Experimental removal of introduced slider turtles offers new insight into competition with a native, threatened turtle

Abstract

The red-eared slider turtle (Trachemys scripta elegans; RES) is often considered one of the world’s most invasive species. Results from laboratory and mesocosm experiments suggest that introduced RES outcompete native turtles for key ecological resources, but such experiments can overestimate the strength of competition. We report on the first field experiment with a wild turtle community, involving introduced RES and a declining native species of conservation concern, the western pond turtle (Emys marmorata; WPT). Using a before/after experimental design, we show that after removing most of an introduced RES population, the remaining RES dramatically shifted their spatial basking distribution in a manner consistent with strong intraspecific competition. WPT also altered their spatial basking distribution after the RES removal, but in ways inconsistent with strong interspecific competition. However, we documented reduced levels of WPT basking post-removal, which may reflect a behavioral shift attributable to the lower density of the turtle community. WPT body condition also increased after we removed RES, consistent with either indirect or direct competition between WPT and RES and providing the first evidence that RES can compete with a native turtle in the wild. We conclude that the negative impacts on WPT basking by RES in natural contexts are more limited than suggested by experiments with captive turtles, although wild WPT do appear to compete for food with introduced RES. Our results highlight the importance of manipulative field experiments when studying biological invasions, and the potential value of RES removal as a management strategy for WPT.

Plastic ingestion by freshwater turtles: a review and call to action

“…We captured and euthanized a total of 65 Trachemys scripta elegans from the UCD Arboretum, including 29 hatchlings (CL 32–44 mm), 25 juveniles (CL 50–98 mm), 8 adult females (CL 123–180 mm), and 3 adult males (CL 100–160 mm; size classes follow Ernst and Lovich, 2009 58 ). The strong representation of young turtles in our sample is likely attributable to an earlier project that removed most of the adult T. s. elegans from this waterway 32 ; the turtle carcasses from that study were unavailable to us, having been either incinerated or used for teaching dissections. Following recent recommendations for reporting ingested plastics in marine turtles 20 , we report frequency, quantity, and normalized quantity of ingested plastic debris.…”

“…Due to these conservation concerns, lethal sampling of E. marmorata or E. pallida for gastrointestinal tract dissections is inadvisable, and invasive methods like stomach flushing are also potentially problematic—making the proxy method appropriate. Although T. s. elegans and E. marmorata show some spatial and behavioral segregation in the UCD Arboretum 31 , 32 , both species co-occur throughout the waterway and are roughly comparable in body size 58 . Furthermore, both T. s. elegans and E. marmorata are dietary generalists, and both species shift to more herbivorous diets as adults, although this shift is much more pronounced in T. s. elegans 58 .…”

“…We trapped for T. s. elegans from 27 May–2 June and from 24–30 September, 2012. The May/June trapping was part of an unrelated experiment assessing body condition change in E. marmorata following prior T. s. elegans removal efforts 32 . In decreasing order of frequency, we captured turtles using dipnets, submersible traps, hoop nets, by hand, and in a basking trap.…”

Historical museum collections and contemporary population studies implicate roads and introduced predatory bullfrogs in the decline of western pond turtles

“…3). For E. pallida, data sets came from trapping data provided by the USGS, San Diego office (R. (Lambert et al, 2019). We extracted the midline carapace length (a standard index of body size), sex, date of capture, location and population size from each study.…”

Peer Review #2 of "Historical museum collections and contemporary population studies implicate roads and introduced predatory bullfrogs in the decline of western pond turtles (v0.1)"

“…The Nature Conservancy gave permission to sample on the Dangermond Preserve which was also approved by the CDFW (CDFW SC-2480). For E. marmorata, data sets included two sites near Chico, Butte County, CA (T Engstrom, 2019, personal communication), Sacramento County, CA (L Patterson, 2019, personal communication), and the University of California Davis Arboretum, Yolo County, CA (Lambert et al, 2019). We extracted the midline carapace length (a standard index of body size), sex, date of capture, location and population size from each study.…”