Sympatry or syntopy? Investigating drivers of distribution and co-occurrence for two imperiled sea turtle species in Gulf of Mexico neritic waters

Abstract

Animals co-occurring in a region (sympatry) may use the same habitat (syntopy) within that region. A central aim in ecology is determining what factors drive species distributions (i.e., abiotic conditions, dispersal limitations, and/or biotic interactions). Assessing the degree of biotic interactions can be difficult for species with wide ranges at sea. This study investigated the spatial ecology of two sea turtle species that forage on benthic invertebrates in neritic GoM waters: Kemp's ridleys (Lepidochelys kempii) and loggerheads (Caretta caretta). We used satellite tracking and modeled behavioral modes, then calculated individual home ranges, compared foraging areas, and determined extent of co-occurrence. Using six environmental variables and principal component analysis, we assessed similarity of chosen foraging sites. We predicted foraging location (eco-region) based on species, nesting site, and turtle size. For 127 turtles (64 Kemp's ridleys, 63 loggerheads) tracked from 1989 to 2013, foraging home ranges were nine to ten times larger for Kemp's ridleys than for loggerheads. Species intersected off all U.S. coasts and the Yucatán Peninsula, but co-occurrence areas were small compared to species' distributions. Kemp's ridley foraging home ranges were concentrated in the northern GoM, whereas those for loggerheads were concentrated in the eastern GoM. The two species were different in all habitat variables compared (latitude, longitude, distance to shore, net primary production, mean sea surface temperature, and bathymetry). Nesting site was the single dominant variable that dictated foraging ecoregion. Although Kemp's ridleys and loggerheads may compete for resources, the separation in foraging areas, significant differences in environmental conditions, and importance of nesting location on ecoregion selection (i.e., dispersal ability) indicate that adult females of these species do not interact greatly during foraging and that dispersal and environmental factors more strongly determine their distributions. These species show sympatry in this region but evidence for syntopy was rare.

Keywords: Caretta caretta; Gulf of Mexico; Lepidochelys kempii; foraging; principal component analysis; satellite tracking; sea turtle; switching state‐space modeling.

Figure 1

Satellite‐tagged loggerhead (Caretta caretta) female heading back to the water after nesting (left panel) and a female Kemp's ridley (Lepidochelys kempii) during nesting (right panel). Images taken with permission (MTP 176 issued to KH and USFWS permit TE840727‐3 issued to DS) under conditions not harmful to turtles

Figure 2

Tagging locations in the Gulf of Mexico (yellow stars) for Kemp's ridley (Lepidochelys kempii) and loggerhead (Caretta caretta) sea turtles. Blue lines represent 100 m bathymetry contour intervals; darker blue line is −200 m

Figure 3

Kernel density estimates (KDEs) for 39 Kemp's ridley (Lepidochelys kempii; 58 KDEs) and 49 loggerheads (Caretta caretta; 51 KDEs color‐coded based on subpopulation/tagging location) sea turtles tagged in the Gulf of Mexico. 50% KDEs represent core‐use areas and 95% KDEs represent home ranges. Red areas highlight where home ranges co‐occur for the two species. DRTO: Dry Tortugas National Park

Figure 4

The number of individual turtles with home ranges (95% Kernel Density Estimates [KDEs]) in 10 km grid cells throughout the Gulf of Mexico. Both Kemp's ridley (Lepidochelys kempii) and loggerhead (Caretta caretta) KDEs are included: If a turtle had more than one KDE, these were merged so that every turtle was only counted once throughout all grid cells

Figure 5

The number of subpopulations with intersecting home ranges (95% Kernel Density Estimates) in each 10 km grid cell throughout the Gulf of Mexico: Turtles from the same subpopulation were combined and could include loggerheads (Caretta caretta) tagged in Dry Tortugas National Park, loggerheads tagged in the Northern Gulf, or Kemp's ridleys (Lepidochelys kempii), for a possible maximum of three subpopulations represented per grid cell. However, we did not find all three in any grid cell. Therefore, dark blue cells indicate where the two loggerhead subpopulations co‐occur, and red indicates where Kemp's ridleys co‐occur with a single loggerhead subpopulation

Figure 6

(a) Box plots of six habitat variables (latitude, longitude, distance to shoreline, daytime mean sea surface temperature, and net primary production) at an identified foraging location (centroid of 50% Kernel Density Estimate [KDE]) of 88 nesting turtles (39 Kemp's ridleys [Lepidochelys kempii] and 49 loggerheads [Caretta caretta]) satellite‐tracked in the Gulf of Mexico. Note other sea surface temperature variables used for comparing the habitat similarity are not shown because they are correlated with the mean SST. Star marks indicate that species difference was significant (* <0.05, ** <0.01). All variables that were significant at an α‐level of 0.01 were also significant after the Bonferroni correction. (b) The scatter plot of first two principal component scores calculated from 11 habitat variables (latitude, longitude, daytime and nighttime mean, minimum, and maximum sea surface temperatures, net primary production, bathymetry, and distance to the shoreline) at an identified foraging location (centroid of 50% KDE) of 88 nesting turtles (39 Kemp's ridleys [black dots] and 49 loggerheads [open circles]) satellite‐tracked in Gulf of Mexico. The polygons indicate 95% confidence ellipses.

Figure 7

Identified final foraging locations (centroid of 50% Kernel Density Estimate [KDE]) of 88 nesting turtles (39 Kemp's ridleys [Lepidochelys kempii] and 49 loggerheads [Caretta caretta]) satellite‐tracked in the Gulf of Mexico. The ecoregions were hand‐digitized based on coastal regions in Wilkinson et al. (2009). Inset: Final model of categorical regression tree to predict foraging ecoregions (NGOM: Northern Gulf of Mexico; SFLBAH: South Florida/Bahamian Atlantic; SGOMCA: Southern Gulf of Mexico/Caribbean Sea) from nesting area (DRTO: Dry Tortugas National Park; GS: Gulf Shores; PAI: Padre Island National Seashore; SJP: St. Joseph Peninsula)