ETD system

Electronic theses and dissertations repository


Tesi etd-05192009-195209

Thesis type
Tesi di laurea specialistica
Patterns of spatial variability in the abundance of exotic algal species in shallow subtidal assemblages exposed to different levels of anthropogenic stress
Corso di studi
Relatore Dott. Bulleri, Fabio
Relatore Prof. Benedetti Cecchi, Lisandro
Parole chiave
  • Caulerpa racemosa
  • Codium fragile
  • Asparagopsis taxiformis
  • Spatial scales
  • Anthropogenic disturbance
  • Biological invasions
Data inizio appello
Data di rilascio
Riassunto analitico
Habitat degradation and biological invasions are two widespread and closely linked components of global environmental change, with serious ecological and economic consequences. There is a vast literature documenting a positive relationship between invasibility and disturbance, mainly for terrestrial systems. In particular, stressed environments are generally less resistant to invasions. Many studies have, in fact, shown that introduced species tend to take advantage of environmental alterations and successfully spread within communities altered by human perturbations.
The primary aim of this study is to assess whether the abundance of the exotic macrophytes, Caulerpa racemosa var. cylindracea, Codium fragile spp. Tomentosoides and Asparagopsis taxiformis, varies among coastal environments exposed to different levels of anthropogenic disturbance. According to the theory predicting that anthropogenic stress provides opportunities for invasion to introduced specie, we expected communities more exposed to human pressure to be more heavily invaded.
To test this hypothesis, we carried out an observational study, along the coast between the urban centers of Livorno and Rosignano Solvay (about 25 km), between July and September 2008. In both localities, on the basis of distance from urban centers, we qualitatively identified a gradient of anthropogenic disturbance, consisting of three different conditions: Urban (high disturbance), Extra-urban (medium disturbance) and off-shore Shoal (low disturbance). we then built a hierarchical sampling design, including, for each condition, a spatial replication across four scales, ranging from a few centimeters to a maximum of about 25 km. The percentage cover and presence/absence of C. racemosa, C. fragile and A. taxiformis, were chosen as response variables, running both linear and logistic regressions. We also created a further dichotomic variable, named Invasive, grouping together the three exotic species.
This sampling design allowed us to test the disturbance-invasibility hypothesis, as well as to assess the spatial variability in the abundance of C. racemosa, C. fragile and A. taxiformis. Investigation of spatial variations in the distribution of invasive species is a key issue in invasion biology, as it represents the first step towards the comprehension of the mechanisms generating the observed patterns.
Furthermore, in order to identify the factors driving the spatial distribution of the exotic algae, we investigated various physical and biological attributes of the recipient systems, which we considered as influential in regulating their invasibility. A number of biotic (composition and spatial configuration of benthic habitats, abundance and diversity of organisms belonging to different trophic levels) and abiotic (depth, seabed topographic complexity at two different scales) variables were registered. Including these variables as covariates into the analysis, we estimated their relative contribution to the observed spatial variability in the abundance of C. racemosa, C. fragile and A. taxiformis.
Data analysis, performed by means of hierarchical linear regression models (HLM), did not reveal any significant difference between the three levels of anthropogenic stress for none of the response variables. Spatial variability in the distribution of exotic species was mostly concentrated at intermediate spatial scale (few hundreds of meters). Covariate inclusion produced different results, depending on the response variable. They contributed to explain the spatial variability in the percentage cover of C. racemosa and in the probability of occurrence of C. racemosa and Invasive.
These results show that the process of invasion of resident subtidal assemblages by C. racemosa could have reached an advanced phase. The alga is well established along the coastline of Livorno and was present in great abundance at all study sites, regardless of environmental conditions. C. fragile and A. taxiformis, on the contrary, despite being commonly acknowledged as highly invasive species, were recorded in low abundance (only at Livorno, in Urban and Extra-urban areas) and displayed an irregular patchy distribution in our study system. However, no significant difference was found among levels of anthropogenic impact, probably because of the limited number of observations for these two species. Such contrasting distribution patterns, suggest that invasions are context-specific processes, depending on both properties of recipient system and traits of the invaders. Moreover, our results highlight the importance of distinguishing among introduced species characterized by different morphological and functional traits.
The present study also represents a first important step towards the comprehension of factors and processes driving the distribution of C. racemosa, C. fragile and A. taxiformis in temperate shallow subtidal assemblages. After identifying the spatial scales where variability in the abundance of these species occurs, we selected biotic and abiotic variables able to explain the observed spatial patterns of distribution. Basing on the results here reported, further analysis of this dataset may show the importance and the direction of the effect of each of these variables on the abundance of C. racemosa, C. fragile and A. taxiformis. Thus, this work can be considered as a starting point for future investigations, aiming at providing new insights into prevention and management of invasions in coastal habitats.