Functional biodiversity loss along natural CO2 gradients

The effects of environmental change on biodiversity are still poorly understood. In particular, the consequences of shifts in species composition for marine ecosystem function are largely unknown. Here we assess the loss of functional diversity, i.e. the range of species biological traits, in benthic marine communities exposed to ocean acidification (OA) by using natural CO2 vent systems. We found that functional richness is greatly reduced with acidification, and that functional loss is more pronounced than the corresponding decrease in taxonomic diversity. In acidified conditions, most organisms accounted for a few functional entities (i.e. unique combination of functional traits), resulting in low functional redundancy. These results suggest that functional richness is not buffered by functional redundancy under OA, even in highly diverse assemblages, such as rocky benthic communities.

that the functional richness does not deviate from a random expectation. As the random selection is done from the total pool (of which the ambient pH zone participates greatly), it is consistent that the ambient pH zone falls within the random scenario. However, for the extreme low pH zone, functional richness is much smaller than expected by chances as the mean observed value is lower than the confidence interval of expected values indicating a drastic selection of a limited combination of traits (i.e. environmental filtering). Number of species = 72, number of FEs = 68. Figure 3. Sensitivity analyses based on a decreased number of categories for each trait. We tested whether functional diversity estimates were robust to the resolution of the categorization of functional traits. We reduced the number of categories for each trait (See Table S4 for the description of the reduced number of categories) and re-ran all the analyses. The sensitivity analyses indicated no major changes compared to the results obtained with the finer categorization (Main text, Figure   1). Number of species = 72, number of FEs = 32.

Morphological form
The growth form (morphology) of a benthic species determine its competitive ability for space and light, through maximizing photosynthetic production, disturbance (both biotic and abiotic), and food supply 1 . Growth form was coded as a categorical trait, with 13 categories: from boring species to tree-like forms.

Maximum longevity
Longevity of species indicates resistance to repeated disturbances as well as nutrient storage 2 . Longevity was coded as ordinal along a continuum, with 7 categories: from weeks to more than 20 years.

Growth rates
Growth rate, like longevity, is a prominent indicator of the dynamics of benthic communities, environmental stress and disturbance regimes. Typically, long-lived species exhibit slow growth rates whereas short-lived species grows faster. Growth rate was coded as ordinal along a continuum, with 5 categories: from extreme slow (<1 cm/year) to very high (>100 cm/year).

Size (height and width)
Size (height and width) is associated with energy demand, competition for space, and resistance to predation. Height and width were coded as ordinal along a continuum, both with 6 categories: from extreme small (up to 1 mm) to very large (> 500 mm for height, and > 200 mm for width, respectively).

Epibiosis
Some benthic species depend on other species to attach. Epibiosis was coded as ordinal along a continuum, with 3 categories: obligate, facultative, and never.
Energetic resource, major photosynthetic pigments and feeding Energetic resources, feeding and major photosynthetic pigments determines species impact on ecosystem functioning through tropic interactions and on nutrient cycling.
Energetic resource was coded as ordinal, with 3 categories: photosynthetic autotroph, photo-heterotroph, and heterotroph. Feeding was coded as categorical, with 6 categories: from no-feeding to carnivores. Major photosynthetic pigments were coded as categorical, with 8 categories: from absence of pigments to a mixture of different pigments.

Age at reproductive maturity
Age at reproductive maturity is an important component to characterize population dynamics of communities. Age at reproductive maturity was coded as ordinal, with 7 categories along a continuum: from weeks to more than 5 years.

Potential of asexual reproduction
Potential of asexual reproduction (as fragmentation, regrowth of existing colonies, internal gemmules, and external propagules) has implications regarding the costs and benefits of energy allocation under different regimes of mortality and the recovery of populations. Potential of asexual reproduction was coded as a binary trait with 2 categories: no and yes.

Physical defenses (calcification)
Physical defenses have a primary role in defense against predation (e.g. from sea urchins and/or fishes). Physical defense was coded as categorical, with 5 categories: from without to continuous carbonate shells and skeletons.

Chemical defenses
Chemical defenses have an important role for predation deterrence, prevention of fouling, inhibition of overgrowth, and protection from ultraviolet radiation 3 . Chemical defense was coded as a binary trait with 2 categories: no and yes.

Mobility
Benthic communities include sessile and vagile species. Mobility was coded as ordinal trait with 2 categories: sessile and vagile.