Transcript ORN_chap3
CHAPTER 3 Avian distributions Avian distributions Disjunct ranges • breeding ranges of some birds fall in two or more discrete areas, separated by hundreds or thousands of kilometers • as distances greatly exceed normal dispersal distances and subpopulations become genetically isolated, there is the possibility for independent evolution • may arise by splitting of continuous distribution (vicariance) or by long-distance dispersal and colonization • in Palaearctic region about 20% of all landbirds species have at least one population separated more than 1000 km Avian distributions Examples of discontinuous distribution patterns Avian distributions Disjunct ranges species fall into three main categories • (i) throughout their history confined to rare and patchily distributed habitats, relying on continual dispersal to find and colonise suitable patches • (ii) once occurring in widespread habitats, now fragmented • (iii) disjunct distributions within an expanse of continuous and apparently suitable habitat Avian distributions Disjunct ranges species of rare or patchy habitats • some birds of rare or patchily distributed habitats must always have had patchy distributions (e.g. Lesser flamingos on saline lakes with rich algal food supplies; very few breeding colonies) Avian distributions Disjunct ranges Avian distributions Disjunct ranges species of rare or patchy habitats • wetland birds species in general have good dispersive powers and often turn up at suitable, remote sites (e.g. Mediterranean gull, Little egret, Eurasian spoonbill, Little gull) Avian distributions Disjunct ranges species of once-continuous but now disjunct habitats • species confined to mountain ranges expected to show naturally disjunct distributions; more widespread in glacial times when habitats occupied low ground and moving up when climate warmed again; e.g. White-winged snowfinch, Alpine accentor, Yellow-billed chough Avian distributions European and North American bird species restricted for breeding to high montane areas Avian distributions Disjunct ranges Albania Andorra Armenia Austria Azerbaijan France Georgia Germany Greece Italy Lichtenstein Macedonia Russia Serbia and Montenegro Slovenia Spain Switzerland Turkey Avian distributions Disjunct ranges species of once-continuous but now disjunct habitats • some species occur more continuously in similar habitat elsewhere (tundra, boreal forests), e.g. Rock ptarmigan (tundra), Spotted nutcracker, Three-toed woodpecker (coniferous forests) • during glaciations, other Palaearctic species may have spread south to establish populations in Africa, now isolated in montane areas (e.g. Black-billed magpie in S Arabia, 1600 km; Red-billed chough in Ethiopia, 2000 km; Horned lark in Columbian Andes and Altlas mountains of NW Africa) Avian distributions Disjunct ranges species of once-continuous but now disjunct habitats • apart from upslope retreat of former lowland habitat, dispersal between montane ranges could also be involved, e.g. birds of isolated mountains of NE South America originating from dispersal from the Andes in W Avian distributions Number of bird species in mountain conifer forest areas in California and Baja California Avian distributions Relationships between species numbers and forest area (left) and barrier width (right) of mountain conifer areas in California and Baja California Avian distributions Disjunct ranges species of once-continuous but now disjunct habitats • mountains are important, not only in maintaining distinct species, but also in separating lowland avifaunas on either side; promote separate evolution and sometimes result in pairs of closely related species with one member on each side Avian distributions Disjunct ranges species with disjunct distributions in continuous habitat • some species have discrete populations within large expanses of apparently suitable habitat (Marsh tit, Goldcrest, Twite, White stork); probably because they disappeared from some areas for natural reasons; e.g. Azure-winged magpie (gap of 9000 km) Avian distributions Disjunct ranges Two separate populations. Western distribution limited to Spain and Portugal. May once have been linked with eastern population, which is distributed widely from Mongolia and eastern Russia through most of China and Korea to Japan. Avian distributions Disjunct ranges species with disjunct distributions in continuous habitat • Asian birds were introduced to Europe (16th century) returning from far east; yet fossilized bones (Gibraltar) dated 44.000 years old confirms historical existence in Europe • analysis of mt DNA suggests that W and E populations separated ca. 1.2 million years ago, presumably during a glaciation; fossils in intermediate areas still lacking Avian distributions Disjunct ranges species with disjunct distributions in continuous habitat • comparable situation with Corsican and Chinese nuthatch Avian distributions Disjunct ranges species with disjunct distributions in continuous habitat • likely explanation for present discontinuities: during past climatic extreme, species were confined to remaining isolated refuges of habitat; with improvement in climate they spread less rapidly than habitat (some tropical species) • some species still show idiosyncratic patterns of patchiness (e.g. New Guinea) possibly due to (i) habitat unsuitability, (ii) presence of competitors (e.g. competitive exclusion on islands or tropical areas with high species richness) Avian distributions Patchy distributions of various species in New Guinea Avian distributions Disjunct ranges species with disjunct distributions in continuous habitat • human impact may also have contributed to disjunct ranges (e.g. White-backed woodpecker) Avian distributions Disjunct ranges distributional gaps in continuous habitat may be due to • gap areas containing no suitable habitat in the past; habitat only developed recently, giving insufficient time for colonization (as in glacial refuge explanation) • interspecific competition preventing species from persisting in gap area • human persecution or an unknown pathogen or predator preventing species from persisting in gap area • a serious decline of the species, gaps representing areas of recent withdrawal • explanations are not mutually exclusive nor independent Avian distributions Range dynamics • study of factors that influence current range dynamics by measurement and mapping of bird ranges, study of relationships between bird abundance and distribution, and study of factors that limit population spread • range dynamics may depend on (i) conditions of climate, habitat and food a species requires; (ii) geographical extent of suitable conditions, (iii) area of origin and dispersive powers of the species and historical opportunities to reach areas with suitable conditions • actual ranges may be smaller than potential ranges, as species may be restricted to regions where they evolved (islands) or have been eliminated by human action from suitable areas Avian distributions Range dynamics • ranges are intrinsically dynamic, both because of characteristics of the species and of the habitat; sometimes spectacular concentrations • Red-billed quelea: nightly roosts of +1.000.000 ind; Bramling: night roosts in Beech crops in central Europe +20.000.000 ind; Dickcissel: world population winter in Venezuela in only four roosts (many millions of birds); Spectacled eiders: world population of 400.000 in moult at only four sites; Eared grebes: 1.000.000 ind gather at Great Salt Lake Avian distributions Range dynamics measurement of ranges • interpolation problem with maps in handbooks; based on museum specimens, depict distribution of observers, generally too optimistic • most maps not depicted on equal area projection (exaggerating high-latitude land areas in relation to low ones) • atlas projects (both survey and mapping done on grid basis) represent major advance in range assessment • range boundaries seldom sharp but rather appear as zones of progressively decreasing abundance and increasing patchiness Avian distributions Range dynamics Avian distributions Range dynamics measurement of ranges • questions about current bird distributions can be addressed at several levels of scale • large-scale issues not distinct, but rather intergrade with those concerned with local distribution, abundance, and individual behaviour Avian distributions Range dynamics size of geographical ranges • majority of landbird species breed over remarkably small areas (17% restricted to oceanic islands; 93.6% of continental species restricted to single continent) Distribution of range sizes among 1434 African landbirds Avian distributions Range dynamics size of geographical ranges • among African birds, median geographical range size corresponds to only 1% of the continental area S of Sahara • broad positive relationship between body size and geographical range (some small birds have large ranges, but not vice versa) Avian distributions Range dynamics size of geographical ranges • large species generally live at low overall densities, each individual requiring large area; overall abundances in small areas too low to be viable • combination ‘large body size – low density – small range’ will not persist long through evolutionary time • may also be due to longer dispersal distances of large species, preventing genetic isolation and speciation Avian distributions Range dynamics size of geographical ranges • tropical species in general have smaller geographical ranges than high-latitude counterparts • also tendency for smaller range sizes with increasing species richness (e.g. tropical areas); possibly due to strong competition Avian distributions Range dynamics variation in abundance within ranges • for many species, population density tends to be greatest near centre of range and to decline towards the edges; possibly reflects environmental suitability of the conditions Avian distributions Abundance of the Scissor-tailed Flycatcher over its breeding range in the United States Avian distributions Range dynamics variation in abundance within ranges • in some cases more patchy patterns, leading to source-sink dynamics at different spatial scales (Sparrowhawk, Eurasian dotterel, Barn owl) • source areas enable species to occupy wider distributions than would otherwise be possible, i.e. by replenishing (rescuing) sink populations near edges Avian distributions Range dynamics variation in abundance within ranges • most bird species now live in patchily distributed habitats; smaller and/or more isolated habitat patches usually support smaller populations which are more likely to die out • regardless of other limiting factors, spatial configuration of habitat patches within a landscape can influence population persistence, abundance and distribution • in some cases, metapopulations may be in operation (European nuthatch, Northern spotted owl) Avian distributions Range dynamics Avian distributions Range dynamics Avian distributions Range dynamics variation in abundance within ranges • species found in habitat patches produced by fragmentation of formerly continuous habitats often represent patterned subsets of the original pool of species • together they may form nested series, i.e. increasingly depauperate fauna in small patches making up subsets of more species-rich fauna of larger patches • suggests specific sequence of local extinctions governed by habitat area Avian distributions Range dynamics relationship between abundance and distribution • between species, local abundance and spatial distribution often correlated, i.e. species with high average densities tend to inhabit high proportion of sites within region (Europe, North America, Australia) Avian distributions Range size in relation to average within-range abundance for 70 species of North American landbirds in winter Range size in relation to average within-range abundance for 65 species of North American seedeaters in winter Avian distributions Range dynamics relationship between abundance and distribution • 3 main explanations: (i) sampling artifact (locally rare species more difficult to detect); (ii) generalistic species more abundant and widespread; (iii) positive correlations between population density within sites and number of sites occupied (metapopulation models) • greater number of species, smaller geographic ranges, lower average densities in tropics; latitude explains 47% of variance in population density with average densities increasing lineary from equator to poles • smaller ranges and densities make tropical birds more vulnerable Avian distributions Range dynamics implications for conservation • most bird species occupy small geographical ranges, both continental as oceanic • geographically restricted species tend to have small local populations as well • species at low population densities unlikely to occupy all suitable habitat at any one time • correlation between abundance and range means that reduction in number of populations will lower densities in remaining sites and hence reduce the overall population (cfr. threshold level in Northern spotted owl) Avian distributions Crossing barriers • most range expansions of landbirds involve progressive spread through more or less continuous habitat and require demographic excess • in contrast, crossing of oceans and other barriers involves colonization events by small number of individuals that undertake sustained long flight, the outcome of which depends largely on chance Avian distributions Crossing barriers constraints to cross-barrier colonisations • more than half of ca 570 species on British list are vagrants; many species thus continually reach new areas (even as remote as New Zealand) • main problem is difficulty of establishment (low on body reserves, migratory state, low numbers involved, …) Avian distributions Crossing barriers constraints to cross-barrier colonisations • given favourable climate, habitat and food, colonization chances affected by (1) isolation, weather conditions, dispersive abilities (2) numbers of simultaneous arrivals (3) chances of subsequent arrivals (4) breeding success and rate of population growth (5) amplitude of population fluctuations (6) ultimate population size achievable (7) competition, parasitism and predation from established species Avian distributions Crossing barriers constraints to cross-barrier colonisations Probability of arrival • importance of distance from source area to arrival of non-native species shown by (i) close versus remote islands, decline with latitude; (ii) relative contributions of resident, migrant and vagrant species; (iii) patterns of vagrancy Avian distributions Relative contributions of resident, migrant and vagrant species to the regional bird lists of western North America Avian distributions Total numbers of sightings of different species of eastern North American warblers in California in relation to distance from the breeding range Avian distributions Crossing barriers constraints to cross-barrier colonisations Probability of arrival • influence of wind conditions to vagrancy during autumn and spring (overshoots) • other vagrants may appear because of directional or navigational flaws (e.g. reverse migration); if deviations from usual migration direction genetically controlled, may give rise to new migratory habits and range extensions Avian distributions Crossing barriers constraints to cross-barrier colonisations Founder populations • as birds travel in group, multiple arrivals more likely in birds than in most other organisms • in natural colonizations, numbers of founder members mostly unknown; importance can be illustrated from birds released in introduction schemes • New Zealand: 83% species where >100 ind were released within 10-year period became established, 35% species with 11-100 ind, and 7% species with 2-10 ind Avian distributions Crossing barriers constraints to cross-barrier colonisations Founder populations • similar relationships between numbers released (or persistence of release) and probability of establishment in other regions (e.g. gallinaceous birds in North America, Red-legged partridge and Little owl in UK) • (repeated) natural invasions by large groups of birds do not necessarily result in establishment of breeding populations (e.g. Northern lapwings in Newfoundland) • (semi-)natural colonisation events very well documented in UK because of geographic position and density of skilled birdwatchers Avian distributions Crossing barriers constraints to cross-barrier colonisations Founder populations • at least 27 bird species recorded breeding once/sporadically during 200-year period; no regular breeding population established • during same period establishment of 30 other species; some after history of population increase/range expansion in continental Europe (Cetti’s warbler, Savi’s warbler, Eurasian collared-dove) • in addition establishment of 27 non-native species (mainly waterfowl) through escapes/releases of captive birds • initial breeding attempts that were subsequently successful mainly close to source populations at mainland Europe (eastern part of UK) Avian distributions Crossing barriers constraints to cross-barrier colonisations Population growth and fluctuation • following initial breeding attempts, factors likely to favour establishment of a permanent population are (1) larger numbers of founders (2) higher speed of achieving high rate of population growth (3) smaller fluctuations in population numbers (4) large population size (density) at carrying capacity • causes relationships with body size, trophic level, ecological specialism Avian distributions Crossing barriers constraints to cross-barrier colonisations Interactions with other species • possible interactions involve competition, predation, parasitism; often act in a density-dependent way Avian distributions Crossing barriers constraints to cross-barrier colonisations Interactions with other species • invading/introduced species more successful in areas with impoverished avifaunas (e.g. New Zealand) or man-made habitats (e.g. cultivated land) • colonists may be more susceptible to predators or parasites than locally-adapted species • widely distributed species from continental areas with diverse communities most successful in small, isolated, species-poor areas; may explain unidirectional colonisations (mainland to islands; Asia to Australia) Avian distributions Sea barriers Decline in occurrence of (sub)families of breeding landbirds from New Guinea eastwards on various Pacific Islands Avian distributions Land barriers Avian distributions (Sub)species differences between the two sides of the Rift Valley systems of East Africa Avian distributions Successful colonisations Spread of the Cattle egret, both westward and eastward Avian distributions Successful colonisations Spread of the Common starling in North America