The ecological dynamics of Inselberg rock pool communities: from local to global scale

In resistant bedrock, eroded depressions can form which may periodically hold water after rain. Such freshwater rock pools form a peculiar habitat for aquatic organisms. These rock pools are often associated with inselbergs : isolated rock outcrops in the landscape that have been inspiring scientists and adventurers through out the ages. Rock pools can be harsh environments where organisms need to cope with drought, high irradiance, and unpredictable and fluctuating environmental conditions. This has led to a surprisingly high floral and faunal biodiversity. The most common aquatic invertebrates are large branchiopods, waterfleas, copepods, flatworms, nematodes, mites, snails, tardigrades, rotifers, mosquito larvae, bugs, beetles, and dragonfly & damse l fly larvae. Some of these (i.e., most insects), can disperse out of the rock pool when conditions become unfavorable . Others, such as the crustaceans, do not have this option and instead rely on dormant stages to bridge the gap between inundations . We call these groups active and passive dispersers , respectively .
Thus far, rock pool research has been largely limited to specific case studies in restricted areas and descriptive studies or reviews. This PhD provides a first attempt to integrate patterns across spatial scales from case studies on local inselbergs to an alyses at global scale to fill four major knowledge gaps.
In the first study, we contrasted rock pools with other temporary aquatic habitats in the same landscape in a National Park in Ivory Coast. We demonstrated that inselberg aquatic biodiversity was higher than and strongly differentiated from the other aquatic systems that exist in this region. In addition, we identified several species that exclusively used rock pools. Rock pool communities were also functionally different because they rely more on phytoplankton as primary producers whereas the other aquatic ecosystems depend more on input of organic matter. This study shows how unique rock pools can be, compared with other temporary wetlands, supporting their conservation value.
In the second study, we used rock pools as model systems to answer macro ecological questions. Rock pools are easy to sample, can be found on every continent as a similar habitat template, are discrete entities with clear boundaries, can be old ecosystems that have been isolated from strong human influence, and can be seen as a double insular system, with clusters of rock pools on isolated inselbergs. Because of this clear hierarchical striucture , we could quantify variables that affect biodiversity and e cosystem functioning (i.e., biomass) on two scales: the local (rock pool) and regional (inselberg) scale. We showed that, contrary to our hypothesis, not regional but local biodiversity was generally associated with higher ecosystem function ing . This relationship was quite variable but more positive in active dispersers . We also demonstrated that climate i s an important driver of gamma diversity in rock pools at a global scale.
In the third study, we contrasted the rock pool communities in a single rock pool cluster with the same communities that were sampled a second time 25 years later. Over this period, we found strong taxonomic and functional shifts in the communit ies, which became more differentiated. We found that this was the result of a strong decline in passive dispersers both in terms of diversity and occupancy Active dispersers, on the contrary, di d better than 25 years ago. We demonstrated that these changes were at least partially related to climate driven changes in rock pool hydrology.
In the final study, we assessed the influence of an invasive mammal on rock pool communities. Results showed that feral goat dung tended to accumulate in deeper rock pools and that this eutrophied these relatively small freshwater ecosystems. While this eu trophication did not result in dramatic reductions in invertebrate richness just yet, it did alter the community structure, seemingly benefitting specific groups like large branchiopods.
The first two studies illustrate and support the value of rock pool biodiversity while the last two studies give a first confirmation that rock pool communities are indeed changing under global change Together, these results show that some anthropogenic changes are impacting specialized rock pool invertebrates and most likely also rock pool ecosystem functioning.