A global analysis of 355 alien bird species (Dyer et al., 2020) showed that established alien birds display a Rapoport-like latitudinal gradient — latitudinal range extents increase with latitude outside the tropics — and that a higher proportion of introduced species persists nearer the equator. The mechanisms behind these patterns remain unattributed: they could reflect per-introduction establishment probability that varies with latitude, the number and latitudinal placement of introduction events, or post-establishment spread, and it is unknown whether the patterns are robust to transient post-introduction range dynamics, geographic variation in recording effort (including documentation of failed introductions), the choice of latitudinal-extent metric (Stevens' band-span method), and the influence of a few very wide-ranging high-latitude species. The problem is to decompose the observed latitudinal patterns into these components using temporally explicit, population-level introduction and establishment data, and to test the robustness of the patterns to the identified dynamical and methodological caveats.

Background

Two of the most discussed large-scale patterns in biogeography are the decline of species richness from the tropics toward the poles and Rapoport's rule, the tendency for species' geographic range sizes to be larger at higher latitudes. The rule was named and promoted by Stevens (1989), in honor of Eduardo Rapoport's earlier climatological observation, as part of an explanation for how so many species coexist in the tropics; its generality and mechanism have been debated ever since, with Gaston, Blackburn & Spicer (1998) arguing in part on methodological grounds that the rule deserved an 'epitaph'. Because manipulative experiments are essentially impossible at the continental and global scales where these patterns are expressed, alien species — species introduced by human activity beyond the limits of their native ranges, either accidentally or deliberately — have been used as a form of natural experiment: introductions sample species and environments in new combinations, so the similarity or difference of alien and native patterns can reveal which patterns are driven by environment, history, or by people. Sax (2001) used this logic for alien birds and fishes in North America, and Guo et al. (2012) showed that species found at high latitudes in their native ranges also tend to occur at high latitudes in their alien ranges. Birds are a particularly suitable alien taxon for such tests because they have been introduced to essentially all ice-free latitudes and are comparatively well documented. The Global Avian Invasions Atlas (GAVIA; Dyer, Redding & Blackburn, 2017) compiled worldwide records of alien bird introduction events — including failed ones — together with the distributions of established alien populations; Dyer et al. (2017, PLoS Biology) used it to identify the environmental and anthropogenic drivers of global alien bird species richness. Building on this, Dyer et al. (2020) combined introduction and establishment locations for 355 bird species with established alien populations (establishment being the formation of a self-sustaining population from an introduction; native range extents measured in 5-degree bands of latitude) and asked whether alien birds conform to Rapoport's rule. To quantify latitudinal extent they used Stevens' method — assigning each species the total span from its lowest to highest occupied latitude band — rather than the alternative 'midpoint method' (plotting range extent against the latitude of the range midpoint), because many alien bird distributions are spatially disjunct (populations in several separated regions), which makes range midpoints uninterpretable; the cost is that mean extents of adjacent latitude bands are not statistically independent, since neighbouring bands share many of the same species. Dyer et al. (2020) found: (1) latitudes with more established alien bird species have also had more species introduced; (2) mean latitudinal extents of established alien species increase with latitude poleward of the tropics — a Rapoport-like gradient that is weaker for native ranges — while tropical alien extents are uniformly small; (3) species are mostly introduced within the latitudinal confines of their native ranges and tend to contract away from the more extreme latitudes to which they were introduced; and (4) a higher proportion of introduced species persists nearer the equator, while the poleward increase in extent is driven by a small number of very successful wide-ranging species. They concluded that latitudinal patterns in alien birds are largely a consequence of where species were introduced, and hence not informative about equivalent patterns in native species — but the attribution itself could not be completed with their band-level data, and the paper's Discussion leaves the decomposition and robustness questions open. Relevant machinery now exists on the sides of the problem: establishment-success research distinguishes propagule pressure (the size and number of founding populations) from location- and species-level determinants of per-introduction success, and Redding et al. (2019) modelled establishment success per introduction event worldwide; invasion-lag research (Häkkinen, Hodgson & Early, 2023) shows many naturalised species (species with established alien populations), especially birds, have far from filled their potential ranges, so current alien ranges are transient snapshots rather than long-term outcomes.

Problem Statement

Determine the mechanism and robustness of the latitudinal patterns documented for the global alien avifauna by Dyer et al. (2020), by decomposing them into their candidate causes and testing whether they survive the dynamical and methodological caveats the authors identified. Specifically, using temporally explicit, population-level data on introduction events and their outcomes (e.g., the GAVIA introduction-event records together with subsequent range-dynamics observations): (1) decompose the two observed gradients — (a) the higher relative persistence of introduced alien bird species at lower latitudes, i.e. the higher proportion of introduced species that persist (through establishment or spread) nearer the equator, and (b) the Rapoport-like increase in established latitudinal range extent with latitude outside the tropics (following Dyer et al., 2020, tropical and temperate latitudes are divided by the Tropic of Cancer and Capricorn, and the three most poleward low-sample bands are excluded, leaving bands spanning 0°-65°N and 0°-55°S) — into the contributions of (i) per-introduction establishment probability varying with latitude, (ii) the number and latitudinal placement of introduction events, and (iii) post-establishment spread to latitudes beyond those of introduction; and (2) test whether these gradients are robust to (iv) transient post-introduction dynamics, since many populations are still expanding and current established ranges may not represent long-term outcomes, (v) geographic variation in recording effort, including under-documentation of failed introductions in poorly surveyed (especially tropical) regions, and (vi) the choice of latitudinal-extent metric — the non-independence among adjacent bands introduced by Stevens' band-span method versus alternatives that accommodate disjunct distributions — together with the influence of the few very wide-ranging species at high latitudes. All components serve one objective: establishing whether the alien-bird latitudinal patterns reflect latitude-dependent ecological processes (establishment probability, spread capacity) or the human geography of introductions and methodological artifacts, and hence whether established alien distributions can legitimately serve as a natural experiment on the mechanisms of latitudinal gradients.

References

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