Demography of the red gorgonian Paramuricea clavata (Anthozoa, Octocorallia)damaged by repeated anomalous mortality events

Diseases and mass mortality events of corals and other suspension feeders are becoming a main concern for conservation of biodiversity at the whole community level. The Western Mediterranean coralligenous assemblages have been recently affected by mass mortality events which involved several ecosystem engineer species (Cerrano et al. 2000, Perez et al. 2000, Bramanti et al. 2005). The population of the gorgonian Paramuricea clavata (Risso, 1826), dwelling in the Gulf of La Spezia (NW Mediterranean, Italy), supplies a paradigmatic example of the response of a population living near the edge of the summer thermocline (16-25 m depth, a bathymetric range shallower than usual) to the 1999 and 2003 mortalities associated to anomalous temperature increases. By following a demographic approach, this study performed the comparison between the short-term versus the multi-year assessments of the consequences of these events, thus providing useful data to determine the capacity for resilience of the species. Thanks to the availability of pre-events data (1998), we were able to compare findings of the post-mortality’ period (2004-2009) with the demographic features of the healthy population (1998). During the first three years (2004-2006) after mortalities, a dramatic reduction in the density of healthy colonies (more than 90% suffered total or partial mortality), a shift in the dominant size class towards smaller size and a significant reduction in recruitment were recorded (Chapter I). Larger colonies of P. clavata were more frequently affected than small ones, causing a sharp reduction of the larger size classes (>36 cm high). In the following years (2007-2009), a significant recovery of injured colonies was found, together with a reduction in mortality and a fourfold increase in recruitment. Following the disappearance of the gorgonian living canopy, the increase in sedimentation on the vertical substrata where P. clavata population dwells seemed not to negatively affect larval settlement and recruit survival (Chapter II). In fact, in 2008 recruitment doubled pre-mortality values (Chapter II). Recruitment increased only after the massive, delayed detachment of dead colonies, an event that probably increased the availability of free space for larval settlement (Cupido et al. 2008, Chapter II). P. clavata damaged colonies revealed an unexpected ability to recover with a significant decrease in the extent of injury over time after mortalities (from 56 % in 2004 to 10 % in 2008, Chapter V). The high growth rate of primary branches and the great production of new branches, never reported before, together with the unexpected ability to regenerate lost tissues, greatly counterbalanced negative colony growth (Chapter V). Since primary branches are recognised as being the most fecund, their high yearly production (14 branches y-1, on average) rapidly increased the number of reproductive modules (polyps) and therefore the total colony reproductive output. Some years after the last mortality event, damaged colonies did not show any negative effect on their reproductive traits. Both healthy and damaged colonies showed neither significantly different fertility, nor fecundity (Chapter IV). Reproductive output increased exponentially with colony size due to the exponential increase of reproductive modules. Larger colonies accounted for 99% of the total population reproductive output; the other size-classes, even if more represented in the population, being less fecund, led to smaller birth coefficients. Consequently, the disappearance of the large, highly reproductive colonies, as happened in the studied population, could have a disproportionate effect on population growth rate and resilience (Gotelli 1991, Fujiwara and Caswell 2001, Linares et al. 2007b). Colony growth rate spanned a broad range (0 -14 cm y-1) with a mean value of 2.7 ± 0.7 cm y-1 (Chapter V). A fast growth represents a useful tool in recovering after disturbances, especially in the early life history stages, as they suffer greater natural mortality due to competition and predation (Connell 1973, Hughes and Jackson 1985). The accurate estimates of population reproductive and demographic parameters (Chapter IV and V) enabled us to construct reliable life history tables, which summarised the main demographic parameters of the population (Chapter VI) and allowed the description of the population structure. In 2009, five years after the last mortality event, population had a 14 year life-span and showed a monotonic decreasing distribution indicating that was in steady-state (Chapter VI). The high density of the first age-class suggested a high recruitment success, partially overridden by mortality occurred after settlement. In fact, recruits showed the lowest survival rate (38%), in accordance with the peculiar dynamics of gorgonians (Coma et al. 2004, Linares et al. 2007b). On the whole, these findings indicate clear-cut restoration trends of the population, in which population recovery and canopy re-establishment could start some years after a high-mortality event, driven by the high fecundity and high growth rate of colonies. Population could recover its original density in some years or, alternatively, reach a new equilibrium, characterized by lower density or smaller colonies (Ricklefs and Miller 1999, Bramanti et al. 2009). Therefore, recovery after extensive mortality could be faster than predicted by our current knowledge of gorgonian population dynamics