Predicting time-at-depth weighted biodiversity patterns for sharks of the North Pacific

Depth is a fundamental and universal driver of ocean biogeography, but it
is unclear how the biodiversity patterns of larger, more mobile organisms
change as a function of depth. Here, we developed a predictive
biogeography model to explore how information of mobile species’ depth
preferences influence biodiversity patterns. We employed a literature
review to collate shark biotelemetry studies and used open-access tools to
extract 283 total records from 119 studies of 1,133 sharks from 35
species. We then matched field guide reported depth ranges and IUCN
habitat associations for each shark species to use as covariates in a
hurdle variant of Ensemble Random Forests. We successfully fit this model
(R2 = 0.63) to the noisy time-at-depth observations and used it to predict
the time budgets of the Northeast Pacific shark regional pool (n = 52). We
then assessed how occurrence diversity patterns, informed by minimum and
maximum depth of occurrence, compared to time-at-depth weighted diversity
patterns. Time-at-depth weighted richness was highest between 0 and 25 m
and at the upper part of the mesopelagic zone, 250 – 300 m; resulting in
little similarity to common depth or elevational biodiversity patterns
while the occurrence-weighted richness pattern was similar to the
“low-plateau” pattern. In the phylogenetic and functional dimensions of
biodiversity and over three different distance metrics, we found strong
but haphazard differences between the occurrence- and time-at-depth
weighted biodiversity patterns. The strong influence of time budgets on
biodiversity led us to conclude that occurrence data alone is likely
insufficient or even misleading in terms of the depth-driven biogeographic
patterns in the open ocean. Utilizing the increasing amount of
time-at-depth information from biotelemetry studies in predictive
biogeographic models may be critical for capturing the preferences of
pelagic, mobile species occupying the largest biome on the planet.