The Implications of Habitat Management on the Population Viability of the Endangered Ohlone Tiger Beetle (Cicindela ohlone) Metapopulation

1. What they did

Insects are often overlooked in conservation planning. The Ohlone tiger beetle lives only in the coastal prairies of Santa Cruz County, California, where five populations remain from a group of 10 to 15 habitat patches in the last 25 years. Adults hunt prey on open ground, and larvae wait at the mouths of burrows flush with the soil to catch passing arthropods, so both need bare ground. Grazing, scraped bare-ground plots and recreational trails now keep the ground open, but fast cycling can disturb adult mating and hunting.

To find out how management affects the beetles’ chances of survival, the researchers:

• Counted adults once or twice a week from late January to early June, 2010 to 2012, at all five sites.
• Marked larval burrows. They found first-stage larval burrows (and in 2012, egg-laying burrows), told the stages apart by burrow diameter, and took a burrow as active when its entrance was clean and clearly outlined. Each group of burrows was mapped under a gridded quadrat, marked with metal washers and nails, and located with a Global Positioning System (GPS) unit.
• Followed survival. They revisited the burrows in spring and summer. A wider entrance showed the larva had moulted to the next stage. The next winter, an irregular exit hole of 6 mm or more showed an adult had emerged.
• Tested crowding. They checked whether the number of burrows in a quadrat affected larval survival.
• Built population models for each site and for all sites together, then tested the effect of more bare ground, slower cycling (8 to 12 kilometres per hour) and different rates of migration between the two central sites.

2. What they found

Survival and crowding

• Numbers and survival rates varied between sites and years. Egg survival was lowest at Wilder Ranch (0.06 in 2012).
• Larvae never stayed in the first stage between visits.
• Quadrats held 1 to 13 larval burrows (average 3.3). Crowding had no effect on larval survival.

Population growth

• Four populations were growing: Glenwood (1.41, the highest), Moore Creek (1.164), the campus (1.163) and Lower Marshall (1.03). Wilder Ranch was shrinking (0.598).
• Wilder Ranch was certain to go extinct within 21 years in every model; every other population had zero risk in all time frames modelled.
• Growth was most sensitive to egg survival. At Lower Marshall it was also sensitive to the growth of third-stage larvae.

Management

• More bare ground and slower cycling both raised growth rates at every site, bare ground more than slower cycling. Neither did much for Wilder Ranch.

Metapopulation

• Any combination of sites that included Glenwood, Moore Creek and the campus had zero risk of falling to very low numbers.
• Wilder Ranch and Lower Marshall alone had a 52% chance of falling to 10 individuals per stage within 100 years with no migration. Migration of 2%, 4% and 10% of adult females lowered this to 49%, 46% and 43%.

3. What they concluded

• The Ohlone tiger beetle has four viable populations that, with habitat management, could recolonize extirpated sites and avoid extinction.
• Glenwood shows how good habitat and management can lower the risk for an isolated patch. Wilder Ranch shows how a well-connected patch can fail as habitat declines.
• Protecting occupied habitat is not enough on its own: bare ground must be kept open and accidental deaths reduced. As little as 9 m² of bare ground per site would be beneficial.
• Three years of data are not enough to predict insect population swings, and how far the beetles disperse between patches is unknown. Future studies should focus on dispersal, to judge whether the beetles can recolonize on their own or need to be moved.

4. How tracking was or was not used

The study relied on reading the beetles’ signs in the soil rather than seeing the larvae themselves. The researchers identified larval burrows, told the three larval stages apart by entrance diameter, judged burrows active by a clean, clearly outlined entrance, read a widened entrance as a moult to the next stage, and took an irregular exit hole of 6 mm or more as a sign that an adult had emerged. Adults were counted by sight. No trailing was used.

5. Tracker or observer certification

Not reported. The survival estimates rest on burrow interpretation, but no training or skill check is described. Tara Cornelisse and Michelle Bennett carried out the fieldwork, with field help from M. Hash.

6. Other technologies, skills or innovations

• GPS and metal washers and nails (found again with a metal detector) to relocate marked quadrats without disturbing the larvae.
• Gridded quadrats and matching datasheets to map individual burrows.
• Population viability modelling in Matlab, adding management actions as a multiplier on survival and egg numbers.

7. Implications for conservation (from the paper)

• Making bare ground and slowing cyclists in occupied habitat can raise beetle survival and population growth.
• Habitat should also be managed at recently extirpated sites and in potential coastal prairie habitat, so beetles can recolonize.
• Wilder Ranch needs more habitat management; the area between Wilder Ranch and Lower Marshall is protected, but improving extirpated patches there is a necessary first step.