Data from: Resource partitioning facilitates coexistence in sympatric cetaceans in the California Current

1. Resource partitioning is an important process driving habitat use and
foraging strategies in sympatric species that potentially compete.
Differences in foraging behavior are hypothesized to contribute to species
coexistence by facilitating resource partitioning, but little is known on
the multiple mechanisms for partitioning that may occur simultaneously.
Studies are further limited in the marine environment, where the spatial
and temporal distribution of resources is highly dynamic and subsequently
difficult to quantify. 2. We investigated potential pathways that foraging
behavior may facilitate resource partitioning in two of the largest
co-occurring and closely related species on Earth, blue (Balaenoptera
musculus) and humpback (Megaptera novaeangliae) whales. 3. We integrated
multiple long-term datasets (line-transect surveys, whale-watching
records, net sampling, stable isotope analysis, and remote-sensing of
oceanographic parameters) to compare the diet, phenology, and distribution
of the two species during their foraging periods in the highly productive
waters of Monterey Bay, California, USA within the California Current
Ecosystem. 4. Our long-term study reveals that blue and humpback whales
likely facilitate sympatry by partitioning their foraging along three
axes: trophic, temporal, and spatial. Blue whales were specialists
foraging on krill, predictably targeting a seasonal peak in krill
abundance, were present in the bay for an average of 4.7 months, and were
spatially restricted at the continental shelf break. In contrast, humpback
whales were generalists apparently feeding on a mixed diet of krill and
fishes depending on relative abundances, were present in the bay for a
more extended period (average of 6.6 months), and had a broader spatial
distribution at the shelf break and inshore. 5. Ultimately, competition
for common resources can lead to behavioral, morphological, and
physiological character displacement between sympatric species.
Understanding the mechanisms for species coexistence is both fundamental
to maintaining biodiverse ecosystems, and provides insight into the
evolutionary drivers of morphological differences in closely related
species.