Historical biogeography supports Point Conception as the site of turnover between temperate East Pacific ichthyofaunas

The cold temperate and subtropical marine faunas of the Northeastern Pacific meet within California as part of one of the few eastern boundary upwelling ecosystems in the world. Traditionally, it is believed that Point Conception is the precise site of turnover between these two faunas due to sharp changes in oceanographic conditions. However, evidence from intraspecific phylogeography and species range terminals do not support this view, finding stronger biogeographic breaks elsewhere along the coast. Here I develop a new application of historical biogeographic approaches to uncover sites of transition between faunas without needing an a priori hypothesis of where these occur. I used this approach to determine whether the point of transition between northern and southern temperate faunas occurs at Point Conception or elsewhere within California. I also examined expert-vetted latitudinal range data of California fish species from the 1970s and the 2020s to assess how biogeography could change with the backdrop of climate change. The site of turnover was found to occur near Point Conception, in concordance with the traditional view. I suggest that recent species- and population-level processes could be expected to give signals of different events from historical biogeography, possibly explaining the discrepancy across studies. Species richness of California has increased since the 1970s, mostly due to species’s ranges expanding northward from Baja California (Mexico). Range shifts under warming conditions seem to be increasing the disparity between northern and southern faunas of California, creating a more divergent biogeography.

Historical biogeography supports Point Conception as the site of turnover between temperate East Pacific ichthyofaunas   S1: Range shifts of nearshore California fishes since the 1970s Table S2: New species of nearshore fishes in California waters since the 1970s Table S3: Species with unclear range information Table S4: Region-of-origin inferred for California ray-finned fish species Table S5: Region-of-origin inferred for California cartilaginous fish species

Note S1: Post-hoc analyses of range shifts given the null expectation
In this study I report that most new species in California waters since the 1970s dispersed from Baja California, Mexico (northward dispersal, n=69 species) as opposed to dispersal from Oregon (southward dispersal, n=15 species; Table S2) (excluding new species added from recent species descriptions).This is important because dispersals northward are consistent with an effect of anthropogenic climate change on range shifts.However, this is complicated by the fact that species richness is higher towards the equator (i.e. the latitudinal diversity gradient).Since there are more species in Baja California than in Oregon, the pool of species that can potentially disperse into California is also greater.Therefore, we would expect more species to disperse northward under a null model if dispersal was random.
I simulated potential dispersals to test the hypothesis that the number of northward dispersals since the 1970s was even greater than expected.I determined the pool of species that could have potentially dispersed from Oregon to California since 1978 to be 293 species.This is the number of species known to be restricted to North of California today, plus the number reported to have dispersed to California in this study (these species would have been restricted to North of California in 1978).The same logic was applied to determine the pool of species that could have potentially dispersed from Baja California to California to be 728 species in 1978.I performed trials where I randomly sampled 84 species (the observed number) from the total pool of potential dispersers to disperse to California.I then calculated a ratio of dispersers from the southern pool to dispersers from the northern pool.I performed 100,000 trials to determine a null distribution of ratios.
The median ratio in the simulated distribution is 2.5 dispersal events from Mexico for every 1 dispersal from Oregon.The observed ratio of 4.6 (69 divided by 15 species) is outside of the 95% quantile of the simulated distribution (Figure S2).These results suggest the observed number of species entering California from Mexico is significantly greater than expected at random (given the latitudinal diversity gradient), supporting the assertion that recent dispersals are influenced by anthropogenic climate warming.

Figure S2
. Simulated ratios of species dispersing into California from Baja California (Mexico) versus species dispersing from Oregon (USA).The observed ratio is shown by the red dashed line.
Table S1.List of range changes among inshore marine species between 1978 dataset (Horn and Allen 1978)

Figure S1 :
Figure S1: Biogeographic affinities of offshore deep-sea fishes by latitudinal band

Figure S1 :
Figure S1: Turnover of northern-and-southern origin species with latitudinal band in California waters.This figure shows results for deep-sea species only; see Tables S4-S5 for a list of species included.All range data for these species were based on modern sources (Love and Passarelli 2020; Love et al. 2021).

Table S2 .
and this study.List of new inshore marine species added to California waters since 1978 dataset (Horn and Allen 1978).Ref. [1] = Love and Passarelli 2020.Ref. [2] = Love et al. 2021.Ref. [3] = Horn et al. 2006.Species with a (*) were newly described or revised since the 1978 study, so their addition cannot be attributed to dispersal (see Love and Passarelli 2020 and references therein).

Table S4 :
Region-of-origin of California ray-finned fishes as inferred from BioGeoBEARS analyses.Blue rows= southern origin species; orange rows= northern origin species; unshaded rows= species with unclear origin.

Table S5 :
Region-of-origin for California cartilaginous fishes as inferred from BioGeoBEARS analyses.Blue rows= southern origin species; orange rows= northern origin species; unshaded rows= species with unclear origin.