Similar functional composition of fish assemblages despite contrasting levels of habitat degradation on shallow Caribbean coral reefs

Functional trait-based approaches provide an opportunity to assess how changes in habitat affect the structure of associated communities. Global analyses have found a similarity in the composition of reef fish functional traits despite differences in species richness, environmental regimes, and habitat components. These large-scale patterns raised the question of whether this same stability can be observed at smaller spatial scales. Here, we compared the fish trait composition and their functional diversity in two Caribbean shallow coral reefs with contrasting levels of habitat degradation: Limones (>30% cover), constituted mainly by colonies of Acropora palmata and Bonanza, a reef with extensive areas of dead Acropora structures, dominated by algae. To characterize the functional structure of fishes on each reef, we calculated the community-weighted mean trait values (CWM), functional richness, functional evenness, functional dispersion, and functional originality. Despite the differences in habitat quality, reefs exhibited a similar proportion and common structure on fish functional traits. Functional richness and functional evenness differed significantly, but functional dispersion and functional originality did not show differences between reefs. The greater niche complexity driven by the high availability of microhabitats provided by A. palmata may explain the higher functional richness in Limones, whereas the reef degradation in Bonanza may contribute to a higher functional evenness because of a similar distribution of abundance per fish trait combinations. Our results suggest that widespread degradation on Caribbean reefs has limited the type, variety, and range of traits, which could lead to a functional homogenization of fish communities even at local scales.


Trait
Ecological implications Response/effect on habitat*

Body size
Body size acts as a primary driver of ecological processes that directly impact almost all basic anatomical, physiological, and behavioral parameters [1].Body size also captures the variation of functions relating to feeding, movement, home range and energetic requirements [2].
Body size has been closely tied to ecosystem functions such as nutrient cycling [3], bioerosion [4] or growth [5].Body size determines energy needs through the amount of energy required per unit of body mass [6] and small fish species are a major contributor to reef energy turnover [7].
The consequences of coral habitat degradation are particularly severe on small body size individuals [9].Many fishes that feed or shelter among live coral colonies are small bodied, and disturbances that reduce coral cover and their rugosity have detrimental effect on their abundance [10].In addition, habitat loss also results in fewer small-bodied juveniles and prey that replenish stocks and provide dietary resources for predatory target species [11].In some cases, the decline of larger bodied fish relates partially to a higher dependence on live coral and associated structure Body size also affects the fish ability to swim.For example, larger fishes are expected to be faster and have greater endurance than small ones, but small fishes have better maneuverability and can thus move in topographically complex environments such as coral reefs [8].
during the early life history, when fish are smaller [12].
Thus, reef habitat availability of shelter of different sizes has the potential to control the abundance of fish body sizes that can use available holes, suggesting that the absolute availability of holes for refugedependent, small, and mid-sized fish declines as the reef degrades, and that more uniform fish size distributions are associated with homogenous versus heterogeneous and complex environments [13,14].

Home range
Home range is influenced by fish mobility and determines energy needs, with mobile species requiring a lot of energy by mass unit compared with sedentary species [6].
Home range also affects the spatial extent at which fishes control their resources and transfer nutrients, especially between habitats around reefs [6].
Traditionally, the study of species home range has been used to identify key components of habitat such as important foraging and shelter areas as well as migration paths [16].Thus, declines in the abundance of fishes following habitat degradation may reflect the large-scale migration of fishes to nearby and relatively unaffected habitats but also can limit the ability of certain species to find Some coral reef fishes move daily between sites used for feeding and those used for reproduction or for resting.Often, these activities occur in different habitat types and species with mobility divided between two habitat types tend to have larger home ranges than species with only a single habitat type [15].
Home range scales positively with body size, and the degree of exclusivity habitat use declines with increasing body size due to the metabolic scaling of energy acquisition and as larger fishes tend to also increase in mobility [13].
alternative habitats depending on their home range [9].In addition, some reef fish exhibit ontogenetic shifts in habitat use and may relocate their home ranges in response to habitat changes [15].

Period of activity
Period of activity or diel activity has implications on the trophic role a species plays in the food web through both bottom-up controls (i.e., the set of resources it can target) and top-down controls (i.e., the susceptibility it has for being preyed upon) [6].
Diurnal fish are active in the day, retreating to the reef matrix at night, Diel period variation in habitat usage by fish remains understudied.
However, some studies have found that habitat associations displayed by fishes during the day are not always maintained at night [18].As dusk approaches, some species migrate out onto areas such as deep reefs, rhodolith, sand, and seagrass habitats when the nocturnal fish emerge [17].
These diurnal and nocturnal migrations may be driven by feeding or reproductive behavior and further influenced by the presence of predators [18].
Period of activity also describes temporal fish species turnover within the reef fish assemblages [19].For example, many grunts (Haemulidae) rest during the day on coral reefs but feed at night over soft substrates [15].Gregariousness Gregariousness or schooling influences predation vulnerability, nutrient cycling, and resource depletion [6].
Schooling behavior describes species social strategies that minimize predation and energetic costs while feeding [19].Particularly, many fishes exhibit a schooling behavior when young, whereas older individuals are solitary [22].This indicates the presence of a range of schooling types, depending on age, physiological state, and factors such as habitat type, The appearance / absence of schooling behavior in fishes of the same species may be caused by changes in the ecological conditions.
There is a clear tendency to schooling in homogeneous habitats but an individual mode of life in more complex heterogeneous habitats [22].
Particularly, non-aggregators (e.g., non-social, and often territorial species) may occur at high densities where resources and shelter are abundant, but do not shoal [23].In activity, environmental conditions, etc.
Most nocturnal species get away predation from active predators during the day (e.g., finding shelter in preserved habitat).

Position in water column
Position in the water column is critical to determining fish ecological niche and influences species distributions, mobility, nutrient transfer between vertical strata, and habitat requirements [6].
Water column position also strongly influences vulnerability to predation [25].For example, red snappers are known to be active, opportunistic predators consuming a variety of prey species including benthic crustaceans, reef-associated fishes, pelagic fishes, squids, and zooplankton [26].
The position in the water column in which the species usually forage is a good indicator of their swimming ability and can indicate the dependence of a species on the substrate [27].Although water transports particles (e.g., zooplankton), maintaining water column position in high flows is energetically costly.Thus, structural complexity provides fishes with refuges against both water flow and predators [5].

Diet
Diet determines fish impact on ecosystem functioning through trophic interactions with other food resources, contributing to nutrient cycles [6].
Nutrient acquisition depends on diet composition, i.e., which resources are consumed, and the nutrient content of these resources [8].
The top trophic position fish on coral reefs typically have diverse fish prey drawn from all trophic levels, including the bottom of reef pyramid [28].Likewise, numerous smallbodied, coral associated planktivorous fishes perform the transfer of pelagic nutrients to the benthic community [29].
Diet mediates habitat requirements of fishes because some resources are restricted to specific habitats [30].
For instance, some epilithic algal feeding pomacentrids also show a preference for habitats with skeletons of branching corals, whilst invertivores tend to be habitat generalists and are therefore expected to be less susceptible to habitat disturbance [10].
Reefs with greater structural complexity support longer food chains with more predator-dominated fish communities compared to flatter reefs with low coral cover [13,14].
Therefore, responses of fishes to coral loss and habitat degradation do vary with trophic roles, often showing a negative relationship, especially with coral-dependent species [12].
to feed at night.For example, nocturnally active Haemulidae and Lutjanidae migrate from sheltered sites during the day to seagrass beds at night[20,21].Therefore, day and night activity patterns provide an ecologically meaningful unit for scaling the environment in habitatuse studies and are important in defining suitable seascapes for some fish species that move across spatially heterogenous patches.