Science N’Stuff

When Biodiversity Masks Ecosystem Fragility

Why do some ecosystems collapse after species loss and others remain stable? Increasingly, ecologists suspect that the answer lies not only in how many species ecosystems contain, but in the ecological roles those species perform. This idea, known as functional diversity, describes the range of traits and behaviours through which organisms influence ecosystem processes such as seed dispersal, pollination and predation. In many ecosystems, birds play particularly important roles, controlling insect populations and dispersing seeds across large distances (Şekercioğlu, Daily and Ehrlich, 2004).

Closely related to functional diversity is functional redundancy, the presence of multiple species performing similar ecological functions (Figure 1). Ecologists have long suggested that functional redundancy acts as nature’s insurance policy: if one species disappears, others can compensate and maintain ecosystem processes that might otherwise be lost (Elmqvist et al., 2003). In this way, biodiversity can buffer ecosystems against environmental disturbances and species loss (Tilman, Reich and Knops, 2006).

Figure 1: Functional redundancy in bird communities
Conceptual diagram showing how bird species occupy functional trait space in ecosystems with high (left) and low (right) functional redundancy. Coloured circles represent ecological niches defined by a combination of traits. In communities with high redundancy, multiple species share overlapping roles, providing resilience to species loss. In communities with low redundancy, fewer species perform each role, making ecosystem functions more vulnerable to disturbance.

However, human-driven land-use change is dramatically reshaping ecosystems worldwide. As forests are converted into agricultural landscapes and urban environments expand, specialist species often decline whilst adaptable generalists persist. Although these altered communities may retain many species, whether they also maintain sufficient functional redundancy to sustain ecosystem stability remains uncertain. Addressing this question requires large-scale analyses that link species traits, biodiversity patterns and ecosystem functioning, an approach taken by Weeks et al. (2026) in their new global study of bird communities.

Drawing on one of the most extensive global datasets of bird biodiversity and traits assembled to date, Weeks et al. (2026) analysed 3,696 bird species recorded across 1,281 ecological sites spanning a wide range of habitats, from relatively intact ecosystems sites to landscapes heavily modified by agriculture and urbanisation. This broad geographic slope allowed the authors to compare how bird communities respond to different levels of human disturbance.

Rather than focussing solely on species richness, the researchers examined functional traits that reflect how birds interact with ecosystems. Morphometric characteristics such as body mass, beak morphology and wing shape provide insights into feeding behaviour dispersal ability and other ecological roles. By positioning species within a multidimensional functional trait space, the authors were able to quantify both diversity of ecological roles present within communities and the degree of functional redundancy among species.

To explore how resilient these communities might be to further biodiversity loss, Weeks et al. (2026) conducted simulation extinction experiments. Species were progressively removed from each community in computational models, allowing the researchers to track how rapidly functional diversity declined as species disappeared. By comparing these patterns between natural and human-modified environments, the study tested whether land-use change alters the ability of bird communities to maintain ecosystem functions when additional species are lost.

The analysis revealed clear and consistent shifts in the structure of bird communities across human-modified landscapes. In areas strongly affected by land-use change, communities were increasingly dominated by widespread, adaptable generalist species capable of thriving in disturbed environments, while many specialists became less common or disappeared altogether. Although species richness did not always decline dramatically, the functional composition of these communities changed substantially.

One key consequence of this shift was a reduction in functional redundancy. Bird communities in human altered landscapes contained fewer species performing similar ecological roles, meaning that ecosystem functions were supported by a smaller number of species.

The extinction simulations reinforced this pattern. When species were progressively removed from communities, functional diversity declined more rapidly in human modified ecosystems than in relatively undisturbed ones. In other words, disturbed communities were less able to absorb additional species losses without experiencing substantial functional change. These findings suggest that ecosystems shaped by human activity may appear stable while in reality becoming increasingly fragile.

Together these results suggest that land use change can produce ecosystems that retain species yet lose ecological insurance provided by functional redundancy. As specialists disappear and generalists dominate, ecosystem functions may become increasingly dependant on a limited number of remaining species. This concentration of ecological roles could make ecosystems more vulnerable to future disturbance and biodiversity loss.

These findings have important implications for how biodiversity loss is understood and managed in human altered landscapes. Conservation strategies often prioritise maintaining species richness, but this study highlights that preserving functional diversity may be equally critical. Ecosystems that appear species rich may nevertheless be vulnerable if the remaining species perform a narrow range of ecological roles.

This issue is particularly relevant for the ecosystem services provided by birds. By dispersing seeds and regulating insect populations, birds contribute to the regeneration of plant communities and help control agricultural pests (Şekercioğlu, Daily and Ehrlich, 2004). If these functions become concentrated in only a few species, their loss could have disproportionate ecological consequences. Protecting functionally diverse bird communities may therefore be essential for maintaining stable ecosystem processes in increasingly disturbed environments.

The study also raises several important questions for future research. Similar patterns of declining functional redundancy may occur in other taxonomic groups such as insects or mammals, which also play crucial roles in ecosystem functioning. In addition, understanding how landscape management and habitat restoration influence functional diversity could help identify ways to strengthen ecosystem resilience.

Taken together, the findings of Weeks et al. (2026) highlight an often overlooked consequence of human driven environmental change. Bird communities in modified landscapes may retain a similar number of species, yet their functional structure can be profoundly altered. As ecological roles become concentrated among fewer species, the redundancy that once provided resilience is eroded.

These results emphasise that the stability of ecosystems cannot be understood by simply counting species. Instead, attention must also be paid to the diversity of ecological roles species perform and the redundancy that safeguards these roles against future losses. As land use continues to reshape ecosystems worldwide, maintaining functional diversity may be critical for preserving the ecological processes on which both natural systems and human societies depend.