First record of naturalization of Scaevola taccada (Gaertn.) Roxb. (Goodeniaceae) in southeastern Mexico

Scaevola taccada (Gaertn.) Roxb. is native of Asia and eastern Africa but has been introduced into the Americas as an ornamental urban plant. This paper reports, for the first time, the presence of Scaevola taccada in natural environments from southeastern Mexico. Several populations of S. taccada were identified during a botanical survey of the coastal dunes of the Cozumel Island Biosphere Reserve (State of Quintana Roo, Mexico) aimed at recording the most common plant species. Scaevola taccada is considered as an invasive species of coastal areas in this region. Evidence of its invasiveness is suggested by the fact that populations consisting of individuals of different size classes are found distributed throughout the island. Furthermore, they appear to belong to different generations since we found individuals of different sizes and with and without flowers and fruits. Our observations suggest that the population of Scaevola taccad a can be in a process of expansion. Further studies are necessary to understand the potential effect on the native flora.

Species from the genus Scaevola L. are commonly found in environments that are either arid or with prolonged dry periods (Grande and Nozawa 2010).Only three native species, S. wrightii M. Gómez, 1890; S. plumieri (L.) Vahl, 1791 andS. rialagartensis Cast.-Campos, 2019 are known from the American continent and the Caribbean, all of which mainly inhabit the beach and coastal dunes (Thieret and Brandenburg 1986;Howarth et al. 2003;Gutiérrez-Báez et al. 2013;Castillo-Campos et al. 2019).Scaevola wrightii is endemic to Cuba (Liogier 1962;Howarth et al. 2003), whereas the other two species are native to Mexico.Scaevola plumieri is found in some states of the Mexican Pacific coast, the Gulf of Mexico and the Yucatan Peninsula (Flores Guido 1983;Téllez Valdés and Cabrera Cano 1987;Avendaño-Reyes 2008;Gutiérrez-Báez et al. 2013;Parra-Tabla et al. 2018), although it is also distributed in other tropical and subtropical regions of the Northern and Southern hemispheres (Howarth et al. 2003).Scaevola rialagartensis is a recently described species, endemic to the northern part of the Yucatán Peninsula (Castillo-Campos et al. 2019).Scaevola taccada (Gaertn.)Roxb., 1814 is a species native to Asia that currently has a pantropical distribution (Emura et al. 2014).In the Americas, the distribution range of S. taccada covers from the USA to Venezuela, crossing the Caribbean basin (Grande and Nozawa 2010;Howarth et al. 2003).In Mexico it is often used as an ornamental plant in hotel and urban gardens in coastal locations (personal observations); however, to date, populations growing in natural conditions have not been recorded.
As part of a botanical survey of the coast of the Cozumel Island Biosphere Reserve, Quintana Roo, Mexico, we observed specimens of the genus Scaevola (Goodeniaceae), which we thought might belong to the recently described S. rialagartensis (Castillo-Campos et al. 2019).Under closer examination, we noticed that the specimens showed characteristics that were different from those of S. rialagartensis.A review of type specimens of Scaevola species available in the JSTOR Global Plants (JSTOR 2019) and Tropicos (Tropicos.org 2019) databases showed that the characteristics of our specimens corresponded to Scaevola taccada.
This paper reports, for the first time, the presence of Scaevola taccada in natural environments in the Cozumel Island Biosphere Reserve, municipality of Cozumel, State of Quintana Roo, Mexico.The species was found naturalizing on sand dunes and beaches where only S. plumieri had previously been recorded (Flores Guido 1983;Téllez Valdés and Cabrera Cano 1987;Collantes-Chávez-Costa et al. 2019).

Materials and methods
A wide literature search for floristic and vegetation studies carried out in the Yucatán Peninsula and/or the Island of Cozumel was conducted by surveying the major academic electronic databases (Journal of Citation Reports [JCR], SCOPUS, Google Scholar), aiming to locate records of Scaevola taccada.
Because the island is a Biosphere Reserve, the collection of specimens for herbaria is not permitted.Thus, in order to make an in-depth comparison of the characteristics of Scaevola plumieri and S. taccada, we examined voucher specimens of the genus Scaevola collected in Mexico and deposited in major Mexican herbaria (CHAPA, CICY, ENCB, MEXU, and XAL, see Index Herbariorum NYBG for more information about herbaria).The characteristics of the voucher specimens were compared to those recorded in at least 20 specimens in the field using a hand lens (20x Coddington, Bausch & Lomb, USA), measurements made with a calliper, and detailed photographs.Afterwards, scientific illustrations were drawn based on high-quality photos taken with a Canon SX270 HS 20x zoom lens and the characteristics recorded in the field.Species characteristics recorded in the field and those observed in the herbarium specimens were used to make the taxonomic description presented here.

Spatial distribution
To determine the distribution of this species throughout the island of Cozumel, in August and November 2019, and again in February 2020, we thoroughly explored the coasts of the island using all the available coastal roads and via boat where there were no roads available, on the northeastern part of the island.Transects 1 to 2 km long, covering most of the coastline of the island, were walked to record as many plants as possible.Points were taken for each individual, using a Magellan GPS.The largest and smallest diameters of the individual were measured to estimate the plant's area using the ellipse formula.When it was not possible to distinguish between individuals, the cluster of individuals were recorded as one single plant.Geographical data were used to generate a distribution map, and with the individual plant cover records we built a frequency histogram with the different plants size classes.

Results
A literature search through the major academic electronic databases (JCR 2020; SCOPUS 2020; Google Scholar 2020) showed that Scaevola plumieri is found along the coast of the Mexican Pacific Gulf of Mexico, and Yucatan Peninsula (Flores Guido 1983;Téllez Valdés and Cabrera Cano 1987;Avendaño-Reyes 2008;Gutiérrez-Báez et al. 2013;Parra-Tabla et al. 2018), whereas S. rialagartensis is endemic to the Yucatan Peninsula (Castillo-Campos et al. 2019).No previous scientific records of the presence of S. taccada in Mexico were found.

Plant communities and spatial distribution
Scaevola taccada is usually found in the shrub stratum of coastal dune scrub vegetation, growing on white sandy soils, at elevations from 0 to 7 m above sea level, mainly on beaches in the northeast part of Cozumel Island (Figure 3).The prevailing climate type at Cozumel island is Am(f)(i), Köppen's classification, i.e., warm, humid with summer rains, mean temperature of 25.5 °C and mean annual rainfall of 1,570 mm (García 1988).The species commonly associated with S. taccada include: Bidens pilosa L. ( Asteraceae We recorded 1,565 plants and plant clumps throughout the Cozumel Island coastal area (Figure 3).Despite the fact that S. taccada is used as an ornamental plant in urban and hotel gardens in the central western and south-western coastal areas, respectively, most individuals of this species are located in the areas specifically destined for the protection of terrestrial and maritime flora and fauna in the north and northeastern side of the island (DOF 2012; SEMARNAT 2016).We also observed different sizes of S. taccada, ranging from very small with only 0.10 m in diameter (0.008 m 2 of coverage), with the largest clumps reaching more than 15 m in diameter (> 200 m 2 of coverage) (Figure 4).Most individuals were found to be between 1 and 5 m in diameter (0.40 m 2 and < 20 m 2 ), which are mature and reproductive plants.This suggests that the S. taccada population has been expanding in the coastal zone, the smaller plants being of a later generation from the larger, more established plants.

Discussion
Scaevola taccada was introduced to the Island of Cozumel as an ornamental plant for urban and hotel gardens.It blooms almost all year round; however, flowering peaks occur between May and November and produces  fruits from July to December.The plant escaped from these sites and has spread widely to the entire coastal area of the island.The fact that we observed populations consisting of individuals of various size classes distributed throughout the island is evidence of this species' invasiveness.Furthermore, the species seems to have become naturalized along the Mexican coasts, but mostly concentrated along the Mexican Caribbean, as indicated by the occurrence information found in the database from the Global Biodiversity Information Facility (GBIF.org2020).Here 26 records are reported along the Mexican Caribbean, with 4 additional occurrences on the Island of Cozumel, and three more in northern Yucatan Peninsula (Figure 5).As mentioned earlier, to our knowledge, these findings have not been verified with field observations and are not reported in the published scientific literature.Thus, this is the first report on the invasion by S. taccada along the coasts of the Island of Cozumel, with field corroboration.Apparently, the effective invasiveness of S. taccada is associated with the dispersal attributes of its fruits, rather than by the occurrence of a longlasting seedbank.Although local studies in Mexico do not exist, studies elsewhere can help understand the dispersal modes of the species.In this sense, Emura et al. (2014) observed that fruits collected from the Okinawan islands in Japan had two morphs: one floats on seawater and thus can be dispersed by ocean currents, and the other is eaten and dispersed by birds.On the Island of Cozumel we observed seeds of S. taccada in small vertebrate droppings in the northeastern part of the island.It is also possible that birds are eating the fruits.However, we do not have information on which species might be acting as dispersers in Cozumel.Ocean currents are also likely to have promoted the local dispersal of the species, especially because sea water facilitates seed germination by breaking the hard-coat dormancy (Lesko and Walker 1969).
In contrast with the above, there is no evidence that a permanent longlasting seed bank of S. taccada seeds is inducing its invasiveness, since fruits and seeds are predated in the soil and do not accumulate (Louda and Zedler 1985;O'Dowd and Lake 1991).
The broad expansion of S. taccada along the coasts of the Island of Cozumel is probably affecting the natural ecosystems of this Biosphere Reserve, especially those from the beach and coastal dunes.In fact, on-going studies in Cozumel are showing an impact in community composition and structure of beach and coastal dunes vegetation (Castillo-Campos et al. unpublished).In other tropical locations, such as Florida, Gordon (1998) found ecosystem effects of S. taccada, but did not observe a competitive advantage of this species over the native flora.
The potential impact of this broadly expanding invasive calls for more detailed studies on the local reproductive biology and dispersal forms of the species, as well as the effect that the expansion has had on the local flora.This information is highly relevant to help local authorities to implement control programs and tighter restrictions on using S. taccada as an ornamental plant.

Figure 2 .
Figure 2. Scaevola taccada (Gaertn.)Roxb.a, branch with flowers and fruits; b, detail of the indument; c, flower; d, detail of the flower with the corolla, two sepals and two stamens removed; e, stamen; f, stigma; g, detail of the infructescence.Illustration by Edmundo Saavedra.

Figure 4 .
Figure 4. Frequency of Scaevola taccada (Gaertn.)Roxb.individuals in to the different size categories according to their coverage (m 2 ) registered on the Island of Cozumel, Quintana Roo, Mexico.

Figure 5 .
Figure 5. Occurrence records of Scaevola taccada found in the Global Biodiversity Information Facility (GBIF) along the coasts of the Yucatan Peninsula (map from GBIF.org).