On 4 June 1930, the participants of the VIIth Ornithological Congress at Amsterdam had the opportunity to visit estate Gooilust near 's-Graveland, where Frans Ernst Blaauw had settled after his marriage with Lady Louise Digna Catharina Six, and had built himself a large private zoo. The extensive lawns, stately trees and ponds were particularly suited for keeping waterfowl. In its heyday, the collection contained hundreds of birds and mammals, all in all some 100 species. Although much smaller than the other internationally renowned private Garden of Eden, i.e. the one of Jean Delacour in Normandy (3000 individuals and 500 species; Delacour's first collection in Picardy was utterly destroyed during the Great War; Mayr 1986), it was a fine collection with a large variety of waterbirds, including Cygnus buccinator, C. columbianus, C. melanocorypha, Coscoroba coscoroba, Anser canagicus, A. caerulescens, A. rossii, all the common Anser and Branta species, Branta sandvicensis, B. ruficollis, all Chloephaga species, Neochen jubata and a plethora of duck species. Blaauw, like Delacour and Peter Scott, was a waterfowl aficionado with the means to purchase and breed rare species in a private collection (Blaauw 1913). In the not yet embanked and unspoiled wetlands of The Netherlands in his days, many fewer waterbird species were breeding, and these were in dire straits. Greylag Geese were all but gone, Garganey and Shoveler already in decline, Mute Swans just gaining a toehold.
At present, the situation is of an entirely different order. Take a summer trip along one of the rivers, the IJssel for example, and the eye meets a bewildering array of waterbirds. Apart from the omnipresence of indigenous Cormorants, herons, White Storks, Mute Swans, Gadwalls and Tufted Ducks (all of them scarce or absent half a century ago), exotic species abound. Indeed, the overwhelming impression is one of being out-of-place. Large flocks of Egyptian Geese let themselves be heard and seen, Greater Canada Geese roam the grasslands, Bar-headed, Swan, Snow, Greater White-fronted and Upland Geese add to the confusion, Mandarin Ducks weave their way across nearby woodland edges. And what to think of the Greylag Goose, a breeding bird which suffered into oblivion in the early 20th century, habitat loss and hunters having no mercy with this — then — shy wetland species. Substantial efforts were made in the 1960s, to restore Greylag Geese as a breeding bird in The Netherlands. Clipped birds — partly from Danish stock but who knows what else — were released in De Rottige Meenthe in Frisia in the 1960s, and elsewhere. Although it is unclear whether these reintroductions were responsible for the subsequent boom, there can be no doubt that continued releases are a decisive factor in any successful establishment. In fact, for exotic birds this is often the most parsimonious explanation for success (Blackburn et al. 2009), but it probably also holds for reintroduced species. Bringing exotic birds to a novel environment is a distinctly non-random event, driven by mundane decisions connected with taxonomic biases in the bird trade industry (waterfowl being a clear favourite), from where birds are extracted, locations where to introduce (affluent societies) and the number of individuals escaped or introduced (and number of repeats after failures). Is this another reason why exotic waterbirds swamped our country from the 1970s onwards? As prosperity increased after several decades of struggle to recoup from the ravages caused by World War II, many people started private aviaries with exotic birds. Formerly a privilege of the rich, nowadays many a farm is adorned with Flamingoes, cranes, geese, ducks, even Wallabies and what not. Inevitably, birds started to escape and propagate in the wild. An important parallel development, substantially boosting reproductive success, was the steep increase in fertilizer usage (>250 kg manure and fertilizer/ha/year) and the conversion of grassland into cropland with oilseed rape, potatoes, sugarbeets and winter cereals (van Eerden et al. 2005). High quality food! The Netherlands has become a huge Garden of Eden for avian herbivores, especially swans and geese. Where formerly private initiatives, like Blaauw's, created small pockets of non-indigenous waterbirds for the happy few, nowadays EU-funded agriculture has created a similar phenomenon on a country-wide scale, for all to see and rejoice in. Who could have foreseen that the combined population of breeding geese in The Netherlands would increase from 200 pairs in 1970 to 40 000 in 2005 (among which 25 000 Greylag Geese and 6000 Barnacle Geese; Voslamber et al. 2007)? The change was almost apocalyptic in the experience of Albert Beintema (2007), when he revisited an island in the Naardermeer after an absence of >40 years: trampled and mud-covered vegetation, tens of deserted nests with hundreds of rotting eggs, a scattering of loose eggs, interspecific egg dumping… In his words: “Complete madness. A multicultural orgy of geese.” So, rejoice in? No, not really. The organisations and governmental agencies involved in the attempts to bring the Greylag Goose back into the Dutch landscape in the 1960s are at present — although on paper a bit more reserved after public outcries — outdoing themselves to eradicate breeding geese, using shooting, gass (to kill captured geese), nest destruction, outfencing foraging grounds (starving goslings to death)… No lack of imagination here. The double turnaround from eradication to protection/reintroduction to culling makes one wonder about the value of Red Lists and scientific research.
Invasions, especially of exotic species, are generally considered as unwanted, either perceived as a threat to indigenous species, or disrupting local ecosystems and causing considerable damage to economic interests. But many invasions are of indigenous species, which — in one way or another — have suddenly found conditions suitable to expand and increase. Many such opportunities have been created by mankind, particularly so in the last few centuries when man's impact on the environment has led to loss and fragmentation of pristine habitats in favour of highly productive monocultures. The Barnacle Goose in The Netherlands is a nice example. Another one are the Great-tailed Grackles, which suddenly arrived at a Biological Station in a pristine dry forest reserve in Mexico (see Ian MacGregor-Fors et al. in this issue of Ardea). This successful, aggressive non-exotic invader may well prove a nuisance (to say the least) to the bird population of the tropical dry forest. What to do? Remove them? But at what cost? Given the fact that the landscape surrounding the reserve has changed in favour of this species, the chances are that a continuous trickle of grackles will invade the reserve. And is the very presence of a Biological Station within the reserve not an open invitation for intruders in itself? In short, golden opportunities for students interested in invasion ecology.