Last Thursday marked the final group project week in the Wild Davis class! Following this week, students will be working on their individual projects, culminating in a paper and presentation over a campus ecological interaction they find interesting. Also, I missed posting about our wasp gall day a few weeks ago, so I’m including a latergram of that exercise here, too. Both of these exercises related to published research that has been done on the UC Davis campus, which provided the students with a background of data on the system we were observing.
Impacts of Wasp Gall on Herbivory of Oak Trees
The valley oak trees along the Arboretum sport numerous galls (commonly called ‘oak apples’) created by Andricus quercuscalifornicus, a gall-forming wasp, which lays its eggs in twigs of the oak trees. As the eggs develop into larvae, they elicit the development of galls which house and feed the larvae until they are mature and tunnel out of the gall. While the larvae are developing, the galls are reddish-green and resemble small apples (hence their nickname); however, once the mature wasps leave the gall, it dries out (‘senesces’ in scientific terms) and remains on the tree for several years before eventually falling to the ground and decomposing. Consequently, a given tree may carry hundreds of galls at various stages of development and senescence.
In March of 2013, the EVE 180 class (Experimental Ecology and Evolution in the Field) performed an experiment on valley oak trees in the Putah Creek Riparian Reserve in which they documented arthropod communities prior to and following experimental removal of senesced galls from trees. The team removed over 5,000 galls from 52 different trees and found that gall removal resulted in a 59% increase in density of herbivorous arthropods. This difference was driven by the fact that senesced wasp galls are frequently occupied by Salticid spiders (my favorite! so cute!), which in turn reduce the arthropod community (and alter its composition) through their own predation. The study was later published in the journal Ecology, and is a great example of research opportunities for undergraduates at UC Davis.
Our class studied a different population of valley oaks – those occupying the Valley Oak Grove in the Arboretum. Instead of removing the galls and tracking changes in arthropod density through time, we observed gall density and arthropod density per tree. Students worked in pairs to count and identify arthropod communities on the trees, and to estimate the quantity of herbivore damage to leaves. We were also interested in whether trees near the waterway in the Arboretum contained more galls than those further away (since anecdotally, this appeared to be the case). While we didn’t find any strong correlations between oak galls and arthropod densities, leaf damage, or proximity to the waterway, we did find an impressive diversity of herbivorous and parasitic activity on the trees, and even some evidence of spider presence on senesced galls. One of the things I enjoyed about this session was seeing the quantity of parasitism and herbivore damage that an individual tree can sustain without it being obvious to a casual observer. My personal favorite was a leaf found by Isabelle Gilchrist, which had sustained leaf gall damage that altered the growth of the primary vein, compromising the entire size and shape of the leaf.
Tracking Native and Introduced Turtle Populations in the Arboretum Waterway
Our last week focused on finding, identifying, and documenting the behavior of turtles in the Arboretum waterway. Though a number of species have been found in the waterway over the years, only two are common: the native western pond turtle, Emys marmorata, and the red-eared slider Trachemys scripta elegans, introduced to the waterway via release of unwanted pet turtles. I love turtles, and I spent probably more time than was strictly necessary doing recon (wandering the Arboretum with my long-lens SLR) for this exercise, which means I have a *lot* of adorable turtle photos.
Our goals for the class exercise were to track how many turtles we saw of each species (and what this might mean for their relative population sizes) and whether or not there were differences in the types of basking perches they chose (which might indicate competition between the two species). Turtles present a few particular problems for this kind of project. First, given the clarity of the water in the Arboretum, we can only really observe turtles that are basking, or swimming near the surface. Second, the two species can be difficult to distinguish, especially from afar. Third, basking turtles tend not to…. do much… which can make 3-hour class observations somewhat boring.
I countered these problems by 1) extensive recon (cute turtle photos!!) to identify the areas of the waterway where we are most likely to find turtles basking and swimming, 2) a crash course in turtle ID for the Wild Davis students, and 3) a short-term ethogram (behavioral table) based primarily around choice of basking habitat and “turtle plops,” which represented the ‘disturbability’ of the turtles (when they abandon their basking habitat due to human or other turtle interference).
This exercise would have been impossible without the generous equipment loans of Santiago Ramirez, who let me borrow binoculars without a neck strap (the trust level!), Marcel Rejmanek, who might not even know I have his two pairs of binoculars, and Gail Patricelli, who handed me her THREE THOUSAND DOLLAR sighting scope and tripod with a casual “Don’t break it!” and a chuckle. I continue to be impressed by and proud of the generosity and collaborative nature of my colleagues. Gail’s sighting scope was perfect for the lake section of the waterway, which is the widest, and also where turtles like to bask on the cement embankment, out of view of the path on their own side, and the furthest possible distance from viewing on the opposite path. The photos below show the same Emys through my zoom lens SLR (which at any other part of the waterway is more than sufficient) and through Gail’s sighting scope. I also want to say a special thanks to Robyn Screen and Bob Thomson (both now at University of Hawai’i, Mānoa) who have studied these turtles (see links to studies below) and who could provide input on basking locations, turtle ID, and general fun facts about the turtles and their history.
We found fewer turtles during class than on any of my recon visits, and particularly fewer native Emys. Still, we could do some good observations. We noted few differences in behavior or basking preference between the two species, though we observed more Trachemys in the water, and mostly only saw the Emys basking. We also noted that the juvenile turtles we saw (including the teensiest turtle I have ever found!) were all Trachemys – which might suggest this species recruits offspring better than Emys. Fortunately, on my recon trips, I saw numerous juvenile Emys, so the native turtles are also recruiting, although perhaps not as much as the Trachemys.
After our observations, the students had a number of questions about the interactions between invasive and native turtles. Even if they have similar basking habitats, could the Trachemys outcompete Emys for food? With seeing so few juvenile Emys and so many juvenile Trachemys, should we be worried about the native turtles’ population size? What could we do, in management terms, to remove the Trachemys from the waterway? How can we prevent more non-native turtle species from being introduced to the waterway?
Fortunately, some of the research done on the turtles has looked at a few of these questions. A 2003 study by Spinks et al quantified population sizes of the native Emys and nine other introduced species, finding that the Emys population had declined by 40% throughout the study period of six years. “Headstarting” the turtles (rearing juveniles in captivity and then releasing them to the waterway) did improve population sizes, but does not address the root causes of decline. A study by Lambert et al in 2013 did find differences in basking habitat, which recommended management practices the Arboretum could undertake to provide more spaces for Emys (namely areas with low human presence, steep slopes, shallow water, and a concrete basking substrate). Another study currently under review by Lambert et al tracked the effect of Trachemys removal from the waterway on Emys basking preference. The team removed nearly 200 Trachemys turtles and then observed Emys behavior in response to this reduction in competition. Interestingly, the Emys primarily used the same basking locations they had used previously (abandoning some basking sites, but not expanding into others). This study also documented a pronounced east-west gradient in basking preference, with Trachemys primarily occupying the eastern end of the waterway and Emys more prevalent in the western end. It is worth noting here that our turtle observations were primarily in the western end of the waterway, where this study would have predicted our observations to include more Emys. These studies might lead us to some concern about the future of the native Emys turtles, but another study assessing population structure throughout the Sacramento Basin provides some hope. In 2010, Thomson et al reported relative distributions of Trachemys and Emys, finding that while the introduced Trachemys are present throughout the basin, they are concentrated in more urban areas with high human traffic (like the UC Davis Arboretum waterway) and are far less common throughout portions of the Emys range with less human impact, where the native turtles can still be found in abundance. Still, Emys marmorata is still listed as a Species of Special Concern in California, making continued observations of these populations important.