Cambios en la vegetacion asociados a la urbanizacion costera turistica.

• Autor: Faggi, Ana M.

RESUMEN

Este estudio señala procesos de homogeneización y diferenciación en la flora vascular de 18 localidades balnearias ubicadas en el extremo norte de la costa atlántica argentina. Las variables se estimaron a lo largo de un gradiente de uso de suelo y de actividades extendiéndose por la franja costera desde los centros urbanos hasta 10 km alejados del centro. Se realizaron 90 censos en 38 sitios de muestreo en los centros, a 2 km, 5 km y a 10 km del límite urbano. Comparamos similitud espacial y temporal entre los sitios usando datos actuales e históricos.

En las áreas céntricas el número de especies actuales (114) duplica la riqueza de épocas previas al desarrollo turístico (40). Muchas especies inmigrantes han sido introducidas deliberadamente o se instalaron en forma espontánea proveniendo de comunidades vegetales vecinas y/o de áreas más distantes.

A lo largo del gradiente las plantas exóticas fueron más sensibles que las nativas para indicar algunos procesos. Se reconocieron dos "Eutrofización" y "Verde Urbano" como aquellos que más influyeron en la distribución y abundancia de las especies indicadoras.

Palabras clave: Exóticas, impacto humano, comunidades vegetales, homogeneización florística,turismo, dunas

SUMMARY

This study shows evidences of plant homogenization and differentiation in the vascular flora of 18 beach resorts located along the northern end of the Argentinean Atlantic coast. Data were collected along a gradient of urban land-uses and activities, extending away from city centres along a line 10 km-long. Ninety plant surveys were carried out in 38 sampling sites at resort centres, 2 km, 5 km and 10 km from the urban edge. We compared spatial and temporal similarities among sites using current and historical data.

Our results showed that the current number of species (114) more than doubled the number recorded in pre-tourism times (40) at the resort centres. Many immigrant species were introduced--whether deliberately or not--from both neighbouring and far away plant communities. Clearly, exotics were more useful than natives to exposing some preexisting drivers along the gradient. "Eutrophication" and "Urban green" were the two major driving forces affecting the distribution and abundance of indicators species.

Keywords: Exotics, anthropogenic impact, plant assemblages, floristic homogenization, tourism, dunes

VEGETATION CHANGES ASSOCIATED TO COASTAL TOURIST URBANIZATIONS

INTRODUCTION

Evidences from many cities around the world show that urbanization is a drastic and widespread process that homogenises biota as communities become more similar to each other by the introduction and extinction of species (Wittig, 1996; Mc Kinney, 2005). This process is a big concern for the conservation of local biodiversity because it reduces it through the coupled effects of stimulating--intentionally or not--the extinction of rare species and the spread of exotic ones having a potentially high risk of invasiveness.

As a city spreads and its land is allotted to multiple and diverse uses, the new abiotic conditions thus created very likely impact on the status of thermal range and regime, water resources, and soil environment that can create habitability conditions favourable for the establishment and spread of immigrant species (Wittig et al, 1985; Kowarik, 2003). In parallel the habitat quality for natural vegetation decreases and many taxa, the most vulnerable and sensitive species, behave as "avoiders" of the new environmental conditions so that they decrease or disappear.

Changes in the plant composition affect attributes of the community such as structure, productivity, resilience, etc. Changes in competitive capability can modify species dominance and thereby the structure of the community. Resources "exploiters" species also called "synantropic" may increase their abundance because they benefit from the new environmental conditions. Some other species of flora and fauna that tolerate disturbances may become adapted to modified urban habitats and can remain in the perturbed environment as "adapter" species. (Mc Kinney, 2006).

In beach resorts changes in land use resulting from urbanization and activities associated with tourism are usually recognised as a principal driving force behind biodiversity change (Lemauviel & Rozé, 2003). Coastal ecosystems are highly dynamic (Rust and Illenberg, 1996) and complex. Land use, watershed processes and biodiversity are often intricately linked to each other, and affected by several inputs (Stoms et al, 2005). The stability of dunes is much imperilled when changes in land use like e.g. planting exotic trees and shrubs or recreational activities such as off-road driving, riding and hiking are carried out (Curr et al, 2000; Levin & Ben Dor, 2004).

Coastal urbanization is regularly preceded by the stabilization of dunes with fast-growing plants, often exotics that strongly impact on local biodiversity (El-Keblawy & Ksiksi, 2005). Domestic gardening, urban forestry and green area stewardship are other important sources of increasing diversity in exotics. In a number of cases some of these species can grow spontaneously in remnant vegetated patches; occasionally some of them can become invasive.

The aim of this work is to study the anthropogenic impacts on the vegetation of several beaches located along a strip of Argentina's Atlantic coast. In particular we analyse temporal and spatial changes in floristic composition of fore dune vegetation due to urban development and human activities related to tourism.

We build upon the work of Dadon (1999, 2002), who postulated a model for the expansion of tourist uses of the coastal zones in the South Atlantic. In that model, tourism pressures are not confined to the boundaries of urbanised resorts; on the contrary, tourism-related activities extend gradually from urban to remote natural beaches. The spreading of beach uses and activities generates an impact gradient. Some of those are linked to a recreational urban beach--e.g. beaches with public services such as tents and parasols, dressing rooms, showers and restaurants--, others are linked to the dunes--e.g. camping and surfing--and others take place in both sectors but with different intensity and frequency--e.g. trekking, motorcycling, off-road driving, fishing, etc.--. Different impact intensities are expected along this gradient.

Mac Donnell and Pickett (1990), Mac Donnell et al (1997) have applied the environmental gradient paradigm to urban environments. This approach has been very much used by many authors, linking flora and fauna to the intensity of urbanization (Blair, 1996; Jokimaki and Suhonen, 1993; Faggi et al, 2006).

Differences in the composition of vegetation along the urban-natural dunes gradient would be caused by the advance of the tourist frontier; plant assemblages would reflect the growing pressures due to novel uses and activities. Consequently, some of the changes occurring in plant assemblages could be coupled to planting (afforestation, gardening), unintentional anthropocore and zoocore seed dispersal, trampling, vehicular traffic and eutrophication.

We focus our study on the vegetation change driven by the addition and/or loss of species to dunes derived from human activities and land use change along a strip of beach. Therefore, our work deals with the biotic homogenization effect described by Lockwood and Mc Kinney (2001), Mc Kinney (2005, 2006).

We hypothesise that because of tourist induced temporal and spatial changes:

1. The vegetation of dunes fairly away from town centres will remain similar to the pristine original communities. In areas subjected to intermediate impact intensities plant assemblages will reflect transitional situations between city cores and rural areas.

2. Exotic species and extradune natives coming from neighbouring communities will displace many of the original native dune assemblages.

3. Plants indicating humidity are expected to be more frequent in urban areas because of irrigation coupled with the redistribution of run-off water by increased impervious surfaces.

MATERIAL AND METHOD

DESCRIPTION OF THE BEACH VEGETATION

The vegetation under study covers the dunes and comprises Panicum grasslands and forests. Woodlands are composed mainly by two exotic trees: Acacia melanoxylon and Tamarix gallica, which were planted for stabilizing the dunes. Historically the pristine dune vegetation was an open grassland composed of ca. 40 species, dominated by Panicum racemosum, and a relatively few alien plants. This grassland community was thoroughly described by Cabrera (1941); it can be considered the typical plant community previous to the advent of massive tourism to the beach. The intensification of human activities since the second half of the last century brought with it the deliberate introduction of specific plants for binding the dunes, creating woods for amenity uses and domestic gardening. These actions gradually decreased the population of sun-loving...