South Africa's population of Kelp Gulls (Larus dominicanus vetula) numbered about 10,000 pairs during 1976/1977–1980/1981, increased to 21,000 pairs from 2000/2001–2004/2005 and then decreased to 17,500 pairs from 2009/2010–2013/2014. The increase in the late 20th century and the decrease in the early 21st century were mainly attributable to large increases and decreases in numbers breeding at islands off the west coast of South Africa. The increases followed cessation of controls on Kelp Gulls at the islands in the 1970s and were associated with supplementary food provided by fisheries and landfill sites, whereas the decreases were influenced by substantial predation of chicks by Great White Pelicans (Pelecanus onocrotalus). The decreases of Kelp Gulls at islands off the west coast of South Africa in the early 21st century were offset to some extent by an increase in numbers breeding on mainland sites, especially around greater Cape Town and along the south coast. The proportion of Kelp Gulls breeding on the south coast increased from 15% in 2000/2001–2004/2005 to 44% in 2009/2010–2013/2014. As there are fewer islands off the south coast than the west coast of South Africa, the proportion of Kelp Gulls breeding at mainland sites as opposed to island localities increased from 12% in 2000 to 31% in 2014. Mainland colonies are more susceptible to disturbance by humans and predation by mainland carnivores, but, in spite of this, the generalist Kelp Gull has performed better in recent years than several specialist, endemic seabirds. At Dyer Island and Bird Island (Algoa Bay), where Kelp Gull numbers increased, predation by Kelp Gulls on eggs and chicks of threatened seabirds is thought to have contributed to decreases in these seabirds, and controls on Kelp Gulls have been reintroduced.
The conservation status of many seabird species has given cause for concern in recent years with 97 (28%) of 346 species listed as globally threatened (Croxall et al. 2012). Of the 15 species of seabirds that breed regularly in southern Africa (Cooper et al. 1984; Whittington et al. 1999; Crawford et al. 2014), nine are included on the South African Red Data list as Threatened and one as Nearthreatened (BirdLife South Africa 2014). In contrast, many gull (Laridae) species have increased in numbers (Conover 1983; Blokpoel and Spaans 1991; Hatch 1996; Yorio et al. 1998), probably as a result of their ability to adapt to human-altered environments and take advantage of food sources such as fishery discards and organic waste at landfill sites.
The Kelp Gull (Larus dominicanus) is widespread in the Southern Hemisphere, breeding in southern Africa, South America, Antarctica, Australia, southern Madagascar, New Zealand and on subantarctic islands (Jiguet 2002). Its International Union for Conservation of Nature Red List status is Least Concern, with the global population estimated to number 3.3 to 4.3 million individuals and to be increasing, although some populations have unknown trends (BirdLife International 2014). Kelp Gulls breeding in southern Africa are recognized as a distinct subspecies L. d. vetula (Brooke and Cooper 1979), representing approximately 2% of the global Kelp Gull population (Wetlands International 2006). Crawford et al. (1982) estimated that 8,906 pairs (approximately 80% of the population) of L. d. vetula bred at 36 localities in South Africa in 1981. Of the 8,906 pairs, 7,444 (84%) bred on offshore islands. No clear overall trend in numbers was recognized (Crawford et al. 1982). Whittington et al. (2006) recorded a 71% increase in numbers of Kelp Gulls breeding from Plettenberg Bay eastward along the south coast of South Africa between 1982 and 2004, while Crawford et al. (2009a) recorded an increase in numbers on 11 islands in the Western Cape between 1978 and 1999/2000 followed by a subsequent decrease. This study reports trends for the L. d. vetula subspecies throughout South Africa and within five regions around the South African coast between the 1976/1977 and 2013/2014 surveys.
Methods
To investigate a possible change in the distribution of Kelp Gulls breeding around the South African coastline, we split the coastline into the five regions similar to those defined in Crawford et al. (2013): Northern Cape, West Coast, Cape Town-Cape Agulhas, South Coast and Eastern Cape (Fig. 1). The West Coast, Cape Town-Cape Agulhas and part of the South Coast regions fall within the provincial boundaries of the Western Cape Province. A distance of approximately 400 km separates the Kelp Gull colonies of the Northern Cape from those in the West Coast region, and trends in other seabird species have shown differences between the northern and southern parts of the Western Cape (Crawford et al. 2014). The provincial boundary of the Eastern Cape falls within the Garden Route National Park, so for the purposes of this study the Eastern Cape is defined as the area between St. Francis Bay and the most easterly breeding site near Hamburg (Fig. 1).
Nest counts were carried out at 74 localities between 1976 and 2014; three in the Northern Cape, 20 in the West Coast region, 14 in the Cape Town-Cape Agulhas region, 23 in the South Coast region and 14 in the Eastern Cape. Breeding sites were located between the Orange River mouth at 38° 37′ S, 16° 28′ E (Anderson et al. 2003), which straddles South Africa's border with Namibia (Fig. 1A), and a location near Hamburg in the Eastern Cape at 33° 21′ S, 27° 22′ E, where an isolated single breeding attempt occurred (Tree 2006) (Fig. 1E). The total number of Kelp Gulls breeding in South Africa during 1976/1977–1980/1981 was estimated by Crawford et al. (1982) by summing the highest counts of numbers of pairs obtained at colonies visited during this period. To examine trends in the numbers of Kelp Gulls breeding in South Africa, we selected two 5-year periods (2000/2001–2004/2005 and 2009/2010–2013/2014) that had reasonably complete census information for the main breeding localities and similarly summed the maximum counts of breeding pairs at each locality visited in a 5-year period. The colonies that were counted in the three different 5-year periods were not always the same. Therefore, for each period we again summed the maximum counts at colonies that were not counted in the period (but omitted colonies that were known to have been formed after the period being considered) to gauge the possible extent of undercounting for the period. We also compared estimates between pairs of periods, which were based on the sums of the maximum counts at colonies that were visited in both periods.
In addition to examining changes between periods in the numbers of Kelp Gulls breeding in each of the five regions, we also examined the proportions breeding at offshore islands (including rocks and stacks surrounded by the ocean) and on the mainland (including islands in salt works and estuaries). Two of the mainland sites were formerly islands: Bird Island adjacent to Lambert's Bay and Marcus Island in Saldanha Bay, which were joined to the mainland in 1959 and 1976, respectively, by causeways to form sheltered harbors (Jarvis and Cram 1971; Shelton et al. 1984). Many years of data were available from 11 localities in the Western Cape and at the Swartkops Estuary in the Eastern Cape (Crawford et al. 2009a, 2009b). Long-term changes in numbers of Kelp Gulls were investigated using these 12 localities.
Counts were made either from vantage points using 10x magnification binoculars after breeding Kelp Gulls had settled at nests, or by walking tightly spaced grids and marking nests (see Crawford et al. 1982, 2009a; Martin and Randall 1987; Whittington et al. 2006 for details). Counts took place from late October-early November, when birds at most nests were incubating eggs (Crawford et al. 1982). It was not always possible to visit all colonies before chicks had hatched, which may have resulted in some undercounting, because chicks are able to leave nests shortly after hatching, and their cryptic plumage and behavior make them easy to overlook (Crawford et al. 1982). Some gull species are able to change their breeding locations (Cramp et al. 1974), and the formation of new colonies of Kelp Gulls in South Africa has been recorded (Calf et al. 2003; Whittington et al. 2006; Crawford et al. 2009a; Ryan and Dyer 2014). If Kelp Gulls changed their breeding location within a 5-year period, it is possible that this method overestimated the population because the same birds may then have been counted at more than one breeding locality within that 5-year period.
Figure 1.
Locations of breeding colonies of Kelp Gulls in South Africa. Numbers refer to those given to each colony in Table 1. A refers to colonies in the Northern Cape, B to the West Coast, C to Cape Town-Cape Agulhas, D to the South Coast and E to the Eastern Cape.
Results
At four sites that each had less than five pairs at some stage prior to 1976, no subsequent breeding was recorded. Six sites, each with a single pair, were counted between the 1976/1977 and 2013/2014 surveys but not in one of the 5-year periods, and a single pair was discovered at a new site in 2014. Another new site containing 16 nests was discovered during a survey of the Northern Cape in November 2014 (Table 1). Numbers of counts per breeding site ranged from 1–36 and averaged 7 (median of 3), while at 27 well-established colonies the number of counts ranged from 3–36 and averaged 16 (median of 9). A maximum count of over 1,000 nests was recorded from seven colonies, four of which were in the West Coast, two in the Cape Town-Cape Agulhas region and one in the South Coast (Table 1). The bulk of the Kelp Gull breeding population was situated in the West Coast region with numbers getting progressively smaller to the south and east (Fig. 2).
National Population Changes
Between 1976/1977–1980/1981 and 2000/2001–2004/2005, the South African population of Kelp Gulls was estimated to have doubled to over 21,000 pairs (Table 1), and the proportion breeding in the West Coast region rose from 70% (Fig. 3A) to 85% (Fig. 3B). Restricting the comparison to the 24 colonies with at least one count in both of these 5-year periods suggests that the increase was of the order of 141%. Of these 24 colonies, eight were in the West Coast region, three in the Cape Town-Cape Agulhas region, four in the South Coast region and nine in the Eastern Cape. Unfortunately, no data were available for the Northern Cape during the period 2000/2001–2004/2005. Although numbers increased at colonies in the Cape Town-Cape Agulhas and South Coast regions, their proportion of the total population decreased, as did the proportion of birds breeding in the Eastern Cape (Fig. 3B). During 2009/2010–2013/2014, numbers of Kelp Gulls breeding in South Africa decreased by approximately 3,500 pairs (17%) from their peak in the late 1990s and early 2000s (Table 1). Restricting the comparison to the 26 colonies with at least one count in each of the periods 2000/2001–2004/2005 and 2009/2010–2013/2014, there was a decrease of 24%. Of these 26 colonies, nine were in the West Coast region, four in the Cape Town-Cape Agulhas region, five in the South Coast region and eight in the Eastern Cape. No Kelp Gulls were recorded breeding at the Orange River mouth in 2011/2012 (or in 2014/2015), previously the largest colony in the Northern Cape, and only eight nests were found at the other two known breeding localities in the Northern Cape during 2009/2010–2013/2014. Numbers breeding in the Cape Town-Cape Agulhas and South Coast regions and in the Eastern Cape increased during the 2009/2010–2013/2014 period. The West Coast region bore the brunt of the decline, only accounting for 56% of the breeding population in 2009/2010–2013/2014, the lowest proportion during this study (Fig. 3C). A direct comparison between 31 colonies with at least one count in the periods 1976/1977–1980/1981 and 2009/2010–2013/2014 indicated an increase of 6,653 pairs or 67%. Of these 31 colonies, one was in the Northern Cape, 10 in the West Coast region, five in the Cape Town-Cape Agulhas region, nine in the South Coast region and six in the Eastern Cape.
Table 1.
Maximum count of Kelp Gull nests at each colony in South Africa in three 5-year periods. The maximum count from all records, including those outside the selected periods going back to 1755, is also shown. Blank cells indicate that no count was carried out in that 5-year period. M = mainland colony, I = maritime island. Sources: Crawford (1983), Crawford et al. (1982, 2009a, 2009b), Kriel et al. (1980), Steele and Hockey (1990), Whittington et al. (2006) and unpublished records of authors. * indicates sites combined as one locality on Figure 1.
Continued.
Continued.
Figure 2.
Maximum numbers of pairs of breeding Kelp Gulls recorded in each region in each 5-year period.
Nine colonies thought to have contained breeding birds were not counted in the 1976/1977–1980/1981 period. The sum of the highest counts obtained in other periods for these colonies was 101; therefore, the maximum underestimate of the Kelp Gull population in 1976/1977–1980/1981 was thought to be 101 breeding pairs. The maximum undercount for the period 2000/2001–2004/2005 was estimated to be 2,632 pairs resulting from 19 colonies that were not counted, and for the period 2009/2010–2013/2014 the estimate was 313 from 11 colonies that did not get counted.
Regional Population Changes
The breeding Kelp Gull population in the Northern Cape declined between 1976/1977–1980/1981 and 2009/2010–2013/2014, principally due to the loss of the Orange River mouth as a breeding site. The largest changes occurred in the West Coast region where numbers rose by about 10,000 between 1976/1977–1980/1981 and 2000/2001–2004/2005 and then dropped by about 9,000 pairs in 2009/2010–2013/2014. Although numbers of pairs involved were considerably less than in the West Coast region, the greatest growth occurred in the Cape Town-Cape Agulhas region where the direct comparison between 1976/1977–1980/1981 and 2009/2010–2013/2014 showed an increase of over 1,000% (Table 2). A more modest but still substantial increase was shown by colonies in the South Coast region and the Eastern Cape (Table 2).
Breeding Locality Changes
In 1976/1977–1980/1981, about 80% of Kelp Gulls nested on offshore islands, rocks or stacks (Table 3). This proportion increased slightly by 2000/2001–2004/2005, but decreased to 69% in 2009/2010–2013/2014 (Table 3). On a regional basis, most Kelp Gulls in the West Coast region nested on offshore islands, although this proportion decreased in 2009/2010–2013/2014 (Table 3). In the Cape Town-Cape Agulhas region, between 60% and 68% of Kelp Gulls bred on offshore islands, the proportion changing little during the study. In contrast, over 80% of Kelp Gulls in the South Coast region bred at mainland sites, and by 2009/2010–2013/2014 virtually all those recorded were on the mainland. A different pattern emerged in the Eastern Cape, where 85% of Kelp Gulls bred on islands in 1976/1977–1980/1981, but by 2000/2001–2004/2005 89% were nesting on the mainland. Numbers on the Algoa Bay islands recovered a little after their low counts in 2000/2001–2004/2005 such that the proportion of Eastern Cape birds nesting on islands rose to 21% in 2009/2010–2013/2014 (Table 3).
New and Extirpated Colonies
A number of new breeding colonies formed during the study period while others disappeared. Three new colonies were discovered in salt works on the West Coast, two of them in the last 3 years of the study. Dwarskersbos salt works (Fig. 1B) was developed during the study period, and no suitable habitat for breeding by Kelp Gulls existed at this site in the 1970s. In the Cape Town-Cape Agulhas region, new colonies were found at Monwabesi close to the preexisting Wolfgat colony in 2012 and at Steenbras Dam, an inland site where the birds nest among montane fynbos vegetation (Ryan and Dyer 2014), in 2013, as well as on an unnamed island covered in dense pine trees in 2014 (Fig. 1C). In the South Coast region, six birds were discovered nesting at Sterkfontein and another small colony was found at Paalneskop in 2012 (Fig. 1D). In the Eastern Cape, a new colony was found at Coega salt works in 2003 and has grown rapidly, most now moving into the nearby Port of Ngqura (Fig. 1E). There were 25 isolated breeding attempts, including five in the West Coast region, three in the Cape Town-Cape Agulhas region, 13 in the South Coast region and four in the Eastern Cape. Most of these breeding attempts related to single pairs, which either did not breed again or were in relatively remote areas that were not subsequently checked. Breeding at one colony in the West Coast region (Marcus Island) and another in the Cape Town-Cape Agulhas region (Rondevlei) ceased in 1993/1994 and 1984/1985, respectively.
Trends in Intensively Monitored Colonies
The 12 most intensively monitored colonies included three mainland sites (Bird Island at Lambert's Bay, Marcus Island and Caspian Island, a low, muddy, estuarine island in Langebaan Lagoon), and six islands (Malgas, Jutten, Meeuw, Schaapen, Vondeling and Dassen) in the West Coast region (Fig. 1B), Robben Island and Dyer Island in the Cape Town-Cape Agulhas region (Fig. 1C) and the Swartkops Estuary in the Eastern Cape (Fig. 1E). Schaapen Island and Dassen Island housed the largest colonies of Kelp Gulls throughout the period until they were overtaken by Robben Island in 2010. All colonies in the West Coast region, with the exception of Marcus Island, showed a steady increase in numbers followed by a decline, the timing of which varied from about 2001 through to 2011 (Fig. 4). The colony at Marcus Island began a steep decline in 1990, and Kelp Gulls ceased breeding there by 1993 (Fig. 4C). Caspian Island was first colonized in 1993 (Crawford et al. 2009a) and grew until 2000, but then decreased steadily beginning in 2006 (Fig. 4H). Robben Island in the Cape Town-Cape Agulhas region has been monitored annually since 1978, but Kelp Gulls were not recorded breeding there until 2000 (Calf et al. 2003). Both Robben Island and Dyer Island showed a similar pattern to colonies in the West Coast region, but did not decrease until after 2010 (Figs. 4J and 4K). The Swartkops Estuary population in the Eastern Cape increased steadily from 1978/1979 to 1986/1987 and then more gradually and was still growing in 2013/2014 (Fig. 4L).
Table 2.
Percentage changes between 5-year periods in numbers of pairs of breeding Kelp Gulls in five coastal regions using colonies that were counted in both periods.
Discussion
The Kelp Gull was one of 18 species of gulls with an increasing trend in at least one population at the turn of the 21st century; 10 species or populations were listed as stable and nine as declining (Wetlands International 2002). Both L. d. vetula and the nominate subspecies of Kelp Gull in South America were listed as increasing (Wetlands International 2002). In southern Africa, two main reasons probably account for the increasing numbers of Kelp Gulls. Prior to the 1960s, numbers of Kelp Gulls on offshore islands were controlled to protect the guano and egg producing species (Crawford et al. 1982), because Kelp Gulls are predators on the eggs and small chicks of guano-producing seabirds. The virtual cessation of these control measures in the 1960s and 1970s allowed island Kelp Gull populations to recover (Crawford et al. 2009a). The other factor promoting population growth has been the provision of additional food from fishing activities and waste disposal. Bycatch and offal discarded by the fishing industry and organic material dumped at landfill sites provide Kelp Gulls with easily available food (Steele 1992; Whittington et al. 2006). An increase in inland records of Kelp Gulls, especially at slaughterhouses and landfill sites, was noticed in the 1980s and may be related to the increased Kelp Gull population at this time (Suter and Ryan 1984; Hockey et al. 1989). The expanding Cape fur seal (Arctocephalus p. pusillus) population, including individuals that specialize in predation of penguins, gannets and cormorants (Marks et al. 1997; Makhado et al. 2006, 2013), provides additional feeding opportunities for Kelp Gulls, which often scavenge scraps at seal kills (du Toit 2001). Kelp Gulls on the west coast also feed on the introduced snail (Theba pisana; P. G. Ryan, pers. obs.), which is common in agricultural lands in the Western and Eastern Capes (Herbert 2010).
Table 3.
Numbers and percentages of Kelp Gull pairs nesting on maritime islands in each region in each 5-year period.
Figure 4.
Trends in numbers of Kelp Gulls breeding in the 12 most intensively monitored colonies, 1976/1977–2013/2014.
The recent decrease in the South African Kelp Gull population resulted from losses at islands off the West Coast. These were in part due to easterly shifts in the centers of distribution of sardine (Sardinops sagax) and rock lobster (Jasus lalandii), resulting in lower abundance of these prey species to seabirds breeding on the west coast and a subsequent easterly movement of some breeding seabirds (Crawford et al. 2008a, 2008b, 2008c). However, probably the most important factor was predation of Kelp Gull chicks by Great White Pelicans (Pelecanus onocrotalus; hereafter pelicans). Numbers of pelicans nesting on Dassen Island in the southern part of the West Coast region increased considerably in the latter part of the 20th century (Crawford et al. 1995; du Toit et al. 2003; Crawford 2005). Pelicans walk through Kelp Gull colonies feeding on gull chicks (Crawford et al. 1995; de Ponte Machado 2007; Mwema et al. 2010), resulting in near breeding failure in some years (Mwema et al. 2010). Only four Kelp Gull chicks were raised by 4,600 pairs at Dassen Island in December 2007 (Crawford et al. 2009a). Predation spread to nearby islands in Saldanha Bay, resulting in complete breeding failure of Kelp Gulls at Jutten Island and Schaapen Island in 2006/2007 (de Ponte Machado 2009). Prior to this time, Schaapen Island housed the largest Kelp Gull colony in South Africa. In spite of this decline in the Western Cape, the Kelp Gull population remained higher than it had been during the period 1976/1977–1980/1981.
The absence of breeding Kelp Gulls on a survey of the Orange River mouth in 2011 may have been due to flooding of the lowlying islands where they had previously bred (B. M. Dyer, pers. obs.). In the West Coast region, the decline and extirpation of the colony at Marcus Island in Saldanha Bay may have been due to terrestrial predators gaining access to the island after it was connected to the mainland via a causeway in 1976 (Crawford et al. 1994), although other mainland colonies have continued to thrive. The increase in Kelp Gull populations east of Cape Town probably owes much to being beyond the range of pelicans (Crawford et al. 2009a), although pelicans have been seen in the Kelp Gull colony at Strandfontein, apparently taking chicks, and five were chased off Dyer Island in October 2005 (L. J. Waller, pers. obs.).
The increase in the proportion of islandnesting Kelp Gulls in the Cape Town-Cape Agulhas region can be attributed to the growth of the colonies at Robben Island and Dyer Island, but there was also growth in the mainland colonies at Strandfontein and Wolfgat and establishment of an inland colony at Steenbras Dam. Kelp Gulls did not begin nesting on Robben Island until 2000/2001 (Calf et al. 2003), yet it was the largest colony in the country in 2011/2012. The initial increase at Robben Island was largely driven by immigration (Crawford et al. 2009a). Numbers of Kelp Gulls at Dyer Island increased to the point where concern was raised over the negative effects of their predation on the eggs and chicks of other seabird species, notably African Penguins (Spheniscus demersus) (Pichegru 2012) and Cape Cormorants (Phalacrocorax capensis) (Voorbergen et al. 2012), and on adult Leach's Storm-petrels (Oceanodroma leucorhoa) (Taylor and Whittington 2015). A culling program began in 2012 and continued in 2013. Subsequently, a 65% drop in the numbers of Kelp Gulls breeding on Dyer Island was recorded (Fig. 4K). The increase in numbers in the South Coast region was largely due to the growth of the mainland colony at the Keurbooms Estuary. This site is in a nature reserve and is within 7 km of the Robberg landfill (Whittington et al. 2006). There are few island sites available for Kelp Gulls to nest in this region. The reasons for the change from island to mainland breeding in the Eastern Cape are not fully understood. St. Croix Island and Seal Island housed the largest Kelp Gull colonies in the Eastern Cape in 1976/1977–1980/1981, but the Swartkops Estuary had taken over by 1982. Kelp Gulls from St. Croix Island probably relocated to Redhouse saltpan in the Swartkops Estuary, possibly to be closer to sources of supplementary food at the Port Elizabeth harbor (14 km from the estuary) and at the Arlington landfill site (20 km away). Redhouse saltpan is ~10 km closer to these locations than St. Croix Island. However, St. Croix Island is free of predators and human disturbance, whereas Kelp Gull nests at Redhouse saltpan were frequently robbed by mongooses, as well as by dogs and people from Motherwell (Martin and Randall 1987; Whittington et al. 2006). By 2006, most Kelp Gulls from Redhouse saltpan had relocated to the saltmarsh of the estuary proper.
It is likely that new colonies of Kelp Gulls formed on the mainland of the West Coast region in response to the heavy predation of their chicks by pelicans at island colonies. The growth of some mainland colonies, such as at the Swartkops Estuary, and the founding of the colony at Coega saltpans (most of these breeding birds have now moved into the port of Ngqura; A. P. Martin, pers. obs.) in the Eastern Cape may be in part due to immigration of Kelp Gulls from islands in Algoa Bay (Whittington et al. 2006). Single pairs nesting between Kariega Point and the Kasouga Estuary in 2003 (Whittington et al. 2006) and east of the Ngculura River in 2006 (Tree 2006) represent the most easterly recorded breeding attempts of this species in South Africa.
The Kelp Gull population in South Africa grew following cessation of control methods, aided by fishery discards and poor waste management, as has been noted for gulls elsewhere (Cotter et al. 2012). Environmental changes influencing the distributions of Kelp Gull prey (both natural and humansupplemented) have probably encouraged a redistribution of breeding by Kelp Gulls, and their varied diet (Steele 1982) and flexible breeding behavior have allowed them to avoid the marked population decreases exhibited over the last decade by some of the more specialized seabirds that breed in the region (Crawford et al. 2007, 2008a, 2011).
Acknowledgments
PAW acknowledges support from the Nelson Mandela Metropolitan University (NMMU) and the National Research Foundation (NRF); RJMC from the Department of Environmental Affairs (DEA) and NRF; RMR from NMMU and South African National Parks (SANP); MB from BirdLife Plettenberg Bay and Bird-Life South Africa's Ornithological Trust Fund; and MW from NRF, BirdLife Plettenberg Bay and BirdLife South Africa's Ornithological Trust Fund. CapeNature, DEA, Robben Island Museum, SANP and the South African Defence Force provided logistical support and assisted with counts at several localities. We thank everyone who assisted with field work and those who reviewed and improved earlier versions of the manuscript.
