OJF  Vol.10 No.2 , April 2020
Plants Diversity of the Burigi-Chato National Park: Rare and Invasive Species
Abstract: This study was conducted within a distance of three to five kilometres around Lake Burigi as a study area in the Burigi-Chato National Park for the purpose of documenting the diversity of plants, identifying invasive and rare species for enhancing conservation in the park. Lake Burigi was purposively sampled as a study area for plant identification due to its potential as a tourist destination. No study has been conducted in the study area to reveal plants diversity, with a focus on documenting invasive and rare species, thus necessitating undertaking of this study to inform the park authority to take urgent control measures in avoiding widespread of invasive species and proper planning for conservation of rare species. The methods involved included field observation; plant identification using field guide books, indigenous and professional knowledge; field mapping using GPS receiver and literature review. Analysis of meteorological data and soil sample were also used among other methods. Meteorological data were analysed using Microsoft Excel and SPSS v.20 to determine the relationship between variables. A total of 102 plant species of different growth forms of trees, shrubs, grasses, sedges, forbs, and herbs were identified and recorded. Tegetes minuta and Argemone mexicana were recorded as invasive plant species, while six rare plant species potential for pharmaceutical industry; Zanthoxylum usambarense, Gardenia ternifolia, Faidherbia albida, Harrisonia abyssinica, Anona senegalensis, and, Pappea capensis were also recorded. Scars of wildfires were observed in the study area. The study area received an average rainfall of 964.36 mm per year, the highest peak recorded in 1951. The trend of rainfall showed that many years had rainfall below the average while the temperature was found to increase from year to year, the situation which suggests the existence of climate change in the study area. There was a weak negative relationship between temperature and wind speed. Soil nutrients and disturbances in the area were found to favour the growth of Tegetes minuta. Findings of this study would help ecological unit in the park to conduct regular ecological assessment for the purpose of controlling invasive plant species, which if left to flourish are likely to reduce habitat suitability for ungulates to utilize the area. Moreover, knowing which plant species are rare, adds value to the area as a destination to visitors interested in plants. The findings also allow the park authority to keep record of species rarity and thus easily take control of them to avoid unintended exploitation. Based on the findings, it was concluded that a number of disturbances, meteorological and edaphic factors favour the growth of invasive species. Urgent measures are to be taken to control the observed invasive plant species before they are left to spread in the park to avoid destruction of the habitat suitability and incurring unnecessary expenses and time in combating them. Furthermore, rare plant species especially those with pharmaceutical and industrial values are to be conserved with special attention to ensure their continued survival. It was recommended that human-induced activities should be prevented through regular ecological assessment, field patrols and public awareness programs. A similar study needs to be conducted during dry season to make comparison of the capacity of plants in adapting to different changes of the weather.

1. Introduction

Plants are the most important components of the environment for the immense roles they perform. They are the producers of nutrients and energy to support the trophic levels in wildlife conservation areas. Lack of enough knowledge on plants identification and documentation of types of plant species available in protected areas has resulted in drastic decline or extinction of certain species or upsurge of invasive species in a particular area. Biological invasion has grown tremendously in the past 50 years (Pysek et al., 2006) and has become an important multi-dis- ciplinary in the field of ecology. Biological invasions threaten the integrity of many ecosystems and are considered second only to habitat destruction in their effects on biodiversity and on landscapes as the whole (Runyon et al., 2012).

From a vegetation management perspective, it is important to know a plant’s identity to determine if it is a weed and the level of risk it poses to desired vegetation. Identification is especially important for early detection of new invasive species that have never been documented in an area before and can be targeted for eradication (Mangold, 2018). Plant identification (Mangold, 2018) is also important for people who raise livestock and are concerned about their animals eating toxic plants. Additionally, knowing what plant, you are about to eat something from a wild plant could become a matter of life or death.

Like many areas globally, invasive species in the dry forests and rangelands (which are hereafter jointly referred to as drylands) of East Africa have been introduced both intentionally and accidentally and are damaging the natural and man-made ecosystems. In East Africa, and particularly Kenya, pastoralists have been adversely affected by invasive plant species and is a disaster registered in many communities (Obiri, 2011). Recent information revealed that over 60 invasive species have been recorded in different National Parks in Tanzania (TANAPA, 2020).

By definition, invasive alien species refer to alien species whose establishment and spread threaten ecosystems, habitats or species with economic or environmental harm. These are addressed under Article 8 (h) of the Convention on Biological Diversity (CBD, 1992). Invasive alien species were also defined (CBD News, 2001) as species introduced deliberately or unintentionally outside their natural habitats where they have the ability to establish themselves, invade, out-compete natives and take over the new environments. Invasive plant species largely flourish in conditions resulting from the complex interactions (disturbed areas) among natural and anthropogenic factors such as native and non-native pests, fires, droughts, wind storms, climate warming, management practices, human travel, and trade (Dix, 2009).

Invasive species (Pyšek and David, 2010) are a major element of global change and are contributing to biodiversity loss, ecosystem degradation, and impairment of ecosystem services worldwide. Strong allelopathic effects of invasive plant species on native plants are frequently observed when the plants in the introduced range did not co-evolve with the invader (Chen et al., 2017). Exotic invasive plants exclude/replace the native plant species by direct interference, that is to say, using competition and allelopathy mechanisms (Chen et al., 2017).

The overwhelmed management of the Burigi-Biharamulo-Kimisi (BBK) Game Reserves (currently, Burigi-Chato National Park) created an opportunity for the refugees to freely enter the reserves en-masse plundering and depleting the resources with no respect for Tanzanian laws (MNRT, 2005). Environmental impacts of refugees on forests resulting in deforestation and degradation of surrounding woodlands were due to the high demand for building poles and fuel wood (Jambiya et al., 2007). As a result of environmental consequences caused by refugees from neighbouring countries, the probability of deteriorating rare plant species, especially those with renown for medicinal values and perhaps introduction and distribution of invasive species, would have increased in the area.

Lack of previous documentation of vegetation types and status in the former BBK Game Reserve which has recently upgraded status of Burigi-Chato National Park makes this study important in establishing a baseline on the type and status of vegetation present in the park that would require close monitoring and management. Such baseline is imperative in the conservation arena as it allows management of habitat for its suitability to ungulates; management of rare plant species that are vulnerable to extinction due to human activities if carefully monitored; and planning for prevention and control measures against invasive species that are typically invaders and therefore not palatable to grazers.

The management of Burigi-Chato has done little in resource inventory. Although several studies being conducted in the study area, such as Jambiya et al. (2007), Mutagwaba (2010) and Masalu (2008), neither of them has reported on plant identification, rare species nor presence of invasive species. This study was therefore, concerned with plant identification; documentation of rare plant species and invasive plant species in the area.

2. Materials and Methodology

2.1. Study Area

The study was conducted in Burigi-Chato National Park, around Lake Burigi at Burigi One Camp moving about three to five kilometres (in some places) in the terrestrial ecosystem (Figure 1). The study area lies between the grid reference 9,760,000N and 9,770,000N and between 300,000E and 310,000E. Burigi-Chato National Park is among twenty two national parks in Tanzania. The park is one of the five newly gazetted National Parks in the country, following upgrading of some of the existing Game Reserves, managed by the Tanzania Wildlife Management Authority (TAWA).

Geographically, Burigi-Chato National Park (formed by Burigi, Biharamulo and Kimisi) is situated in the North-western part of Tanzania covering Kagera and Geita regions, and its total area coverage is 4702 km2 (Jambiya et al., 2007). The park was established in July 2019 after upgrading three Game Reserves of Burigi-Biharamulo-Kimisi (BBK), which were established in the 1970s (Assistant Conservation Commissioner, 2019).

Burigi-Chato National Park, is located within the boundaries of Biharamulo, Karagwe and Chato Districts in Tanzania and lies at an altitude of 1000 - 1500 m from the sea level (Masuki & Mbogoni, 2016). Several lakes found in Burigi-Chato National Park are Burigi, Ngoma, Nyamalebe, Kasinga/Nyarwambaire and small portions of Lake Victoria, which are permanent water sources, which supports wildlife as well as a variety of aquatic species (Assistant Conservation Commissioner, 2019).

Lake Burigi (the largest of lakes in the park) has water with approximately maximum of 4 km wide, 27 km length and at least 8 m deep. The water is saline as one approaches the shore and becomes fresh as one moves interior the lake (Kulekana, 2004). The pH for water in this lake ranges between 8.5 - 8.3 (Kiss, 1977; Kulekana, 2004; Mutagwaba, 2010). In terms of topography, the area has undulating to rolling plains, while the dominant soils are Haplic Ferralsols, which is the low soil fertility. The park has diverse vegetation types including forests, thickets, woodlands, shrublands, bushlands, grasslands and swamps.

Administratively the Burigi-Chato National Park is located in Kagera region within Muleba, Ngara, Biharamulo districts and Geita region, within Chato district and is ranked the third National Park in Tanzania after Ruaha and Serengeti National Parks which ranks number one and two respectively in terms of the size (Wild Taina Safaris, 2019).

Figure 1. A study area used for plants identification in Burigi-Chato National Park.

2.2. Methods

This study was conducted using site visit as a ground truthing to allow image data (in case used in other studies within the study area) to be related to real features and materials on the ground. Digital photographs were also taken for identified plant species of potential significance to help in a checklist preparation for the park management. This study was undertaken between October and December 2019, during the rainy season climate. The study area was selected using a purposive sampling approach, based on the criterion of the potentiality of the area in harbouring biological diversity, both flora and fauna which need special conservation measures as one of the tourists’ destination in the park.

Several methods were employed during this study including field observation and identification; coordinates recording using a hand-held global positioning system (GPS) device to record invasive and rare plant species at some locations thus, allowing close follow up by park ecologists, researchers, park authority or tourists interested in plants. Methods also included analysis of meteorological data, which were analyzed using Microsoft Excel and SPSS v.20 for establishing relationship between variables and facilitating easy interpretation of meteorological phenomena in the area.

Another method was literature reviews using online resources and printed materials to acquire information that would confirm scientific names for some plant species, their families as well as their pertinent common names. Analysis of soil samples collected by other studies were also used to establish the relationship between edaphic factors and growth of invasive species. A number of different plants field guides were used to identify species encountered within the sampled area to help close comparison of the already documented plant species with those observed in the field. Professional knowledge was also employed in identification some common plants species.

3. Results

Plants of different growth forms including trees, shrubs, grasses, sedges, forbs, and herbs were identified. A total of one hundred and two (102) plant species were recorded in a period of two months. Each identified plant was recorded for its English name (common name), scientific name, a family name, local name(s) for some plants as well as the status, whether the plant was a common in the area, invasive or rare.

During the study, six (6) rare species recorded. A total of six rare plant species, equal to 5.8% of the identified plants namely, the Knob-wood tree (Zanthoxylum usambarense), Large-leaved common gardenia (Gardenia ternifolia), Faidherbia albida, Harrisonia abyssinica, Anona senegalensis, and, Pappea capensis were recorded.

Apart from rare species, two hazardous invasive plants species were also identified, including; Tegetes minuta and Argemone mexicana whose proportion was 1.9% of the total identified plants. Tegetes minuta was recorded with GPS coordinates (in some places) including 36M TN 80169, MGRS 11672 and 36M TN 80188, MGRS 11690.

Scars of wildfires were observed at different sites indicating presence of human-induced activities previously conducted in the area. Additionally, many plants that were observed to be affected by wildfires were at their young stages of growth. If no consideration of conservation measures to control such unplanned fires there is a great likelihood of extinction of some plant species especially which are rare.

Through field observation, the area was found to be susceptible to wildfires during drought seasons as many plants were found struggling to recover from tissues damages caused by fires. Black scars resulting from such fires were observed on tree stumps of many identified plants thus justifying previous wildfire incidences in the park.

3.1. Plant Species Identified in Burigi-Chato National Park

During field visit, 102 plant species (see Table 1) of different growth forms such as grasses, herbs, forbs, shrubs and succulent plants were identified and recorded. Although professional knowledge and experience were vital in plant identification, indigenous knowledge was of paramount importance during the exercise for easy identification of plants using traditional or local names. Some of these tribes were Kara, Jita, Kurya, Luo and Sukuma (Lake Zone), Masaai, Pare, Chagga and Iraq (Northern Tanzania) and others such as Nyaturu and Gogo (Central Tanzania), Hehe, Bena and Matengo (from Southern regions of Tanzania) as well as Fipa and Nyamwezi in the Western part of the country. Nevertheless, some literatures were also accessed for familiarizing with English names, scientific names and family names: (Dharani, 2006; Roodt, 2005; Yumpu, No Date).

3.2. Analysis of Rainfall Data in the Study Area

Rainfall data (Figure 2) for a period of 67 years from 1923 to 1994 collected at Biharamulo whether station was analysed in Excel. However, some data for five years (1979, 1987, 1988, 1989 and 1990) were either not complete or missing, therefore they were not analysed. The analysis was meant to explain the climate conditions for the growth of Tegetes minuta in the study area.

Figure 2. Trend of Rainfall (mm) in Burigi-Chato National Park from 1923-1994. Source: Tanzania Meteorological Authority (TMA), cited in Oosterom et al. (1999).

Table 1. A Checklist of plant species identified in around Lake Burigi.

Based on the 67 years data analysis, it was revealed the study area receives an average of 964.36 mm per year. The highest peak of rainfall was recorded in 1951, about 1600 mm. High amount of rainfall was also recorded in 1937 and in 1961 (about 1400 and 1300 mm, respectively). On the other hand, there was a drastic drop in rainfall in 1949 and 1976 where the record indicated about 600 mm. As reported by another study, the park receives bimodal rainfall pattern with a mean annual rainfall of 800 mm and the temperature regime is intermediate (Masuki & Mbogoni, 2016).

Rainfall in the study area is generally high although there was some drop down in some years. The study area therefore, provides conducive environment for the Tegetes minuta to survive. This explanation was supported by some studies (CABI International, 2019). For example, it was reported that Tegetes minuta grows well under high soil moisture conditions, although can tolerate low rainfall (CABI International, 2019).

Through analysis of the deviation from the mean monthly rainfall for a period of 69 years (1921 to 1994), months of April and March were observed to receive more rainfall, with November and December also receiving more precipitation. On the contrary, June, July, August and early September are normally dry months. These findings are important when conducting studies that intend to make comparisons of plant diversity and behaviour in response to precipitation. More importantly, this analysis gives a cue for the tourism department in the park to promote the area as a destination for visitors during these dry months for its enormous tourism attractions.

There was observed a small decline of 1.09% variance around the mean annual rainfall (964.36 mm) since 1923 to 1994. Such decline in the trend of rainfall possesses significant negative consequences on biological diversity and rural livelihoods. With unreliable rainfall, the area may experience frequent human-wildlife conflicts as a result of increased water stress, thus augmenting a great demand for water and pastures for both wildlife and livestock. Encroachment of local community for grazing may also intensify if not controlled, the situation which therefore, calls for a focused conservation planning with an emphasis in improved community conservation approach.

Correlation analysis was performed at 0.377 significance level (2-tailed) to explore the relationship between the mean annual temperature (˚C) and the mean maximum wind speed (Km/h). The results revealed a weak negative relationship between the two variables with the Pearson correlation coefficient of −0.296. However, the coefficient of determination resulted in a small variance (8.7%) between the two variables. Due to the small sample size of the collected data, the relationship seemed not to be statistically significant as it would be at the traditional p < 0.05 level. A small rise in temperature in the study area has therefore, been associated with small decrease in the wind speed.

3.3. Soils Factors for the Growth of Tegetes minuta

The analysis of Soil samples (Table 2) collected from 15 sites in Biharamulo District in which Burigi-Chato National Park is situated, at an average depth of 25 cm deep varied quantitatively in terms of soil pH and different organic matters elements as follows:

Table 2. Composite topsoil samples in the study area.

Source: Oosterom et al. (1999).

Trace elements such as Aluminium and Hydrogen were only recorded in five sites whose mean values were 0.958 (cmol/Kg) and 0.064 (cmol/Kg) respectively. The observed measurements of the composite top soil sample were sufficient to favour the growth of Tegetes minuta in the area. Seven anthropogenic activities that lead to environmental degradation as a result of refugees’ influx in Burigi- Chato National Park (Masalu, 2008) were identified as farming, encroachment for settlements, poaching, wildfires, tree cutting, and grazing.

As reported by some studies (Holm et al., 1997), Tegetes minuta colonizes many places in the tropics and subtropics with the soil pH ranging from 4.3 to 6.6. It also prefers under high nutrient and high soil moisture conditions. GIS analysis in Burigi protected area (Masalu, 2008) indicated that four wildlife habitats were impacted by refugees namely; riverine forest, woodlands, scrubland and grasslands.

Another study revealed that Tegetes minuta which is a drought tolerant weed (although it prefers areas with enough rainfall) and can establish in waste areas, neglected rangeland and poorly managed fields and survives easily in poor soils hence it colonize waste ground, roadsides, and gardens (CABI International, 2019).

A study conducted in India (Rengasamy et al., 2013) found that most of the plants studied along the road side were invasive plants. According to the study, higher carbon gain (both in biomass and carbon content), larger proportion of photosynthetic tissues, thicker leaves, larger stomatal size, higher stomatal density, and larger leaf vascular tissues were associated with the exotic species.

3.4. Impact of Climate Change on Invasive Plant Species

Climate change is altering vital aspects of the environment such as temperature and precipitation, the frequency of extreme weather events, as well as the atmospheric composition and land cover. The temperature, atmospheric concentration of CO2 and available nutrients are the key factors that will drive species survival; changes in these factors will most likely stress the ecosystems and the chances of invasions (Marambe et al., 2009).

The overall impact of existing invasive species may increase or decrease under several scenarios of global change driven by greenhouse-gas influenced climate change, increasing carbon dioxide (CO2) concentration, increasing nitrogen (N) deposition, and altered disturbance regimes (Dukes and Mooney, 1999; Bradley et al., 2010). Bioclimatic envelope modeling is a valuable tool for predicting species response to climate change, but these models assume that species distribution is static under a given set of climatic conditions (Runyon et al., 2012).

Analysis of the deviation of mean annual rainfall (963.36 mm) from 67 years rainfall data (Figure 3) indicated a clear cut between years which had more precipitation above average and those with shortage of rainfall below average. The analysis revealed that many years have continued to receive low amount of rainfall, at least 50 mm below average (964.36 mm). Surprisingly, recent years had shown a significant drop in precipitation as compared to the previous years. By observing the trend of rainfall in the area, it is evident that there is no indication of experiencing more rainfall in the future as it was in the past, instead there is a welcoming drought, which is likely to threaten life in the study area and neighbouring places indiscriminately. With such temperature extremes, there is no doubt that climate change is real taking place in the area, whose negative consequences ought to be realized from an individual family level to policy makers.

Consequences of climate change have been reported by different scholars. Climate change could open up new opportunities for introduced species that could devastate native flora and fauna. Thus concepts of global change need to include consideration of the behaviour and distribution of invasive species. It seems highly likely that invasive species are going to have even more opportunities in the changed future climate than they have at present (McNeely, 2001). Climate change is therefore a pervasive element of the multiple forcing functions which maintain, generate and threaten biodiversity and induce biological invasion. Between 1900s and 1970s, a series of droughts of different magnitudes were experienced in the Serengeti ecosystem with a minimum return period of ten years (FAO, 2010).

Figure 3. Analysis of Deviation from the Mean Annual Rainfall (mm) from 1923-1994. Source: TMA also cited in Oosterom et al. (1999).

Due to these extreme events and associated diseases (e.g. rinderpest), the wildebeest population dropped to 200,000 in 1950s and the migration stopped due to smaller and fragmented wildebeest population (Mduma et al., 1999). Climate indirectly affects nature-based tourism by impacting the physical resources (i.e. land-cover) that define the nature and quality of natural environments on which tourism depends (Scott, 2005).

Normally in extreme climate situations, women (especially widows), children and elders are more vulnerable than men due to their little coping capacity. The observed climate change may therefore, create more difficulties to women living in villages adjacent to the study area than men. In Bangladesh (Terry, 2009), the increased hazard impacts due to climate change have affected more women. The link between poverty and vulnerability is clearly crucial, and affects women disproportionately. If there is no serious progress in reducing poverty in Bangladesh, then it can be assumed that women will become increasingly affected by the impact of intensified hazards, in terms of their ability to resist and recover from them (Terry, 2009).

While there is a slow decrease of rainfall (Figure 2), generally, there is sharp upsurge of variation around the mean annual temperature (19.92˚C) for eleven years (Figure 4) in the study area as revealed by the coefficient of determination of 0.663. With such trend, climate adaptation and mitigation measures pertinent to drought are to be incorporated into conservation planning, otherwise, conservation efforts in the study area (even in the nearby protected areas) will be in jeopardy.

In explaining the impacts of extreme temperatures, scholarly views (USAID, 2012), found that mixed rain-fed and highland perennial systems in the Great Lakes region and other parts of Eastern Africa are expected to be severely affected by climate change, with increased variability and warmer temperatures of greatest concern, resulting in crop yield declines for these areas. Another study estimates that an increase in temperature of 5˚C in Eastern Africa may lead to a production decline of nearly 20 percent by the 2090s (USAID, 2012).

Figure 4. The Trend of Temperature at Burigi-Chato National Park from 2009-2019. Source: World Weather Online.

The increasing temperature in the study area, also warns about the probability of future proliferation of a variety of invasive species colonizing many places of our protected areas, thereby worsening the suitability of habitats which wildlife species depend on. Essentially, the management of such biological invasion needs to be considered from the pathways point of view before their establishment, otherwise it will allow unnecessary cost and time implications to the park authority.

The highest mean annual temperature in the study area for a period of eleven years (2009-2019) was recorded to be 20.67˚C in 2018, this temperature is sufficient for the growth of Tegetes minuta in the park. Some studies have reported on the impacts of temperature on the spread of invasive species. For example, it was observed that the Tagetes minuta is propagated by seeds, which germinate over a period of 48 hours; most seeds germinate temperature between 20˚C and 30˚C (optimal 25˚C), with such temperature, germination of fresh seed may be as high as 95%. At 36˚C, the thermo-inhibited proteins may result in the prevention of radical emergence at unfavourable temperatures (Holm et al., 1997).

Other possible effects of invasive plant species due to climate change may include: Longer growing season, thus increasing seed production and biomass; higher environmental temperatures improve plants’ fertility resulting in increased population sizes (Marambe and Amarasinghe, 2002) and increased fruit and seed set due to enhanced activity of insect pollinators’ activity (Marambe et al., 2009).

Increasing seasonality and marked wet and dry cycles therefore, benefitting aquatic invasive alien plants where fluctuations of water levels help expansion of the free-floating invasive species such as Eichhornia crassipes (Silva and Kurukulasuriya, 2010) that would likely to benefit from rising temperatures, and prolonged droughts supporting enhanced insect pest incidence (Wijesekara, 2010).

Other possible effects include higher CO2 levels—supporting the prevalence of C3 invasive species that are reported from Sri Lanka such as Mimosa pigra (Marambe et al., 2009); and changes in atmospheric circulation that affects the dispersion pathways of invasive plant species making warmer-water alien species become more abundant where established and helping their expansion (Marambe et al., 2009).

3.5. Pathways of Plant Invasion

An invasive plant can either be exotic (a plant living outside its native distribution range) or native to the area (Bernard, 2017). Invasion process of exotic invasive plants comprises of three stages namely; introduction from a donor region (whether intentional or accidentally), establishment in the recipient region (plants have chances of surviving but not spreading); spread (through naturalization) and invasion through a range expansion in the recipient region (Bernard, 2017; Simberloff, 2013). The pathways can be natural or man-made. Natural pathways (i.e., those not aided by humans) include for example wind and other forms of natural dispersal that can bring species to a new habitat. Human-induced activities are characteristically of two types which are intentional, or unintentional (USDA, No Date).

Another study (Lockwood et al., 2013) revealed that biological invasion is a process, which has been realized to pass through five stages or pathways namely; transport, introduction, establishment, spread and impact. In the cause of transport, invasive species can experience two scenarios, death or remain in captivity and may be introduced in a new area. When introduced, the invasive plant has the probability of failure or establish. Being established, it can remain local or spread. When spreading, it will have impacts, which can be low or high depending on the magnitude of the impact as perceived by people (Lockwood et al., 2013).

Alien species may arrive and enter a new region through three broad mechanisms: importation of a commodity, arrival of a transport vector, or spread from a neighbouring region. Pathways for invasive alien species are broadly categorized into three means, those related to transport of a commodity, related to a transport vector and those related to natural spread from a neighbouring region.

One of such pathways is release in nature (the intentional introduction of live invasive species for the purpose of human use in the natural environment, e.g. for fishing or hunting in the wild). Escape—the movement of (potentially) invasive alien species from confinement, e.g. from agriculture into the natural environment. The third one is transport-Contaminant referring to the unintentional movement of live organisms as contaminants of a commodity that is intentionally transferred through international trade. This may include seeds, other products of agriculture, forestry to mention but a few.

Those pathways related to a transport vector include the transport-stowaway. This refers to the moving of live organisms attached to transporting vessels and associated equipment and media, for example, various transportations, boats and other water vessels, fishing equipment etc. Stowaways of any other vehicles and equipment for human activities, such as waste dispersal, recreational boating, tourism (e.g., tourists and their luggage) are also included under this pathway.

The pathways related to natural spread from a neighbouring region include, corridor referring to movement of invasive species into a new region following the construction of transport infrastructures in whose absence spread would not have been possible. Such trans-biogeographical corridors include, for example, international canals (connecting river catchments and lakes or seas). Another pathway is unaided, which refers to the secondary natural dispersal of invasive alien species that have been introduced by means of any of the foregoing pathways.

The means by which these species have moved around the globe varies by taxonomic group, by geographic region, and over time. The bulk water (Ruiz et al., 1997) seems to be the single largest source of unintentional transfers of aquatic nonindigenous species throughout the world. While the major pathways of introductions of invasive species are now well recognized, there is not yet any comprehensive understanding of the many and varied routes by which introductions occur (D’Antonio et al., 2001). In East Africa, Tegetes minuta is abundant following fires or other clearing operations (CABI International, 2019).

During this study, it was observed at different locations that Tegetes minuta prefer growing near the road and places where previously were used for settlements. Disturbances of the study area as a result of roads construction, campsites establishment in the park and high chances of release of invasive plant species due to encroachment for fishing at Lake Burigi and livestock grazing (especially in the past) suggest high probability of the presence of invasive species in the study area.

Among other locations, Tegetes minuta was recorded at Birigi one camp (close to Lake Burigi), previously used for tourist hunting and “Msitu wa Tembo” (few kilometres from Nyungwe entrance gate) which have been used by wildlife rangers during their field patrols. Another disturbance could be caused by fumes originating from vehicles that frequently move in the park, which not only release CO2 fumes but also may act as pathways for releasing the weed in the area (unintentional).

The observed plants (Rengasamy et al., 2013) reached maturity rapidly and produced large quantities of seed that were easily transported by different methods such as vehicles, animals, water and came under the category of weeds due to the adaptation in absorbing food and water effectively under stress conditions.

It was observed that since Burigi-Chato is a protected area with a variety of wild animals, there is high probability of animals and birds being pathways of the Marigold (Tegetes minuta) seeds. Apart from scholarly findings reporting on encroachment of local communities for grazing in the study area, there was observed a dog (which either used to accompany pastoralists or fishermen who poach at Lake Burigi) frequently coming at Burigi one camp. Such domesticated animals could also be agents of the weed spread in the study area.

One study (SFGATE, 2018) found that seeds of Tegetes minuta may be blown by wind, carried by small animals, birds or can be transported away by rain. In areas where Tegetes minuta is cultivated, man can also contribute to the spread of the species.

3.6. Factors Determine the Success of an Invasive Plant Species

Climate: All species are adapted to a particular climatic range where temperature and rainfall (amount and distribution) play a significant role in tropical areas. It is possible to find similar regions of climate in different parts of the earth (biomes) (Medawatte et al., 2008). Microclimate: Habitats differ in their features such as light intensity, vegetation cover, wind, drainage and other edaphic factors and those can influence survival of a species in a specific habitat (Zhang et al., 2006).

Site and land use Patterns: Especially for plants, site-specific variables like edaphic properties play a key role in determining establishment and survival of a species in any particular area (Sheng and Bao, 2006). Species characteristics: Every species is genetically adapted to thrive on a set of specific environmental conditions. The genetic makeup defines its capacity to grow, to resist pests or predators, to resist diseases, and reproductive fecundity. Invasive species exhibit a wide genetic pool, which enhances their invasive potential (Sheng and Bao, 2006).

3.7. Characteristics of Invasive Plant Species

Invasive species exhibit a particular ecological profile rather than a biological profile. A species’ traits determine their success or failure in the transition between different stages of the invasion process (Ratnayake, 2014). Invasive plant species possesses characteristics that make them especially suited for colonizing new ecosystems and those allow them to out-compete native plants for resources. Hence, successes of the control methods are impeded by vegetative, reproductive, life history, biochemical, genetic and ecological characteristic of invasive plant species (Ratnayake, 2014).

A combination of the characteristics of invasive plant species include: Biochemical and genetic characters (i.e. allelopathy), Vegetative and growth characters (Ratnayake, 2014); reproductive characteristics (species with multiple reproductive strategies, long or extended flowering and fruiting periods, production of large quantities of seeds or offspring, short life cycle, early maturity, high initial capacity of germination, efficient seed dispersal mechanism and long seed dormancy and staggered germination) (Witkowski & Wilson, 2001; Sakai et al., 2001; Ratnayake, 2014); ecological characters; tolerance characters and seed dispersal (Ratnayake, 2014).

Some characteristic features of invasive plant species are: their rapidly growth and early maturity; many species are capable of vegetative reproduction via stolon, rooting at the tips of stem and root fragments; they are highly adapted to wind and insect pollination; their seeds get widely dispersed by winds, water, birds and other means, enabling them to colonize in new areas at distances far from their original home; they often have a different phenology for leafing to dormant stages that provide better opportunities to take nutrients from soil; and they usually are not attacked by parasites, diseases, herbivores, etc. in the newly introduced area. Attributes and categories to classify a plant species as invasive are given below (Table 3).

3.8. Tagetes minuta and Its Ecological Impacts

Tagetes minuta is a member of the Genus Tagetes classified under Asteraceae family (formerly, Compositae). The genus Tagetes is composed of about 56 species, out of which 27 are annual and 29 perennial species (Soule, 1996).

Table 3. Attributes and categories to classify a plant species as invasive based on the threat it offers in a particular biogeographical region.

Adopted from Durigan et al. (2003).

Tagetes minuta originated in South America, and has been deliberately distributed across the tropics, subtropics and several temperate countries as an ornamental, medicinal or perfume plant as well as accidentally as a weed (Stadler et al., 1998).

Tegetes minuta was observed to be abundant in some locations within the park (Figure 5). The plant lasts for 4 to 6 months depending upon the time of sowing, crop practice followed and the prevalent climatic conditions (CABI International, 2019). In another study, it was reported that T. minuta was originally restricted to the higher altitudes, but has since spread to lower altitudes as a result of increasing agricultural activities (Stadler et al., 1998).

A number of studies have been conducted to investigate the negative ecological consequences of invasive species on ecosystem processes (Levine et al., 2003; Dukes and Mooney, 2004). Invasive weeds have been reported to possess the ability of changing plant community composition by suppressing the abundance of indigenous species (Vasquez et al., 2008).

Biological invasions (Millennium Ecosystem Assessment, 2005) are one of the main causes of declines of biodiversity, which translates into reduced ecosystem services worldwide. Synergistic interactions between invasive plant species and other elements of global change (Pyšek & David, 2010) make it difficult to assign a rank to specific causes of biodiversity decline; however, invasions are regarded as a fundamental driver of ecosystem degradation in many parts of the world.

Among other ecological impacts, invasive species affect the functioning of ecosystems by changing the availability of resources and the disturbance regimes of invaded ecosystems (Richardson et al., 2000) impacts of invasions on soil processes (Liao et al., 2008); impacts on native species richness (Gaertner et al., 2009), and competition from aliens with native plants (Vila et al., 2004).

Figure 5. Booming of the Tegetes minuta, an invasive plant species in Burgi-Chato National Park as observed at “Msitu wa Tembo” area, few kilometres from Nyungwe gate.

3.9. Adverse Effects of Tegetes minuta on Agricultural Crops

Seeds (Holm et al., 1997) do not require light for germination; however, they respond to it very positively, so that germination only occurs from seeds near the soil surface and most seedlings emerge from soil depths of less than 6 mm.

Tagetes minuta may leave allelopathic residues in soil. The roots exude a polyacetylene derivative which delays germination and reduces the yield of crops grown in soil previously infested with the species (Meissner et al., 1986). Tegetes minuta oil possesses phytotoxicity (a delay of seed germination, inhibition of plant growth) toward test weeds. Radicle growth was affected more compared to the plumule. The effect of Tegetes minuta oil varied from species to species with maximum allelopathic inhibition of Amaranthus viridis and Amaranthus tricolor (Arora, 2017).

Tegetes minuta is a fast-growing annual weed which competes with crops and interferes with their management or harvest. It has been reported as a weed of 19 crops in 35 countries (Holm et al., 1997). Its presence in a crop may also lead to skin irritation to agricultural workers (CABI International, 2019). Teteges minuta is a significant crop seed contaminant in East Africa (Holm et al., 1997).

The weed is an alternative host to the bean fungus Ascochyta phaseolorum as revealed in Australia (Holm et al., 1997). This invasive plant may be also found to leave allelopathic residues in soil (Meissner et al., 1986). The roots exude a polyacetylene derivative which delays germination and reduces the yield of crops grown in soil previously infested with the species (CABI International, 2019).

3.10. Management of Invasive Plant Species

Effective management of invasive plant species is clearly a priority for biological conservation worldwide. Manual uprooting, burning and controlling through uses and weeding, hoeing and ploughing were applied as controlling measures and plantation of fodder trees and grasses was used as controlling and preventive measure. Multiple methods were often applied for controlling, however almost were controlling focused for Ageratum conyzoides in Nepal.

Effective management of biological invasion should follow three main steps from prevention; early detection and eradication; and control backed up by integrated management. Prevention is the best form of invasive species management (Rejmanek, 2005). If prevention is no longer possible, it is best to treat infestations when they are small to prevent them from establishing. Controlling the weed before seeds will reduce future problems. Control is generally best applied to the least infested areas before dense infestations are tackled (Rejmanek, 2005).

A number of integral prevention and control practices methods have been applied to prevent the spread of invasive plant species in the US. Such methods include (California Invasive Plant Council, 2012) provision of prevention training to staff and contractors prior to starting work; scheduling activities to minimize potential for introduction and spread of invasive plants; surveying for invasive plants and evaluate risks before activities begin; Clean clothing, footwear and gear before leaving infested areas; and lastly, after activities, monitor worksites for invasive plants using monitoring plans.

Other measures (California Invasive Plant Council, 2012) include designation of waste disposal areas for invasive plant materials; planning travel routes to avoid areas infested with invasive plants; cleaning tools, equipment, vehicles and animals before transporting materials and before entering and leaving worksites; prepare worksites to limit the introduction and spread of invasive plants; and minimize soil and vegetation disturbance).

Tagetes minuta like other invasive plant species particularly, can be controlled by a variety of methods such as cultural control, mechanical control, and chemical control. Using cultural control (CABI International, 2019), Tegetes minuta can be easily uprooted or removed by hand or mechanical cultivation. However, this should be done at early stages of plant growth before the flowers form to prevent the return of viable seeds to the soil. Using mechanical control (CABI International, 2019), tillage and hand pulling are very effective in controlling the plant in agricultural fields and in cultivation processes. Nonetheless, agricultural machines should be cleaned to prevent seed dispersal among fields.

Tagetes minuta revealed susceptibility to acifluorfen, ametryne, bentazon, bifenox, bromacil, cyanazine, dicamba, diphenamid, diquat, diuron, 2,4-D, glyphosate, imazaquin, linuron, metribuzin, molinate, oxadiazon, oxyfluorfen, par- aquat and simazine, the study revealed during in screening trials in Brazil, (Lorenzi, 1986). Current Australian registrations for the control of Tagetes minuta include 2,4-D, MCPA, norflurazon, prometryn, pendimethalin, atrazine, 2,4-D + picloram, linuron, and bromacil + diuron (Hamilton, 1997). At the moment, no biological control has been attempted against Tagetes minuta (CABI International, 2019).

Generally, in the course of implementing efforts to control biological invasion, such invasive species and pathways are to be identified and prioritized; priority species need are controlled or eradicated, and measures are put in place to manage pathways to prevent their introduction and establishment. Thus, with relation to pathways, the Target contains three elements: to identify pathways; to prioritize pathways; and to manage pathways (UNEP & CBD, 2014). Strategies used to control invasive species (Ratnayake, 2014) include keeping potential invaders out, eradicating potential invaders soon after invasion, biological control, chemical control, and mechanical control.

3.11. Rare Plant Species Identified in Burigi-Chato National Park

Six rare plant species were identified and recorded during the study. The rare species were found to be the most important due to its scantiness and great medicinal and industrial value by humans, which if not controlled, will put the rare plants in jeopardy of extinction in the future. Of the identified rare species, two of them were considered the most important medicinal pressures from human. The plant species were the Knob-wood tree or Kokwaro (Zanthoxylum usam- barense) and Large-leaved common gardenia (Gardenia ternifolia).

Most Zanthoxylum species produce pungent alkamides derived from polyunsaturated carboxylic acids, stored in the pericarp. The plants have been used to treat backache, pneumonia, malaria, and rheumatism by local Maasai people of Tanzania. The pharmaceutical industry has captured the use of this tree by exuding/oozing materials such as methanol and aqueous extracts to produce anti-malaria, and canthin-6-one have shown effective as an anti-fungal drug.

3.12. Medicinal Value of Zanthoxylum usambarense

Zanthoxylum usambarense (Figure 6) is grouped under the family Rutaceae (Citrus family). The genus Zanthoxylum comprises about 549 species distributed worldwide mainly in tropical and temperate regions (Global Biodiversity Information Facility, 2010). Zanthoxylum usambarense has traditionally been used for the treatment of malaria, upper respiratory tract infections, cough, rheumatism, tooth decay and sore gums in Kenya and other African countries (Ozkan et al., 2012). The present findings demonstrated that Zanthoxylum usambarense could be a potential source for new cytotoxic compounds for possible anticancer drug development (Ozkan et al., 2012).

The dichloromethane fraction of the roots and the bark of Zanthoxylum usambarense were proved to have medicinal significance in inhibition of different types of fungi and also against the housefly (Musca domestica). The test also showed positive results in insecticidal activities (Van Puyvelde & Bosselaers, 2002). Traditionally, some tribes in Tanzania including Wakara and Wajita use dried powder of the Z. usambarense barks to treat flu, headache and stomach pains.

3.13. Medicinal Value of Gardenia ternifolia

Gardenia (Figure 7) is classified under family Rubiaceae. Extracts of anthocyanins and organic acids from Gadenia ternifolia (Ngbolua et al., 2015) were reported to be the major anti-sickling agents used in Congolese folk medicine for the management of microbial infections, especially those due to bacteria are recurrent pathologies of the Sickle Cell Disease (SCD).

Traditional medicine (Ngbolua et al., 2015) continues to play a very significant role in the medical primary health care implementation in developing countries. Administration of the aqueous leaves extract of Gardenia ternifolia plant (ALEGTP) offered more improvement on preventing the severity of damage to the liver by carbon tetrachloride (CCl4), a toxic chemicals, which has ability of causing liver damage in mammals as it was revealed as a result of a test conducted using rats (Yunana & Dahiru, 2015).

Figure 6. Zanthoxylum usambarense plant—leaves and young twigs.

Figure 7. Gardenia ternifolia plant.

3.14. Management of Rare Plant Species

Species are considered rare if their area of occupancy or their numbers are small when compared to the other species that are taxonomically or ecologically comparable (Flather & Sieg, 2007). In another study, a species is considered to be “rare” if it exhibits any one of the following attributes: The species is naturally occurring in a narrow geographical area; and/or occupies only one or a few specialized habitats and forms only small population(s) in its range (Isik, 2011).

Many rare and/or endemic species reveal one or more of the following aspects, which make them especially prone to extinction: If a species inhabits a narrow (and single) geographical range; has one or a few populations; has small population size and little genetic variability; is over-exploited by people; its population sizes are declining; has low reproductive potential; the need for specialized ecological niches; and that its growth needs steady and almost constant environments (Isik, 2011).

Species that experience any of the above attributes must be given priority, monitored and managed carefully in an effort to promote genetic conservation (Isik, 2011).

In conservation biology, a species’ rarity is defined most based on its distribution and abundance (Gaston, 1994). Species considered threatened with extinction will more than likely also be considered rare (Gaston, 1994). The IUCN Red List of Threatened Species is based on risk of extinction criteria (IUCN, 2001, 2005).

The main criteria used in assessing extinction risk include: population size; geographic range (both extent of occurrence and area of occupancy) and; population size trajectory. Other assessment criteria are: degraded habitat quality, levels of exploitation, and the effects of introduced taxa, hybridization, pathogens, pollutants, competitors, or parasites (IUCN, 2001).

Conservationists are highly concerned with rare species due to that those which are rare will have a greater extinction risk than those that are common (Johnson, 1998; Matthies et al., 2004). Small populations are more likely to be impacted by chance demographic and environmental events, such as failure to reproduce, diseases, floods, and fires (Boyce, 1992).

Species rarity can be contributed by two broad categories of natural or intrinsic causes defined by a species’ inherent biological or ecological characteristics; and anthropogenic or extrinsic causes defined by harmful human activities that have resulted in limited distribution and abundance, independent of their biology (Partel et al., 2005). Habitat loss and habitat degradation have been revealed as the major anthropogenic factors that increase species rarity (Wilcove et al. 2000).

Management of rare plants requires that populations are monitored. Monitoring generally includes counts or estimates of population size, and often includes demographic data that provide detailed information with which to manage populations and meta-populations (Bevill & Louda, 1999). Due to small populations and scattering of rare plants, a multistage sampling approach that allows efficient and effective monitoring has been developed (Glitzenstein et al., 2001). Management plans can include habitat restoration and species re-intro- duction. Also habitat prediction models can be used to identify suitable habitats for species re-introduction or increase efficiency of rare plant survey on large sites (Imm et al., 2001).

4. Conclusion

A total of 102 plant species within the sampled area. Out of 102 species so identified, two (2) species which are Tegetes minuta a member of family Asteraceae and Argemone mexicana belonging to family Papaveraceae were observed to be dangerous invasive species. The area was also recorded to have six (6) rare species, which suggests the area to be potential for conservation in western part of the country. The rare species were identified as Faidherbia albida, Harrisonia abyssinica, Zanthoxylum usambarense, Anona senegalensis, Pappea capensis, and, Gardenia ternifolia. Findings indicated that the study area receives an average of 964.36 mm per year. The highest peak of rainfall was recorded in 1951 with almost 1600 mm. Such rainfall is sufficient to support the growth of invasive species. Soil nutrients and pressures that cause disturbance of the area have contributed to the spread of Tegetes minuta. The trend of rainfall, however, indicated that many recorded years had rainfall below average, suggesting that climate change exists in the study area. On the other hand, the temperature was found to increase from year to year. There was also no statistical significance difference between the mean annual temperature and maximum annual wind speed. A number of disturbances, meteorological and edaphic factors were found to contribute to the growth of invasive species. In combating invasive species, urgent measures need to be taken before they are left to thrive in the park to avoid destruction of the habitat suitability and eventually incurring unnecessary expenses and time in combating them. Elimination of invasive plant species from the park is thus, vital before their widespread to avoid competition with native plant species, cost and time. The documented rare plant species, in particular those with pharmaceutical and industrial values need to be conserved with special attention to ensure their survival.

5. Recommendations

Based on the findings of this study, it was recommended that:

· Prominent (eye-catching) trees in camp sites should be labelled with scientific, English (and vernacular) names for fascinating visitors and researchers.

· Ecological assessment within the park should be conducted regularly to maintain the ecological health by eliminating invasive plant species and protecting biological diversity such as rare plant species their extinction.

· Human-induced activities such as grazing, deforestation/forest degradation etc. should be prevented through regular field patrols and public awareness programs to ensure existence of plants in the area.

· Nature trails should be designed for tourists interested in walking safaris to make easy identification and viewing smaller plant species.

· Since this study was conducted during wet season, it is suggested that another similar study be conducted during dry season to investigate the performance of plants, as the way of making comparison of the capacity of such plants in adapting different changes of the climate.

· Collaboration between the park authority and training institutions is essential for undertaking more studies to discover other unknowns as the way of improving conservation of natural resources in the area.

Cite this paper: Makunga, J. and Gobolo, A. (2020) Plants Diversity of the Burigi-Chato National Park: Rare and Invasive Species. Open Journal of Forestry, 10, 232-263. doi: 10.4236/ojf.2020.102016.

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