Abstract: To explore the effects of invasive plant Mikania micrantha H.B.K invasion on plant community and diversity in farming systems, the composition, density, importance value, species richness, diversity indices and evenness index were analyzed under five different M. micrantha cover classes (0, 1-25, 26-50, 51-75 and 76-100%). The study took place in farm fields in Longchuan County, Northwest of Yunnan, China. A total of 20 plant species from 20 genera and 10 families were identified. Within communities where M. micrantha occurred, M. micrantha was the most dominant species with the highest population density. Population density and importance values of some dominant species, Ageratum conyzoides, Bidens pilosa, Borreria latifolia, Digitaria sanguinalis and Galinsoga parviflora clearly declined as M. micrantha cover increased and their importance values were significantly negative correlated with M. micrantha cover (p<0.05). The cover of M. micrantha clearly was also negatively correlated with Eleusine indica, Phyllanthus urinaria and Siegesbeckia pubescens (p<0.05). By contrast, population density and importance values for Commelina communis and Kyllinga cylindrical, increased substantially with increased M. micrantha and there were significant positive correlations between their importance values and M. micrantha cover (p<0.05). Maximum values for species richness (17.00), Simpson index (0.86), Shannon-Wiener index (2.10) and Pielou index (0.73) occurred in 1-25% cover of M. micrantha; the next highest values occurred with 0% cover of M. micrantha. Most species richness, diversity and evenness values within M. micrantha cover ranges of 1-25 and 0% were not significantly different but as M. micrantha cover increased, species richness, diversity and evenness values significantly declined, going from 26-100% cover of M. micrantha. Overall, it was concluded that M. micrantha invasion had profound effects on plant community and species diversity in farming systems which must be taken account as we attempt to manage its invasions.
INTRODUCTION
The rapid spread of invasive alien species has become a major concern among ecologists, naturalists, biologists and land managers worldwide. Invasive alien species are now considered one of the most serious threats to global biodiversity (Mack et al., 2001; Ricciardi, 2004; Porte et al., 2011). Moreover, much evidence indicates that as international trade and economic industrialization increase, the magnitude of the threat posed by invasive species increases globally (Lodge, 1993; Hulme, 2009). Invasive alien species alter ecosystem processes (Powell et al., 2011), decrease native species abundance and richness via competition, predation, hybridization and indirect effects (Gaertner et al., 2009; Sugiura and Taki, 2012), change community structure (Hejda et al., 2009) and alter genetic diversity (Ellstrand and Schierenbeck, 2000; Roux et al., 2013). Thus, the effect of invasive alien species on local plant communities and biodiversity has become a major research focus worldwide.
Mikania micrantha H.B.K. (Asteraceae), a perennial herb or semi-woody vine, is native to Central and South America (Zhang et al., 2004). The vine has been listed among the top 100 worst invasive species and as one of the top 10 worst weeds in the world (Lowe et al., 2001; Zhang et al., 2004). Mikania micrantha is present in tropical Asia, parts of Papua New Guinea, Indian Ocean islands, Pacific Ocean islands and Florida in the U.S. (Zhang et al., 2004; Manrique et al., 2011). It has colonized a broad range of farming systems and forest lands, banks of streams and rivers, roadsides and railway tracks, pastures and open disturbed areas (Zhang et al., 2004). In invaded habitats, due to rapid growth, vegetation smothering habit and action of allelopathic chemicals, M. micrantha has caused serious economic loss, biodiversity loss and negative environmental impacts (Zan et al., 2000; Zhang et al., 2004; Shen et al., 2013a).
Some reports have shown that M. micrantha threatens crops production, native species and biodiversity of invaded ecosystems (Zhang et al., 2004; Manrique et al., 2011; Day et al., 2012). In natural ecosystems, it grows quickly and overtops vegetation, blocking sunlight to these species, as well as forming dense monospecific stands on the ground, smothering or suppressing growth of grasses and herbaceous species (Zhang et al., 2004; Day et al., 2012). Species richness, native seedlings, adult plants and native species diversity have decreased due to the presence of M. micrantha (Kaur and Malhotra, 2012). This weed can also affect species composition through its ability to alter the soil chemistry and mineral recycling which in turn affects soil microbial communities (Wong, 1964; Li et al., 2007; Shen et al., 2015). In farming systems, M. micrantha may completely smother crops or trees, blocking sunlight, thus reducing growth, flowering and yield or even killing the plant (Wong, 1964; Wang et al., 2008; Day et al., 2012; Shen et al., 2013a). However, there is scant literature available on the effects of M. micrantha on plant community diversity in invaded farming systems.
The present study examined the effects of the invasive plant M. micrantha on plant community diversity in farming systems in Longchuan County, Northwest Yunnan, China. These findings are valuable for increasing our understanding of the relationship between M. micrantha and associated native plants/plant communities in infested habitats and providing useful suggestions for M. micrantha management in agricultural systems.
MATERIALS AND METHODS
Study site: The study site was located in Longchuan County (24°08-24°39'N, 97°17-97°39'E), Northwest Yunnan, China. This area is characterized by a typical tropical climate, having a rainy season featuring heavy rainfall with 90% humidity alternating with a dry season (Shen et al., 2014). Rainfall averages 1595 mm year-1 and the annual mean temperature is 18.9°C. Recently, the range of M. micrantha has been expanding rapidly within Longchuan County, invading agricultural areas and forest margins. It has infested sugarcane, orange, banana, coffee, bamboo, sweet potato, maize crop, as well as artificial pasture and secondary forest in Longchuan County (Shen et al., 2013a).
Methods: In October 2013, the field investigations were conducted in Zhangfang Township in Longchuan County. According to cover values of M. micrantha based on visual assessment (Shang and Cai, 1992), the surveyed communities were divided into five groups based on cover class: Group I, M. micrantha cover of 0%; group II, M. micrantha cover of 1-25%; group III, M. micrantha cover of 26-50%; group IV, M. micrantha cover of 51-75% and group V, M. micrantha cover of 76-100%.
For each M. micrantha cover class, there were four 30×30 m plots (replicates) established in farming systems, with an attempt to encompass all types of farming crops, such as sugarcane, maize crop, banana, vegetables and others. All plots were located in the same local area, with similar climate and altitude. Then, fifteen 1×1 m quadrats were randomly selected in each plot for statistical analysis. Thus, a total of 300 quadrats were surveyed. For each quadrat, plant species, plant cover, plant number, frequency and plant height were surveyed and the locations of all quadrats were recorded with GPS.
Data analyses: Species density of each plant was determined by dividing the number of individuals or tussocks by the plot. Species cover was determined as the proportion of the quadrat area covered by the canopy of a given species and the species frequency was summarized as the number of quadrats with at least one individual divided by the total number of the sampled quadrats in a plot. The importance value for a given species was calculated by computing the sum of its relative density, relative cover and relative frequency. Relative values were obtained via dividing species specific values by the sums of the densities, cover proportions and frequencies of all species in a plot, respectively.
Species richness, diversity and evenness were estimated as follows: (1) Simpson diversity index (D) was calculated as:
D = 1-∑[Ni(Ni-1)/N(N-1)]
where, Ni is the total number of individuals from species i in a plot and N is the total number of individuals from all species in a plot. D ranges from 0-1, with 1 being the maximal diversity, (2) Shannon-Wiener diversity index H (Ma and Liu, 1994) was measured as:
H = -∑pilnpi
where, pi is the proportion relative to the total number of species per plot and (3) Pielou evenness index (J) (Ma and Liu, 1994) were calculated as:
J = H/lnS
where, S is the species richness of each plot.
Statistical analysis: Data was analyzed by analysis of variance (one-way ANOVA). If significant differences were detected with the ANOVA, Duncan’s multiple range tests were used to detect differences among treatments at a 5% level of significance. The relationship and significance of M. micrantha cover values and plant importance values were determined by Pearson’s coefficient (R) using SPSS.
RESULTS
Plant species and densities: A total of 20 plant species belonging to 20 genera and 10 families were recorded within the study plots (Table 1). All plants were herbaceous, among which 13 annual plants accounted for 65%, 1 annual/perennial plant accounted for 5% and 6 perennial plants occupied 30% of all species, respectively. In our study plots, 10 plants were invasive alien species and the other 10 species were native (Table 1).
There were 13 main species found in the study plots of which ten plants appeared in all five M. micrantha communities (Table 1). Within communities M. micrantha occurred, population densities of M. micrantha were 61.29, 160.84, 297.28 and 568.15 corresponded to M. micrantha cover ranges of 1-25, 26-50, 51-75 and 76-100%, respectively. There were five species, four species, two species, two species and three species absent from the 76-100, 51-75, 26-50, 1-25 and 0% M. micrantha cover communities, respectively. Eight plants, Ageratum conyzoides, Bidens pilosa, Borreria latifolia, Commelina communis, Digitaria sanguinalis, Echinochloa hispidula, Galinsoga parviflora and Kyllinga cylindrica occurred in all five M. micrantha communities, exhibited high population density and dominance. Of these, population densities of Ageratum conyzoides, Bidens pilosa, Borreria latifolia, Digitaria sanguinalis and Galinsoga parviflora clearly declined as M. micrantha cover increased, however population density of Commelina communis and Kyllinga cylindrica, increased substantially with increasing M. micrantha cover.
| Table 1: | Plant species and densities under different Mikania micrantha cover (individual/m2) |
| Values are given in Mean±SD, Different letters in the same row are significantly different at the 0.5 level, AH: Annual herb, PH: Perennial herb, I: Invasive species and N: Native species | |
With cover increases in M. micrantha, total density of both all plants collectively and invasive alien plants were significantly increased but total density of native plants declined substantially (Fig. 1).
Effects of Mikania micrantha on plant importance values: Individual species responded differently to increased cover of M. micrantha. Importance values of Ageratum conyzoides, Bidens pilosa, Borreria latifolia, Commelina communis, Digitaria sanguinalis, Galinsoga parviflora and Kyllinga cylindrica were higher within M. micrantha communities.
| Fig. 1: | Total density for all plants, invasive plants and native plants under different levels of Mikania micrantha cover |
The cover of M. micrantha was positively correlated with Commelina communis and Kyllinga cylindrica (p<0.05), whereas, it was negative correlated with Ageratum conyzoides, Bidens pilosa, Borreria latifolia, Digitaria sanguinalis and Eleusine indica, Galinsoga parviflora, Phyllanthus urinaria and Siegesbeckia pubescens (p<0.05). For other species, a general trend was not discernable because their frequency varied across the M. micrantha categories (Table 2).
Effects of Mikania micrantha on species diversity: This data showed that the 1-25% cover class for M. micrantha had the highest species richness (17.00), Simpson index (0.86), Shannon-Wiener index (2.10) and Pielou index (0.73), followed by the 0% cover of M. Micrantha class (Table 3). Overall, most species richness, diversity and evenness values within M. micrantha cover ranges of 1-25 and 0% were not significantly different from each other. Going from 26-100% cover of M. micrantha, as M. micrantha cover increased, species richness, diversity and evenness values significantly declined. This evidence indicates that invasion of M. micrantha has decreased the species richness, diversity indices and evenness index of local communities substantially.
| Table 2: | Plant importance values under different Mikania micrantha cover |
| Different letters in the same row are significantly different at the 0.05 level, values are in Mean±SD | |
| Table 3: | Biodiversity indices of plant communities under different Mikania micrantha cover |
| Different letters in the same column are significantly different at the 0.05 level, values are Mean±SD | |
DISCUSSION
Biological invasion is considered one of the greatest threats to species diversity and community structure and recognized as the primary cause of global biodiversity loss (Mack et al., 2001; Hulme, 2009; Porte et al., 2011). Many studies have shown that invasive alien plants decrease species richness, species diversity indices, evenness index and alter native community structure and function (Ding et al., 2007; Ehrenfeld, 2010; Vila et al., 2011). This study found that the invasive alien plant M. micrantha is no exception.
Mikania micrantha causes serious yield loss of crops and biodiversity loss through both aboveground and underground communities because of its rapid growth, vegetation smothering habit and action of allelopathic chemicals in infested habitats (Zhang et al., 2004; Chen et al., 2009; Li et al., 2012; Shen et al., 2014). In farming systems, M. micrantha frequently suppresses growth and yield of food crops and cash crops or even kills them (Day et al., 2012; Shen et al., 2013a). Shen et al. (2013a) reported that M. micrantha invaded cash crops and was mostly distributed in sugarcane, bamboo, lemon, banana and orange in Longchuan County. The crop yield was reduced quickly with M. micrantha cover increasing, especially for sugarcane, lemon, banana and orange. As M. micrantha cover increased, the density, biomass and cover of native plants were reduced and the biomass and cover of invasive plants were increased (Shen et al., 2013a). The present study indicated that M. micrantha had the highest population density and was the most dominant species within communities where M. micrantha occurred. Many species are devastated by M. micrantha invasion in farming lands. Population density and importance values of some dominant species, Ageratum conyzoides, Bidens pilosa, Borreria latifolia, Digitaria sanguinalis and Galinsoga parviflora clearly declined, as M. micrantha cover increased. Conversely, the population density and importance values of Commelina communis and Kyllinga cylindrical, actually increased substantially with increasing M. micrantha cover, because these two plant species prefer wet habitats and are shade-tolerant.
In natural ecosystems, species richness, native seedlings, adult plants and native species diversity have been shown to decrease in the presence of M. micrantha (Kaur and Malhotra, 2012). Our findings showed that most species richness, diversity and evenness values were not significantly different between M. micrantha cover ranges of 1-25 and 0%. Within 26-100% cover range for M. micrantha, as M. micrantha cover increased species richness, diversity and evenness values declined substantially. This evidence demonstrates that the invasion of M. micrantha has reduced species richness, diversity and evenness of local communities. The species richness of invasive plants and native plants is equal in study plots but the total density of invasive species is significantly higher than native species. Moreover, with increases in M. micrantha cover the total density of invasive species increases substantially but the total density of native species decreases. Thus, M. micrantha has higher negative impacts on native species than on invasive alien species and invasive alien species as a whole (including M. micrantha) are actively excluding native species in the study area. Thus, some appropriate control measures should be taken immediately.
Compared with other ecosystems, agricultural ecosystems are relatively fragile and lower in diversity. The study county shares a 51 km border with Myanmar, which increases the risk of invasive alien species spread and invasion. Shen et al. (2013b) studied the characteristics of seed banks and seedling banks under different habitats invaded by M. micrantha and found that seed bank plants accounted for 57.5% of all invasive alien plant species in Longchuan County. Moreover, the study area belongs to subtropical and tropical agricultural ecological system and features farming practices with 2-3 crops in succession per year. The results show that the species richness in farming lands is lower compared to other ecosystems. Firstly, this is because more intensive farming production decreases population density, species richness and diversity indices of communities. Secondly, higher growth and competitive abilities of invasive alien species and numerous disturbances facilitate the establishment of invasive species and suppress native species in farming lands. Our findings are consistent with those of Bellingham et al. (2005), who found that invasive alien species often quickly (re)colonize after a disturbance and may dominate during early succession.
CONCLUSION
In conclusion, the results show that M. micrantha invasion has reduced population density and importance values of plants substantially and ultimately changed the structure and function of plant communities in farming lands. Moreover, species richness, species diversity and evenness of local communities are also decreased with increasing cover of M. micrantha. Due to particular local geological and ecological conditions and intensive farming production besides the invasive plant M. micrantha, the study area has been invaded by many other invasive alien species. Thus, determining how to promote the restoration and management of agricultural ecological systems through increasing native species diversity or using some ecological measures to suppress invasive species represents an urgent priority.
ACKNOWLEDGMENTS
This research was supported by Yunnan Provincial Key Fund Program (2010CC002) and International Science and Technology Cooperation Program of Yunnan Provincial Science and Technology Department (2014IA009). We wish thank Yang Jian, Dong Jianping and Gao Rui, from the Plant Protection Station of Longchuan County, Dehong Prefecture of Yunnan Province for their great field support.