Abstract
Southern bull kelps (Durvillaea spp., Fucales) are ‘primary’ foundation species that control community structures and ecosystem functions on temperate wave-exposed rocky reefs. However, these large foundation species are threatened by disturbances and stressors, including invasive species, sedimentation and heatwaves. It is unknown whether ‘alternative’ foundation species can replace lost southern bull kelps and its associated communities and networks. We compared community structure (by quantifying abundances of different species) and attachment-interaction networks (by quantifying which species were attached to other species) among plots dominated by Durvillaea spp. and plots where Durvillaea spp. were lost either through long-term repeated experimental removals or by recent stress from a marine heatwave. Long-term experimental removal plots were dominated by ‘alternative’ foundation species, the canopy-forming Cystophora spp. (Fucales), whereas the recent heatwave stressed plots were dominated by the invasive kelp Undaria pinnatifida (Laminariales). A network analysis of attachment interactions showed that communities differed among plots dominated by either Durvillaea spp., Cystophora spp. or U. pinnatifida, with different relationships between the primary, or alternative, foundation species and attached epiphytic ‘secondary’ foundation species. For example, native Cystophora spp. were more important as hosts for secondary foundation species compared to Durvillaea spp. and U. pinnatifida. Instead, Durvillaea spp. facilitated encrusting algae, which in turn provided habitat for gastropods. We conclude that (a) repeated disturbances and strong stressors can reveal ecological differences between primary and alternative foundation species, (b) analyses of abundances and attachment-networks are supplementary methods to identify linkages between primary, alternative and secondary foundation species, and (c) interspersed habitats dominated by different types of foundation species increase system-level biodiversity by supporting different species-abundance patterns and species-attachment networks.
Highlights
Total degrees were significantly greater in plots dominated by Cystophora spp. compared to plots dominated by Durvillaea spp. or U. pinnatifida (Figure 1F, F2,29 = 5.145, p = 0.013)
Cystophora compared to plots dominated by Durvillaea spp.encompassed or U. pinnatifida the three habitat types (Figure 4D) highlighted in a single graph (a) the central nodes around B. hirsuta, coralline turf and encrusting corallines, (b) the absence of Durvillaea spp. from attachment interactions, and (c) the many links to mobile species that included snails, limpets, and chitons
When southern physically removed over six years, or recently lost following a marine heatwave, communities changed bull kelps were physically removed over six years, or recently lost following a marine heatwave, dramatically resulting in replacement by other large canopy-forming macroalgae with different associated epiphytes and understory species, macroalgae with different associated epiphytes
Summary
Ecologists have traditionally studied the ecological importance of foundation species by combining species abundance data with species removal or addition-experiments to quantify per capita effects [12,13,14,15,16,17]. These classical methods can be supplemented with network analyses to identify how foundation species may modify networks of different types of ecological interactions [18,19]. Similar host-epiphyte interactions are prevalent in marine ecosystems [9,13,14,35], but we are not aware of studies that have analysed these attachment-interactions with community-wide network metrics
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