Fig 1.
Current ranges of giant kelp and purple urchin across the Eastern Pacific Ocean with bounded study area containing 500 km buffer from the coast (N 19.5277594–61.064468, W 173.391988–106.863603). Section delineation lines mark where range was split into northern, central, and southern sections. Dotted lines indicated marginal habitat where there are known occurrences but non-continuous populations.
Fig 2.
Urchin southern range occupancy.
Urchins occupied less of the southern range edge under all climate change scenarios by 2100 than at current day (Scenario × Section Tukey p-values = 0.04, 1.2E-5, and 0.005 for each RCP respectively; S2 Table in S1 File). See supplement (S3 Fig in S1 File) for mid-century predictions.
Fig 3.
Purple urchin distribution maps.
Purple urchin is modeled with giant kelp at RCP 8.5 for end of century; (a) shows the current day projection; (b) shows the RCP 8.5 scenario for 2100. Pullouts focus on the far southern edge of the species range. Visual reductions in geographic spread are seen at the farthest southern edge by end of century.
Fig 4.
Range occupancy is shown for both (a) kelp and (b) urchins under current day and climate change scenarios in the northern range edge. Kelp and urchins demonstrated potential for a northern range expansion under all future climate scenarios by end of century (Section × Scenario Tukey p-values for kelp were 6.1E-8, 0.0013, and 5.1E-8 for each RCP respectively [S3 Table in S1 File]; Section × Scenario Tukey p-values for urchins were 8.3E-9, 2E-8, and 9.8E-8 for each RCP respectively [S2 Table in S1 File]). See supplement (S4 Fig in S1 File) for mid-century urchin predictions.
Fig 5.
Giant kelp is modeled at RCP 8.5 for end of century; (a) shows the current day projection; (b) the RCP 8.5 scenario for 2100. Pullouts focus on the far northern edge of the species range. Visual increases in geographic spread are seen at the farthest northern edge by end of century.
Fig 6.
Purple urchin distribution maps.
Purple urchin is modeled with giant kelp at RCP 8.5 for end of century; (a) shows the current day projection; (b) shows the RCP 8.5 scenario for 2100. Pullouts focus on the far northern edge of the species range. Visual increases in geographic spread are seen at the farthest northern edge by end of century.
Fig 7.
Conceptual southern food webs.
Conceptual diagrams of simplified food webs for the southern range edge of purple urchins and giant kelp, demonstrating both the modeled contraction of purple urchins and hypothesized community dynamics that may result from this change. (a) Current day food web with the purple urchin represented in the upper right corner of the herbivore box, and the giant kelp represented in the primary producer box. (b) Southern range food web with the modeled removal of purple urchins and the addition of another urchin species filling the abandoned niche shown by the blue box. Interaction strength between groups, as indicated by arrow widths, is expected to remain stable due to the continuity in number of species per trophic group. (c) Southern food web with purple urchin removed. This niche is filled by another urchin species shown in the blue box as in panel (b). In addition, hypothesized tropicalization could result from the movement of other tropical generalist species moving into the range, shown by the black and orange boxes. The addition of these species could increase consumer pressure (thicker arrows) on lower trophic levels due to the increase of consumer species. Please see S5 Table in S1 File for species represented in the diagram. All drawings are original by M. Cortese.
Fig 8.
Conceptual northern food webs.
Conceptual diagrams of simplified food webs for the northern range edge of purple urchins and giant kelp, demonstrating both the modeled expansion of these species into current day marginal habitat, and the hypothesized community dynamics that may result from these changes. (a) Current day food web for the northern range edge where both purple urchins and giant kelp are rare. (b) Northern edge food web with the modeled addition of purple urchin and giant kelp, shown by the new species in the herbivore and primary producer boxes. The addition of these species could increase competition among species in the same trophic level due to resource limitations. The addition of another mid-trophic level herbivore may increase consumer pressure on producers (thicker arrows). (c) Far northern edge food web with the modeled addition of purple urchin and giant kelp as in panel (b). In addition, theorized range shifts of other generalist consumers could allow for concurrent expansion, shown by the addition of species highlighted with black, blue, and orange inner boxes. The movement of both mid and high trophic level consumers may lead to food web stability, reducing the herbivore pressure shown in panel (b) by increasing higher level consumer pressure. Please see S5 Table in S1 File for species represented in the diagram. All drawings are original by M. Cortese.