On the origin of the tree Spondias mombin in Africa

Journal of Historical Geography 32 (2006) 249e266www.elsevier.com/locate/jhg

On the origin of the tree Spondias mombin in Africa

Chris S. Duvall

Department of Geography, 384 Science Hall, 550 North Park Street, University of Wisconsin,Madison, WI 53706, USA

Abstract

This paper examines evidence for the origin of the tree Spondias mombin in Africa, where it is generallydescribed as introduced from the Neotropics. Based on biogeographical, ecological, and historical evidence,this paper argues that the tree reached Africa via natural, long-distance dispersal, and should be consideredan African native plant. The idea that it is introduced appeared in 1868, when it was a scientifically justifi-able conclusion reflecting the limitations of nineteenth-century biogeographical knowledge. By the earlytwentieth century, representing Spondias mombin as a human introduction in Africa was no longer scien-tifically justifiable because of advances in knowledge of the African flora and plant dispersal ecology. None-theless, most authors continued to represent the tree as introduced, in part because such representation wasconsonant with dominant images of the African environment: (a) that the continent had suffered extensivedeforestation in the recent past, and (b) that Africa’s flora was depauperate in fruit trees prior to the comingof Europeans. Recent authors continue to describe Spondias mombin as introduced because they have notsufficiently considered the historic contexts of primary and secondary sources on Neotropical plant intro-ductions in Africa. This paper concludes that the human role in creating the African portion of the trans-Atlantic tropical flora has been overemphasized. Natural plant dispersal across the Atlantic may be morefrequent than generally accepted.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Spondias mombin; Columbian exchange; Africa; History; Agriculture; Biogeography; Plant invasions

The origins of plant taxa found in both Africa and the Neotropics have interested biogeogra-phers since the early 1800s. At the species level, this amphi-Atlantic flora is bipartite: those speciestransported by humans since 1492, and those found naturally in both hemispheres. Africa andSouth America share at most 350 species, of which several hundreds are probably not introduced.

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250 C.S. Duvall / Journal of Historical Geography 32 (2006) 249e266

About 108 species are present only in Africa and South America, and around 20 of these representexamples of successful chance dispersal1dlong-distance dispersal having an extremely low prob-ability of occurrence by primarily non-coastal, non-aquatic, and non-ruderal plant species.2 Iden-tifying such taxa is necessary for reconstructing African cultural history, where diets are nowdominated by Neotropical crops, and also for assessing the frequency of chance dispersal betweenthe hemispheres, a crucial aspect of plant evolution in the Atlantic basin.

Recognizing Neotropical plants in Africa seems to present little difficulty since African popu-lations do not differ significantly from American source populations, and historical documentsattest many introductions. Also, many introduced plants belong to Neotropical taxonomic groupswith no or few Paleotropical representatives. Few terrestrial Neotropical plants have proven in-vasive in tropical Africa,3 and the distribution and ecology of most plants indicate their historicand continuing association with humans.4 The introduced flora mainly comprises: (1) non-foodcrop plants, such as rubber (Hevea brasiliensis); (2) ornamentals, such as Thevetia neriifolia;and especially (3) food crops, including groundnut (Arachis hypogaea), cassava (Manihot escu-lenta), maize (Zea mays), and many others.5

However, the origin of the tree Spondias mombin (abbreviated Sp.mo.) in Africa is anomalous.6

Sp.mo. is cultivated in the Neotropics and Africa,7 primarily for fruit or as live fencing. The treealso occurs in seemingly natural situations in both areas, in riparian forests and as an early suc-cessional species in upland forest canopy gaps or clearings in sub-humid to semi-arid areas. Bot-anists agree that the species originated in the New World, and most sources describe Sp.mo. asa human introduction in Africa.8 Others are uncertain. For instance, Burkill considers that‘[w]hether or not it is indigenous or introduced. is open to conjecture’, while Aubreville suggeststhat Sp.mo. may be ‘one of the last representatives in Africa of the ancient flora common to[Africa and South America]’.9

Why have most authors considered Sp.mo. a human introduction in Africa? Much of theliterature on Neotropical plant introductions to Africa is remarkably ahistorical despite thefundamentally historical nature of the problem. The strongest evidence for Sp.mo.’s originin Africa is historical. However, this evidence must be used cautiously because the historiccontexts of primary and secondary sources limit their accuracy as either chronicles or historiesof plant introduction. An examination of how knowledge of Sp.mo.’s origin in Africa has beengained reveals three broad, historic factors that have shaped understanding of the amphi-Atlanticflora:

1. Europeansdthe primary transporters of plants from the Neotropics to Africadhave nevervalued Sp.mo. highly. The lack of historical evidence for its introduction should be seen asa lack of evidence, and not as a curious omission by historical observers.

2. The idea that Sp.mo. was introduced first appeared in 1868, when it was scientifically justifi-able only because knowledge of African vegetation and plant dispersal ecology was limited.This scientific context had changed by 1917 to the extent that several researchers concludedthe tree is native to Africa.

3. Reconsideration of Sp.mo.’s origin in Africa was discouraged through the 1900s because itsrepresentation as introduced was consonant with the dominant views that: (a) poor Africanland management has caused extensive deforestation of the continent; and (b) European co-lonialism was fundamentally beneficial to Africans.

251C.S. Duvall / Journal of Historical Geography 32 (2006) 249e266

Recent authors have insufficiently considered the historic contexts of sources used in historicalplant geography in Africa, thereby unintentionally maintaining a representation of Sp.mo. that isunreasonable in light of the available, albeit circumstantial, evidence. There is insufficient evidenceto consider Sp.mo. a human introduction in Africa.10 It is more reasonable to consider it an ex-ample of successful chance dispersal from the Neotropics to Africa.

Biogeography and ecology of Sp.mo.

Biogeography

Spondias is a tropical genus of the family Anacardiaceae with about 14 species worldwide.Sp.mo. is the only widespread species in Africa (Fig. 1); Neotropical S. purpurea is planted occa-sionally.11 A Neotropical origin for Sp.mo. is suggested by its wider distribution (Fig. 1) and thegreater number of its congeners in the Americas. Including Sp.mo., there are four Spondias speciesnative to the Americas.12 In Asia, Sp.mo. and S. purpurea are known only in cultivation, while 10other species are native.13 Since several closely related genera occur only in Asia and several Asianspecies are narrow endemics, Spondias may have originated in Asia.14

Spondias arrived in the Americas probably either via an Eocene North Atlantic land bridge orvia long-distance dispersal across the Pacific, since there have been no tropical linkages directlybetween Asia and the Americas since before the evolution of angiosperms.15 Prehistoric humantransport of Spondias across the Pacific is unreasonable since the two species found in bothAsia and the New World are an unequivocally recent introduction.16

The distribution of many Spondias species indicates a widespread capacity for water dispersalwithin the genus. Dispersal by ocean currents must account for the presence of various species onislands because their pits are too large for bird dispersal to be likely. Many Spondias speciesdaswith other anacardiaceous generadare characteristic of littoral or riparian habitats, and haveclear adaptations for water dispersal.17

While humans have certainly affected the distribution of Sp.mo. in Africa, its distribution ap-pears to have been broadly constant for the past several hundred years.18 When the first detailedbotanical descriptions of Africa appeared in the 1800s, Sp.mo. was abundant inland, unlike allknown European introductions (except pineapple [Ananas comosus] and various staple crops),which were common only near the coast.19 Sp.mo. occurs in settlement sites, fallows, and secondaryvegetation, as well as in remote, apparently undisturbed and unfarmable sites in sub-humid to semi-arid parts of westernAfrica.20While animal dispersal certainly accounts for some transfer ofSp.mo.between anthropogenic and undisturbed vegetation, no introduced, animal-dispersed tree (includ-ing invasive Neotropical guava [Psidium guajava]) is similarly abundant in undisturbed sites.21

Ecology

Sp.mo. fruits are edible, and animal dispersal is its primary dispersal mechanism. However,Sp.mo. produces a relatively small amount of pulp in relation to total fruit volume, and its corkypit is an adaptation for water dispersal.22 Sp.mo. occurs in riparian forests and upland communi-ties in the Neotropics23 and Africa. Many other amphi-Atlantic species occur in riparian habitats,


and there is strong evidence that water may be a primary or secondary dispersal mechanism formost of these species.24

Chance dispersal from the Neotropics to Africa is probably a rare but plausible event for plantspecies with seeds that can survive in seawater for several months. Sp.mo. seeds can survive in theprotective pit for 2e3 months in seawater, and few fruits are more abundant in Neotropicalflotsam.25 Based on extensive study of plant dispersal and distribution around the tropical Atlantic,Guppy argued that Sp.mo. migrated to Africa via the Equatorial Counter Current.26 This currentruns east along the West African coast, originating between the North and South Atlantic Equa-torial Currents near the Guayanas (Fig. 1). Guppy hypothesized that this current periodically ex-tends farther west than usual to affect the American landmass directly, although there is apparentlyno literature on how frequently this may happen. Hedges suggests tropical storms are probablysignificant in transporting flotsam against generalized current patterns around the West Indies,27

and storms may be a more likely cause of Neotropical drift coming into the Counter Currenteven though most storm systems are easterly.

Humans have transported Sp.mo. beyond its natural range. However, the tree has not proveninvasive anywhere, despite widespread planting.28 In many parts of its range, Sp.mo. occurs in sec-ondary vegetation in forest clearings including old fields, but this ecology does not in itself indi-cate that humans have introduced the tree to a given site or area.29 Early successional species arenot necessarily pre-adapted to colonizing new areas, and water- or animal-dispersed plants com-prise but a small proportion of known invasives.30 Nonetheless, some consider Sp.mo. human-introduced in parts or all of the Antilles partly because it characterizes disturbed vegetation inthese islands.31 Conversely, Guppy showed that a combination of water and wind dispersal of des-iccated fruits could account for the tree’s presence on most of the Caribbean islands where it oc-curs.32 In any case, recent reviews and databases list Sp.mo. as ‘naturalized’ in Curacao, and not

Fig. 1. Distribution of Spondias mombin. Neotropical distribution from Francis (Note 28). African distribution based on various sour-

ces; see Notes 19, 20, 95 and 102.


invasive or ‘naturalized’ elsewhere, including many tropical islands with ecosystems threatened byother introduced plants.33

Biogeographical and ecological evidence for Sp.mo.’s origin in Africa is equivocal. What doesthe historical record indicate about the tree’s origin? The following sections summarize the scien-tific history of Sp.mo. in Africa, and suggest that depictions of the tree as introduced representreceived wisdom born of limited, nineteenth-century knowledge, and sustained by past politicalenvironments and recent poor scholarship.

Another, coarser kind of fruit

Written records provide evidence for the presence of Sp.mo. in Africa in the early 1500s, andgive no indication that Europeans ever introduced the tree to Africa. In 1506e1510, the Portuguesesailor Valentim Fernandes probably observed Sp.mo. in Guinea-Bissau, referring to it as ‘mani-peyro’.34 This name may be an earlier form of the Pepel name ‘mupila/n’pilo/umpilo’ or theMankanya name ‘n’pela/umpela’35 and the root of the modern Crioulo name ‘mandiple’.36 In1594 the Portuguese sailor Andre Alvares d’Almada similarly described the ‘manıpulo’ tree inGuinea-Bissau as having flavorful, yellow, plum-like fruits.37 Another Portuguese sailor, DuarteLopez, also probably observed Sp.mo., at the mouth of the Congo River in 1578e1579. In the1597 English translation of this work, the pertinent passage reads (with modernized orthography):‘there are [trees here]. called Ogheghe which bear a fruit. like a yellow Plum.. [Africans plantthese trees] so that when they are grown great, they make a strong fence or wall about their houses’.38

‘Ogheghe’ can be referred to theKikongo name ‘mungyenge’ forSp.mo.39 Lopez observed towering,lush ‘arbors’ of Sp.mo., suggesting it must have been present at least several decades before his visit.

The earliest possible introduction date for Sp.mo. would have been about 1500, when the firstofficial Portuguese trans-Atlantic voyage occurred.40 However, introduction of Sp.mo. prior toany of the sixteenth-century reports is unlikely for several reasons. First, Sp.mo. may not havebeen known from Portuguese possessions in the New World (eastern tropical South America):all sixteenth-century reports of the tree in the Americas come from observations made in Spanishpossessions (the Antilles and Central America).41 Although Katz states Sp.mo. had been intro-duced ‘directly from Brazil’ prior to Lopez’s visit, she provides no indication that the tree wasknown in Brazil before 1578.42

Second, it is unlikely Europeans would have exported Sp.mo. from the Americas because it hadlittle value or potential in their estimation. The Spanish chronicler Gonzalo Fernandez de Oviedoy Valdes considered ‘hobo’ fruit tasty and easy to digest, but warned that ‘the pit is very large. sothat [the fruit] offers little to eat, and it is. damaging to eat it with the teeth, when using much ofit, because of. the hard pits’.43 Fernandez appreciated other fruits more, especially pineapple.The maker of a 1537 map of Colombia found Sp.mo. noteworthy only because feral pigs common-ly ate ‘hobo’ fruits.44 The Spanish missionary Joseph de Acosta, in his 1590 natural history of ‘theIndies’, described the Neotropical fruits Europeans most appreciated. None of his fruits can bereferred to Sp.mo. except ‘hobos’, one of several ‘other, coarser kinds of fruits’ he mentions with-out describing.45 Through the early 1800s, Sp.mo. was consistently listed among the least valuableNeotropical fruit plants, mainly because its fruit has little pulp and its taste was not consideredpalatable.46


Finally, it is unlikely Europeans would have expended as much (undocumented) effort tointroduce Sp.mo. to Africa as is documented for more appreciated species. Prior to 1800, Euro-peans unequivocally introduced to Africa only plants with proven utility; there is no evidencethat Europeans experimentally planted non-African species with low perceived value duringthis time.47 Amongst fruit plants, Sp.mo. would not have been considered an equivalent choicefor introduction to pineapple or sweet orange (Citrus sinensis). Oranges were probably themost highly valued fruit for Europeans outside Europe. The Portuguese frequently planted sweetorange along the African coast as early as the 1400s, and by 1506e1510 it was abundant in SaoTome.48 Moreover, Europeans had widely introduced sweet orange in the Neotropics by the early1600s.49 Pineapple was quickly and widely introduced around the world in the sixteenth century,but was first reported in western Africa only in 160250dthough it was present probably severalyears before this.51

No other Neotropical fruit plants are mentioned in European sources on Africa until the lateseventeenth century. Although European trade records for this period do not indicate formaltrade in fruits, plants, or seeds other than grains,52 the Portuguese grew fruits and vegetableson Sao Tome for sale in continental Africa.53 The Dutch traveler Pieter de Marees recounted ap-parently oral history that Africans ‘gave out a lot of money for some Fruits’ when the Portuguesefirst offered these in Ghana.54 Nonetheless, no Neotropical fruit plants were recorded in sixteenth-century sources for western Africa, and during the 1600s only pineapple (1602), guava (1686),cashew (1686; Anacardium occidentale), and Annona squamosa. (1686) were recorded.55 By1602, demand in Ghana for fruits from Sao Tome was low because the imported plantsdpineapple, Paleotropical sugar cane (Saccharum officinarum), and Paleotropical plantain (Musaspp.)dwere cultivated and locally abundant.56 In 1578e1579 along the lower Congo, maize,called ‘Portuguese corn’, was the only Neotropical introduction Lopez mentioned; he did notmention pineapple or sweet orange, and no other plants were called ‘Portuguese’.57 In 1611,the Swiss surgeon Samuel Brun observed that Africans in this area ‘have very good and sweetfruit’ including oranges and pineapples (the only Neotropical fruit plant mentioned), as well as‘other species, which are totally unknown in our country’.58

Many travelers in the 1600se1700s carefully detailed agricultural systems around Europeancoastal outposts in western Africa, where introduced plants were most likely to occur.59 Onlythe Dutch traveler Olfert Dapper, the French naturalist Michel Adanson, and the Swedish bota-nist Adam Afzelius clearly recorded observations of Sp.mo. between 1600 and 1800, describing thetree as being planted by Africans but also found in natural vegetation.60 Contemporaneous trav-elers mentioned only a small number of fruit plantsdgenerally just banana (Musa spp.), sweetorange, and the Neotropical trio pineapple, guava, and papaya (Carica papaya).61 In many sour-ces, Neotropical plants were accurately recognized as introductions, often apparently based onoral histories recorded in Africa.62 Such historical knowledge seems to have persisted into the1800s in many areas, and Neotropical introductions were, for the most part, recognized assuch. In contrast, plantains, bananas, lemons, and limes (Citrus spp.), which came from Asia inpre-Columbian times,63 puzzled many Europeans who recognized that they occurred only at set-tlement sites, but had no sources to indicate their origins.64

In the early 1800s, pineapple, papaya, and guava were still the only widespread fruitplants regularly recognized as Neotropical introductions by Europeans in western Africa, andfew other identifiable fruit plants were consistently mentioned.65 All authors who clearly


described Sp.mo. before 1850 considered it a notable indigenous plant and not introduced,even though some of them66 knew taxonomists had described the species from Neotropicalspecimens.

Reinterpreting Sp.mo. in Africa

Despite this history, in 1868 the English botanist Daniel Oliver concluded that Sp.mo.had been introduced to the continent.67 Understanding Oliver’s conclusion requires examinationof early nineteenth-century biogeography because he did not provide data or analysis to sub-stantiate his view of the tree. Within its scientific context, Oliver’s representation of Sp.mo.was a scientifically justifiable assessment that reflected the limits of contemporaneous biogeo-graphical theory about the origin of widespread species and empirical knowledge of Africa’sflora.

Centers of origin

During the 1700s, increasing knowledge of global biodiversity caused European scholars to dis-mantle their earlier, Biblical framework for understanding the natural world. Naturalists slowlyrealized that all organisms could not have dispersed from the landing site of Noah’s Ark, andthat there were multiple centers of origin across the Earth.68 Buffon catalyzed this paradigm shiftby observing that different regions have different endemic organisms, a concept often called ‘Buf-fon’s law’.69 While Buffon’s original observation pertained specifically to amphi-Atlantic mam-mals, botanists soon recognized that ‘the law of Buffon seems to be exact with respect to thespecies furnished with two cotyledons’.70

Acceptance of Buffon’s law brought with it the need to more rationally understand disjunct dis-tributions than earlier naturalists had done.71 Possible causes of disjunct distributions were easilyhypothesized: vicariance explanations were prominent,72 and most biogeographers accepted thepossibility of long-distance dispersal by animals, wind, water, and humans, especially for usefulplants.73 The more vexing problem was identifying the centers of origin for widespread species.In 1818, the English botanist Robert Brown proposed one of the first solutions to this problem.He suggested that the centers of origin of widespread species correspond to those regions wherethe genera in question are most diverse, and that species found in areas where no congeners alsooccur are probably introduced.74 Based on this criterion, now called the ‘location of the greatestdifferentiation of a type’,75 Brown deduced that ‘the greater part of the plants. cultivated on thebanks of the Congo. have probably been introduced from other parts of the world, and do notoriginally belong even to the continent of Africa’.76 Brown’s criterion became perhaps the mostimportant means of identifying centers of origin for nineteenth-century biogeographers,77 andhis conclusion about the introduced status of most cultivated plants along the Congo certainlyinfluenced later scholars.

Brown recognized that it would be incorrect to apply the criterion of the location of thegreatest differentiation in type in all cases and in opposition to other evidence.78 Others pointedout that even if a genus has its center of origin in a certain area this does not mean speciesfound outside this area are necessarily human-introduced.79 However, nineteenth-century


biogeographers placed great significance on the location of the greatest differentiation in type inattempts to identify centers of origin, in part because they usually had few data to consider be-side broad, imperfect outlines of the global distribution of taxa. Few early European sources onAfrica were widely available in the mid-1800s, and few scholars had personal experience inAfrica.

Botanical exploration in Africa

Another factor that contributed to Oliver’s representation of Sp.mo. was the spatially constrainednature of botanical exploration. Until the late nineteenth century, nearly all scientific botanical obser-vations inwesternAfricawere limited to a handful of areas: the coast fromSenegal toAngola, the low-er Senegal valley, the Niger Delta, and the lower Congo River. Within these areas most observationsweremade in humanized environments. Thus, sinceSp.mo.was knownmainly just in agricultural sitesprior to the late 1800s, its known ecology suggested that it was present only in cultivation in Africa.

Duringmost of the1800s, plant collectors inAfricawere spatially tied to settlements for anumberoflogistical reasons.80 Most collectors depended on European trading companies or governments fortransportation, accommodation, and protection, which limited their abilities to travel far from Euro-pean outposts. SometimesAfrican hosts restricted collectors, including theEnglishmanThomasBow-dich: ‘[d]uring the earlier part of our residence at [Kumasi, Ghana] the seasonwas tolerably favorableto the gatheringof plants butwewere allowed [by the ruler ofKumasi] to go out but seldom, andneverbeyond the town’.81 Finally, some plant collectors were employees ofmetropolitan governments, whodirected their travels toward generally well-populated, commercially or militarily important areas.82

These factors are evident in all major accounts of botanical explorations in western Africa publishedbefore about 1850.83 Additionally, secondary accounts of the itineraries of other botanists suggest thattheyalsovisitedmainlyhumanized environments.84Thisunintentionalbias led to skewedunderstandingof plant ecology in western Africa. For instance, the GermaneEnglish botanist Theodore Vogel, at-tached to the 1841 British expedition to the Niger River, spent most of his time in and around occupiedor abandoned settlement sites.When given the opportunity to voyage substantially inland inGhana, hewas surpriseddafter several weeks along the coast west of Ghanadto discover that plant diversity in-creased away from towns. In the end, Vogel observed Sp.mo. only in towns, fields, or fallows.85

Additionally, European botanists found that indigenous people planted Sp.mo. throughoutAfrica and the Neotropics. In Africa, Sabine, in Sierra Leone, and Welwitsch, in Angola, foundSp.mo. widely cultivated, while Hooker considered the tree ‘cultivated’ based on Vogel’sobservations.86 Botanists also recognized that Sp.mo. was cultivated in the Neotropics.87 Such ob-servations, combined with the limited knowledge of natural vegetation in western Africa before thelate 1800s, gave botanists little reason to suspectSp.mo.was anythingother than cultivated inAfrica.This perception seems to have been especially significant inOliver’s reinterpretation of the tree,88 be-cause he did not consider any non-cultivated amphi-Atlantic plant to be introduced.89

By the mid-1800s, Spondias was known to be more diverse in the Neotropics than Africa.Sp.mo. was known to be present only in these two tropical areas,90 and the most authoritativeworks on African botany described it as ‘cultivated’. Furthermore, biogeographers had widelyconcluded that most plants cultivated in Africa were introduced. If Oliver assessed these facts us-ing Brown’s criterion of the location of the greatest differentiation of a type, his representation ofSp.mo. as introduced in Africa was reasonable within the scientific context he worked.


A changed scientific context

The various bits of theoretical and empirical knowledge that composed this context changedduring the late 1800s and early 1900s. With increased knowledge of the African flora andlong-distance plant dispersal, representation of Sp.mo. as introduced in Africa was no longer sci-entifically tenable by the 1910s. Nonetheless, most scientists maintained this representation be-cause it was consonant with dominant images of the African environment.

Expanded botanical knowledge

In the late 1800s, Europeans gained military and administrative control over Africa, allowingmore individuals to explore the continent more extensively than previously possible. Advances intropical medicine also allowed Europeans to work for longer periods of time. As a result of thesefactors, knowledge of the distribution and ecology of plants in Africa greatly increased, especiallyafter about 1880.91

Many botanists found that Sp.mo. was widespread and abundant in non-humanized vegetationthroughout western Africa, unlike trees Europeans had introduced along the coast. Welwitsch de-scribed Sp.mo. as ‘very common, as much silvicolous as cultivated’, throughout Northern Angola,while all other European-introduced fruit trees were rare inland, and common only near the coast.92

Similarly, in Guinea, Pobeguin observed that ‘[a]s soon as one penetrates into the interior, the fruittrees [introduced by Europeans] diminish in number, and one finds only some rare examples hereand there in villages’, but Sp.mo. was ‘very common just about everywhere’.93 de Wildemanwrote that Sp.mo. was widely abundant in the Congo basin, although he believed it ‘seems [tobe] originally from. tropical America’.94 In the following decades, many others found the treecharacteristic of riparian, upland, and secondary forests in semi-arid to sub-humid westernAfrica.95 While many such observations of ‘natural’ vegetation certainly refer to unrecognizedfallows and abandoned settlements, the tree’s abundance, distribution, and ecology was clearlydifferent from comparable, known introductions, and more like known native species.

These observations led a handful of scholars to question whether Sp.mo. was really introducedin Africa.96 For instance, Chevalier observed that Sp.mo. has ‘in tropical Africa the appearance ofa spontaneous plant [because] it is found in. the forests all the way to the heart of the conti-nent’.97 Aubreville concluded that ‘[i]t is difficult. to know these days if this species belongs[to] Africa., or if it is a question of a very ancient introduction’.98 The nineteenth-centuryidea that humans had introduced Sp.mo. was not consonant with the more detailed knowledgeof the African flora available in the twentieth century.

However, the idea that Sp.mo. is an African native plant was not consonant with two images ofthe African environment that became increasingly dominant in the twentieth century. First, mostEuropean natural resource scientists regarded African land management as inherently destructiveof woody vegetation, and the primary factor in the destruction of the continent’s supposed, exten-sive, aboriginal forests.99 In the environmental change narrative that arose from this culturallysubjective viewpoint, virtually all vegetation were heavily disturbed by unsound agricultural andpastoral practices.100 For instance, colonial foresters called the dominant, natural vegetation inmuch of West Africa ‘derived savanna’ rather than simply ‘savanna’, ‘forest’, or ‘woodland’ in or-der to stress human impact on the environment rather than environmental influence on society.101


Similarly, most twentieth-century botanists dismissed observations of Sp.mo.’s widespreadabundance in undisturbed vegetation by asserting it was introduced, but ‘naturalized’ or ‘sub-spontaneous’ in Africa.102 Its presence in fields and fallows was emphasized to suggest that it isweedy and occurs only in anthropogenic vegetation. Colonial-era botanists interpreted their in-creasing knowledge of Sp.mo. within the dominant framework of vegetation history, and thusportrayed the widespread tree as evidence of past deforestation. For instance, Schnell contrivedthe tree’s ecology to support his view that African agriculture had deforested the continent. Hewrote that ‘Spondias mombin., originally from tropical America, [now] penetrates even in the de-forested regions of the forest domain,dpropagated by man’. Its supposed capacity ‘to naturalizeand diffuse into spontaneous vegetation’ makes it comparable to the notorious weed water hya-cinth (Eichhornia crassipes).103 There is no evidence that Sp.mo. is invasive anywhere,104 and itsoccurrence in secondary vegetation does not indicate a historic human introduction.105

While agriculture has certainly altered vegetation throughout Africa, it is unreasonable to as-sert that Sp.mo.’s African distribution indicates the environmental impact or areal extent of agri-cultural disturbance since its supposed introduction. Depicting the tree as ‘naturalized’ isanalogous to depiction of savanna vegetation as ‘derived’. Recent descriptions of Sp.mo. as intro-duced derived from colonial-era secondary sources on historical plant geography inconspicuouslymaintain an outdated narrative of African environmental change, though this has not been theintent of most recent authors who have written such descriptions.

Understanding long-distance dispersal

In the mid-1800s, human activities were the only dispersal mechanism that seemed reasonableto account for the presence of Sp.mo. in Africa. Although the possibility of plant migration viaocean currents was recognized early, few data on the dispersal ecology of tropical plants existedbefore the 1880s. By 1917, however, plant dispersal ecology was known well enough to concludethat Sp.mo. had probably reached Africa naturally.106

In the early 1800s, botanists knew water could transport seeds long distances, but they tendedto see potential for water dispersal only in fruits with conspicuous adaptations. By the 1830s, bot-anists realized some fruits have less obvious secondary adaptations for water dispersal that in-crease their capacity to traverse oceanic barriers, including seed dormancy and durability.107

Additionally, the geography of ocean currents was well known from both maritime transportationand scientific experiments.108 Thus, by the mid-1800s, botanists could roughly assess the proba-bility of plant dispersal between landmasses. Particularly, A. de Candolle concluded that the At-lantic current system had great potential for carrying plants between the hemispheres, because theequatorial currents ‘unite regions [that are] equally hot, where seeds find upon arrival conditionsapproximately similar’ to those on the other side of the ocean.109

Despite this general understanding, there were few data on the actual or potential dispersalecology of species until the late 1800s. The study of long-distance dispersal flourished followingthe Darwinian revolution.110 Darwin believed that trans-oceanic migrations occur commonly inevolutionary time, which he sought to prove through experiments showing that many plantshave seeds that remain viable after floating in seawater for long periods.111 Others recognizedthat many riparian plants have adaptations for water dispersal, including durable, corky pitsthat have the potential to cross oceans.112 While many considered long-distance dispersal by


ocean currents undoubtedly a ‘chance’ occurrence,113 the vegetative recolonization of Krakatauafter 1883 proved sea dispersal was not so rare as to be negligible.114

These developments led naturalists to study the actual dispersal ecology of plants, includingSp.mo., by analyzing observations of drift seeds and fruit around the world. Researchers on theChallenger expedition (1872e1876) observed that Sp.mo. pits were amongst the most abundanttypes of plant matter washed ashore in the Antilles.115 Later researchers showed that Sp.mo. hasgreat potential for oceanic dispersal since its pits can remain buoyant in seawater and protectthe seeds for 2e3 months.116 By the 1920s, two researchers concluded separately that many am-phi-Atlantic species, including Sp.mo., probably arrived in Africa via natural oceanic dispersal.117

Nonetheless, no botanist working in Africa reconsidered the tree’s origin based on dispersalecology. A natural origin for Sp.mo. in Africa was not consonant with the image that Africa’s flo-ra was impoverished in terms of useful species, especially fruit trees, before Europeans began in-troducing plants in the 1400s. Beginning with Brown’s conclusion about the introduced status ofmost cultivated plants along the Congo, biogeographers consistently noted the paucity of Africannative plants that were economically important (from a European perspective).118 In the late1800s, such observations increasingly emphasized that (European) plant introductions had beencrucial in changing the African environment from miserably impoverished to tolerably livable.119

For instance, Schonck de Goldfiem stated that in Guinea ‘the totality of plants useful to man,[and] all edible plants, except Fonio [Digitaria spp.], Guinea Sorghum [Sorghum bicolor], andmaybe some others of infinitesimal importance have been brought in by man’, primarily Euro-peans.120 Milder versions of this premise were common, and many scholars believed Africa hadbeen singularly depauperate in fruit trees before the coming of Europeans.121

This environmental change narrative arose from, and was presented as evidence for, thepurported beneficence and wisdom of the colonial enterprise. Nineteenth-century biogeographywas an obviously imperial science in an increasingly imperial time.122 Biogeographers often con-sidered cultivated plants uniquely characteristic of (European) civilization, and introduced plantssymbolized the human progress many believed inherent in (European) colonization. For instance,Hehn argued that ‘as cultivated plants migrated from nation to nation. so migrated in the samedirection, and at the same time, freedom and culture in every shape’. Moreover, by introducingvaluable and ‘ennobling’ plants outside Europe, Europeans could hope to civilize the ‘inferior,coloured races’.123 Indeed, the ‘zeal of certain missionaries’ was later celebrated as having spread‘fruit trees that the Blacks did not yet possess’ throughout Africa,124 and the current dominance ofNeotropical food crops represented past European benevolence and foresight.125

While Europeans did introduce many useful plants to Africa, the view of plant introduction asjustifying colonization led botanists to contrivedly interpret historical and other data.126 Some ar-gued that introduced Neotropical plants ‘gave birth to agriculture’ in some parts of Africa,127 de-spite knowledge of the pre-Columbian antiquity of African agriculture.128 Others concluded that‘the rapidity of the diffusion of fruit trees. proves that Africans [are] not insensible to the im-provement of their existence [i.e. welcome agricultural colonization]’,129 despite the fact thatmost fruit plants introduced since the 1500s had limited distributions prior to aggressive plant in-troduction efforts in the early 1900s. Botanists similarly overemphasized the human role in creat-ing the African portion of the amphi-Atlantic flora. For instance, Chevalier estimated thathumans introduced to Africa 99.5% of amphi-Atlantic species,130 a gross overestimate accordingto later analysis.131


Undeniably, Europeans introduced many economically important plants to Africa. However,colonialist views of colonialism often overrode data in contemporaneous historical plant geogra-phies.132 Secondary sources from the early to mid-1900s on plant introductions must be usedcautiously, because the dominance of certain images of the African environment, and associatedenvironmental change narratives, limited the ability of their authors objectively to considerevidence for the origins of widespread, useful plants present in Africa. It is unreasonable to labelSp.mo. a human introduction in Africa considering its adaptation and potential for water dispersal,and the lack of supporting historical evidence.

Conclusion

The scientific history of Sp.mo. in Africa supports two main conclusions. First, although thestudy of historical plant geography in Africa is not new, critical reassessment of primary and sec-ondary sources on Neotropical introductions is needed. Many colonial-era works, in whichSp.mo. is represented as a European introduction in order to validate outdated views of Africanagriculture, remain valuable to historical geographers despite their limitations. Uncritical use ofthese works has apparently led several recent authors to also describe Sp.mo. as introduced,even though many of these authors have contributed to challenges to colonialist stereotypes ofAfrican agriculture. Recent authors have also failed to consider the historic context of primarysources. Europeans have never valued Sp.mo., highly and it is unreasonable to assume they intro-duced it to Africa simply because it is found on both sides of the Atlantic.

Finally, more strictly botanical sources have been similarly uncritical of representations ofSp.mo. as introduced in Africa, despite strong evidence that the tree reached Africa naturally.The frequency of amphi-Atlantic, chance dispersal may be higher than previously believed, becauseno assessment of floristic relationships between Africa and the Neotropics has considered Sp.mo.native to Africa. Ceiba pentandra also came to Africa via chance dispersal,133 even though its rep-resentation as an European introduction persists.134 If Thorne’s estimate of the number and ecolog-ical types of amphi-Atlantic species is accurate,135 these two increase the number of species thatcrossed the Atlantic via chance dispersal by 10%da substantial amount in an evolutionary context.

The historical factors that have influenced interpretation of Sp.mo. (and Ceiba pentandra) inAfrica have certainly affected other species. Other plants described as ‘introduced’ or ‘naturalized’may also be native to Africa; recent genetic studies suggest that amphi-Atlantic, chance dispersalis more common than often assumed.136 To fully understand plant evolution and historicalbiogeography in the Atlantic basin, species-level disjunctions must not be summarily disregardedas human introductions or the expectable spread of ruderal, littoral, or aquatic species.137 Ifchance, oceanic dispersal is accepted for the spread of plants in the deep past, historical biogeog-raphers must also accept it as a process active in the recent past and the present.

Acknowledgements

I thank Allison Miller for providing documents used in this research, and Tom Vale and threeanonymous reviewers for helpful comments on earlier versions of this paper.


Notes

1. R. Thorne, Floristic relationships between tropical Africa and tropical America, in: B. Meggars, E. Ayensu andW.D. Duckworth (Eds), Tropical Forest Ecosystems in Africa and South America, Washington, DC, 1973, 27e47.

2. See: R. Berg, Plant distribution as seen from plant dispersal, in: K. Kubitzki (Ed.), Dispersal and Distribution,Hamburg, Germany, 1983, 13e36; S. Carlquist, Chance dispersal, American Scientist 69 (1981) 509e516.

3. ‘Invasive’ plants are non-native species spreading in natural habitats without direct human assistance, causingsignificant change in vegetation characteristics or ecosystem processes. Q. Cronk and J. Fuller, Plant Invaders,London, 2001.

4. A. Zeven and J. de Wet, Dictionary of Cultivated Plants and Their Regions of Diversity, Wageningen, The Nether-lands, 1982; J. Hutchinson and J. Dalziel, Flora of West Tropical Africa, London, 1954e1972; M. Arbonnier,Arbres, arbustes et lianes des zones seches d’Afrique de l’Ouest, Paris, 2000.

5. Taxonomic nomenclature from: Hutchinson and Dalziel, Flora of West Tropical Africa; Zeven and de Wet, Dic-tionary of Cultivated Plants; Arbonnier, Arbres, arbustes et lianes; A. Kostermans, Kedondong, ambarella, ambra:the Spondiadeae (Anacardiaceae) in Asia and the Pacific Area, Bogor, Indonesia, 1991.

6. Sp.mo.’s common names include yellow hog plum, monbin or mombin, and hobo or jobo: J. Morton, Fruits of

Warm Climates, Winterville, USA, 1987.7. It is rarely planted in Asia. Kostermans, Kedondong, ambarella, ambra; H. Airy Shaw and L. Forman, The genus

Spondias L. (Anacardiaceae) in tropical Asia, Kew Bulletin 21 (1967) 1e19.

8. A.Nyerges, Coppice swidden fallows in tropical deciduous forest,HumanEcology 17 (1989) 379e400; B.Roussel andD. Juhe-Beaulaton, Les plantes americaines en Afrique tropicale, Cahiers d’Outre-Mer 45 (1992) 373e385;D. Juhe-Beaulaton and B. Roussel, A propos de l’historicite des forets sacrees de l’ancien cote des esclaves, in:

M. Chastanet (Ed.),Plantes et paysages d’Afrique, Paris, 1998, 353e382; E. Katz, Plantes americaines au sud Congo,in: Chastanet (Ed.),Plantes et paysages d’Afrique, 283e322; J.-L.Guillaumet,Migrations et transports de plantes, in:Chastanet (Ed.), Plantes et paysages d’Afrique, 31e44; R. Schnell, La phytogeographie des pays tropicaux, Paris,

1970e1974; R. Schnell, La flore et la vegetation de l’Amerique tropicale. Tome I, Paris, 1987; J. Mitchell, Anacardia-ceae, in: A. Gorts-Van Rijn and M. Jansen-Jacobs (Eds), Flora of the Guianas, Kew, UK, 1997, 1e79.

9. H. Burkill, The Useful Plants of West Tropical Africa, Kew, UK, 1985, 92; A. Aubreville, Flore forestiere soudano-guineenne, Paris, 1950, 403. All translations are by the author.

10. cf. D. Webb, What are the criteria for presuming native status? Watsonia 15 (1985) 231e236.11. Kostermans states S. purpurea is ‘naturalized’ throughout Nigeria; no other sources confirm this. Gossweiler’s re-

port of S. purpurea as common in Angola certainly refers to Sp.mo. Hutchinson and Dalziel, Flora of West Trop-

ical Africa; Kostermans, Kedondong, ambarella, ambra; J. Gossweiler, Flora exotica de Angola, Luanda, Angola,1950; R. Keay, Trees of Nigeria, Oxford, UK, 1989.

12. Kostermans, Kedondong, ambarella, ambra; Mitchell, Anacardiaceae; W. Blackwell Jr. and C. Dodson, Flora of

Panama, Anacardiaceae, Annals of the Missouri Botanical Garden 54 (1967) 351e379; T. Croat, A reconsiderationof Spondias mombin L. (Anacardiaceae), Annals of the Missouri Botanical Garden 61 (1974) 483e490; J. Rzedowskiand G. Calderon de Rzedowski, Flora del Bajio y de Regiones Adyacentes. Familia Anacardiaceae, Patzcuaro,

Mexico, 1999; H. Lorenzi, Arvores brasileiras, Nova Odessa, Brazil, 1992; A. Barfod, Anacardiaceae, in:G. Harling and B. Sparre (Eds), Flora of Ecuador, Goteborg, Sweden, 1987, 9e50.

13. Kostermans, Kedondong, ambarella, ambra.14. S. Pell, Molecular systematics of the cashew family (Anacardiaceae), Ph.D. dissertation, Louisiana State Univer-

sity, USA, 2004.15. cf. Pell, Molecular systematics; A. Hallam, An Outline of Phanerozoic Biogeography, Oxford, UK, 1994; P. Raven

and D. Axelrod, Angiosperm biogeography and past continental movements, Annals of the Missouri Botanical

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l’Academie des Sciences 225 (1947) 213e216; Humans have also affected Sp.mo.’s distribution in the Neotropics;

see Note 31.19. A. Chevalier, Mon exploration botanique du Soudan francais, Bulletin du Museum d’Histoire Naturelle 5 (1900)

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78. Brown, Observations Systematical and Geographical, 53.

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Hooker (Ed.), Niger Flora, London, 1849, 285. Welwitsch did not consider Sp.mo. introduced, although Ficalhoconcluded this based seemingly on a misunderstanding of one of Welwitsch’s specimen labels. Oliver probably sawthis specimen, and may have also misunderstood its label. Hiern’s summary of this label reads: ‘Cultivated in the

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On the origin of the tree Spondias mombin in Africa

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