A Study on Competition Between Hydrilla verticillata and Mayaca fluviatilis

Mayaca fluviatilis is an exotic ornamental aquatic introduced to the country via the ornamental aquatic plant industry. The plant is now identified as a potential invasive in the country. The plant morphologically resembles Hydrilla verticillata and, a native aquatic while growing submerged in the water. Hydrilla verticillata has been recorded for its invasive behavior in the Western Province of Sri Lanka while M. fluviatilis is still at its early stages of spread with restricted distribution. Therefore information on M. fluviatilis is lacking as it has only been recently recognized as a problematic plant. The present study was conducted with the objective of evaluating inter-specific competition between M. fluviatillis and H. verticillata . A competition experiment was designed with five treatment combinations viz. terminal shoots of M. fluviatilis only (control), terminal shoots of H. verticillata only (control) and different combinations of M. fluviatilis and H. verticillata . The experimental plots were arranged in Completely Randomized Design and plant growth parameters were recorded after eight weeks of planting. Data were analyzed by using Minitab statistical software (version 16). With the introduction of H. verticillata , all the growth parameters of M. fluviatilis were reduced of which, shoot number root number and root dry weight were significantly (p M. fluviatilis , all the growth parameters of H. verticillata were increased of which, length of newly formed shoots, shoot dry weight and shoot number were significantly (p M. fluviatilis , the growth performances of H. verticillata were increased. Based on the present study, in a situation where H. verticillata and both species coexist, H. verticillata will have a competitive advantage over M. fluviatilis . Hence, the result suggest that despite the similar ecology, H. verticillata may outcompete M. fluviatilis in many situations.

suggest that despite the similar ecology, H. verticillata may outcompete M. fluviatilis in many situations.

Introduction
Ornamental aquatic plant trade is a blooming industry in Sri Lanka. Currently, 368 plant species including cultivars and hybrids have been listed as traded under ornamental aquatics plant industry in the country and out of which 76% are exotic species. Interestingly 30% of these plants have been recorded for their invasive behavior elsewhere in the world (Yakandawala et al., 2013). Once escaped from the controlled environment some of these plants are capable of invading into natural ecosystems causing irreversible damage to the biodiversity (Yakandawala and Dissanayake, 2010). Many such events have been recorded globally (Maki and Galatowitsch, 2004). Mayaca fluviatilis Aubl. is one such plant that has been introduced into the country via the ornamental aquatic plant trade and now has started invading the natural environment. Mayaca fluviatilis belongs to the monogeneric monocot family Mayacaceae. It is a perennial herbaceous submerged plant and commonly known as stream bog moss. Recent studies recorded it as naturally occurring in a water body in the Gampaha district in the wet zone of Sri Lanka (Yakandawala and Yakandawala, 2007). Now, M. fluviatilis has recorded in 22 other pools/ditches close proximity to the initial location in wet zone of the country (Yakandawala et al., 2014). Certain countries recognized it as a problematic plant such as a quarantine weed (European & Mediterranean Plant Protection Organization, 2007) or as a problematic weed in Florida Lakes in USA (Hanlon et al., 2000). Currently Sri Lanka categories it under potential invasive species.
Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae) is a native aquatic that has been identified as an invasive aquatic plant in the Western Province (Bambaradeniya, 2001 and2008). It is a rooted, submersed perennial monocot often grows abundantly and has become a serious aquatic weed globally. In particular, it has caused substantial ecological and human-related problems in many aquatic systems around the world (Murphy, 1988). Hydrilla reproduces mainly from fragments of stems and it also reproduces by growth of axillary buds (turions) and subterranean tubers (Langeland, 1996).
Mayaca fluviatilis while growing as a submerged plant still is capable of growing on wet soil during the dry periods. While growing submerged the plant superficially resembles Hydrilla. According to Keane and Crawley (2002), competitive exclusion by native plant species seems to be a major force resisting exotic invasions. Studies on the control of invasive species suggest that competition can reduce invasive plant growth more than herbivory (Lonsdale andFarrell 1998, Mu¨ller-Scha¨rer 1991). An understanding of the role of competition in plant invasions requires the study of both the ability of the invader to grow and increase in population size in the recipient community, and the recipient community's tolerance of the invading species (Vila` and Weiner, 2004).
A large body of evidence supports rapid contemporary adaptation of introduced plants to novel environments (Prentis et al. 2008) and the competitive ability of Hydrilla is well documented (Mony et al., 2007;Spencer and Ksander, 2000 ). However, information on Mayaca is lacking as it has only recently recognized as a problematic plant. Therefore, this study was conducted with the objective of evaluating inter-specific competition between M. fluviatillis and H. verticillata.

Experimental Site
The experiment was carried out in a net house (70% shade) during the period from September to November in 2014 at the Faculty of Agriculture and Plantation Management, Wayamba University of Sri Lanka, Makandura.

Arrangement of Plant Material
Terminal shoots of both species were cut into 10 cm length fragments and existing branches, roots and flower buds were removed and planted in plastic containers (diameter x height: 35 × 15 cm) filled with 1500 g of media (Top soil: sand 2:1). The containers were filled with 6.5 L of water and placed inside a net house. The water level of each container was maintained at a constant level throughout the experiment.

Experimental Design
The experiment included five treatment combinations viz. terminal shoots of Mayaca only (control), terminal shoots of Hydrilla only (control) and three different combinations of Mayaca and Hydrilla as given in Table 1.
Forty terminal shoots were planted per container and the method of planting (arrangement of plants in the container) is given in Figure 2. The containers were laid out in a Completely Randomized Design (CRD) representing three replicates per treatment. Hence altogether 600 terminal shoots 300 each from Mayaca and Hydrilla were used in the experiment.

Data Collection and Analyses
Plant growth parameters viz., length of the main stem (cm), length of the newly formed shoots (cm), shoot number, root number and shoot and root dry weight (g) (oven dried at 80 0 C for 48 hours) were recorded after eight weeks of planting. Data were analyzed by General linear model and Tukey method in the Minitab statistical software (version 16).

Results
The experiment conducted to investigate the inter-specific competition between M. fluviatilis and H. verticillata, clearly showed a significant change in growth parameters of both species under different treatments.

Length of the Main Stem
Mean stem length of Mayaca varied between 10.60 cm to 13.84 cm (Figure 3). However, there were no significant differences (P > 0.05) among treatments. The treatment with only Mayaca plants (T1) recorded the highest mean stem length compared to other treatments. The mean stem length of Hydrilla varied between 12.46 cm to 13.78 cm and there were no any significant differences among different treatments. Compared to other treatments the control treatment with only Hydrilla plants (T5) showed the lowest stem length. The highest stem length was recorded in Mayaca:Hydrilla 3:1 (T2) treatment.
Means with same letters represent non-significant differences (p<0.05)

Length of the Newly Formed Shoots
The average length of the newly formed shoots of Mayaca was decreased with the introduction of Hydrila, where the highest shoot length was recorded in T1 (16.24 cm), while the lowest was recorded in T3 (3.21 cm). However, a significant difference was not observed (Figure 4). The average length of the newly formed shoots of Hydrilla was increased with the introduction of Mayaca. A significantly high shoot length of Hydrilla was recorded in the Mayaca : Hydrilla 3:1 (T2) treatment (53.83 cm) compared to other treatments and there were no any significant differences among other treatments (T3, T4 and T5). Compared to Mayaca, shoot lengths were higher in Hydrilla in all the treatments.

Figure 4. The average length of the newly formed shoots of M. fluviatilis and H. verticillata Shoot Dry Weight
In Mayaca, the mean shoot dry weight varied between 0.024 g to 0.017 g and there were no any significant differences among different treatments ( Figure 5). The highest stem dry weight was recorded in the control treatment with only Mayaca (T1). Significantly high (P < 0.05) mean stem dry weight (0.101 g) of Hydrilla was recorded in the Mayaca : Hydrilla 3:1 treatment (T2). While significantly low stem dry weights were observed in control treatment with only Hydrilla (T5) and Mayaca : Hydrilla 1:3 (T4) treatment (0.063 g respectively).
Means with same letters represent non-significant differences (p<0.05)

Shoot Number
The highest average shoot number (2.4) in Mayaca was recorded in the control (T1). (Figure 6). The shoot number was significantly reduced with the introduction of Hydrilla and it was significantly low in the control treatment with only Hydrilla (T5) and Mayaca : Hydrilla 1:3 (T4) treatment. With the introduction of Mayaca, Hydrilla produced more shoots and significantly high shoot numbers were recorded in the T2 and T3 compared to T4 and T5. Compared to Mayaca, number of shoots were higher in Hydrilla in all the treatments.
Means with same letters represent non-significant differences (p<0.05)

Root number
In Mayaca, the highest mean root number (6.2) was recorded in the control where only Mayaca plants were used (T1) (Figure 7). In the presence of Hydrilla, root numbers of Mayaca were significantly reduced. However, there were no significant differences in the root numbers among treatments T2, T3 and T4. In Hydrilla, root numbers were not significantly different among treatments. However, an increase trend in root number was noted with the increase in the number of Mayaca plants. Compared to Mayaca, the total numbers of roots were higher in Hydrilla in all the treatments.
Means with same letters represent non-significant differences (p<0.05)

Root Dry Weight
In Mayaca, a significantly high root dry weight was recorded when only Mayaca was present (T1) (Figure 8). However, with the introduction of Hydrilla, root dry weight was significantly (P<0.05) reduced but there was no significant difference among treatments T2, T3 and T4. The mean root dry weight of Hydrilla was not significantly different in control (T5) and T2. The lowest root dry weight of Hydrilla was recorded in T4 which was significantly different from T2 and T5. Compared to Mayaca, root dry weights were higher in Hydrilla in all the treatments.
Means with same letters represent non-significant differences (p<0.05)

Discussion
The success and impact of alien species depend on their biological attributes, the environmental characteristics of the invaded ecosystem and the biotic interaction with the receptive community (Keane and Crawley, 2002). When an exotic is introduced, competition for limiting resources is probably the first interaction the species encounter with the recipient community (Crawley, 1990) and high competitive ability of the exotic species has been mentioned as a key factor promoting successful invasive potential (Roy, 1990).
In aquatic plants, the most intense competition may be expected to occur between plants occupying the similar position in the water column (Tilman, 1982). During the present study, with the introduction of Hydrilla, all the growth parameters of Mayaca were reduced of which, shoot number root number and root dry weight were significantly reduced. Though stem length, length of newly formed shoots and shot dry weights were reduced, there were no any significant differences. Hence, we could conclude that due to the competition with Hydrilla, the growth performances in Mayaca were reduced.
On the other hand, with the introduction of Mayaca, the growth parameters in Hydrilla viz., length of newly formed shoots, shoot dry weight and shoot number were significantly increased. Though there is no any significant effect, stem lengths, root number and root dry weight were increased. Therefore, due to the competition, with the introduction of Mayaca, overall growth performance of Hydrilla was increased.
Studies conducted by Spencer and Ksander (2000) has also indicate a strong competitive ability of Hydrilla in the presence of American pond weed Potamogeton nodosus. Further, in another study (Mony et al., 2007) also indicates that, H. verticillata may outcompete Egeria densa in many situations, probably due to its higher plasticity.
The inter-specific competition affects Mayaca in a negative way and it affects Hydrilla in a positive way. Hence there is a possibility to control Mayaca using Hydrilla. However, as these species are perennials and as they were in their early stages of growth it is difficult to conclude which species would over compete the other in long term. Distinctly fewer studies have been carried out on interference among submerged plants. Therefore, further field studies should be conducted to see whether it is possible to eliminate Mayaca using Hydrilla. The present study shed some light to the inter-specific competition between two submerged aquatic plants and this information will be useful in further studies.

Conclusions
Hydrilla verticillata and M. fluviatilis both occur in similar habitats though they do not occur together naturally in the country. Hence, if coexist, both plants will compete for the similar resources. Based on the present study, in a situation where Hydrilla and Mayaca coexist, Hydrilla will have a competitive advantage over Mayaca as growth parameters are significantly increased in Hydrilla, whereas it was significantly decreased in Mayaca. Hence, Hydrilla could out compete Mayaca in an event of a competition.