1 °C warming increases spatial competition frequency and complexity in Antarctic marine macrofauna

Abstract

Environmental conditions of the Southern Ocean around Antarctica have varied little for >5 million years but are now changing. Here, we investigated how warming affects competition for space. Little considered in the polar regions, this is a critical component of biodiversity response. Change in competition in response to environment forcing might be detectable earlier than individual species presence/absence or performance measures (e.g. growth). Examination of fauna on artificial substrata in Antarctica's shallows at ambient or warmed temperature found that, mid-century predicted 1°C warming (throughout the year or just summer-only), increased the probability of individuals encountering spatial competition, as well as density and complexity of such interactions. 2°C, late century predicted warming, increased variance in the probability and density of competition, but overall, competition did not significantly differ from ambient (control) levels. In summary only 1°C warming increased probability, density and complexity of spatial competition, which seems to be summer-only driven.

Fig. 1. Location of deployment site in Antarctica.

Study area. Red dot within this is study site adjacent to Rothera Research station.

Fig. 2. Spatial competition on artificial substrata at 15 m depth in Antarctica.

Colonists of panels at ambient (A) and +1 °C (B) treatments (9.8  ×9.8 cm), together with image analysis frames used to highlight less abundant species (right images). Rarer species are shown in different colours. Probability of individuals encountering competition (C), competition density (D) and complexity (E). Letters above plots C–E show significant differences between treatments. Degrees of freedom (DF) = 4, n = 20 for C–E; (C) Welch’s one-way F = 28.326 p = 0.0001, Games–Howell post hoc tests p < 0.01; (D) Welch’s one-way F = 6.5527 p = 0.017, Games–Howell post hoc tests p < 0.1; (E) ANOVA F = 5.467 p = 0.006, Tukey HSD p < 0.1. Key to treatment symbols (F), + 1: warmed to +1 °C; + 2: warmed to +2 °C; controls: no warming; previous: deployed previously without warming; summer: warmer to +1 °C in summer (Sep–Mar) only.

Fig. 3. Composition, richness and competition in Antarctic encrusting assemblages with differing warming treatments.

Similarity of assemblage composition (A, ANOSIM R = −0.041, p = 0.632, 1000 permutations) and similarity of species involved in competitive encounters by treatment (B; ANOSIM R = 0.4, p = 0.002, 1000 permutations) presented using non-metric multidimensional scaling (key to treatment symbols Fig. 2F). Species richness (C) and counts of colonies of rare species (D). Probability of individuals encountering intraspecific competition (E) and probability of species encountering competition with spatially dominant F. rugula (F). Letters above plot E indicate significant differences between treatments. Degrees of freedom (DF) = 4, n = 20 for C–F; (C) ANOVA F = 2.659 p = 0.073; (D) ANOVA F = 2.632, p = 0.076; (E) Welch’s one-way F = 5.6086 p = 0.022, Games–Howell post hoc tests p < 0.1; (F) Welch’s one-way F = 2.6113, p = 0.13. Treatments did not significantly influence species richness (C), rare species (D), or probability of competition with the key species Fenestrulina rugula (F). Key to treatment symbols (Fig. 2F).

Fig. 4. Example panel area showing spatial competition by bryozoans.

Competitive encounters were each marked with a number as they were counted to ensure none were missed or counted twice. The left image is zoomed into one quarter of a panel (image size 4.9 × 4.9 cm) and the right one further zoomed into one-sixteenth of a panel.