LAND Are indigenous expansionists cause for concern? INVASIONS

warwick tarboton

As extinctions occur at a rate unprecedented in recent history, the eyes of the world’s conservationists are firmly focused on the planet’s diminishing biodiversity. In the case of birds, the path to extinction is most often paved by fragmentation, degradation and subse- quent loss of habitat. Whilst these concerns are of paramount global importance, what is sometimes forgotten is that the loss of any habitat has, as a necessary corollary, its replace- ment with another. In some cases, these changes are extreme – such as the transformation of wetlands to residential areas, or grasslands to conifer plantations. In others, such as the replacement of Zanzibar forests with clove plantations, structural changes are less dramatic. In this article, Phil Hockey considers how changing landscapes may have resulted in range

expansions of indigenous birds, and whether such range shifts are cause for concern. 

october/november 2003 range expansions 35 irds respond more to the physi- ed shallow, phosphate-rich pans on the cal structure of the environment tops of large mine dumps. Another habitat Some predictions of how than they do to its botanical specialist, Sclater’s Lark, is dependent on landscape-level changes could make-up. Obviously, there are water. The construction of small farm result in range expansion exceptionsB to this generalisation, such as dams in the western Karoo has increased nectar-feeding birds that depend on one the species’ potential breeding range (as • Proliferation of farm dams or a small suite of flowering plant species. it has done for other water-dependent spe- favours waterbirds But at the other extreme, man-altered cies, such as sandgrouse). habitats create environments that can be It is not difficult to imagine how the • Increasing tree cover (plantations, exploited by opportunists. Red-winged­ landscape-level changes to the environ- suburban gardens) favours wood- Starlings and Speckled (Rock) Pigeons, for ment that result from agriculture and land- and forest-adapted species example, find the ledges of high-rise build- other human activities can cause large- ings perfectly adequate substitutes for their scale changes to the distributions of native • Buildings and other structures natural nesting sites on cliffs. birds. For species that respond posi- favour species that breed on ‘hard’ Not only ‘generalists’ have made such tively to such changes, there could be far- surfaces, such as cliffs or caves adaptations. The Chestnut-banded Plover reaching ramifications as they come into • Electricity pylons provide nest sites is an extreme specialist, dependent almost contact with ‘new neighbours’. Southern for tree-nesting species in other- Previous page Black Sparrowhawk has entirely on hypersaline pans. Commercial Africa provides an excellent canvas to wise treeless landscapes used the proliferation of exotic plantations, saltworks have provided an excellent alter- examine such changes, firstly because it is such as eucalypts, to its advantage as native habitat and, as a result, the plover’s a region of dramatic natural gradients, and • Increasing spread of humans nesting sites. range and numbers have increased. Other secondly because it has experienced sub- favours man’s commensals. wetland specialists have been similarly stantial changes at the hand of man. The Below The ability of Sacred Ibis to live in oppor­ tunistic.­ Near Phalaborwa, adjacent gradients in question are primarily longi- close proximity to man aided its colonisa- to the Kruger National Park, Cape Teals, tudinal rather than latitudinal: the east tion of arid areas. flamingos­ and many waders have colonis­ experiences higher rainfall than the west, supports more structurally complex vege- Western Cape during the 20th century, 30 tation, and has a richer bird diversity. The (32 per cent) are associated with freshwater west, by contrast, is dry, dominated by wetlands. In contrast, of 30 species whose short, scrubby vegetation, and supports range/numbers have decreased, only seven relatively few bird species. Because species are wetland species (all requiring large, richness is greatest in the east, it could be vege­­tated wetlands or pristine rivers). mathematically predicted that westward The past 50 years have seen a number of range expansions (from areas of high to colonisa­ tions­ of the Western Cape by wet- low diversity) will be more frequent than land species from the east or north, includ- vice versa. ing Blacksmith Plover and White-faced Man-made changes to local environ- Duck. One of the most dramatic expan- ments have either increased the structural sions has been that of the Glossy Ibis. complexity­ of the habitat (such as the Between the 1950s and 1970s, this species replacement­­ of highland grasslands with more than doubled its regional range, commercial forestry), or decreased it (such expanding westwards as far as the Free as the replacement of fynbos with cereal State and central Botswana. Not long crops, or lowland forest with sugar-cane). afterwards, it made its way across the Similarly, the plethora of farm dams built Karoo to colonise the Western Cape, for either stock-watering or irrigation has where it is now common, probably num- dramatically changed the distribution of bering in the thousands. Hadeda and  surface water, especially in the arid west.

Effects of the changes Proliferation of surface water Globally, wetlands are one of the planet’s most threatened habitats, yet, in southern Africa, thousands of small wetlands have been created in the arid regions for irriga- tion and stock-watering. The effect of this 1950s has been to create a more uniform distri- 1970s bution of wetlands across the region. Wet­ 1990s land birds have most definitely responded to this change: of 92 species thought to Sacred Ibis Glossy Ibis warwick tarboton (2) have increased in numbers or range in the

36 range expansions africa – birds & birding october/november 2003 range expansions 37 Sacred ibises are also recent colonists of raptor in Cape Town, breeding and hunt- the west. The former is thought to have ing in suburban gardens. Similarly, until used irrigated lands as stepping stones the 1950s, Burchell’s Coucal was barely through the dry country, while the latter known in Cape Town; today it is a com- probably exploited­ a combination of mon garden bird of the greener suburbs. man-made wetlands and other human At least one fynbos endemic – the Cape artefacts, such as refuse dumps. Bulbul – has also adapted well to garden life and has used gardens as stepping 1950s Afforestation and gardens stones to colonise Namaqualand. Even in 1970s 1950s The regional demand for timber, and the Karoo, gardens have affected bird dis- 1990s 1970s especially pulp, has led to afforestation in tributions. Rufous-chested Sparrowhawks early 2000s 1990s habitats ranging from the lowlands of have moved into eucalypt and poplar KwaZulu-Natal to highveld grasslands and groves around farmsteads, and the spread South African Cliff-Swallow Black-chested Prinia fynbos. Pines and eucalypts are the domi- of the African Harrier-Hawk has been little nant trees of plantations and, whilst such short of meteoric. stands are generally sterile habitats for birds, a handful of species, especially rap- Man-made structures and tors, have used them to great advantage man’s commensals as nesting sites. Two such opportunists Worldwide, birds use man-made structures are the Black and Rufous-chested sparrow­ as substitutes for natural nesting and roost­ hawks. As recently as the mid-1970s, ­ing habitats: falcons, owls, storks, starlings Black Sparrowhawks were unknown in and swifts are but a few of the birds that Cape Town; today, they are not only com- breed on buildings. In the case of the mon, but increasing. Purple Martin Progne subis of America, it is Gardens too have paved the way for suspected that almost the entire world colonists. In Zimbabwe, Harare has be­come population breeds in nest boxes. In south- a green ‘island’ and new species have ern Africa, road bridges and culverts have arrived in response. Purple-crested Turacos facilitated the spread of at least one spe- have been resident for more than 30 years. cies, the South African Cliff-Swallow. More recently, both Variable Sunbirds and Anyone who has passed under Karoo Thick-billed Weavers have moved in from bridges in summer might wonder where the Eastern Districts. these birds bred historically, and whether In Gauteng, there has been a similar indeed they continue to breed in natural response by the Grey Go-away-bird. Forty habitats. By the mid-1990s, the cliff-swal- years ago, its range stopped well north of lows had arrived within a few hundred Pretoria; today, it is one of the first birds a kilometres of Cape Town; five years later, visitor to Johannesburg is likely to encount­ ­ this distance had been reduced to a mere er. At present, African Olive-(Rameron) 50 kilometres. In Zambia, construction of warwick tarboton Black-chested Prinia – is its range Pigeon appears to be undergoing a similar roads and culverts has produced a similar expansion indicative of climate change? invasion of highveld suburbia. response by Red-breasted Swallows. In the Western Cape, the African Gos­ Electricity pylons have also affected dis- hawk was almost unknown 30 years ago. tributions. Used as nest sites, they attract Climate change - the curved ball Today, it is probably the most common crows to otherwise treeless landscapes. 

chris van rooyen In north-western Europe, several birds have expanded their breeding ranges northward. This is thought to be a consequence of global warming – the same phenomenon that may be respon- Three expansionist top predators – sible for the northward retreat of breeding Ruffs to the fringes of the Arctic Ocean. In southern African Goshawk (above), Black Africa, in contrast to most range expansions which have been westwards, some dry-country spe- Sparrowhawk and African Harrier- cies have expanded their ranges to the south and east. The reasons for these changes are not Hawk – have all colonised areas where immediately obvious, but they do suggest that the border of the ‘arid zone’ itself may be moving. no equivalent predator existed before. That changes are happening in the arid zone is supported by broad-scale increases in the mortal- ity rate of kokerboom Aloe dichotoma trees. Mortality rates of these trees have increased in the hottest parts of their range, but populations in the cooler south are increasing. Both Pririt Batis and Black-chested Prinia increased their ranges more rapidly between the 1970s and the 1990s 1950s than between the 1950s and the 1970s, suggesting that the rate of aridification might be accel- 1970s erating: this accelerating rate of change is also supported by meteorological data. 1990s In southern Africa, climate change is predicted to have dramatic impacts on vegetation within the next 50 years. It is highly that many range shifts will occur among birds; some of these are likely to become apparent within the next decade. African Goshawk Black Sparrowhawk African Harrier-Hawk

38 range expansions africa – birds & birding october/november 2003 range expansions 39 1950s 1950s 1990s 1970s (1970s data poor) 1990s

Lesser Honeyguide Southern Grey-headed Sparrow

no means wholly, explain the westward expansion of the latter’s range. Expansionists – bonuses or liabilities? Several range-expanding species are large birds at the tops of food chains, the most obvious being raptors. Almost 20 years ago, it was noted that bird communities in the vicinity of Karoo farmsteads support- ing Rufous-chested Sparrowhawks were depauperate compared to those around albert froneman The expansion of the Southern Grey-headed Sparrow’s range seems to farmsteads without hawks. Whether the be fairly closely tracked by that of its parasite, the Lesser Honeyguide. small birds had been scared away or eaten was never established, but it is certain that these birds were suddenly faced with a the Cape Peninsula have decreased since sity. predator that hunted in a manner they the hawks’ arrival, with the remaining AUTHOR’S ACKNOWLEDGEMENTS had never previously experienced. It was populations being confined to areas of I thank Odette Curtis, Andrew Jenkins, Penn suggested at the time (Bokmakierie, 38(1):3– dense cover. Lloyd, Guy Midgely, Warwick Tarboton and 4) that it might be considered a conserva- On the coast, Sacred Ibises have become Tony Tree for their input to this article. tion coup if Rufous-chested Sparrowhawk regular predators at seabird colonies, eat-

chris van rooyen could be removed from the list of birds ing eggs and chicks of African Penguins, occurring in the Karoo National Park. Swift Terns and cormorants. Coupled with The African Olive-Pigeon is currently under- These nests in turn are taken over by other Impacts of most other ‘range-expanders’ greatly increased numbers of predatory going a range expansion in Gauteng, where new colonists, including Lanner Falcons, have never been studied. In Cape Town, Kelp Gulls, which themselves have bene- the greening of the suburbs has provided it and Greater and Rock kestrels. both African Goshawks and Black fited from supplementary food provided SOURCES FOR RANGE MAPS with ideal habitat. Some species are highly successful at liv- Sparrowhawks undoubtedly eat many by man, this begs the question of whether 1950s ing with man, certain sparrows, starlings birds, mostly caught in and around subur- these expansionists do, or soon will, pose McLachlan, G.R., Liversidge, R. 1957. Roberts’ and mynas being cases in point. Indeed, ban gardens. Both species may be indige- a significant threat to breeding seabirds. Birds of South Africa. 2nd ed. Trustees of the the record for the fastest ever spread of a nous to southern Africa, but are they A conservation mindset exists that South African Bird Book Fund, Cape Town. bird species in southern Africa is held by indigenous to the Cape Town area? Some ‘indigenous is good and alien is bad’. Wars the alien House Sparrow. In the 1950s, it believe that the recent appearance of are justifiably waged on alien House 1970s was confined to a fairly small area in the Black Sparrowhawk represents a re-coloni- Crows. House Crows hitch-hiked on man- McLachlan G.R., Liversidge, R. 1978 Roberts’ south-east of South Africa; by the 1990s, it sation of the Cape Peninsula following a made boats, but did Black Sparrowhawks Birds of South Africa. 4th ed. Trustees of the had colonised the entire region bar a small past extinction. These birds may be per- island-hop through man-made stepping John Voelcker Bird Book Fund, Cape Town. portion of north-western Botswana. The forming an ecological role in controlling stones of alien trees? Is there such a big indigenous Southern Grey-headed Spar­ populations of other species (such as doves, difference? Perhaps the time has come to 1990s row has also spread. This range expansion Feral Pigeons and European Starlings) look more closely at how our activities Harrison, J.A., Allan, D.G., Underhill, L.G., started in a small way in the 1960s and whose numbers have been artificially may be affecting the distribution patterns Herremans, M., Tree, A.J., Parker, V., Brown, C.J. 1970s, but has subsequently progressed inflated by man’s urban activities. A flip- of indigenous species and to question rig- (eds). 1997. The Atlas of Southern African rapidly. This species is one of the hosts of side of this, however, is that the range and orously whether they do or do not make Birds. BirdLife South Africa, Johannesburg. Lesser Honeyguide and may partly, but by numbers of Lemon (Cinnamon) Dove on valuable additions to local biodiver- 

40 range expansions africa – birds & birding october/november 2003