Population ecology of seabirds in Mexican Islands at the California Current System

The Baja California Pacific Islands (BCPI) is a seabird hotspot in the southern California Current System supporting 129 seabird breeding populations of 23 species and over one million birds annually. These islands had a history of environmental degradation because of invasive alien species, human disturbance, and contaminants that caused the extirpation of 27 seabird populations. Most of the invasive mammals have been eradicated and breeding colonies have been restored with social attraction techniques. We have systematic information for most of the breeding populations since 2008. To assess population trends, we analyzed data and present results for 19 seabird species on ten island groups. The maximum number of breeding pairs for each nesting season was used to estimate the population growth rate (λ) for each species at every island colony. We performed a nonparametric bootstrapping to assess whether seabird breeding populations are increasing or decreasing. San Benito, Natividad, and San Jerónimo are the top three islands in terms of abundance of breeding pairs. The most widespread species is Cassin’s Auklet with 14 colonies. Twenty-three populations of 13 species are significantly increasing while eight populations of six species are decreasing. We did not find statistical significance for 30 populations, however, 20 have λ>1 which suggest they are growing. Seven of the 18 species for which we estimated a regional population trend are significantly increasing, including three surface-nesting species: Brown Pelican, Elegant Tern and Laysan Albatross, and four burrow-nesting species: Ainley’s and Ashy Storm-Petrels, and Craveri’s and Guadalupe Murrelet. Our results suggest that the BCPI support healthy and growing populations of seabirds that have shown to be resilient to extreme environmental conditions such as the “Blob”, and that such resilience has been strengthen from conservation and restoration actions such as the eradication of invasive mammals and social attraction techniques.


174
We used the maximum number of breeding pairs for each nesting season to estimate the population 175 growth rate (λ, lamda; S2 Table). For surface nesting-species (i.e., cormorants, pelicans, terns, and  Spatial diversity patterns of seabirds 195 We analyzed spatial diversity patterns of seabirds on the BCPI through diversity indices (Koleff 196   (1) (2)  We estimated the annual population growth rates (λ) for each species nesting on each island or 213 archipelago using the relationship described in equation (4): (4) 215 0 is the number of breeding pairs at the time when the first count of the period was made, λ is the 216 annual population growth rate and is the time interval between the first and last counts of the 217 period. This was done for 19 seabird species (S2 Table). With results from equation (4) we 218 calculated the percent of change in the population using equation (5): We calculated a regional population growth rate (λR) and its percent of change using equations (4)

232
Nonparametric bootstrapping makes no assumptions on data distribution and sampling is done 233 with replacement (Berrar 2019). We generated a bootstrap set * by randomly sampling 234 instances with replacement from our data set of maximum number of breeding pairs (S2 Table), 235 , where is the total number of records (i.e., monitored breedings seasons; e.g., n= 5 for This is the Tukey fences criterion, where 1 is the first quartile, 3 the third quartile, = 3 − 242 1 is the interquartile range, and is a positive constant that defines the outlier tolerance level.

243
Here we used = 1.5 as is customary. After this, we calculated the 95% bootstrap percentile

Status of the seabird breeding populations 249
The BCPI current seabird assemblage is 23 species and a total of 129 breeding colonies on 17 250 islands.    with two colonies each (S1 Appendix Fig A). This is not surprising since the three tern species and breeding taxa, respectively, than two decades ago (S3 Table). These recent breeding records have  Table).

290
The island with the lowest number of breeding species is Guadalupe (5 species), despite being the 291 biggest (24,171 ha) although the most oceanic island (S1 Appendix Figs B and C). Two species 292 are extirpated from this island due to predation by feral cats: Black-vented Shearwater and Cassin's Auklet, which are restricted to the nearby cat-free Zapato and Morro Prieto islets (S3 Table). There  The seabird species' richness in the BCPI increases with island area (e.g., San Benito and San

312
With respect to beta diversity, the islands that are most similar are Natividad-Asunción, Coronado-

313
Todos Santos, and Guadalupe-Morro Prieto/Zapato (S1 Appendix Fig D and E). This means that 314 these islands share many species and thus have similar species composition, possibly because of 315 their proximity. Seabird population trends 321 We were able to assess the population growth rates of 61 colonies of 19 species of the BCPI.  trend but rather that we need to continue monitoring them for subsequent years to be able to find  Table). We 332 also found that all taxonomic groups evaluated except that of boobies and cormorants (Suliformes) 333 are growing, and that populations of murrelets, auklets, gulls, and terns (Charadriiformes) show 334 the fastest growth (Fig 3). The median growth rate for all seabird populations in the BCPI was 335 λ=1.08.      Brandt's Cormorant, respectively, in 2015, between the "Blob" and "Godzilla" marine heatwaves.

504
Nonetheless, despite these severe environmental conditions in recent years, the seabird breeding 505 abundances and population growth rates that we report here indicate that the BCPI seabird