Corresponding author: Aurelio Ramírez-Bautista ( firstname.lastname@example.org )
Academic editor: Franco Andreone
© 2017 Jorge Luis Becerra-López, Cristina Garcia-De la Peña, Ulises Romero-Méndez, Aurelio Ramírez-Bautista.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Becerra-López JL, García-De la Peña C, Romero-Méndez U, Ramírez-Bautista A (2017) Plant cover effect on Bolson tortoise (Gopherus flavomarginatus Legler 1959, Testudinidae) burrow use. Nature Conservation 17: 57-69. https://doi.org/10.3897/natureconservation.17.11582
The Bolson tortoise, Gopherus flavomarginatus, occurs within a restricted geographical area in the Mexican Chihuahuan Desert. We analyzed the variation in surface microhabitat with relation to the burrow occupancy for this tortoise at the Mapimí Biosphere Reserve, Mexico. In summer 2010, we monitored burrow activity (active, inactive, or abandoned) and measured environmental factors that might influence the burrow’s occupancy by tortoises (air temperature, relative humidity and substrate temperature, both inside and outside the burrow, and the plant cover around it). Discriminant analysis was used to identify the importance of these variables influencing burrow occupancy. Correlation and linear regression analyses were performed to quantify the relation between environmental factors in the sampled burrows.
Results. Sixty-one burrows were identified at the Tortugas locality. The first function’s auto-value analysis indicates that this function explains 97.9% of the variation in burrow activity status; high occupancy scores were associated with low substrate temperature inside the burrow. Plant cover was inversely proportional to substrate temperature inside the burrow. These results suggest the importance the density of plants surrounding the tortoise’s burrow as a key factor influencing the burrow microclimate and occupancy by the tortoises.
Conclusions. Gopherus flavomarginatus inhabits burrows, in part, based on microhabitat structure, with plant cover being a main factor influencing burrow occupancy. Our findings indicate that human land use and vegetation management are important for conserving Bolson tortoises, and for understanding habitat conditions necessary for the successful establishment of populations elsewhere.
Gopherus flavomarginatus, burrow, plant cover, habitat, temperature, microclimate
Research on the ecology of ectothermic organisms has established the importance of vegetation structure for their microhabitat selection (
Population ecology theory predicts that in a changing environment, a population can adapt to new conditions, migrate to a place that favors its survival, or become extinct (
The Bolson tortoise, Gopherus flavomarginatus (Figure
Therefore, if Bolson tortoise requires specific microclimatic conditions to inhabit burrows and survive, variations in microhabitat are expected to influence either their use or abandonment. An analysis of microhabitat variation is shown here in relation to the occupation of burrows of G. flavomarginatus. Our objectives included: 1) characterization of the environmental factors of air temperature, relative humidity, substrate temperature and pH; physical factors of width and height of burrows and 2) determine how these factors are related to plant cover and occupancy of burrows. This information can increase understanding of this species’ response to variation among microhabitats, and support conservation efforts for this species.
The 100 hectare study site, Tortugas, is located in the south-central portion of the Mapimí Biosphere Reserve, in Mexico (26°00', 26°10'N and 104°10', 103°20'W;
Study site. The black circle show Tortugas locality, in dotted lines is show The Mapimí Biosphere Reserve and continuous lines the state limits.
At Tortugas, we followed the monitoring protocol established by CONANP to find adult tortoise burrows (
In every burrow, was measured microhabitat structure considering the variables width (W) and height (H) of the entrance and the substrate’s pH 30 cm inside. Dataloggers (Datalogger USB-WK057, accuracy: ± 1.0) were used to measure environmental factors continuously, including air temperature (Ta) and relative humidity (RH) inside (30 cm depth) and outside (30 cm above surface) the burrow, except pH, all environmental data were recorded each hour for 24 hours; substrate temperature inside the burrow (Ts) was also recorded using dataloggers (in contact with the surface). Also, we measured plant cover (PC) using an ellipse area formula (π × a × b/4, where a = major axis and b = minor axis), within three meters of each burrow.
Discriminant analysis was used to determine which habitat and environmental factors differentiate burrows categorized by their occupancy status. Normality was not achieved (Kolmogorov-Smirnov tests; P ≤ 0.05) and we transformed the continuous data (W, H, pH, Ta, RH, Ts) with the logarithmic formula (X´ = LOG10(X + 1)), and PC with the arcsine formula (X´= Arcsin√X), according to
W burrow width
H burrow height
Tai air temperature inside the burrow
Tao air temperature outside the burrow
RHi relative humidity inside the burrow
RHo relative humidity outside the burrow
Tsi substrate temperature inside the burrow
PC plant cover
LSD least significant difference
d.f. degrees of freedom
SD standard deviation
We located and measured a total of 61 burrows at the Tortugas study site. There was significant difference in the Tsi among the three types of burrows (F = 32.40, d.f. = 2, 58, P < 0.001; Table
Descriptive statistics of environmental factors for active (n = 26), inactive (n = 7), and abandoned (n = 28) burrows, and means comparison tests among burrows categories (d.f. = 2, 58 for all cases). Air temperature inside the burrow (Tai), air temperature outside (Tao), relative humidity inside (RHi), relative humidity outside (RHo), substrate temperature inside (Tsi).
|Environmental factor/Burrow´s status||Mean||Standard deviation||Min-Max||Wilks Lambda||F||P|
|T si (°C)||0.472||32.40||<0.001|
|T ai (°C)||0.995||0.136||0.873|
|RH i (%)||0.964||1.090||0.343|
|relative humidity outside the burrowo (%)||0.984||0.478||0.623|
|Inactive||59.84||21.71||38.7 - 100|
Results of discriminant analysis were as follows: the first function was statistically significant (ᴧ = 0.241, x²= 76.74, d.f. = 18, P < 0.001; n = 61), while the second function was not (ᴧ = 0.942, x²= 3.25, d.f. = 8, P < 0.917; n = 61). The first function’s auto-value analysis indicates that this function explains 97.9% of the variation in burrow activity status, where Tsi showed the higher scores (Table
Discriminant canonical function 1 scores with relation to burrow entrance width (W), height (H), air temperature inside the burrow (Tai), air temperature outside (Tao), relative humidity inside (RHi), relative humidity outside (RHo), substrate temperature inside (Tsi), plant cover (PC), and substrate pH.
An inverse relationship was observed between PC and Tsi (y = -0.2181x + 41.504), indicating that the higher the plant cover around the burrow, the lower the substrate temperature inside it (Figure
Relation between plant cover (PC) and inner burrow substrate temperature (Tsi) for Gopherus flavomarginatus burrows.
Our analyses provided evidence that an increase in substrate temperature inside the burrows and their consequent abandonment at our Tortugas study site was correlated with vegetation cover at a scale of 3 m.
It is important to note that G. flavomarginatus might not have originated as a desert ecosystems species, they appeared toward the end of the Tertiary, so they could have spent more than 94% of their evolutionary history during the Quaternary (Pleistocene-Holocene) living in non-desert grasslands (
Having in mind that vegetation cover is a key part of burrows occupancy dynamics for this species, preserving the plant life in regions where G. flavomarginatus might potentially colonize or be translocated to in and outside the Mapimí Biosphere Reserve is of critical importance. To achieve this, we need to conceptualize a dynamic reserve (as opposed to a static one that actually exists) that follows ecological succession processes on which this tortoise species survival seems to be strongly dependent.
To members of the Tlahualilo, Durango commonality and the Mapimí Biosphere Reserve administration for all the help and assistance provided to carry out this study; also we thank L. D. Wilson for reading and improving the manuscript.