Unraveling local and regional determinants of high plant diversity at marine rocky outcrops in Uruguay

Prepared by Patricia Mai

Representation of species assemblages along the stress and disturbance gradient in marine rocky outcrops of the Atlantic coast of Uruguay. a) Spatial ordination of species assemblages along the coast-inland gradient, with representative species of each assemblage (Figure credit: Tea Eizmendi in Mai et al. 2024). b-i) Frequency of occurrence of each species assemblage along the distance to the coast as a proxy of the coast-inland gradient

Rocky outcrops of the Atlantic coast of Uruguay are relevant environments since they show high plant richness within relatively small areas and host endemics and numerous species with conservation significance. Unfortunately, these rocky outcrops are currently exposed to multiple pressures that threaten their biodiversity. Due to their proximity to the ocean and its conditions (e.g. tide dynamics, wave regimes, marine spray, frequent winds, storms and high salinity levels), they are harsh environments, which present a coast-inland gradient of stress and disturbances. This gradient shapes both the species richness and the species composition according to their particular adaptations to endure stressful environmental conditions. In this context, our study aims to elucidate the influence of regional landscape environments on species composition along this gradient within marine rocky outcrops. We also explored which environmental factors determine the remarkable plant diversity at the local scale.

Representative images of four marine rocky outcrops of the Atlantic coast of Uruguay. Cabo Polonio (-34.4051°S; -53.7802°W): a) aerial view (Photo credit: Sebastián Decuadro), b) rocky landscape, c) sampling plot. Cerro Verde (-33.9451°S; -53.5071°W): d) aerial view (Photo credit: N. Teryda and G. Velez, A.C. Karumbé), e) rocky outcrop slope, f) sampling plot. Punta Ballena (-34.9132°S; -55.0458°W): g) aerial view (Photo credit: Nicolás Chacón), h) characteristic species among rocky ridges, i) sampling plot. La Pedrera (-34.5913°S; -54.12116°W): j) landscape view, k) rocky slope, l) sampling transect in slope

We identified species from nine distinct regional environments or vegetation types (named species assemblages), such as anthropic environment, grasslands, hydrophilous and humid herblands, native forest and shrubland, maritime environment, psammophilous and rupicolous herblands. This combination of species from the regional pool in particular sites enhances local diversity. Surprisingly, 56% of the species were from non-marine environments, e.g. grasslands or hydrophilous herblands, while coastal assemblages reached 44% of the total richness. This highlights the importance not only of connectivity among remnant vegetation patches but also of a landscape configuration at a regional scale that ensures species dispersal among different environments. The species assemblages coexist and are spatially clustered along the coast-inland gradient, probably due to the high environmental heterogeneity present in these rocky outcrops. Additionally, at the local scale (plot scale), altitude, distance to the coast and vegetation cover allow species richness to increase, while the increase in substrate availability at this scale may foster direct plant competition and thus reduce diversity. In these extreme conditions, functional plasticity and local ecotypes may be essential for the persistence of several species. Ecotypes were evidenced by taxa with physiognomic and morphological variations in comparison to individuals growing in their typical habitat.

The connection with a functionally diverse regional species pool interacting with local heterogeneity, and local conditions that enhance or attenuate the effect of the marine stress and disturbance gradient may explain these high diversities and the local arrangement of species along the coast-inland gradient. The ongoing urbanization and fragmentation of these environments and their surrounding landscape, the incorporation of exotic vegetation in nearby areas, and especially the lack of adequate conservation and management plans might compromise the viability of these diverse habitats.

This is a plain language summary of the paper by Mai et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.13284).