PhD thesis: Functioning and resilience of an ecosystem engineer




Esporles, May 24, 2021. Julia Máñez Crespo has defended his doctoral thesis supervised by the doctors Fernando Tuya from the University of Las Palmas de Gran Canaria, and Fiona Tomàs, from IMEDEA (CSIC-UIB). The event took place online on April 30.


Seagrasses are ecosystem engineers (EE) that create diverse and productive habitats across the coastal areas of the world. While previous research has demonstrated considerable small-scale variation in seagrass abundance and structure, studies that tease apart local from large-scale drivers of seagrass meadow functioning and resilience are scarce. Understanding drivers of population resilience of seagrasses is particularly important, since the type and number of disturbances to which these habitats are subjected to has increased dramatically. The elements that contribute to the resilience of EE (such as seagrasses) are poorly understood and, therefore, currently represent a challenge for ecology. By comparing Cymodocea nodosa seagrass meadows in different regions across its range of distribution, this thesis was able to empirically determine the drivers of variability in 1) the functioning of seagrass and the associated fish assemblage abundance, diversity and structure and 2) the resilience capacity of seagrass to herbivore disturbance.


My results highlight that regional and local-scale variation, as well as temporal variability, explained variation in seagrass attributes. Structural attributes, in particular, morphological characteristics of the seagrass leaf canopy, varied significantly among regions and seasons.  Variation in the allocation of belowground tissues was, however, mainly driven by local-scale drivers. High seed densities were observed in meadows of large genetic diversity, indicative of sexual success, which likely resulted from the different evolutionary histories undergone by the seagrass at each region. The variation in fish abundance, diversity and assemblage structure was mostly accounted for by mean seasonal SST, a descriptor of ocean climate that operates at regional scales, and to a lesser extent by local variability in seagrass structure. Beyond climatic drivers, my results suggest that the lower temporal variability in the canopy structure of C. nodosa meadows in Gran Canaria provide a more consistent source of food and protection for associated fish assemblages, which likely explains the higher fish abundance and diversity at this region.


The resilience capacity of seagrass meadows to perturbation, such as pressure due to herbivory, was found to be highly context-dependent and strongly influenced by both biotic and abiotic factors. Populations of C. nodosa from Gran Canaria and Alicante regions, which are exposed to high levels of herbivore pressure, were found to exhibit a number of tolerance responses; higher rates of shoot recruitment during and after the perturbation phase and high levels of basal fibre. The latter suggests an evolutionary process in which a resistance strategy has evolved into a phenotypic trait characteristic of these meadows. Furthermore, meadows in Gran Canaria that experience relatively stable environmental conditions, were observed to have lower rates of seed production and genetic diversity, suggesting a process of evolution whereby the plant invests more resources in resistance and compensation strategies rather than investing in high genetic variability. Conversely, in Mallorca, where environmental conditions are more variable and resource-limited, the meadows appear to have lower resilience to herbivore pressure, but higher seed production and genetic diversity.


The results of this thesis show that seagrass phenotypic plasticity response is influenced by environmental variability and by the life history traits under which the plant populations have evolved.  Of importance to both management and conservation policies, this work has demonstrated that the functioning and resilience of C. nodosa meadows change throughout its range of distribution of this species. Management and conservation policies should therefore be adapted to different regions in order to reflect the difference in the ability of populations to resist and overcome disturbances.