28 Feb 2018

How alien plant invaders crowd out native birds

It's not just cats and rats: invasive plant species can also uproot ecosystems and drive extinctions.

Pink Pigeon © Chris Moody/Shutterstock
Pink Pigeon © Chris Moody/Shutterstock
By Alex Dale

To mark the centennial of the US Migratory Bird Treaty Act – the most powerful and important bird protection law ever passed – we are declaring 2018 the "Year of the Bird". As part of this, BirdLife International, National Geographic, the National Audubon Society and the Cornell Lab of Ornithology are joining forces to encourage nature lovers to take a small but important step every month to make the world a better place for birds.

March's Call to Action is to plant native plants: when filled with native species, your yard, garden, patio, or balcony becomes a vital recharging station for birds passing through, and a sanctuary for nesting birds.


In the below article, we show the other side of the coin - how alien plants, often introduced to other parts of the world for no reason other than their looks, can destroy ecosystems and even push globally threatened birds towards extinction.


Kahili Ginger

Azores, Portugal


Juvenile Azores Bullfinch © Ricardo Ceia

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More than 60% of the plant life found on the Azores archipelago originates from overseas – introduced to the islands either for agricultural or ornamental purposes. They include rugged species such as the Kahili ginger Hedychium gardnerianum of India and Nepal; an aggressive, shade-tolerant plant which takes over forests by forming dense blankets of foliage that crowd out native seedlings. Today, only two percent of the islands’ native laurel forest remains – a collapse which would have starved the endemic Azores Bullfinch Pyrrhula murina into extinction, were it not for the efforts of SPEA (BirdLife in Portugal). Our Partner has worked to restore pockets of Azorean forest, ringfencing them to ensure alien plant invaders stay out. The Azores Bullfinch, once Europe’s rarest songbird, has rebounded in numbers accordingly.


Read more: Europe's most threatened songbird rebounds


Strawberry Guava

Rapa Iti, French Polynesia


Strawberry Guava © Dick Culbert


Even on Rapa Iti, one of the world’s remotest islands, alien plant life has taken root. Strawberry guava Psidium cattleianum, a South American tree that bears an edible, plum-sized fruit, is a well-known invasive pest that has wrecked havoc on ecosystems from Florida to Australia. When distanced from factors such as native seed-predators in Brazil that keep its growth in check, Strawberry Guava can spread rapidly in dense thickets that prove incredibly difficult to remove. Its presence is a major threat to Rapa Iti’s array of unique wildlife (the island boasts over a hundred endemic land snails alone). The permanent removal of these thickets is one of the key objectives of our 2017 Birdfair project, “Saving Paradise in the Pacific”.


Read more: How we're saving the little paradise in the Pacific



Rudong mudflats, China


Smooth Cordgrass Spartina Alterniflora, the species introduced to Rudong © USDA-NRCS PLANTS Database


Otherwise known as cordgrasses, Spartina is a family of fast-spreading aquatic grasses that grows in tight clusters on mudflats and marshes. A North American species was introduced to China’s coastal estuaries in the 1970s to trap sediment and make it easier to build seawalls, but the species quickly took over the mudflats, aided by its ability to spread rapidly via a network of underground stems. Clumps can even break off and take root elsewhere on the bay, making the invasion difficult to contain. In the process, expanses of valuable mud-feeding habitats that support migratory shorebirds such as Spoon-billed Sandpiper Calidris pygmaea have been lost. This is an additional pressure that this Critically Endangered species, with a mature global population of less than 500, can ill afford.


Act now: Help us protect the East Asian-Australasian Flyway



Indian grasslands


Great Indian Bustard Ardeotis nigriceps (Critically Endangered) © Prajwal KM


The problems affecting India’s grassland birds stem from bad policy. Until recently, India considered grassland habitats to effectively be wasteland, and the Forest Department, where possible, converted them into plantations of tress that could be used for fuel or fodder. Even if that meant planting exotic plants such as the notorious Prosopis juliflora, an aggressively-spreading, thorny tree that creates impenetrable barriers that restrict the movements of land animals. Left unchecked, it creates a hostile monoculture – destroying the grassland habitat of Critically Endangered birds such as Great Indian Bustard Ardeotis nigriceps. In India, there’s an extra complication to weeding out invasives – ground work is dangerous and often has to be performed under armed guard, as rhinos, elephants and wild buffalo are often roaming nearby.


Read more: The search for India's hidden treasures






Pink Pigeon © Chris Moody/Shutterstock


Mauritius, famously, was the home of the Dodo Raphus cucullatus – a stocky member of the pigeon and dove family whose fate requires no further elaboration. But while it’s too late to save the Dodo – or its swan-sized close relative Rodrigues solitaire Pezophaps solitaria for that matter – there’s still hope for the world’s only remaining extant Mascarene pigeon: the Pink Pigeon Nesoenas mayeri.

But we’ve cut it fine. The encroachment of invasive plant life such as Strawberry Guava (again!) and Privet Ligustrum robustum, disrupting the habits of this ground-feeding herbivore, is just one of many factors that saw the Pink Pigeon population dip as low as 10 individuals in 1991. An intensive breeding and wild management program has seen that number grow to over 300 by 2000, and now the Mauritian Wildlife Foundation (MWF, BirdLife Partner) is now working to translocate the Pink Pigeon to new areas of Mauritius, in a bid to expand its limited range.


Coconut palms

Cousin Island, Seychelles


Cousin Island, Seychelles © Tiare Scott



How about we leave you with a feel-good story? In 1968, Cousin Island, part of the Seychelles, didn’t have much going for it. Most of its native vegetation had been cleared to make way for coconut plantations, and farmyard animals were trampling over what remained. But Cousin Island was also home to the very last population of Seychelles Warbler Acrocephalus sechellensis – which by 1968 had hit an all-time low of 30 individuals. With the species seemingly doomed, BirdLife swept in to purchase the island, and with the help of local conservations, created a habitat restoration programme, cutting down the plantations and allowing the native vegetation to grow back. The Seychelles Warbler population responded in amazing fashion, swelling to over 300 by 1982. And thanks to translocation efforts, it is now found on several islands, with a population of 3,000 – and increasing.


Read more: Ever fantasised about owning your own tropical island?