|Action F3. Modelling the demography of Posidonia oceanica.|
Summary of activities carried out:
1. Demographic dynamics of Posidonia oceanica and validation
of demographic projections.
From the data obtained in the sampling campaigns it has been possible to evaluate the chronological evolution of the density of P.oceanica plants in fields situated round the Balearics and also to establish the degree of accuracy of the demographic estimates on the future evolution of the field (provided by Action A4). This has made it possible to assess whether the fields are in expansion, decline, or in dynamic balance and to determine at what rate these processes are taking place. The detailed results are available in the “Action F3-deliverable” document found in the Annexe of the Memoir.
2.-Degree of genetic and clone diversity.
The low clone variability and the reduced allelic richness
may be due to the diminishing demographic density of this field which
also suffers the highest death rate and the lowest rate of recruitment
of all the fields studied in the Balearics (Marba et al. 2005).
Table 1: details on the sample locations, with GPS coordinates, number of bushes collected (N), number o clones/genets (G) identified, clone diversity( R) and genetic diversity in terms of number of allele locus (A), and genetic structure of the population as detected through the analysis of special autocorrelation (Sp). Results obtained for Majorca are detected and for comparison purposes the results for other submarine fields along the Spanish coastline are given.
Further, it was observed that the allelic richness was also very low in the fields of Posidonia off the island of Cabrera, and may be due, at least in part, to the high mortality and the low clone diversity. The expected consequence of the loss of genetic variability, in the medium and long term, is the disappearance of some positive synergies between different population varieties, such as a diminishing possibility of finding an adequate variation in case of extreme environmental change. This limitation in the capacity of the plant to adapt may produce extinction at certain locations.
Figure 1: map of the spatial distribution of the clones situated off Cabrera, Sa Paret and Santa María.
Figure 2: Comparison of the rate of clone and genetic diversity off Cabrera and the Iberian mainland or other islands of the Balearics. Results of the paired t test (an asterix indicates significant contrasts at the level of 5 %, two when significant contrast is at the level of 1 %).
3.-Level of genetic structure of the
Application of the results obtained through Action F3
to the project:
1.SCI Area Marina of the North of Minorca: The Posidonia oceanica field here was in recession between the years 2002 and 2004. The net growth rate of the population in this period was estimated to be minus 8.7 % per annum, and this indicates that the density of the field will decrease by half in 7.9 years.
2.SCI Area of the Bay of Pollensa: field in strong recession during at least the last four years. The demographic evolution of the population is identical to the one foretold during the first year of study, and shows that the speed of recession has remained constant throughout time. This suggests that the cause of the recession observed has remained the same in these four years. The presence of Caulerpa prolifera in this field of Posidonia oceanica and the high rate of sedimentation of organic matter in the field could be the cause of the recession.
3.SCI Area of the Bay of Alcudia: field tending to recede. Recession in this field has not continued at the same rate throughout time, at least during the last four years, due to episodes of high mortality. Due to the low rate of bush recruitment the field has been unable to recover the state it was in five years ago. The results obtained indicate that the stability of this field is very vulnerable to disturbances that slightly increase the death rate of its bushes. The presence of Caulerpa prolifera, a species found in the field although for the moment not abundant, could contribute to an acceleration of the recession observed.
4.SCI Area of the East Coast: During the project the field of Posidonia oceanica in the Bay of Portocolom has gone from a state of recession to one of expansion. The change observed in the state of the field is due mostly to an exceptional increase in the rate of bush recruitment. The high level of bush recruitment and its ensuing increase in clone growth demonstrate that growth conditions for Posidonia oceanica in this area have improved in the last 3 years.
5.SCI Area of Cap Salines: The state of Posidonia oceánica in the SCI Area of Cap Salines is good off the coast of Cap Salines where the field is in demographic balance. In contrast, the field situated off Colonia Sant Jordi has been in recession during the last four years. Nevertheless, during the last year and thanks to an increase in the rate of recruitment, the field tends to recovery. This increase in the rate of recruitment reveals that environmental conditions have improved growth.
6.SCI Area of Cabrera: Most (88%) of the P.oceánica population studied in the SCI area of Cabrera were in recession between 2000 and 2004, and this recession continued throughout 2005 in 55 % of the fields. The great similarity between projected demography and the evolution in time of the density observed at most of the study-stations examined in this SCI, reveals that the disturbances causing regression of P.oceanica fields in this area have persisted during the last 4-5 years. The fact that P.oceanica has tended towards recovery at locations where study-stations are situated in less deep waters indicates that growth conditions have improved recently.
7.SCI Area of Ses Salines d'Eivissa-Formentera: The P.oceánica
fields of the SCI area of Ses Salines Ibiza-Formentera have remained in
a stationary state in the last years, confirming their good state of health.
Evolution in time of the density of these fields is similar to that projected
on the basis of the estimated net growth rate of the population during
the first year. This indicates that conditions favourable to the development
of P.oceánica in this area have continued throughout the last 5
years. The drastic drop in bush survival observed at Illetes during the
last year indicates that some recently occurring disturbing factor has
been affecting the field locally .The low recruitment rate at Illetes
is insufficient to compensate for the loss of bushes caused by death in
the last year. In the same manner the low bush recruitment rate observed
at Es Pujols indicates that this field is vulnerable to disturbances which
slightly increase the death rate of the field’s bushes.
Fig. 3. Relation between the projected density of bushes, based on the modelling of P. oceanica demography during the first year of study, and the density observed in the fields found in SCI areas of the Balearics in the last 4.5 years. The continuous line indicates a relation of 1:1.
This indicates that, should it be decided to carry out periodic controls of the state of fields in SCI areas which are in good condition, these controls should be done every five years. The only SCI among those studied where five-year controls of the health of P.oceanica could be carried out is that of Cap Ses Salines in Majorca. Monitoring controls of fields suffering recession, and of fields identified as vulnerable, should be carried out in shorter time spans, annually or bi-annually, as is the case of fields found at the following locations: SCI Area North Marina of Minorca, Pollensa, Alcudia, East Coast, Cabrera, and in some areas of Ses Salines of Ibiza-Formentera (i.e. Illetas).
It has been observed that during the last year there
has been a tendency towards recovery in some fields situated under shallow
waters in, thanks to an increase of bush birth as a consequence of the
increase in clone growth, indicating an improvement in environmental conditions
in these areas. The wide geographical area in which fields showing an
increase in the birth of bushes are found, (including the Bay of Santa
María in Cabrera, where human pressure is non-existent), suggest
that the origin of the improvement in environmental conditions could be
climatic. The recovery of these fields cannot be attributed to measures
enacted to protect the SCI areas previewed in the project as by the time
Action F3 had been completed, anchoring in SCI areas had not yet been