Non-indigenous plants constitute dominant driver of ecosystem change and a major problem for environmental management worldwide [1] [2] [3] . Majority of invasions are associated with human industry and development [4] [5] [6] . They are either deliberately or accidentally introduced [7] and pose serious threats to the biodiversity economy and human health [1] and their irreversible impact is second only to habitat loss [8] [9] . Invasive Alien Plants (IAPs), in particular, have spread rapidly and extensively in many regions of the world [10] [11] [12] [13] . In any one ecosystem, there is likely to be more than one mechanism to explain invasion success of a species and in fact, no one’s hypothesis may explain invasion in totality [13] . Therefore, any effort to understand the invasive success of a species should take into account an integrative approach, which must be combined with rigorous field experiments in order to evaluate these hypotheses [14] .

Beginning from the UN Summit in Rio de Janeiro in 1992, invasive species are regarded as one of the main reasons for the loss of biodiversity [15] [16] [17] . In addition, such species may alter environmental conditions and resource availability of tangible and non-tangible products from the affected ecosystem [18] . This disruption is accomplished when Invasive alien species (IES) differ from native biota in specific species traits [19] . The United Nation Environmental Programme estimates that alien invasive species cost the global economy an annual US $1.4 trillion. In East Africa, no studies have been done to quantify the cost of invasive plants species. However, studies in South Africa alone show the country spends over $60 million annually to eradicate invasive plants [20] . Nonetheless, not all invading species are equal in their ability to alter larger-scale processes. But those that dominate communities have the greatest impact [18] . The numbers of species transported across biogeographical barriers daily is presumably large but only a few become established [21] agricultural production and ornamental purposes are main reasons [22] .

The era of European colonization encouraged the spread of plant exploration in quest for new species for ornamental plants for botanical gardens, nurseries and private individuals back home some of which have escaped to become invasive [22] . When steam engine came into common use between 1820-1930, many Europeans emigrated with numerous plants and animals [22] . Also the colonial military carried many invasive species in 17^th and 18^th century [23] . For instance, France brought Opuntia monacantha and O. ficus indica to Madagascar in 1768 to provide barrier around fort Dauphin and feed oxen respectively [24] . Both species have colonized much of Madagascar but French colonialists have long left, leaving costs of invasion to be borne by locals. Scientists themselves are responsible for species invasions, too [25] . For example, a field scientific experiment set up to investigate salt excretion of various plants at Mission Bay (California) in the late 1960s resulted in a thriving population of the small tree Avicennia marina a decade later [27] . Although, the French botanist Auguste Chevalier regarded Chromolaena odorata as a weed and had written a paper on man’s role in the dispersal of tropical plants, he nevertheless recommended its introduction to West Africa [26] .

Incorporating the ecosystem services approach into conservation management is becoming increasingly prevalent through better understanding of the connections between ecosystems and their associated products and processes [27] . While much effort has gone into the quantitative valuation of these goods and services, knowledge gaps persist in understanding how invasive alien species impact ecosystem services [28] . Exotic invasive species have the capacity to alter ecosystem equilibrium [29] and to cause serious environmental problems like extirpation of native species [30] In addition, competition, predation, hybridization and several other indirect effects [31] [32] which interfere with community structure [33] and alter genetic composition [34] .

Several studies have shown that invasive alien plants also have positive economic, social and ecological contributions and that these need to be considered when assessing the costs resulting from invasions [35] [36] . For instance, it is estimated that 95% of food production in the USA depends on exotic species of plants and animals [37] . It has also been recorded that invasive alien plant species can be used in agroforestry for functions and services that cannot be provided by native species [38] [39] . These services include rapid biomass accumulation, nitrogen fixation, and reforestation of degraded land, improved fallows and contour hedgerows [40] . Invasive alien plants in South Africa, have become a focal point in the conservation of indigenous plants [39] . These plants incur a tremendous cost in labor expenditure to eradicate them, a cost that a poor rurals cannot afford [40] . Kenya is rich with an enormous diversity of ecosystems and wildlife species [41] . Those are source of livelihood to Kenyans, and central to the economy thereby being indispensable to achieving the aspirations outlined in Vision 2030. There are nearly 34 invasive alien animal and plant species in Kenya [42] . In spite of its immense biotic capital, Kenya experiences severe ecological and socio-economic problems [41] . Drought negatively impacts on the country’s biodiversity as well as the national economy and people’s livelihoods [42] . There are also problems of human-induced environmental degradation, such as destruction of natural landscapes, soil erosion, water pollution and loss of species [41] . Inappropriate policies and political impunity have contributed to nationwide habitat destruction, loss of species and the associated genetic resources [41] . The sustainable management of the country’s biological resources is also hampered by lack of a comprehensive biodiversity policy, a biodiversity inventory and of formal procedures for benefit sharing as well as threats from invasive alien species [41] . All these play a role in proliferation of invasive species [13] .

Cestrum aurantiacum Lindl. is exotic species in Kenya and native to Central America [43] [44] ) and is known as an escape species from living collection and has an invasive potential in many forest areas [45] . It has been identified as a wide-spread and well established invasive species in many countries around the world [11] . C. aurantiacum was identified as an exotic invasive plants causing several adverse impacts including displacement of native plants from their habitat in Malawi and South Africa [46] [47] . Secondly, C. aurantiacum is poisonous to humans and animals [46] [48] [49] [50] . The main concern on the effect of C. aurantiacum as invasive species is the possible risk caused by the species invasion to forest ecosystems and economy [51] . The general understanding and theory of invasion process could be used to conceptualize the process of the invasion [52] [53] . This considers that biological invasion of an exotic species is a series of steps [54] .

Available knowledge on biological invasions is geographically and taxonomically biased toward more developed regions, further complicating accurate species identification [54] . Some regions, such as Asia, South America and Africa (excluding South Africa), are inadequately studied in terms of ecology [55] . Since research intensity is generally related to economic prosperity [56] the same regions are also likely to be taxonomically understudied [57] . A typical example is found in the Usambara Mountains of Tanzania where the Maesopsis eminii (or Msira) tree was declared invasive and a disaster to the forest ecosystem 65 years after it was first introduced in 1913 [58] . More recently Mesquite has been declared a serious invader in Kenyan drylands twenty years after its first introduction in the early 1980s [59] .

As Invasive plants continue to pose a major problem worldwide [60] . Every year, exotic species become more prevalent, extending their ranges into areas designated as nature reserves [61] . In a survey of 24 reserves worldwide, all were found to have exotic species, and some reserves harbored more exotic plant species than native species [62] . Despite these facts, invasive species have rarely been implicated as a significant threat to the conservation of tropical forests [63] and deterioration of global economies. C. auranticum plus five other species of cestrum have been declared category 1 noxious species in South Africa [64] [112] , a Country with the highest knowledge base of invasive species in the continent [65] . Dealing with invasive plant is a challenging task that demands accurate planning and resourcing [66] . Since the last century, hundreds of exotic plants incursion have been recorded in different parts of the world. These have been of sufficient concern to warrant nationally coordinated and funded responses [67] .

Many studies have proved that exotic plant invasions displace native species, disrupt ecosystem processes, and consequently inflict substantial ecological costs worldwide [1] [68] [69] . A number of exotic plant species introduced into Kenya, have escaped cultivation and now threaten biodiversity [42] [70] . Not much has been investigated about this species. Cestrum aurantiacum an invasive species has been cited in many parts of high potential Kenya growing in areas where not deliberately introduced. Previous Studies in New Zealand on invasive species depicted C. aurantiacum as a high invader basing on biological success rating [71] . Cestrum species are also known for their ability for rapid establishment and spread, particularly along river, roads, disturbed sites [72] and more so Cestrum aurantiacum has thus been also listed as potentially invasive in Sri Lanka [73] .

Most of the studies addressing invasive plants have focused on their characteristics [74] [75] , the process of invasion [76] [77] , the effect of disturbance on invasion success [78] and the aspects of invaded systems [79] . Few studies have examined the effects of invasive species across many habitats or regions [80] . In Africa for instance, the Invasive Woody Plant Database (with over 5000 publications on species invasiveness) has only 6% referring to tropical Africa [81] . Of the few reports and studies done in East Africa, Lantana camara (commonly referred to as Lantana) is the most widely studied invader [70] [82] . In Kenya, there is minimal empirical data available on how C. aurantiacum interacts with the local habitats how was introduced and its close relatives, though it’s among the most dangerous species to conserved natural ecosystems in Kenya [107] .

A comprehensive analysis of existing literature may provide insight into how much we understand the impacts caused by Invasive plants on ecosystem services in forests and other lives. Such an exercise could help ecologists, social scientists, and natural resource managers to discover hidden taxes on ecosystem services and could assist in developing effective invasive species management strategies [28] [83] . Although databases accessible on the web are developing rapidly, they are still relatively data-poor in comparison with the data potentially available. Hence their value to invasive management remains limited. Moreover, the limited accessibility of online information in developing countries applies equally to online databases. Nonetheless, resources exist, such as the Global Compendium of Weeds, which contains at least some of the synonyms and common names as well as other information.

To achieve these objectives, this review seeks to fill gaps in existing knowledge on the introduction, biogeographic range and impacts of C. aurantiacum in Kenya. This work will contribute to the global knowledge pool on the presence and distribution of invasive cestrum species and also assist countries in meeting their obligations to various international conventions/treaties. In light of such consequences, it is important to understand the taxonomy, phytogeography and impacts of the same [66] . In this paper I examined the extent and prevalence of the impact, taxonomy and introduction of Cestrum aurantiacum an Invasive species in Kenya. In view of these gaps, I reviewed the dynamic and critical role of taxonomy in the study of plant invasions and specifically examines how can be useful in understanding the phylogeny, causes of introduction and impacts of Cestrum aurantiacum.

To accomplish this goal, I synthesized data on four targeted areas (taxonomy, introduction into Kenya, areas species has been documented and impacts of Cestrum aurantiacum and provided examples from the literature for extended discussion. Due to the level of hybdization possible in the genus cestrum comprehensive review of all IES requires tremendous time and effort which this review couldn’t achieve. Thus I have reviewed manuscripts concerning Cestrum aurantiacum most detrimental IES, as identified by various local studies with little reference to the other five cestrum species from the genus declared as weeds in many parts of the world and also cited in Kenya. I conducted literature searches in journals, books, mass media, fact sheets and garden profiles, from the internet. Most of the literature cited in this review is from the USA, Australia, South Africa, Canada and New Zealand. Which are most affected by invasive and also record highest publications on the same [84] . All accessed literature on cestrum species in Kenya has been referenced. Data from species occurrence reports collected has been used as georeferences and used to model overall species distribution.

The earliest records of Cestrum introduction in Kenya dates back to 1921 (see Table 1), the Colony and protectorate of Kenya government which was under His Majesty of Britain had its nerve centre of operations in London. For instance the publishing of official government documents for Kenya colony and protectorate was done in London by His Majesty’s stationery Implies that there was a lot of communication between London and Nairobi. Also, the shortest route from London to Nairobi was through the Suez Canal. Almost immediately after its opening, the Suez Canal had a significant impact on world trade as goods were moved around the world in record time. In 1875, debts forced Egypt to sell its shares in ownership of the Suez Canal to the United Kingdom. However, an international convention in 1888 made the canal available for all ships

from any nation to use. But from the annual reports of the Colony and protectorate of Kenya 192, 1930, visits by his majesty from London to Kenya started from Mombasa. Therefore, it’s most likely that the pathway of cestrum was through Suez Canal to Mombasa via medterrenean sea. There are several species of the genus Cestrum. However, in the field, speciation within the genus is often uncertain due to hybridization [112] .

Forty publications reviewed on the impacts of Cestrum aurantiacum in Kenya gave 19 with beneficial impacts to nature with 21 having negative effects (Table 4). 14 reports were about livestock poisoning and 7 on ecological destabilization. Medicinal use is the most widely reported beneficial application of Cestrum aurantiacum (See Table 2 & Table 3). A phytochemical screening of Cestrum Species (C. aurantiacum, C. diurnum and C. purpureum) revealed the presence of steroidal saponins, sterols, catechol tannins, carbohydrates and flavonoids.

Sucrose, glucose and fructose were the only sugars detectable in the three species. The unsaponifiable matters of lipids of the three species were found to contain β-amyrin and β-sitosterol. The fatty acids were identified as myristic, palmitic,

stearic, oleic and linoleic. The percentage of these acids in the different species revealed quantitative differences [113] . These compounds are responsible for the medicinal applications of these species (See Table 2).

A remarkably case of cestrum aurantiacum poisoning in Kenya was during March 1950, death in stock of unknown etiology was reported from a farm in Limuru District. Following an investigation by [114] , the cause of death was found to be Cestrum aurantiacum poisoning. Cestrum poisoning was reported for the first time in Kenya in 1948 it therefore, became the intention of the government to schedule it as noxious weed in those districts where conditions allow for its rapid multiplication and it had escaped from cultivation and spread into grazing areas [110] . Another case was when a two and a half year old colt from Tigoni Limuru was admitted at the large animal clinic department of veterinary studies university of Nairobi on 13/6/1990 [105] it had rather protracted history, the owner admitted it fed on cestrum cut from nearby hedge [113] . These cases highlight the significance of understanding the consequences of having cestrum in our living environment.

Cestrum aurantiacum has been reported adversely in Montane forests, in Sri Lanka [5] and is a major invasive alien plants that was imported intentionally through the Royal Botanic Garden at Peradeniya [115] . Among these, human intervention has facilitated the spread of several invasive plant species within the country in Kenya it’s also cited in montane forests like Cherangani [111] and Mount Kenya [106] . The species is a high potential species considering that studies by [70] reported on key invasive species of dry tropical forests of Kenya and Tanzania never listed it. C. aurantiacum has been identified as an exotic invasive plants causing several adverse impact including displacement of native plants from their habitat in Malawi and South Africa [46] [47] . Subsequently, C. aurantiacum is toxic to animal and possibly has similar effect on human [46] [48] .

According to [116] the use of cestrum aurantiacum as an antimicrobial agent was new. The active principle behind this could be alkaloid or saponins, but to prove this, more study has to be conducted. In a study conducted by [49] to identify insecticidal plants of potential value in Sri Lanka, C. aurantiacum recorded very high mortality [101] (See Table 3). Also C. aurantiacum showed stronger antibacterial activity compared to other species and acetone extract of it produced maximum zone of inhibition (18 mm) against K. pneumonia. In case of fungi, the zones of inhibition were highest (16 mm) for butanol extract of C. nocturnum against Aspergillus. Whereas, ethanol extract of C. diurnum showed maximum zone of inhibition (14 mm) against Trichoderma in Bangalore india [94] .

Several beneficial uses of cestrum species have also been documented (see Table 3). The flowers of Cestrum aurantiacum are visited by the bees in the absence of coffee flowers in Kiambu Kenya [108] . In the United States of America it’s cultivated as ornamental [81] and Kenya (C & PK 1921; 1929). Cestrum species are used as food by caterpillars of several Lepidoptera species [117] and the insect Rhodogastria amasis. It is either known or suspected that such Lepidoptera are able to sequester the toxins from the plant, Cestrum species are reported as pesticidal [100] . C. laevigatum is employed by wajacas of the Krahós, Krahô tribe in Brazil. It is used “to see far”, i.e. to aid in divination. Like the other hallucinogenic plants consumed by them, Craós wajacas consider it a potent entheogen [118] . Medicinal application of Cestrum nocturnum has been widely documented by amrita 2016. In Western Cape Province, where poor households in townships and informal settlements are dependent on wood collected from invading Acacia longifolia for construction poles and firewood for cooking and heating [5] . It is clear that in South Africa invasive plants are part of their landscape and their existence is hardly frowned upon, they are commercially exploited and in most communities where they occur they are welcomed [1] .

Although cestrum species appear to arrive back in 1921 in Kenya [119] . The first report of cestrum poisoning was by the Veterinary department (1948) in the annual report of 1948 about sheep poisoning (See Table 1). Further reports were from Limuru [103] [120] . In countries with large populations of livestock like Brazil knowledge base of plant poisoning increased rapidly in the last decade. Data from the Brazil indicate that, on average, 10% - 14% of cattle deaths investigated by laboratories involve plant poisoning, and if this is extrapolated to the rest of the country, between 800,000 and 1,120,000 cattle are lost to plant poisoning every year [121] . Southern Africa on the other hand has about 600 species of plants known to cause poisoning of livestock, and the total cost of plant poisoning and mycotoxicoses to the livestock industry in South Africa was estimated at ZAR 150 million [122] . The expected annual losses due to plant poisoning of domestic ruminants in South Africa in 1996 were 37,665 cattle (10% of expected cattle deaths) and 264,851 small stock per year [122] . Although plant poisoning of livestock is reported to be important in Brazil, it appears to involve fewer plants than have been reported in South Africa possibly because, plant intoxication is under-reported, as many of the poisonings described by [123] .

All parts of Cestrum species are toxic to mammals [125] . Inkberry and Chilean inkberry are extremely harmful to cattle. If large quantities are eaten the animal usually dies immediately. Orange cestrum is often the cause of poisoning in Zimbabwe and East Africa [123] Night-blooming cestrum, C. nocturnum, can cause respiratory problems from the scent and feverish symptoms following ingestion. C. aurantiacum [124] [125] and C. parqui [48] [126] are also primarily hepatotoxic and neurotoxicity. Another peculiar neurotoxicity that has been described in Brazil and South Africa is Prosopis poisoning. Cattle and goats are poisoned when large quantities of the pods of the highly invasive Prosopis juliflora are ingested [123] Lantana poisoning appears to be less frequent, and only a few cases of diplodiosis has been recorded in Brazil [51] and several in Kenya [70] . According to [118] , plant poisoning is one of the leading causes of death in farm animals, the other two being: rabies, transmitted by vampire bats, and epidemic botulism, which is predisposed by phosphate deficiency. parqui (inkberry) is species also cited in Kenya considered to be a potential ecosystem transformer [64] .

Today planting of some Cestrum species is still promoted in some parts of Kenya (See Table 3). This is against background understanding that People with respiratory sensitivities or asthma, have reported difficulty in breathing, irritation of the nose and throat, headache, nausea, or other symptoms when exposed to the blossom’s powerful scent [127] . Some species of Cestrum contain chlorogenic acid, and the presence of this potent sensitizer is responsible for this effect in C. nocturnum. Ingesting plant parts especially fruit results in elevated temperature, rapid pulse, excess salivation, gastritis, hallucinations, nervous irritability, tachycardia and paralysis [128] and while a non-fatal poisoning of a human child was reported by [129] . Just like Cestrum aurantiacum, Cestrum nocturnum is known to aggressively colonize disturbed areas [130] and is capable of forming dense impenetrable thickets in the undergrowth of some forest systems [130] displacing other species of plants and altering natural ecological processes. It has been shown suitable for capturing and using light than native Hawaiian species in greenhouse conditions. Like all Cestrum species, all parts of C. nocturnum are known to be highly toxic either fresh or when dried [129] .

Cestrum is taxa of 246 species, 6 of which have been reported in Kenya. Members have high level of hybridization complicating discrete delimitation of species. Cestrum aurantiacum and diurnum were introduced in Kenya by the government in 1921 and 1929 respectively. The following parts of Kenya have reports of cestrum invasion, Nairobi, Limuru, Kiambu, Mt kenya region, Cherangani forest ecosystem, Nandi forest, Trans Nzoia, Mt Elgon, Kericho and Kakamega. Cestrum aurantiacum and cestrum as taxa are generally highly invasive and toxic to humans and livestock and interfere with stability of natural ecosystems. And it has been discouraged for planting in many parts the world and Kenya. A feasibility study on how to use C. aurantiacum for producing bio-insecticide and other applications should be initiated. Harvesting can be implemented like gradual harvesting until all propagules are eradicated.

A review on management of cestrum species needs to be done.

Makokha, J. (2018) Invasion of Cestrum aurantiacum Lindl. in Kenya. Journal of Environmental Protection, 9, 671-690. https://doi.org/10.4236/jep.2018.96042