How determining a small set of indicator species useful in environmental monitoring? A case study of intertidal boulder fields macrofauna (Northern Basque coast) using the R package "indicspecies".

Ecological  indicators are useful to monitor environmental changes and assess ecological management and conservation. They provide information to understand the environment and its health status while also highlighting changes in the environment by giving early warning signals. Among them, the identification of indicator species add ecological meaning to studied sites and their use is an alternative to sampling the entire biodiversity. The northern Basque  coast (located in  the  South  of  the  Bay  of  Biscay marine sub-area  - France) is  dominated by remarkable rocky habitats as stable platforms and mobile boulder fields exposed to swells and freshwater incomes. Recently, the context of the Marine Strategy Framework Directive emphasized significant deficiencies regarding rocky fauna communities. As it is observed  for other  sub-areas,  boulder  fields  habitat  present a high benthic fauna diversity, but surprisingly, no previous studies had been conducted locally to consider and identify benthic fauna indicator in this habitat. Two independent  Indicator Species Analyses (ISA)  were carried out  on two taxa matrix (mobile and sessile fauna) using the R package “indicspecies”.   It provides a set of  functions to assess the strength and statistical significance of the relationship between species occurrence/abundance and  groups of sites representing habitat types or community types. These functions allow to highlight single and/or combination species; assess significance of the association species pattern/microhabitats with a permutation  test; and select significant species (Indicator Value index) by ordering  them through two components values A (specificity) and B (fidelity). It is therefore possible to describe some species or species groups which could be used to detect changes over time within intertidal boulder fields habitat of this specific marine subregion. These various analyses could allow to monitor rocky foreshore macrofauna in a sustainable way.