Meccanismi di distribuzione e sopravvivenza del Grillaio (Falco naumanni) in un agro – ecosistema del Mediterraneo

Merging patterns and processes about the way animals should be distributed in and among habitats is a cornerstone of spatial ecology and have always attracted researchers' interest. In fact, the spatial distribution of individuals in their environment can influence several important ecological processes as population demography and size and species interactions. These ecological processes, in turn, could affect evolutionary processes such as natural selection, genetic drift and gene flow. Assuming that habitat quality is heterogeneous and that organisms are looking for high-quality patches to optimize their fitness, animal and plant species may experience distinct habitat types, different development rates, life span, birth and death rates. Population size and growth rate may vary as functions of the relative proportion of different habitat types available. However, natural and anthropogenic drivers can interfere and change the availability of habitats and, as a consequence, the proportion of individuals of a given species in any particular habitat type may be altered. Here, I report the results of a experimental research in which I analyzed some biotic and abiotic factors determining distribution and survival probability of a lesser kestrel population breeding in Sicily (Italy). The lesser kestrel population in Sicily is actually the second largest in Italy and, clumped in two areas of the island, one corresponding to the Gela Plain (South-East of Sicily) and the other around Sicani Mountain (North-West). This species has suffered a dramatic decline in the last century in all of the Mediterranean populations. Only recently, it improved its conservation status turning from "vulnerable" to “least concern” due to aimed conservation actions. The first aim of this thesis (Chapter 1) was to describe and understand the connection between coloniality and fitness of colony members in such big population of lesser kestrel breeding in the South-East of Sicily. Particularly, I was interested in evaluating the singular and the interactive effect of spatial proximity among nests, breeder abundance and reproductive synchrony on eggs and nestlings survival probabilities in order to investigate mechanisms explaining fitness variation in lesser kestrel in particular, and in colonial species in general. Coloniality is, in fact, the result of multiple interacting costs and benefits, which may vary according to species, populations and individuals. Among fitness costs, the depression of genetic diversity enhanced by the inbreeding rates is likely the most dangerous for population permanence. Specifically, for colonial species living in human- modified landscape, the intensification of farming practices, causing the increase of habitat fragmentation and the reduction of dispersal behaviour, could be a potential driver of the reduction of genetic diversity of a species to be taken in consideration. Thus, the second aim of the thesis (Chapter 2) was to study the effects of agricultural intensification (i.e. human disturbance), colony size and spatial isolation on a measure of inbreeding rates, i.e. the relatedness between nestlings belonging to different nests in two lesser kestrel subpopulations. In fact, numerous studies and reviews have linked the decline of many European farmland bird populations since the 1970s, to changes in agricultural practices. Anthropogenic modifications of habitats may in fact reduce the resources available for species, leading to populations decline and extinction. In particular, for colonial species living in farmland areas, the interaction between agricultural intensification and density dependent mechanisms could influence population dynamics through the reduction of food availability. Thus, I used demographic data on lesser kestrels (Chapter 3) to understand the importance of land-use changes and density-dependent mechanisms in their survival probability and fecundity, and provided conservation guidelines to improve habitat suitability for one of the largest Italian populations of this small raptor. In order to explain the results obtained from survival analysis and population growth rate described in Chapter 3, I analyzed in details the land-use related diet of lesser kestrels living in a changing and anthropogenic environment (Chapter 4). Despite the effects of agricultural intensification on lesser kestrel demography, other anthropogenic factors can contribute to determine the status of this species in Sicily. A growing body of literature demonstrates the effects of human-induced global warming on the structure and distribution of populations and communities across taxa. However, most of these effects was analyzed only at a macro-scale level. In order to quantify the combination between factors at different scales of observation and their effects on animal fitness, an increasing calls for integrating micro - and macro-habitat characteristics are compulsory. Secondary-cavity nesters (i.e. birds that use pre-existing cavities or holes to nest) are an excellent model to analyze the potential interactive effects between temperatures recorded at micro – and macro – scale. Specifically, the last aim of the thesis (Chapter 5) was to investigate whether micro-scale temperature was determined not only by temperature at macro-scale but also by biotic (e.g. number of nestlings) and/or abiotic factors (e.g. nest type). I also explored the possibility that this species might attempt to mitigate adverse microenvironments by modifying behavioral and/or life-history traits.

La distribuzione spaziale degli animali nel loro ambiente naturale ha da sempre attratto l’interesse degli studiosi di ecologia animale e dei biologi della conservazione. Infatti, i meccanismi di selezione dell’habitat possono influenzare processi ecologici di base come la regolazione delle popolazioni e l’andamento demografico, che in ultima analisi, possono agire sulla selezione naturale, la variabilità genetica e il flusso genico. Le principali teorie sulla distribuzione spaziale degli animali affermano che la qualità degli habitat è eterogenea e che gli organismi vanno alla ricerca di territori di elevata qualità al fine di ottimizzare la fitness. La ricerca da parte degli animali di habitat di elevata qualità rappresenta il cuore dei primi modelli evolutivi di selezione dell’habitat: i) la Ideal Free Distribution; ii) la Ideal Despotic Distribution e iii) la Ideal Pre- emptive Distribution. Il Grillaio (Falco naumanni) è un rapace coloniale che vive in ambienti pseudosteppici caratterizzati da differenti usi del suolo. Il Grillaio rappresenta un modello appropriato per lo studio delle relazioni specie – habitat a causa del suo ruolo di indicatore biologico per il monitoraggio delle dinamiche di popolazione di uccelli pseudosteppici mediterranei. Le pseudo steppe sono ambienti aperti aridi spesso drasticamente modificati dall’’uomo. Lo status di conservazione del Grillaio è migliorato negli ultimi anni dopo i drammatici declini degli anni ‘50 ma in parecchie parti dell’areale è ancora fluttuante e non consolidato. In questa tesi, ho analizzato alcuni fattori biotici e abiotici che determinano la distribuzione e la probabilità di sopravvivenza di due sub-popolazioni di Grillaio in Sicilia (Italia). Tra i molti fattori analizzati, la dimensione della colonia sembra essere un determinante fondamentale delle dinamiche di popolazione di questo rapace coloniale. Infatti, essa agisce aumentando la sopravvivenza dei pulcini e dei giovani per mezzo di una interazione tra la distanza tra nidi e il numero di conspecifici. D’altra parte, un elevato numero di conspecifici determina una diminuzione della parentela tra pulcini e della fecondità delle coppie. Un altro fattore importante è rappresentato dalla tipologia di habitat il quale, determinando una differente disponibilità alimentare durante la crescita dei pulcini, agisce sul tasso di accrescimento della popolazione. In conclusione, agendo sui fattori che determinano la sopravvivenza, la distribuzione e l’andamento demografico del Grillaio si potrebbero intraprendere le più concrete azioni di conservazione per una corretta gestione della popolazione di Grillaio in Sicilia e nel resto del suo areale.