions.

A related issue is the widespread discovery of morphologically cryptic species within many taxa, such as the complex of tube-nosed bats in the Philippines revealed through combined morphological and molecular approaches (Eger et al., 2025). If cryptic entities exist within what is considered a single “good species,” each with distinct dietary preferences, then our understanding of species ecology and specialisation is necessarily incomplete (Loxdale et al., 2016).

Among the studies in this Research Topic, the molecular genetic analysis by Nio et al. of the peach–potato aphid Myzus persicae (Sulzer) sensu stricto in France is particularly relevant. This species has long been regarded as a highly polyphagous pest capable of feeding on plants in over 40 families (Blackman and Eastop, 2000; Blackman, 2010), especially within the Brassicales and sugar beet crops, and is an important vector of plant viruses (Stevens and Lacomme, 2017). Although the study’s data are geographically restricted, its implications are broad. The dominance of a few superclonal lineages across diverse hosts suggests that successful genotypes can exploit multiple hosts through molecular-based phenotypic plasticity (e.g. Mathers et al., 2017) rather than true genetic generalism.

The study by Rafter and Walter provides compelling support for specialism as the prevailing evolutionary outcome. Revisiting frameworks for understanding insects that exploit multiple hosts, they show that apparent generalism often reflects behavioural and neurological constraints rather than genuine flexibility. Host recognition, mediated by sensory and neurological feedback, determines host choice in the field. Because host searching carries energetic costs and predation risks, insects are selective even when physiologically capable of feeding on several plants. These findings suggest that many “generalists” are, in reality, a collection of host-associated specialists maintained through behavioural canalisation.

Such insights have direct implications for pest management and biological control. Understanding the behavioural ecology and host-recognition mechanisms of pest species is essential for effective integrated pest management (IPM) (e.g. Finlay-Doney and Walter, 2012). Knowledge of host specificity also informs risk assessments when releasing non-native biocontrol agents and helps minimise non-target impacts.

Quicke et al. contribute a monumental 38-year study from the Área de Conservación Guanacaste (ACG) in north-western Costa Rica, documenting caterpillar–host plant associations across habitats. This dataset, complemented by their companion study on tachinid parasitoids (Quicke et al., 2025), provides an unparalleled perspective on specialisation and generalism among Lepidoptera. The results reveal a spectrum of feeding strategies, from monophagy to polyphagy, with some groups confined to specific microhabitats or host-plant taxa.

These findings illustrate a long-standing evolutionary arms race between plants and herbivores. Plants, unable to flee, rely on anatomical and chemical defences, while herbivores evolve counteradaptations to exploit them. This interaction dates back hundreds of millions of years to the emergence of terrestrial insects (~480 MYA). Chemical defences such as jasmonate not only deter herbivory but also signal to nearby plants and attract predators of herbivores (Thaler, 1999; Thaler et al., 2001; Wang et al., 2019; War et al., 2020).

As Quicke et al. emphasise, insect host range is tightly linked to biochemical detoxification mechanisms, particularly broad-spectrum enzymes such as carboxyl esterases and cytochrome P450s. Evolving and maintaining such systems is metabolically costly, possibly explaining why true polyphagy is rare. Although a broad diet can buffer species against local food shortages, long-term evolutionary trade-offs may favour specialisation (Loxdale and Balog).

Quicke et al.’s study also highlights the challenge of identifying morphologically cryptic species without molecular tools. Only with high-resolution markers—mitochondrial DNA, microsatellites, or genome sequencing—can we accurately assess genetic uniformity across populations. This Research Topic has been underscored by Janzen et al. (2009), who revealed numerous cryptic parasitoid complexes through DNA barcoding. Such findings reinforce that ecological specialisation is mirrored at the molecular level, where genetic divergence tracks ecological divergence.

Finally, the study by Sipos et al. compares two hornet species in Hungary—the native Vespa crabro and the invasive Asian hornet, Vespa velutina nigrithorax, both predators of honey bees, Apis mellifera—using digital microscopy and micro-CT imaging. They found V. velutina to be darker, smaller, and more agile, with longer legs and greater wing surface area relative to body mass. These traits confer superior flight performance, manoeuvrability, and bee-hunting efficiency, providing a competitive advantage where the two species co-occur. Such morphological and behavioural differences exemplify how ecological specialisation drives invasion success.

Collectively, the papers in this Research Topic underscore the prevalence and importance of ecological specialisation. Even studies not directly focused on diet breadth illuminate the complex, co-evolved interactions that structure ecosystems. Together, they affirm that specialisation—not generalism—is the dominant mode of life, fundamental to both evolutionary process and ecological balance.