Figure 1.
Linear extensibility affects pocket gopher activity on Stanford campus.
The soils in the area photographed have a linear extensibility of 4.5%. A and B depict the area during the rainy winter. Fresh signs of pocket gopher activity are visible. C and D depict the same area with hardened and cracked soils during the arid Mediterranean summer; cracks of this type can reach up to 1 m deep [21]. Megascapheus pocket gophers inhabit this region of California, which is south of our study area. Photos by AEM.
Figure 2.
Flowchart outlining how we prepared the data (in rectangles) for our analyses (in circles). Dataset numbers are referred to in the text.
Figure 3.
Genus Thomomys prefer sandy-loam and loam soils.
Genus Thomomys subgenera plotted by soil texture. Red circles indicate soil types inhabited by Megascapheus, the subgenus with additional tooth-digging adaptations. Blue circles indicate soil types inhabited by the predominantly claw-digging subgenus Thomomys. Circle size indicates the frequency of pocket gopher subgenera or the frequency of soil types available in the study region.
Figure 4.
Combinations of soil attributes sort Megascapheus into harder soils; subgenus Thomomys into softer soils.
Conditional inference trees from the 10 km grid dataset (n = 327). A is of soil attributes extracted over the surface to 20 cm depth; B is of soil attributes extracted over the surface to 1 m depth. The 10 km grid datasets are representative of all the datasets in the sensitivity analysis. Each node represents a split based on a critical value for one soil attribute. The p-value quantifies the degree of certainty by which this node improves the separation of the two subgenera. Branches to the left include pocket gophers that are found in soils below the critical value; branches to the right include pocket gophers that are found in soils above the critical value. This process continues iteratively for each branch until no more statistically significant splits can be made. The resulting plots show the proportion of pocket gophers of each subgenus found at the end of each branch. Red represents subgenus Megascapheus; blue represents subgenus Thomomys. The width of the plot represents the number of pocket gophers at the end of each branch. Plots to the left represent pocket gophers found in softer soils; plots to the right represent pocket gophers found in harder soils.
Table 1.
Conditional inference tree critical values are similar to physically significant soil thresholds.
Table 2.
Sensitivity analysis of conditional inference forest variable importance for foraging tunnel depth.
Table 3.
Sensitivity analysis of conditional inference forest variable importance for entire burrow depth.
Figure 5.
Megascapheus inhabit harder soils more often than expected by chance; subgenus Thomomys show opposite trend.
Null hypothesis for the Chi square tests: random distribution across the study area (n = 684). Expected values are proportional to the percent study area in each soil bin. For bulk density, percent clay, and linear extensibility at both depths, Megascapheus are found in harder soils more often than expected by chance and in softer soils less often than expected. Subgenus Thomomys show the opposite pattern: they are found in harder soils less often than expected and in softer soils more often than expected. Depth to bedrock does not produce as striking results; however, Thomomys are found in shallow soils less often than expected by chance.
Table 4.
Soil bin, subgenus and genus Chi square results.
Figure 6.
Changes in available moisture, and its impact on linear extensible soils, affect species boundaries.
In the absence of severe human disturbance, percent clay and bulk density change on time scales of decades to millennia [16]. Hardness of a soil with high linear extensibility, however, can change in just days [19]. The triangle indicates the location of Samwell Cave, an area that records the presence of subgenus Thomomys during the cooler, wetter Pleistocene [47]. The boundary between subgenus Thomomys and Megascapheus appears to have shifted north over the transition to the Holocene, as the climate in the central valley became Mediterranean. Species boundary “a” is currently in an area of California that has cooler, wetter, continental summers in contrast to the central valley. If this were to change, we would expect the Megascapheus range to expand and the subgenus Thomomys range to contract.