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q-bio.PE · Populations and Evolution

Recorded bird composition became more similar on 185 matched Swedish routes from 2000–2004 to 2021–2025

We tested whether bird counts at the same Swedish survey routes became more similar between two periods. We pooled three annual line-transect counts per route in 2000–2004 and 2021–2025. The sample had 185 routes and 17,020 route pairs. Mean Simpson dissimilarity fell from 0.3120 to 0.2953. The change was -0.0166, with a 95% spatial block interval of -0.0290 to -0.0038. This is a 5.3% decrease relative to the early value. Species presence and relative abundance gave the same direction. A check with 300 birds per route and period also showed a decrease. On 177 routes with an intermediate period, the change after 2010–2014 was unclear. The 29 routes with the same observer across both endpoint periods did not show a clear decrease. These routes also gave no clear decrease with the original survey selections. Thus, this check does not isolate an observer effect. The main result applies to recorded counts on a long-running route sample, mostly in southern Sweden. It does not establish a nationwide biological trend or its cause. The package contains the raw open archive, analysis code, route selections, result tables, figures, and reproduction instructions.

birds · Sweden · beta diversity · biotic homogenization · observer effects · fixed routes · reproducible analysis

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References

7
  1. usesdoi:10.15468/hd6w0r

    Swedish Bird Survey fixed routes, version 1.16, CC0. We analyze the raw line-transect count archive.

  2. doi:10.1111/1365-2656.12035

    Davey et al. reported Swedish bird homogenization in 1998–2010. We test a later endpoint with equal survey numbers and fixed routes. This is an extension with different estimands, not a direct replication.

  3. usesdoi:10.1111/j.1466-8238.2009.00490.x

    Baselga defines the Simpson replacement and Sorensen dissimilarity framework.

  4. doi:10.1111/1365-2656.14145

    Wayman et al. show why aggregate changes can hide different species contributions in British birds.

  5. doi:10.1111/2041-210X.13921

    Tatsumi et al. give methods to partition abundance-based beta diversity changes. Our simpler per-species split applies only to Bray-Curtis on proportions.

  6. usesMean within-Sweden abundance centroids of 154 bird species did not move north with the isotherms in 1998–2025 fixed-route counts, with route and observer effects fixed sci:sha256:54019f16d5bb805257de1779f4f8df9d75e57693c3c95a623b049ce4a36fdc5f

    We use its retained CC0 archive snapshot. Its separate prepared count table also provides an independent preprocessing check. The present analysis starts again from the raw archive.

  7. Swedish bird community temperature index rose 0.06 °C per decade on 714 routes in 1996–2025, about half the long-term warming rate, but did not follow route warming rates or annual temperature sci:sha256:c84dfd7d5e48bd849b12a21d84ef26c02ce0b64d3d4de299884f282a9a9a2f66

    Related SCI analysis of community temperature indices on Swedish routes. Composition dissimilarity is a different outcome.

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