Global increase in the endemism of birds from north to south

Global patterns of diversity and endemism

Alpha diversity peaks in the tropics and generally declines towards higher latitudes in both hemispheres (Figs. 1, 2a). It thus follows the classic pattern of a latitudinal gradient in diversity⁴³, albeit the relationships are slightly bi-modal as alluded to previously^44,45, with a local minimum at about 22.5°—more prominently shown in the northern hemisphere and probably caused by the large proportion of desert sites. Contribution to gamma diversity shows a similar decline with latitude, although the decrease is less steep in the southern hemisphere (Figs. 1, 2a). Contribution to gamma diversity is positively correlated with alpha diversity^19,36 (species richness, R² = 0.59, P < 0.001; functional diversity, R² = 0.67, P < 0.001; phylogenetic diversity, R² = 0.65, P < 0.001; Supplementary Information, Fig. S1). In contrast, complementarity shows opposing patterns in the two hemispheres. In the northern hemisphere, complementarity decreases from the equator towards higher latitudes, whereas in the southern hemisphere, the pattern is reversed, resulting in a global exponential increase in endemism from north to south (taxonomic complementarity, R² = 0.91, P < 0.001; functional complementarity, R² = 0.91, P < 0.001; phylogenetic complementarity, R² = 0.93, P < 0.001; Figs. 1, 2a and Supplementary Information, Table S1). Complementarity is not correlated with alpha diversity³⁵ (species richness, R² = 0.002, P < 0.001; functional diversity, R² = 0.0001, P = 0.24; phylogenetic diversity, R² = 0.07, P < 0.001; Supplementary Information, Fig. S1).

Alpha diversity, contribution to gamma diversity (‘weighted endemism’) and complementarity (‘corrected weighted endemism’) for three facets of diversity (taxonomic, functional and phylogenetic diversity) mapped across the world (n = 14,640 grid cells). Cells are coloured according to rank (14,640 highest, 1 lowest value).

a Rank alpha diversity, contribution to gamma diversity and complementarity against latitude are shown for three facets of diversity (species richness, functional diversity, phylogenetic diversity). Values (n = 13,614 grid cells) are summarised in latitudinal bands of 5° (n = 26 latitudinal bands). Points are coloured according to relative rank and boxplots according to median values. b Absolute range size, latitudinal range and longitudinal range (log-transformed) for units representing the three facets of diversity (species, n = 10,954; functional trait combinations, n = 7929; phylogenetic lineages, n = 19,914). Each unit is assigned to a latitudinal band of 5° (n = 25 latitudinal bands) according to the latitudinal midpoint of its range. Colours represent southern (red) and northern (blue) hemispheres; latitudinal bands inside the tropics are lighter coloured. In the southern hemisphere, ranges decrease towards higher latitudes; in the northern hemisphere, absolute range size and latitudinal range show a peak at mid-latitudes, and longitudinal range increases towards higher latitudes. Boxes show median and 1st and 3rd quartiles; whiskers extend to largest and smallest, respectively, values up to 1.5 x inter-quartile range beyond the quartiles; data points beyond the end of the whiskers are plotted individually.

Global decrease in range size from north to south

The increase in complementarity from north to south is in accordance with increasingly smaller range sizes from north to south (species, R² = 0.74, P < 0.001; functional traits, R² =  0.79, P < 0.001; phylogenetic lineages, R² = 0.83, P < 0.001), smaller latitudinal range sizes from north to south (species, R² = 0.54, P < 0.001; functional traits, R² = 0.39, P < 0.001; phylogenetic lineages, R² = 0.35, P = 0.002), and smaller longitudinal range sizes from north to south (species, R² =  0.72, P < 0.001; functional traits, R² = 0.94, P < 0.001; phylogenetic lineages, R² = 0.89, P < 0.001; Fig. 2b; and Supplementary Information, Table S2). If analysed separately by hemisphere, however, latitudinal range size strongly decreases from the equator towards higher latitudes in the southern hemisphere for functional trait combinations (R² = 0.90, P < 0.001) and phylogenetic lineages (R² = 0.88, P < 0.001), but it shows a hump-shaped relationship with latitude in the northern hemisphere (functional trait combinations, R² = 0.81, P < 0.001, peak at 27.5°N; phylogenetic lineages, R² = 0.79, P < 0.001, peak at 42.5°N).

Complementarity as a measure of irreplaceability

By using an endemism measure that is only influenced by the overlap in assemblage composition, but not assemblage richness (alpha diversity), and by including all global areas, we reveal a global north-south gradient in avian endemism with a peak in the southern hemisphere. These findings are in line with continental-scale studies showing a discrepancy between richness and endemism (11,12). On average, southern hemisphere bird assemblages show the highest proportion of irreplaceable, range-restricted species, functional trait combinations and phylogenetic lineages. This pattern for endemism measured as complementarity is strikingly different from that for contribution to gamma diversity (Figs. 1, 2a, 3). In addition to known hotspots of both richness and endemism, such as the tropical Andes, Madagascar, and New Guinea, we reveal endemism peaks at sites with low local richness, such as oceanic islands, and most notably, the sub-Antarctic islands and the Antarctic continent, i.e. sites that are commonly omitted from large-scale analysis of diversity^15,16,17,18 because of their low alpha diversity and small landmass (Figs. 1, 3). Their conservation significance, therefore, owes not only to the shear abundance of birds found on them^46,47, but also to their global contribution to the retention of the extant avifauna.

Grid cells (n = 14,640 grid cells) are coloured ranging from maximum increase to maximum decrease in rank differences (rank complementarity—rank contribution to gamma diversity). Antarctica, the sub-Antarctic islands, the Arctic, oceanic islands in the tropics, desert regions and the southern Andes show the largest increase in rank; temperate regions in the Palaearctic show the largest decrease in rank.

Potential drivers of the north–south increase in endemism

Our results reveal a striking difference between hemispheres in endemism and range size. To assess the potential significance of these differences for the conservation of northern-hemisphere and southern-hemisphere species, we explored these differences further. First, we determined if the differences in endemism and range size influence patterns of gamma diversity by calculating taxonomic, functional, and phylogenetic gamma diversity for each latitudinal band of grid cells. Since endemism and range size are influenced by the availability of landmass, we also quantified available landmass, approximated by the number of grid cells per latitudinal band, and then calculated density (i.e. gamma diversity/available landmass) for each latitudinal band of grid cells.

Gamma diversity and available landmass against latitude

Gamma diversity shows similar patterns in both hemispheres, with a peak in the tropics and a decline towards higher latitudes, albeit with a slightly faster decrease outside the tropics in the southern hemisphere (extratropical, species richness, north, R² = 0.98, P < 0.001, south, R² = 0.98, P < 0.001; functional diversity, north, R² =  0.97, P < 0.001, south, R² = 0.99, P < 0.001; phylogenetic diversity, north, R² = 0.97, P < 0.001, south, R² = 0.98, P < 0.001, Fig. 4d). In contrast, available landmass shows strikingly opposing relationships in the two hemispheres, especially outside the tropics (Fig. 4a, b): it continuously increases in the northern hemisphere up until 65°N, but it decreases up until 40°S in the southern hemisphere (Fig. 4b, c), resulting in an overall continuous decrease of available landmass from north to south. This opposing pattern across hemispheres necessarily contradicts the idea⁴⁸ that the increase in gamma diversity from the poles towards the tropics can be explained by larger available landmass and larger ranges in the tropics; absolute range size, latitudinal range size, and longitudinal range size all generally follow the decrease in available landmass from north to south (Figs. 2b, 4b).

a Extratropical landmasses (green) in the northern hemisphere (above) and southern hemisphere. b Landmass (approximated by number of grid cells) per latitudinal band (n = 252 bands) in the northern (blue) and southern (red) hemisphere against absolute latitude. © EuroGraphics for the administrative boundaries. c Cumulative landmass (number of grid cells) from the equator to the poles in the northern (blue) and southern (red) hemisphere against absolute latitude. d Gamma diversity (above) and density (gamma diversity/landmass) against absolute latitude in the northern (blue) and southern (red) hemisphere. Grid cells (n = 14,640) are divided into latitudinal bands (n = 245 bands) according to their latitudinal midpoint; gamma diversity is calculated for each latitudinal band. Gamma diversity decreases towards higher latitudes in both hemispheres. Density decreases towards higher latitudes in the northern hemisphere (n = 76 extratropical latitudinal bands); in the southern hemisphere outside the tropics (n = 59 latitudinal bands), density peaks at mid-latitudes. Shading around regression lines (mean) represents the 95% confidence interval.

The uneven distribution of landmass affects latitudinal patterns of density: in the northern hemisphere, density declines continuously from the tropics to the highest latitudes (species richness, R² = 0.99, P < 0.001; functional diversity, R² = 0.99, P < 0.001; phylogenetic diversity, R² =  0.99, P < 0.001; Fig. 4d), whereas in the southern hemisphere, the relationship is hump-shaped outside the tropics, with a peak at mid-latitudes (species richness, R² =  0.94, P < 0.001; functional diversity, R² = 0.87, P < 0.001; phylogenetic diversity, R² = 0.86, P < 0.001; Fig. 4d); for functional and phylogenetic diversity these peaks represent the highest values observed along the entire latitudinal gradient (Fig. 4d). The decrease of available landmass from north to south corresponds with the observed decrease in range sizes, especially the decrease in longitudinal ranges. At the same absolute latitude, species in the northern hemisphere can expand their ranges over a large area of similar climatic conditions^48,49,50, whereas in the southern hemisphere, longitudinal ranges are restricted because landmasses are separated by vast expanses of open ocean (Fig. 4a). The smaller range sizes and the higher isolation of landmasses in the southern hemisphere result in reduced overlap in assemblage composition and, consequently, the higher values of contribution to gamma diversity and complementarity observed in the southern hemisphere.

Relevance for global diversity patterns and conservation

The global north-south increase in endemism markedly departs (Fig. 5) from current understanding of global diversity patterns. In line with recent studies that showed that southern hemisphere ecosystems, especially the sub-Antarctic and Antarctic, are more diverse than previously thought⁵¹, we demonstrate that—on average—southern hemisphere assemblages harbour among the highest percentages of irreplaceable, range-restricted species, functional trait combinations and phylogenetic lineages (Fig. 5), which makes these assemblages potentially most vulnerable to changes^24,42,52. Similarly, while latitudinal gradients of diversity are similar in both hemispheres⁴³, the diversity is distributed over a much smaller and more disconnected area in the southern hemisphere, leading to a global peak of functional and phylogenetic diversity per area in the mid-latitudes of the southern hemisphere. At the same time, several areas in the southern hemisphere—including Antarctica, the sub-Antarctic islands, oceanic islands in the tropics, and the southern Andes—were among the sites with the highest discrepancy between complementarity (endemism measured as irreplaceability) and contribution to gamma diversity (i.e. weighted endemism). The classic focus on alpha diversity and weighted endemism to identify biodiversity hotspots, hence potentially draws conservation actions away from these highly endemic, but species-poor, assemblages.

a, b Extratropical landmasses in the southern hemisphere (S) are smaller and more disconnected than in the northern hemisphere (N; a), resulting in smaller ranges and higher endemism (here exemplified by phylogenetic complementarity) in the southern hemisphere (b). Values for rank phylogenetic complementarity against latitude (n = 14,640 grid cells) are summarised in latitudinal bands of 5° (n = 26 latitudinal bands). Points are coloured according to relative rank and boxplots according to median values. Values for absolute range size (log-transformed) of phylogenetic lineages against latitude (n = 19,914) are summarised in latitudinal bands of 5° (n = 25 latitudinal bands) according to the range midpoint. Colours represent southern (red) and northern (blue) hemispheres; latitudinal bands inside the tropics are lighter coloured. Boxes show median and 1st and 3rd quartiles; whiskers extend to largest and smallest, respectively, values up to 1.5 x inter-quartile range beyond the quartiles; data points beyond the end of the whiskers are plotted individually. c,d Functional and phylogenetic endemism in the southern hemisphere (S) is high because of the occurrence of old phylogenetic lineages with unusual morphologies that have no equivalent in northern high latitudes (N). These include c basal lineages of Australaves (Cariamiformes [green, 2 spp.]) and Passeriformes (e.g. Atrichornithidae [orange, 2 spp.], Menuridae [red, 2 spp.], Acanthisittidae [blue, 2 spp.]) and d the Palaeognathae (Tinamiformes [orange, 47 spp.], Rheiformes, Struthioniformes, and Casuariformes [red, 9 spp.], and Apterygiformes [blue, 5 spp.]), many of which also occupy small ranges. e Several other bird lineages, e.g. penguins (Sphenisciformes [red, 18 spp.]) or albatrosses (Diomedeidae [blue, 22 spp.]) are largely restricted to southern mid and high latitudes, i.e. the Antarctic region. Despite their highly endemic avifauna, sub-Antarctic islands and Antarctica are commonly omitted from global diversity studies. Bird silhouettes by DMD.

Impact of environmental change in the two hemispheres

The observed hemispheric differences in endemism and available landmass suggest potential differences in the way in which northern and southern species might respond to environmental change. For instance, the smaller area and higher discontinuity of available landmass in the extratropical latitudes of the southern hemisphere might affect the ability of southern species to shift their ranges⁵³. The majority of landmass in the southern hemisphere consists of the southern tips of continents (Fig. 4), isolated by vast expanses of ocean, whereas in the northern hemisphere, extensive areas of landmass are connected across latitudes and longitudes^48,49,50, and the two largest separated blocks of land (Palaearctic and Nearctic) are only separated by the comparatively narrow Bering Strait^54,55,56. Moreover, for birds with range limits at the southern tip of continents (e.g. Fig. 5), the nearest poleward landmasses are the sub-Antarctic islands or the Antarctic continent, which have climatic conditions that make them unsuitable as breeding grounds for most birds. If environmental conditions in the current ranges become unsuitable^28,29, many southern-hemisphere species might therefore not be able to reach suitable new breeding areas. In addition, due to the isolation of landmasses, it is not uncommon for southern-hemisphere species to have their most closely related species on a different continent², and local extinctions are therefore less likely to be replaced by the functionally or phylogenetically most-closely related species.

To date, most studies on the potential effects of environmental changes on species assemblages are based on data from the northern hemisphere⁵⁷, and it is not clear in which way species assemblages in the two hemispheres might differ in their responses to climatic changes. Our results demonstrate that, in addition to protecting high-richness areas, conservation actions should be aimed towards protecting the most-highly unique ecosystems that are vulnerable due to their high dependence on endemics, that is, their high dependence on irreplaceable species, functional trait combinations, and evolutionary lineages, and which are most prevalent in the southern hemisphere.