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Intertidal estimates of sea urchin abundance reveal congruence in spatial structure for a guild of consumers.
Ma KCK
,
Redelinghuys S
,
Gusha MNC
,
Dyantyi SB
,
McQuaid CD
,
Porri F
.
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We hypothesized congruence in the spatial structure of abundance data sampled across multiple scales for an ecological guild of consumers that exploit similar nutritional and habitat resources. We tested this hypothesis on the spatial organization of abundance of an herbivorous guild of sea urchins. We also examined whether the amount of local along-shore rocky habitat can explain the observed spatial patterns of abundance. Standardized estimates of abundance of four intertidal sea urchins-Diadema cf. savignyi, Echinometra mathaei, Parechinus angulosus, and Stomopneustes variolaris-were determined by six observers at 105 sites across 2,850 km of coast of South Africa. For each species and observer, wavelet analysis was used on abundance estimates, after controlling for potential biases, to examine their spatial structure. The relationship between local sea urchin abundance and the amount of upstream and downstream rocky habitat, as defined by the prevailing ocean current, was also investigated. All species exhibited robust structure at scales of 75-220 km, despite variability among observers. Less robust structure in the abundances of three species was detected at larger scales of 430-898 km. Abundance estimates of sympatric populations of two species (D. cf. savignyi and E. mathaei) were positively correlated with the amount of rocky habitat upstream of the site, suggesting that upstream populations act as larval sources across a wide range of scales. No relationship between abundance and habitat size was found for P. angulosus or S. variolaris. Within the range of scales examined, we found robust congruence in spatial structure in abundance at the lower, but not the larger, range of scales for all four species. The relationship between abundance and upstream habitat availability in two species suggests that larval supply from upstream populations was probably the mechanism linking habitat size and abundance.
FIGURE 1. Locations of surveyed sites (n = 105 sites after removing sites with potential observer biases; solid black points) and the South African distribution of four intertidal sea urchin species in 2019–2020: Diadema cf. savignyi (dark blue line), Echinometra mathaei (dark brown line), Parechinus angulosus (light brown line), and Stomopneustes variolaris (light blue line)
FIGURE 2. Separate abundance estimates of intertidal sea urchins across coastal South Africa for each observer: (a) Diadema cf. savignyi (n = 4 observers), (b) Echinometra mathaei (n = 4), (c) Parechinus angulosus (n = 6), and (d) Stomopneustes variolaris (n = 4); shore distance of 0 km starts at the mouth of the Orange River at the border of South Africa and Namibia; o.d. = outside of distribution, where the species was not detected by all observers; note that y‐axis ranges are different for each panel
FIGURE 3. Separate average (global) wavelet power spectra of mean abundance of intertidal sea urchins across coastal South Africa for each observer: (a) Diadema cf. savignyi (n = 4 observers), (b) Echinometra mathaei (n = 4), (c) Parechinus angulosus (n = 6), and (d) Stomopneustes variolaris (n = 4); NB: y‐axes are plotted on a logarithm scale (base 2)
FIGURE 4. Relationship between the amount of local along‐shore rocky habitat, separated by rocky habitat downstream and upstream (with respect to the prevailing oceanic current) of the sampling site, and abundance estimates of four intertidal sea urchin species within the distributional range of each species: (a, b) Diadema cf. savignyi (n = 4 observers), (c, d) Echinometra mathaei (n = 4), (e, f) Parechinus angulosus (n = 6), and (g, h) Stomopneustes variolaris (n = 4); spatial window used to calculate the amount of rocky habitat ranged from 5 to 140 km at 5‐km intervals for each direction (downstream and upstream); horizontal dashed lines indicate where Spearman's rho = 0
FIGURE A1. Observed variograms of log10 (x + 1) transformed abundance estimates of Diadema cf. savignyi across coastal South Africa based on data from (a) observer #1, (b) observer #2, (c) observer #3, and (d) observer #4; no data collected for this species from observers #5 and #6; note that y‐axis ranges are different for each panel
FIGURE A2. Observed variograms of log10 (x + 1) transformed abundance estimates of Echinometra mathaei across coastal South Africa based on data from (a) observer #1, (b) observer #2, (c) observer #3, and (d) observer #4; no data collected for this species from observers #5 and #6; note that y‐axis ranges are different for each panel
FIGURE A3. Observed variograms of log10 (x + 1) transformed abundance estimates of Parechinus angulosus across coastal South Africa based on data from (a) observer #1, (b) observer #2, (c) observer #3, (d) observer #4, (e) observer #5, and (f) observer #6; note that y‐axis ranges are different for each panel
FIGURE A4. Observed variograms of log10 (x + 1) transformed abundance estimates of Stomopneustes variolaris across coastal South Africa based on data from (a) observer #1, (b) observer #2, (c) observer #3, and (d) observer #4; no data collected for this species from observers #5 and #6; note that y‐axis ranges are different for each panel
FIGURE A5. Distribution of rocky habitat along the coast of South Africa; moving average of the amount of habitat per spatial window of (a) 25 km, (b) 50 km, (c) 75 km, and (d) 100 km; shore distance of 0 km starts at the mouth of the Orange River at the border of South Africa and Namibia
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