Weather and climate reshape Wadden Sea bird communities in contrasting ways

On a chain of low-lying barrier islands off the German North Sea coast, one of the longest-running bird monitoring efforts in Europe has quietly produced a result that should unsettle anyone who judges the health of an ecosystem by counting species. Across the East Frisian Islands in the Wadden Sea, a UNESCO World Heritage Site that hosts millions of breeding, migrating and wintering birds, the number of bird species has generally risen over the past 25 years. Yet the functional diversity of those communities, the sheer variety of ecological roles and body plans packed into them, has often declined over the same period. The islands’ birds are, in effect, becoming more numerous but more alike.

The finding comes from an analysis of two standardised long-term monitoring programmes run by the Lower Saxony State Agency for Water Management, Coastal Protection and Nature Conservation on the islands of Borkum, Juist, Norderney, Baltrum, Langeoog and Spiekeroog. One programme surveys breeding birds each spring and early summer, focusing on 83 species that nest in dunes, salt marshes and beaches. The other covers 74 coastal waterbirds and waders year-round, with two synchronised counts per island per month at roughly fortnightly intervals. The researchers restricted their analyses to 1996 through 2021, the window in which digital breeding-bird data were available and methodology was consistent across both programmes.

What makes the study technically distinctive is its joint treatment of taxonomic and functional diversity, and its explicit separation of two aspects of climate change that ecologists often conflate. Weather describes short-term fluctuations such as a stormy June or an unusually warm week; climate refers to long-term average conditions and their gradual trends. These operate on different timescales and can pull communities in different directions. A single harsh storm can temporarily strip away species sitting at the extremes of a community’s trait space, while sustained warming can systematically displace cold-adapted specialists in favour of generalists and thermophilic species. Distinguishing the two requires long-term, standardised data of exactly the kind the Wadden Sea monitoring provides.

To quantify functional diversity, the team drew on the AVONET morphological trait database, using eleven measurements including bill length, width and depth, tarsus length, wing length, Kipp’s distance, the hand-wing index, tail length and body mass. From these traits they computed balanced Gower dissimilarities between species, represented the resulting multivariate trait space as a hierarchical dendrogram, and derived two complementary indices. Functional richness, or FRic, measures the total branch length of the trait dendrogram spanned by a community, effectively the size of the trait envelope the species occupy. Functional dispersion, or FDis, captures the abundance-weighted spread of species around the centroid of trait space, making it particularly sensitive to which taxa dominate numerically.

A crucial methodological step was the use of standardised effect sizes against a null model. For each island-by-time community, the researchers retained the observed species richness and abundance structure but randomised species identities within the programme-wide species pool, generating 999 randomisations per community. The resulting SES values express how far the observed functional structure deviates from what would be expected given the community’s richness and abundance profile. Values beyond the ±1.96 threshold mark significant departures at the five per cent level. For the year-round waterbird data, monthly SES time series were decomposed using seasonal-trend decomposition based on LOESS to isolate the multi-year signal from the strong seasonal cycles of migration.

The headline result is a striking divergence in trajectories. Species richness and Shannon species diversity increased over time on the six islands in both programmes, often with statistically significant island-specific trends. But functional diversity, expressed as SES values, remained consistently negative and declined more often than it rose, with the most consistent declines occurring in FDis. In other words, communities increasingly fell below the functional structure expected under the null model. This pattern is the signature of functional homogenisation: the species count holds up, even climbs, but the assemblage loses functional distinctiveness as shifts in dominance concentrate abundance within a narrow band of trait space. Taxonomic metrics alone would have reported good news; the trait-based metrics reveal restructuring that the species counts cannot see.

Species-level contribution analyses explain how this decoupling arises, and they reveal that the two functional indices are driven by almost entirely different sets of birds. FDis was dominated by a handful of abundant taxa: among breeding birds, the Black-headed Gull and Lesser Black-backed Gull led contributions, with the top two to four species accounting for half of the summed mean contribution on each island. Among waterbirds and waders, the Dunlin and Eurasian Oystercatcher played the same role, concentrated within four to six taxa. FRic, by contrast, was steered by less abundant species with distinct traits: the Eurasian Spoonbill across all islands for breeding birds, and the Great Cormorant on five of six islands, with the Whooper Swan leading on Borkum, for waterbirds and waders. Trait space can thus expand through the arrival of functionally distinct species while abundance-weighted dispersion contracts because the dominant taxa are shifting.

The weather-versus-climate comparison produced the study’s most conceptually interesting contrast, and it tracked the seasonal logic of the two monitoring programmes. Breeding-bird functional diversity showed stronger associations with short-term weather variability: temperature was positively linked to species richness and diversity, wind speed negatively linked to richness, and FDis was consistently negatively associated with temperature in both weather and climate models, consistent with reduced dispersion in warmer years. This aligns with the documented increase in hot days in the Wadden Sea region since the 1980s and with evidence that short-term extremes can trigger pronounced trait-based community responses, particularly in species with narrow ecological niches. The gulls that dominate FDis are known to show breeding performance and food availability that vary with environmental conditions, providing an ecological mechanism for why changes in dominance structure register so clearly in this metric.

Waterbirds and waders told the opposite story. For these migratory communities, the climate models, built on the long-term trend components of temperature, wind and precipitation, received the greatest statistical support for species richness, species diversity and FDis, as judged by Akaike Information Criterion comparisons. Species diversity increased with temperature and decreased with wind speed, while FRic showed a positive association with temperature. This fits the biology: migratory birds integrate conditions across entire flyways, so their local abundance and community composition should respond more to gradual climate trends that coincide with shifts in migration timing, resource phenology and stopover use than to any single day’s weather. The west-east gradient also mattered here, with functional dispersion systematically lower on the eastern islands, consistent with reported gradients in benthic prey availability across the Wadden Sea.

Island geography modulated these patterns in programme-specific ways. For breeding birds, island area was positively associated with taxonomic metrics but negatively associated with FDis, indicating greater functional similarity amongst abundant taxa on larger islands, a pattern consistent with habitat filtering favouring functionally similar species. For waterbirds and waders, longitude emerged as a key predictor, entering multiple interactions with temperature and precipitation that suggest the west-east gradient modifies climate-related patterns in functional dispersion. The authors note important caveats: detection probability may vary with island size, habitat and season in volunteer-based monitoring, though long-term standardisation reduces the risk that broad patterns are sampling artefacts; the functional metrics rest on morphology alone, which is closely tied to ecological roles but cannot capture dimensions such as stress tolerance or behaviour; and prey dynamics were unavailable for direct analysis. Migratory species also face pressures outside the study area across their annual cycles, from land-use change to disturbance and disease, which these island data cannot fully disentangle.

The broader lesson is one that conservation planners and biodiversity assessors would do well to internalise. Rising species numbers, whether through immigration, range expansion or recovery of formerly persecuted taxa, can coincide with increasing functional redundancy amongst the most common species, narrowing the range of realised ecological strategies and potentially reducing a community’s capacity to respond to future environmental change. If functional dispersion hinges on a small set of abundant birds, communities become disproportionately sensitive to anything affecting those few species. Realistic assessments of ecosystem stability and adaptability, the study concludes, require considering both taxonomic diversity and functional difference, and both short-term weather and long-term climate dynamics, especially in dynamic coastal systems where the two timescales leave different fingerprints on different seasons of the bird year.

Subject of Research: Long-term monitoring reveals contrasting effects of weather and climate on taxonomic and functional diversity of birds on the East Frisian Islands in the Wadden Sea

Article Title: Long-term monitoring reveals contrasting effects of weather and climate on taxonomic and functional diversity of birds on the East Frisian Islands in the Wadden Sea

Article References: Kalusche, J. B., Scheiffarth, G., Kuczynski, L., Böhning-Gaese, K., Fritz, S. A., & Hof, C. (2026). Long-term monitoring reveals contrasting effects of weather and climate on taxonomic and functional diversity of birds on the East Frisian Islands in the Wadden Sea. Frontiers of Biogeography, 19, Article e168424. https://doi.org/10.21425/fob.19.168424

Image Credits: AI Generated

DOI: 10.21425/fob.19.168424

Keywords: Long-term, monitoring, reveals, contrasting, effects, weather, climate, taxonomic, functional, diversity, birds, East