The ecology of reef fishes explains latitudinal gradients of diversity, but how much?

Stochasticity is largely understood as ‘unpredictability’; but for reef fishes, demographic stochasticity is contingent on species ecological traits, including body size and trophic identity, which may subsequently be selected by humans.

Above: Reef fish assemblages censused in the remote, understudied, Principe Island (Gulf of Guinea) in the Tropical Eastern Atlantic (photo by @Aketza Herrero).

As every ecologist, I have been fascinated by the array of different organisms I used to encounter from an early age whilst wandering the rocky tidal pools of the oceanic archipelago I was luckily born in (the Canary Islands, Spain). However, it was not until several years later when I started to ask deeper questions on the ‘why’ of this pattern: e.g. why were fewer species farther away from the regular influx of the tide and the splash of the oceanic Atlantic swells? And, why did species living there seem to closely resemble one another in some attributes compared to the variety of different forms living further down?

Editors’ choice: (Free to read online for a year.)
Bosch, N. E., Wernberg, T., Langlois, T. J., Smale, D. A., Moore, P. J., Franco, J. N., Thiriet, P., Feunteun, E., Ribeiro, C., Neves, P., Freitas, R., Filbee-Dexter, K., Norderhaug, K. M., Garcıa, A., Otero-Ferrer, F., Espino, F., Haroun, R., Lazzari, N., & Tuya, F. (2021). Niche and neutral assembly mechanisms contribute to latitudinal diversity gradients in reef fishes. Journal of Biogeography, 48, 2683–2698. 

Considering a much larger spatial scale, these same questions have intrigued ecologists for centuries, aimed at untangling the ecological mechanisms that have contributed to the origination and mantainance of arguably the most universal pattern on Earth – the latitudinal gradient of species diversity (LDG). This is the focus of our recent paper in the Journal of Biogeography (see above), an idea that saw birth at an international symposium. Together with some brilliant colleagues, we came to the realisation that the role of species ecological differences in explaining LDG of reef fishes had mostly been based on high diversity ocean basins (Indo-Pacific Ocean). The few existing global data synthesis that incorporated information, not only on the number of species, but also on their relative abundances, a key property mediating how species coexist in local communities, presented large gaps across impoverished eastern Atlantic regions (e.g. the Tropical eastern Atlantic).

School of gadids hovering over frondose kelp forests ecosystems in the cool-waters of the North Sea (Photo by @Kjell Magnus Norderhaug).

Understanding the role of ecological processes in determining spatial patterns of biodiversity, and whether these can be generalized across geographies with markedly different evolutionary histories, forms the basis to predict and adapt to rapidly changing environments as we ‘submerge’ deeply into the Anthropocene. By quantifying niche differences – via species evolutionary histories and ecological strategies – we lend support to the role of temperature and resources availability on setting constraints on the number of coexisting species. Over human timescales, these boundaries set by the environment have major implications for ecosystem functioning, as warming oceans open up ecological opportunities for fishes expanding into new uncharted regions.

Changes in ocean climate, however, is just one part of the story, as these are often accompanied by profound alterations to the biotic environment. Loss of habitat structure and replacement of foundation species can also set important niche constraints on reef fishes, but these often remain ‘out of sight’. Limited funding of research groups challenges comprehensive sampling of marine biodiversity across the spatial and temporal scales that are relevant to the changes we are seeing in the global oceans. An important lesson from this study is the value of expanding scientific diving programs based on the robust training of citizen scientists for the systematic collection of marine biodiversity data in this ocean basin (both mobile and sedentary taxa, e.g. Reef Life Survey Program). Lessons from other regions (e.g. Australia) provide an excellent example on their value to inform management and conservation of changing marine ecosystems (Edgar et al. 2020).

Small-bodied reef planktivore, the Azores chromis (Chromis limbata) at the Webbnesia archipelago (Photo by @Jose Miguel Bosch Benitez).

Science is a cumulative learning process, and we take value on alternative explanations that can arise during the peer review process. This was exemplified in an early version of our manuscript, where we attributed the latitudinal peak of diversity at ~15-20°N solely to higher niche specialization among reef fishes. A closer examination of the pattern revealed a remarkable resemblance between the latitudinal peak of diversity and the variability in the number of individuals encountered across reefs (a process termed demographic stochasticity) for small-bodied planktivorous fishes. As scientific divers, we are often amazed by the abundance of these fishes, which can in some reefs be so high they ‘cloud’ our view. Across global coral reefs, the remarkable diversity of this group of fishes has recently been shown to disproportionately contribute to the bullseye pattern of fish diversity (centered around the Indo-Australian Archipelago). Our results suggest that not only niche partitioning enables the coexistence of species within this trophic guild, but also the highly temporally and spatially variable planktonic production found around oceanic islands, likely enhancing the coexistence of these functionally similar species at regional scales (what is referred as metacommunity dynamics).

Stochasticity is largely understood as ‘unpredictability’. However, we showed that, for reef fishes, demographic stochasticity is contingent on species ecological traits – here body size and trophic identity. Human activities can selectively alter these important traits (e.g. by removing large-bodied individuals; Bosch et al. 2021), with largely unknown consequences for community dynamics. Given the increasing spatial footprint of fishing and habitat loss, understanding the interactions between deterministic and stochastic factors driving community structure is key to forecast and adapt to the future configuration of reefs in the ‘Anthropocene’.

Written by:
Nestor E. Bosch, PhD Candidate, The University of Western Australia

Additional information:
@bosch_nestor (Twitter)
@Nestor Echedey Bosch Guerra (Facebook)
@basanbri (Instagram)

– Bosch, N. E., Monk, J., Goetze, J., Wilson, S., Babcock, R. C., Barrett, N., … & Langlois, T. J. (2021). Effects of human footprint and biophysical factors on the body‐size structure of fished marine species. Conservation Biology.
– Edgar, G. J., Cooper, A., Baker, S. C., Barker, W., Barrett, N. S., Becerro, M. A., … & Stuart-Smith, R. D. (2020). Reef life survey: establishing the ecological basis for conservation of shallow marine life. Biological Conservation252, 108855.

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