Throughout 2020 and 2021, the San Diego Natural History Museum managed teams of in-house and subconsultant specialists to implement and improve upon the monitoring program for the federally endangered Stephens’ kangaroo rat (Dipodomys stephensi; SKR) for an ongoing SDG&E Fire Risk Mitigation Initiative (FiRM) program project. The project included work at over 100 locations in the Warner Springs/Lake Henshaw area, where a robust population of SKR persists.
Each power pole replacement or installation process begins with digging a hole about 11 feet deep and 24 to 36 inches in diameter. Newly dug pole holes often intersect with existing rodent burrows, and when working in areas where SKR are prevalent, you run the risk of trapping SKR in the pole holes until they can be removed by a biologist. To minimize this risk, the wildlife agencies mandated that when working in SKR habitat, a net be installed a few feet below the surface to capture any falling animals.
The design of this net system has slowly evolved over the years to lessen potential impacts to SKR. The design borrowed from the previous year utilized a fall net constructed of wire mesh that fit against the wall of the pole hole. Gaps between the wall and the net were often unavoidable due to unevenness of the hole walls, occasionally allowing SKR and other small vertebrates to find their way to the bottom of the hole.
When we were contracted to oversee the SKR monitoring program for SDG&E’s FiRM project in 2020, lead SKR biologist Scott Tremor quickly recognized the deficiencies of the then-accepted net design. He redesigned the system, incorporating the advice of multiple construction crews and revising the design as needed, and exceeded compliance expectations. The current, improved design has already been applied to other pole installation and replacement projects within the range of SKR.
For the first redesign, a section of a cylindrical cardboard concrete form (e.g., Sonotube) was set flush with ground level and within the upper 3 feet of the hole, where it provided a rigid edge for securing the fall net. To create a frame for the fall net, a 3-inch section of cardboard form was cut, and the cut edges were overlapped slightly and secured, so that the frame for the net fit snugly inside the form. A strong but slightly flexible wire mesh (e.g., hardware cloth) was wrapped around the frame, ensuring full contact with the inside edge of the form. The net (i.e., frame + wire mesh) was then secured to the ground surrounding the hole with guy lines, giving it an extra layer of protection against collapse. A piece of plywood was then placed on top of the hole and covered with plastic sheeting at least 25% larger than the plywood. The edges of the plastic sheeting were covered with dirt to exclude wildlife from the plywood and hole. This system was inspected twice daily (whenever work was suspended at that hole) for any potentially captured vertebrates.
This design proved more effective. However, other small vertebrate still entered through gaps between the form and coverboard. Therefore, this past summer, we instituted a second redesign, where the cardboard form now extends 3-4 feet below and at least 1 foot above ground level, essentially capping the hole and removing the need for the net. Thus far, this has proven to be 100 percent effective in preventing entry of SKR, as they do not have the ability to climb the smooth side of the cardboard form. It has also greatly reduced incidental captures of other small vertebrates, as well as invertebrates, living in the area.
This preventative measure is an excellent example of how simple tools combined with a little ingenuity can achieve conservation of our region’s biodiversity.