Ecological Change, Sliding Baselines and the Importance of Historical Data: Lessons from Combing Observational and Quantitative Data on a Temperate Reef Over 70 Years

Abstract

Understanding the effects of environmental change on ecosystems requires the identification of baselines that may act as reference conditions. However, the continuous change of these references challenges our ability to define the true natural status of ecosystems. The so-called sliding baseline syndrome can be overcome through the analysis of quantitative time series, which are, however, extremely rare. Here we show how combining historical quantitative data with descriptive ‘naturalistic’ information arranged in a chronological chain allows highlighting long-term trends and can be used to inform present conservation schemes. We analysed the long-term change of a coralligenous reef, a marine habitat endemic to the Mediterranean Sea. The coralligenous assemblages of Mesco Reef (Ligurian Sea, NW Mediterranean) have been studied, although discontinuously, since 1937 thus making available both detailed descriptive information and scanty quantitative data: while the former was useful to understand the natural history of the ecosystem, the analysis of the latter was of paramount importance to provide a formal measure of change over time. Epibenthic assemblages remained comparatively stable until the 1990s, when species replacement, invasion by alien algae, and biotic homogenisation occurred within few years, leading to a new and completely different ecosystem state. The shift experienced by the coralligenous assemblages of Mesco Reef was probably induced by a combination of seawater warming and local human pressures, the latter mainly resulting in increased water turbidity; in turn, cumulative stress may have favoured the establishment of alien species. This study showed that the combined analysis of quantitative and descriptive historical data represent a precious knowledge to understand ecosystem trends over time and provide help to identify baselines for ecological management.

Structure and biodiversity of coralligenous assemblages dominated by the precious red coral Corallium rubrum over broad spatial scales

“…For instance, the principal aim of the MSFD is to effectively protect the marine environment across Europe achieving Good Environmental Status (GES) of the EU’s marine ecosystems by 2020 and to protect the resource base upon which marine-related economic and social activities depend. During the recent years, different protocols and indices have been developed to gather key information for the assessment of the health status of coralligenous habitats416505152. These studies converged to assess the macrobenthic biodiversity as a key parameter to determine the ecological status of coralligenous.…”

Integración de el índice ESCA por medio de los macro-invertebrados sésiles

“…These patterns confirm the sensitivity of coralligenous assemblages to human pressure (Balata et al 2007b, Piazzi et al 2012, Gatti et al 2015b, highlighting the suitability of these assemblages to be used as ecological indicators in monitoring survey and impact evaluation studies (Deter et al 2012, Sartoretto et al 2014, Gatti et al 2015a. Also, local protection might be not enough to prevent impacts on the structure and the ecological quality of coralligenous assemblages, as many organisms are more sensitive to large-scale alterations of water quality than to local disturbances (Parravicini et al 2013).…”

“…Thus, depending on which of the two components is dominant in a coralligenous assemblages, the two individual indices considering only animals (ESCA-A) or only algae (ESCA) can be used accordingly and then also compared to understand which of the two components has been mostly affected. On the other hand, the concurrent use of the two components in the integrated ESCA-TA index can be effective in all the most common situations of high biodiverse coralligenous assemblages, as well as in situations where periodical fluctuations between animal-dominated and algal-dominated assemblages occur due to synergistic effects of local and global impacts (Parravicini et al 2013, Gatti et al 2015b). The results of this paper also showed that, although both the ESCA and ESCA-A indices, when used alone, clearly separated the highly urbanized locations from the other ones, the ESCA-TA index detected more finely the three environmental conditions, also revealing those subtle differences between locations under a regime of protection and locations affected by low levels of urbanization.…”

“…A sensitivity level (SL) value was assigned to each taxon/group on the basis of its abundance in each environmental condition, data available in the literature (Hong 1983, Balata et al 2005, Gatti et al 2015b and results from an expert judgement survey (Montefalcone et al 2017), following an approach similar to that reported for the evaluation of shallow subtidal assemblages by the CARLIT index (Ballesteros et al 2007). SL values varied according to a numerical scale ranging from 1 to 10, with minimum values corresponding to the most tolerant organisms and maximum values to the most sensitive ones .…”

“…Conversely, little is known about the response of sessile coralligenous macroinvertebrates to stress, even if some taxa are recognized to be sensitive to human-induced alterations (Bavestrello et al 1997, Garrabou et al 1998, Gatti et al 2015b). The concurrent use of both macroalgae and sessile macro-invertebrates allows a wider spectrum of human-induced alterations to be detected than when a single component is used, thus better evaluating the ecological quality of coralligenous assemblages (Kipson et al 2011, Sartoretto et al 2014, Gatti et al 2012.…”