Successful population establishment from small introductions appears to be less common than believed

Although small populations are at high risk of extinction, there are regular reports in the scientific literature of purported small, isolated, persistent populations. One source of evidence of the viability of small populations comes from the alleged successful introduction of species to areas outside their original range from introductions of few individuals. We reviewed the examples from introduction compendia on deliberate translocations of birds, and the original sources, to identify and evaluate purported examples of successful establishments from small introductions. We found 23 purportedly successful introductions from few (<30) individuals. After assessing original sources, we found that two of the claims were substantiated; the rest were ambiguous or could be rejected as examples, primarily due to a lack of evidence in original sources of the number of birds released and because of supplemental individuals from other releases, releases in nearby regions, and the possibility of natural invasion. Our results suggest that reports of successful establishment of birds from introductions of few individuals have been overstated. These results strengthen the relationship previously reported between propagule pressure and likelihood of establishment, and support the lack of viability of small populations presumed by population theory. We suggest that analyses of introduction failure and success would benefit from excluding studies where introduction effort is unknown or unreliably documented.


Mute swan
Cygnus olor 12 (Australia) Duncan et al. 2001Long 1981:38 "A pair of white swans were released on Phillip Island, Victoria, in 1866 (Jenkins 1977, Balmford 1978 and four white swans arrived in Melbourne… Both imports were probably mute swans. Balmford also records that a single white swan may have been noted in the wild before 1859…Six birds were released in Launceston, Tasmania in the 1920s and progeny from these were released in other areas (Jenkins 1977)." Uncertain data: "Both imports were probably mute swans" (Long 1981:38).
Birds may have been present in the wild before introduction (Balmford 1978:244, Lever 1987. The two swans released to Phillip Island in 1866 were said to not have established (Jenkins 1977:76).
Numbers of birds introduced differs between references.  Veltman et al. 1996Long 1981 "Six Mute Swans were introduced at Canterbury in 1966, nine at Otago in 1968-69, fourteen at Auckland in 1969-71, and several birds were introduced by private individuals and dealers at about the same time (Thomson 1922). Other earlier introductions include some which were made at Christchurch 1866, Auckland in 1867, and Dunedin in 1868, when two or more birds were released at a time (Oliver 1955)". Thomson 1922:107 "The Canterbury Society received two in 1966; the Otago Society three in 1968, and one in 1989; the Auckland Society two in 1869 (from Sire Geo. Grey), and 12 in 1871 (from captain Hutton). Several were also introduced by private individuals and by dealers. It is nowhere common." Population may have been increased by the release of several birds by private individuals (Thomson 1922:107) "about the same time," (Long 1981:37).
Earlier introductions occurred, indicating the possibility for existing populations at the time of the cited release (Oliver 1955 1914/1915and 2 in 1917(Thomson 1922. Birds may have dispersed to New Zealand naturally, and not the result of a human introduction (Falla et al. 1966:67, Williams 1922, Williams 1968 It is likely that the goose is not established in New Zealand (Falla et al. 1966:87, Lever 1987  Actual number of birds that escaped is unreported, and appears to be based on a rough population model rather than data (Carleton and Owre 1975:43).

Farm in the suburb of Kendall in Dade
County…" until reports came in from his neighbors of a new bird…" Information from other literature Carleton and Owre 1975:43 "To produce the present population, a founding population of fewer than five feeding pairs would have had an annual rate of increase of more than 50%which seems excessive. An initial population of more than 10 pairs seems to large from accounts of residents… Thus, from tentative data, we hypothesize that between 5 and 10 breeding pairs founding the population…"  (11), 1865 (5), and in 1871 (?), and three years later they were considered to be thoroughly established there…According to Thomson (1922) many more were released by private individuals and dealers at most of the principal centres of that time." Thomson 1922:170 "The Canterbury Society liberated 11 in 1867, and five in 1868; and three years later reported that they are considered to be 'thoroughly established and to need no further importations.' In 1871 a further lot were introduced." Multiple other releases in nearby locations during the 1860-1870s (Turbott 1990:218), including Auckland (113+ ind., 1864-1869), Otago (99 ind., 1868-1871), Wellington (126+ ind., 1874-1879, and private individuals (Thomson 1922:170 There is uncertainty about the exact species of redpoll that was introduced to New Zealand (Williams 1969:446 Multiple authors reported the same suite of introductions differently. The suite is described to the right (Long 1981:466), and the different reports are shown below Long 1981:466 "The Otago Acclimatisation Society liberated seven birds in 1871 (Thomson 1922) and four birds were liberated at Wellington in 1880 (Oliver 1930). Some eighteen birds were also liberated on Stewart Island in 1879, but the species apparently failed to become established on that island."