Kangaroo Rat Population Response to Seismic Surveys for Hydrocarbon Reserves

Exploration for new oil and gas deposits commonly is conducted using geophysical, or seismic, surveys. In areas with potential hydrocarbon resources, these surveys are conducted by generating energy waves that reflect off subterranean strata and return to the surface where they are recorded using geophones. The resulting data are interpreted to identify oil and gas deposits (Milligan 2004). Two common methods of creating these energy waves are detonating buried explosive charges (“shot-hole” method) and generating strong ground-penetrating vibrations (“vibroseis” method; Milligan 2004).

The effects of drilling and hydrocarbon extraction activities on wildlife have been well documented (Flickinger 1981; Kaplan et al. 1996; Lyon and Anderson 2003; Ingelfinger and Anderson 2004; Trail 2006; Ramirez 2010). Also, the effects of seismic exploration activities on large mammals have received some attention (Hook 1986; Joslin 1986; Reynolds et al. 1986; McClellan and Shackleton 1989; Blix and Lentfer 1992; Bradshaw et al. 1997). However, the effects of seismic surveys on smaller wildlife, particularly on semifossorial species, have not been extensively studied. The southern San Joaquin Valley in central California is a major hydrocarbon production region. This region also supports many species of conservation concern (U.S. Fish and Wildlife Service [USFWS] 1998; Germano et al. 2011), including several endemic species of kangaroo rats (Dipodomys spp.), some of which are listed as endangered under the U.S. Endangered Species Act (ESA 1973, as amended) or California Endangered Species Act (CESA 1970, as amended), or as California Species of Special Concern (California Natural Diversity Database [CNDD] 2015a). These species include the giant kangaroo rat Dipodomys ingens (Federal Endangered, California Endangered; Federal Register 1987; CNDD 2015b), Tipton kangaroo rat Dipodomys nitratoides nitratoides (Federal Endangered, California Endangered; Federal Register 1988; CNDD 2015b), and short-nosed kangaroo rat Dipodomys nitratoides brevinasus (California Species of Special Concern; CNDD 2015a). Another species, Heermann's kangaroo rat Dipodomys heemanni, is widespread and abundant, and it is an important prey item (Nelson et al. 2007) for endangered San Joaquin kit fox Vulpes macrotis mutica (Federal Endangered, California Threatened; Federal Register 1967; CNDD 2015b). Kangaroo rats are considered keystone species (Goldingay et al. 1997; USFWS 1998), and their burrows also provide important refugia (USFWS 1998; Davidson et al. 2008) for other species, including blunt-nosed leopard lizard Gambelia sila (Federal Endangered, California Endangered; Federal Register 1967; CNDD 2015b).

Potential adverse impacts to kangaroo rats from seismic surveys could include direct mortality or impaired fitness resulting in population-level effects through reduced survival or reproduction. Kangaroo rats use relatively shallow burrows (Culbertson 1946; Germano and Rhodehamel 1995) that may be vulnerable to collapse from seismic survey activities. Kangaroo rats also have extremely large auditory bullae (Lay 1993) that might be sensitive to intense energy waves (e.g., subterranean explosions, ground-penetrating vibrations) produced during seismic surveys. Hearing impairment potentially could reduce predator detection capacity or intraspecific communication, thereby reducing survival or reproductive success.

The goal of this study was to assess the effects of a seismic survey on kangaroo rats, including two rare species: giant and short-nosed kangaroo rats. Specific objectives were to determine whether seismic survey activities reduced kangaroo rat abundance, survival, or condition. The study was conducted in the Lokern Natural Area (LNA) in western Kern County, California (Figure 1). A major seismic survey was conducted across the LNA in 2011. The LNA is considered critical for the conservation and recovery of all the species of conservation concern mentioned previously (USFWS 1998).

The LNA encompasses about 17,800 ha at an elevation of 122–200 m, and it lies within the boundaries of the San Joaquin Desert (Germano et al. 2011). The region has an arid Mediterranean climate with hot, dry summers and cool, wet winters (Dallman 1998). At Buttonwillow, 13.5 km east of the study area, average high temperatures in August are 35.8 C and lows are 17.4 C, and average highs in January are 13.0°C and lows were 1.1°C (World Climate 2010). Average yearly rainfall at Buttonwillow is 169 mm (20-yr average; Buttonwillow Water Storage District, unpublished data), with virtually no rain falling from early April through October.

Our study area encompassed approximately 1,000 ha in the central portion of the LNA. The area was a gently sloping (2–5%) alluvial plain with soils classified as Kimberlina sandy loam and Kimberlina gravelly sandy loams, which are derived mostly from granitic and sedimentary rock (Soil Conservation Service 1988). The vegetation was a mosaic of arid shrubland and annual grassland. The predominant natural community was Valley Saltbush Scrub, as defined by Holland (1986). This community is characterized by open shrublands with a forb understory comprised of annual plants representative of Nonnative Grassland (Holland 1986). Common shrubs on the plots included desert saltbush Atriplex polycarpa, spiny saltbush Atriplex spinifera, and common herbaceous plants included red-stemmed filaree Erodium cicutarium, red brome Bromus madritensis ssp. rubens, Arabian grass Schismus arabicus, layia Layia pentachaeta, and tansy-leaved phacelia Phacelia tanacetifolia.

The seismic survey conducted in the LNA was referred to as the Cymric survey and covered approximately 11,750 ha. The survey was conducted by Geokinetics, Inc. Two energy sources were used to generate seismic waves: explosive charges placed in shot-holes and vibrations produced by vibroseis vehicles. Shot-holes were created with small drill rigs mounted on small tractors with balloon tires. Shot-holes were 15 m deep and spaced at 33-m intervals along lines spaced 200 m apart. A 5-kg charge of pentolite was placed in each shot-hole. Lines of shot-holes were detonated in a sequential manner across the survey area. Vibroseis was conducted along selected dirt roads in the survey area. At 33-m intervals, four vibroseis trucks traveling in tandem simultaneously produced high-intensity, ground-penetrating vibrations for approximately 16 s. Seismic energy returning to the surface from both methods was recorded with geophones which were deployed on foot at 33-m intervals on lines spaced 200 m apart.

Numerous mitigation measures were implemented during the seismic survey to reduce environmental impacts. Biologists conducted surveys throughout the project site to locate sensitive resources (e.g., kit fox dens, giant kangaroo rat colonies). All workers received environmental training before conducting fieldwork. Except for the small drilling rigs, all vehicles were restricted to roads. A small portable drilling rig was transported by helicopter to more remote locations. Equipment also was delivered to field sites by helicopter to reduce vehicle traffic. To the extent practicable, shot-holes and geophones were placed 10 m or farther from burrows and dens, and damage to shrubs was avoided. Biologists accompanied all seismic survey teams in the field to assist in avoiding impacts to sensitive biological resources (e.g., burrows, shrubs).

To assess kangaroo rat abundance, survival, and relative condition, we established 18 monitoring plots. Eight plots were located in areas with shot-holes, six plots were located along roads where vibroseis was conducted, and four control plots were located in areas approximately 0.3–0.5 km from the nearest shot-hole or vibroseis point. At each plot, a trapping grid was established consisting of three parallel lines of 10 traps. The lines were spaced 20 m apart, and traps were spaced at 15-m intervals along each line. On the shot-hole and vibroseis plots, the centerline of traps was located directly along a shot-hole line or 1 m adjacent to a road where vibroseis was conducted. We assumed that any adverse effects were most likely to occur along shot-hole or vibroseis lines and that the control plots were sufficiently distant from seismic lines that adverse effects were unlikely.

One Sherman aluminum box trap (7.6 × 9.5 × 30.5 cm; H. B. Sherman Traps Inc., Tallahassee, FL), modified to prevent injury to long kangaroo rat tails, was placed at each trap station. Each trap was provisioned with a handful (∼20 ml) of millet seed for bait and an unbleached paper towel or wad of cotton batting for bedding and thermal insulation. Traps were opened near dusk and checked beginning just before sunrise the next morning. We identified all rodents captured to species and marked each ventrally with a nontoxic felt-tipped marker to identify recaptured animals within a trapping session. For kangaroo rats, we determined sex, estimated age (adult or juvenile based on size and pelage), measured mass at first capture each session, and applied a uniquely numbered tag (Model 1005 size 1 monel; National Band and Tag Co., Newport, KY) in 1 y (Table S1).

On each plot, traps were operated for four consecutive nights during each of three trapping sessions: the presurvey session was conducted in April 2011 approximately 1–2 wk before the seismic survey, the postsurvey session was conducted in May 2011 approximately 1–2 wk after the seismic survey, and the long-term session was conducted in October 2011 approximately 5 mo after the seismic survey.

For each plot within each trapping session, we indexed kangaroo rat abundance by calculating a capture rate consisting of the number of unique individuals captured per 100 trap-nights. We indexed survival by calculating the proportion of marked animals that were captured in one session and then recaptured in a subsequent session. Indices were calculated for the presurvey–postsurvey, presurvey–long-term, and postsurvey–long-term intersessions. Relative condition, based on mass measurements of adult kangaroo rats, was assessed by species and sex (kangaroo rats are sexually dimorphic, with males being larger). We compared mean kangaroo rat capture rate among sessions and treatments (shot-hole, vibroseis, and control) using a 2-way analysis of variance, with session and treatment as main effects and with an interaction term included in the model. We compared mean intersession survival indices between treatments using a 1-way analysis of variance. An arcsine transformation was applied to the capture and survival indices before analysis (Zar 1984). The mean proportion of new individuals captured on the last day of each trapping session was compared among treatments and sessions using a 2-way analysis of variance. For each trapping session, mean mass for each species–sex cohort was compared among treatments using a 1-way analysis of variance. Mass was not compared between trapping sessions due to confounding effects from natural seasonal variations in these life-history parameters. P values less than 0.05 were considered significant.

Rodent species captured during live-trapping included giant kangaroo rat, short-nosed kangaroo rat, Heermann's kangaroo rat, deer mouse Peromyscus maniculatus, San Joaquin antelope squirrel Ammospermophilus nelsoni, and San Joaquin pocket mouse Perognathus inornatus. Deer mice, antelope squirrels, and pocket mice were infrequently captured, and these low capture rates precluded any quantitative analysis. Heermann's kangaroo rats also were only infrequently captured. Data for this species were included in kangaroo rat abundance and survival estimates, but they were insufficient to conduct mass comparisons.

Mean kangaroo rat capture rates (Figure 2) varied among trapping sessions (F_2,45 = 15.4, P < 0.01), but they were similar among shot-hole, vibroseis, and control plots (F_2,45 = 2.56, P = 0.09), and the interaction between session and treatment was not significant (F_2,45 = 0.86, P = 0.49). Mean proportions of recaptured kangaroo rats did not differ significantly among shot-hole, vibroseis, and control plots for the presurvey–postsurvey (F_2,15 = 2.62, P = 0.11), postsurvey–long-term (F_2,15 = 1.89, P = 0.19), and presurvey–long-term (F_2,15 = 2.36, P = 0.13) intersessions (Figure 3). The overall mean (±SE) proportion of new individuals captured on the last day of each trapping session was 31.1 ± 2.4% and did not vary among treatments (F_2,45 = 0.58, P = 0.56). Mean mass of adult kangaroo rats did not differ significantly among shot-hole, vibroseis, and control plots during any of the trapping sessions for any of the species–sex cohorts (Table 1; Table S1).

The methodologies used in the Cymric seismic survey were typical of those commonly used in seismic surveys conducted in oil and gas production areas in the San Joaquin Valley of California. Thus, the environmental impacts associated with this survey were considered to be representative of contemporary regional surveys. Consequently, although impacts on kangaroo rats were only evaluated during this one survey, results for other seismic surveys conducted in the southern San Joaquin Valley likely would be similar. Any significant change in seismic survey methodology in the future would warrant additional evaluation of impacts on kangaroo rats and other species.

We did not detect any adverse impacts to kangaroo rat abundance, survival, or condition immediately after (1–2 wk) and up to 5 mo after the survey. Although new individuals were still being captured on the last day of each trapping session, the proportion of new individuals was similar among treatments. Kangaroo rat abundance was similar among shot-hole, vibroseis, and control plots before the survey, and also was similar among plots 2 wk and 5 mo after the survey. Likewise, kangaroo rat survival, as measured by recaptures of marked individuals, did not seem to be adversely impacted by the seismic survey. Although there were no statistical differences among treatments, mean capture rates and mean recapture rates seemed lower for vibroseis plots. One of the roads used by the vibroseis trucks ran under a high-tension powerline, and two of the vibroseis plots were located along this road. During each trapping session, we found fresh kangaroo rat remains along the road including on the plots, and some had characteristic signs of avian predation (e.g., degloved limbs). During the study, we noticed frequent use of the powerline towers by diurnal raptors, particularly red-tailed hawks Buteo jamaicensis and ravens Corvus corax. The towers also may have been used by nocturnal raptors such as owls. Predation by avian predators may have resulted in reduced kangaroo rat abundance and higher population turnover rates on these two plots. Indeed, when we removed these two plots from our analyses, the mean capture and recapture rates for vibroseis plots for all sampling periods were even more similar to rates for the shot-hole and control plots.

Kangaroo rat condition, as measured by mass, also did not seem to be adversely affected by the seismic survey. This was true for both giant and short-nosed kangaroo rats, and for both males and females of each species. Reduced condition (i.e., lower mass) might have been observed on shot-hole or vibroseis plots if seismic activities had disrupted physiological processes or caused physical impairments. If such effects had occurred, reduced mass would most likely have been noticeable in the postsurvey trapping session (May 2011), but no differences in mass were detected during this or any other trapping session.

Reductions in abundance, survival, or physical condition of kangaroo rats associated with the seismic survey potentially could have resulted from direct mortality due to energy sources (e.g., shot-hole explosive detonations, intense vibroseis vibrations) or burrow collapse, or from physiological or physical impairment that interfered with foraging or predator avoidance. If such impacts occurred, they were not of a sufficient magnitude to produce detectable changes to the population attributes we monitored. Instances of accidental burrow collapse were recorded, but such occurrences were very infrequent with approximately one burrow impacted per kilometer of seismic survey (R. Booher, Robert Booher Consulting, personal communication) and not all burrows may have been occupied.

Noise during seismic surveys is a concern. Several kangaroo rat species use footdrumming to communicate identity and to advertise territory (Randall 1984, 1989, 1997). Giant kangaroo rats may use their acute low-frequency hearing to detect and interpret footdrumming signals from conspecifics and to avoid predation (Webster and Webster 1980; Randall 1984). Brattstrom and Bondello (1983) reported temporary hearing impairment in kangaroo rats that were subjected to simulated off-road vehicle noise for 500 s. In the Cymric seismic survey, the longest a kangaroo rat conceivably would have been exposed to nearby loud noise would have been approximately 16 s during vibroseis operations.

In a previous pilot project in the LNA, a seismic survey was simulated to assess the effects of shot-hole detonations and vibroseis vibrations on kangaroo rats (Fiehler et al. 2014). Only 10 shot-holes and 10 vibroseis source points were used, and all were located along the edges of existing dirt roads. Also, only one vibroseis truck was used instead of four. An avoidance buffer was not used around burrows and some shot-holes and vibroseis points were within 1 or 2 m of kangaroo rat burrows. Kangaroo rat abundance was monitored by live-trapping less than 1 wk before the simulated survey, less than 1 wk after the survey, and again 4 wk after the survey. No effect on kangaroo rat abundance or burrows was detected (Fiehler et al. 2014).

Environmental monitoring after previous seismic surveys that used vibroseis methods in the southern San Joaquin Valley revealed a decline in the number of small mammal burrows within vibroseis corridors immediately after a survey, but no effects on the number of active burrows 6 mo after the survey (Tabor et al. 1995). However, vibroseis vehicles were operating off-road during these seismic surveys, unlike in the current survey. In northern Utah, Wilson (2011) reported that some entrances of pygmy rabbit Brachylagus idahoensis burrows were collapsed when directly contacted by vibroseis pads or truck tires and that some minor damage occurred to burrows within 25 m of vibroseis lines. However, burrows more than 25 m from vibroseis lines exhibited no damage, and no radio-collared rabbits were displaced from home ranges by seismic survey activities.

The lack of detectable adverse effects in this study may have been a result of impact avoidance measures implemented during the seismic survey. In particular, the restriction of off-road vehicular traffic to relatively small, light-weight, tractor-mounted drilling rigs with balloon tires and the requirement to avoid burrows by 10 m likely minimized impacts to occupied burrows. Similarly, Wilson (2011) concluded that 15-m buffers around pygmy rabbit burrows were adequate to avoid impacts. However, the extent of adverse effects in the absence of mitigation measures is unknown. Implementation of the measures was required because of the presence of species of conservation concern, the lack of information on impacts from previous seismic surveys, and the potential for such impacts to occur. Until impacts in the absence of mitigation measures are thoroughly assessed, a conservative approach would be to implement the measures in any future seismic surveys in this region and elsewhere when sensitive burrowing species may be present.

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Table S1. Kangaroo rat Dipodomys spp. live-capture data for three trapping sessions on 18 study plots in the Lokern Natural Area, Kern County, California. Trapping was conducted in 2011 to assess the effects of seismic surveys on kangaroo rats.

Found at DOI: http://dx.doi.org/10.3996/072015-JFWM-066.S1 (14 KB DOCX).

Reference S1. Tabor SP, Thomas RE, Vanherweg WJ. 1995. Evaluation of impacts of the Belridge geophysical exploration project on small mammal burrows and the endangered plant, Kern mallow (Eremalche kernensis) in the Lokern Natural Area, Kern County, California. Bakersfield, California: U.S. Bureau of Land Management.

Found at DOI: http://dx.doi.org/10.3996/072015-JFWM-066.S2 (1228 KB PDF).

Reference S2. [USFWS] US Fish and Wildlife Service. 1998. Recovery plan for upland species of the San Joaquin Valley, California. Portland, Oregon: Region 1, US Fish and Wildlife Service.

Found at DOI: http://dx.doi.org/10.3996/072015-JFWM-066.S3 (36802 KB PDF); also available at http://ecos.fws.gov/docs/recovery_plan/980930a.pdf.

Funding for this project was provided by Chevron U.S.A. through Robert A. Booher Consulting. We greatly appreciate support provided by Scott McGlothlin of Chevron, Bob Booher of Robert A. Booher Consulting, and Peter Nelligan of Geokinetics Inc. We greatly appreciate field assistance provided by John Hodge, Lisa Ashley, Amy Kuritsubo, Alex Brown, Dan Hack, and Sylvia Sjuarez. Listed kangaroo rats were captured and handled in accordance with the terms and conditions established in an endangered species recovery permit from the USFWS and a Memorandum of Understanding from the California Department of Fish and Wildlife. Comments offered by the Associate Editor, Doug Kelt, and an anonymous reviewer greatly improved the manuscript.

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.