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We'd like to understand how you use our websites in order to improve them. Register your interest. Demographic studies of imperiled populations can aid managers in planning conservation actions. We found that recruitment in P. Management strategies focusing on hatchling survival would strongly benefit both populations.
As a consequence, within the constraint of the need to more accurately estimate hatchling survival, managers across the range of species such as P. Effective strategies for management and recovery of imperiled species are a central focus of conservation biology.
Demographic modeling is an important part of this process, with perturbation analyses serving as a standard tool Caswell Prospective perturbation analyses Caswell , such as sensitivity and elasticity, examine the potential effects of changes in a vital rate e. Life-stage simulation analysis LSA , a form of retrospective perturbation analysis Wisdom et al. Intraspecific variation in demographic parameters has received little attention Fredericksen et al.
However, such variation leads to associated differences in the elasticity patterns of those rates Oli and Dobson , with concomitant differences in their relative importance to the population growth rate.
The result is that conspecific or congeneric populations may have different demographic responses to environmental perturbations and stressors, such as climate change e. Intraspecific variation in vital rates also has led to recent conclusions that effective management or conservation may require population- or situation-specific strategies Johnson et al.
For example, Johnson et al. Vital-rate variation among species corresponds to differing life-history strategies, and has long been theorized to represent an evolutionary trade-off between survival and reproduction Stearns , Lizards have been widely used as models to explore trade-offs among life-history traits Dunham and Miles ; Miles and Dunham ; Shine and Charnov ; Bauwens and Diaz-Uriarte ; Clobert et al.
This taxon attracts study because lizards exhibit wide variation in life-history traits such as age at maturity, survival of various life stages, annual and lifetime clutch frequency, and clutch size in relation to body size Tinkle et al. More recently, studies of life-history traits of lizards have focused on intraspecific variation to elucidate the connection between such variation and local environmental factors acting on different populations Sears ; Radder ; Sperry et al.
Among squamates, horned lizards Phrynosoma spp. First, they exhibit considerable variation in life history across their range Ballinger ; Pianka and Parker ; Montgomery et al. Clutch sizes co-vary with body size, with a mean clutch size of Additionally, because many species of horned lizards are endangered, threatened, or in decline Carpenter et al.
Assessing which vital rates are most important for restoring vertebrate populations and whether these rates differ between populations is of immediate application in management contexts. We studied 2 populations of P. Our specific objectives were twofold: 1 to describe vital-rate variation in 2 populations of P.
Based on the known latitudinal variation Ballinger ; Montgomery et al. Finally, we expected that support of our predictions would lead to population-specific strategies for conservation and management of this taxon. Modified from Sherbrooke Mean annual temperature during — was Mean annual precipitation was These areas were dominated by mixed oak-hardwood forests and a mixture of native and non-native grasslands see Endriss et al. Research activities were focused on the population of P.
Wildlife Reserve 3 is a natural area ca. As of , WR3 was nearly completely surrounded by residential housing or military buildings. Habitat restoration since and construction of a military housing development in a 7. Management activities designed to restore prairie habitat have included tree removal, disking, mowing, spraying with herbicides, and seeding with native grasses and forbs.
Although these activities do not seem to affect P. Mean annual temperature at nearby Catarina, TX during — was The site, which was predominantly honey-mesquite woodlands or parklands, experiences managed grazing and prescribed burns Flanders et al. A detailed site description is available in Flanders et al. Field methods were similar between sites unless otherwise noted. Work on WR3 occurred from to Endriss et al. We captured lizards during April—August through intensive visual searching, road cruising, and fortuitous encounters, and recorded basic morphometric information for each lizard, including snout-vent length SVL , total length TL , mass, and sex.
Intensive visual searches consisted of slowly walking back and forth across search areas while looking for lizards. We attempted to evenly and thoroughly search all areas of the WR3 field site except areas where vegetation was so thick and high that we probably would not have been able to detect lizards when they were present. Lizards were hand-captured upon detection. Lizards had a single toe clipped as a secondary mark Hellgren et al. We attached a 0. Kazmaier, unpublished data.
Transmitters were attached by the same methods given in Endriss et al. We monitored the locations of radiomarked lizards on both sites 1—5 times weekly during the active lizard season Apr—Aug. On WR3, we also monitored lizards at least bi-weekly from August until they entered hibernation generally Oct—Dec. We varied the times during which we tracked lizards each day to obtain a representative sample of locations across all daylight hours. We also observed telemetered lizards to gather data on the subvital rates that compose fecundity see below.
We tracked females 2—3 times daily throughout nesting periods, observed nesting activity, and gathered data on clutch frequency. Because horned lizards spend at least a day digging a nest, we observed and recorded nest locations, and monitored nests daily from the date they were laid to their hatching date. Further details on detecting and monitoring nests are available in Endriss et al. Life-stage simulation analysis relies on randomly drawing vital-rate values from a probability distribution based on the estimated mean and variance of said vital rate.
Ideally, the mean and variance for each vital rate should be estimated from empirical data, and estimates of process variance and sampling variance should be separated. Morris and Doak Therefore, we incorporated empirical estimates of variance process and sampling variance combined into our LSA wherever possible Wisdom et al. We note that including sampling variance will over-estimate the variance of each parameter. However, if the variance is too great relative to the mean for a parameter e.
The distribution may become bimodal, or the distribution may have a mode much closer to 0 or 1 than to the mean. This scenario is particularly common when field data yield wide sampling variances, which may or may not reflect process variance accurately. Biologically, these would represent more extreme values or a more skewed distribution than is realistic based on empirical data and known information about the species.
In several cases, we therefore adjusted the variance for some parameter estimates to better match biological expectations.
All means, however, were empirically based. We defined fecundity as the number of female offspring per female per year. Following Endriss et al. For the purposes of LSA, we fixed the sex ratio instead of drawing a random value from a probability distribution for each model iteration, as this vital rate seemed constant over time Endriss et al.
Wolf, unpublished data. We defined nest survival as the probability of at least one hatchling surviving incubation to hatch. Nests did not survive if the nest was depredated, flooded, or laid incorrectly i. Some nests on WR3 had eggs that hatched successfully but whose hatchlings could not escape the nest chamber and subsequently died.
We considered these nests to have survived, and incorporated the deaths of hatchlings into the hatch rate see below. As there was no relationship between hatchling survival which is often heavily affected by environmental conditions such as temperature and humidity and laying date R. Kazmaier, unpublished data , we assumed that nest survival rates were the same for first and second clutches.
We calculated a weighted mean White estimate of nest survival at WR3 using estimates and sample sizes from field data collected there during — pooled; Endriss et al. We used the sample variance, which was calculated as the average variance among years. Nest survival at CWMA was estimated based on nests monitored between and , pooling across years. We estimated the proportion of adult females who were reproductively active using data from the center of the P.
The proportion we calculated, 0. Because of low survival at CWMA, few females were monitored throughout the long nesting period that occurs in southern Texas. However, the length of the nesting period and the abundance of food Pogonomyrmex spp. Assuming some females may be unable to reproduce in resource-poor years, we used a variance of 0.
We determined clutch size after hatching by digging up each nest and counting the number of eggs. We estimated the mean number of eggs in the first clutch laid by females each season using field data from 3 time periods: — combined Endriss et al. We used an unweighted mean of the first clutch size for each time period as the estimated mean for the probability distribution and used the variance among the average clutch size of the time periods.
However, estimates at CWMA were complicated by continual nest-laying throughout the active season. Therefore, we could not tell which nests were first, second, or even perhaps third clutches. Evidence of triple-clutching in south Texas could not be confirmed at CWMA, so we assumed all clutches were first or second nests. To estimate sizes of first and second clutches, we assumed the total sample mean was a weighted mean of first and second clutches.
We used the proportion of females nesting Ballinger , the proportion double-clutching on CWMA see below , and the ratio of first: second clutch size on WR3 to solve algebraically for first and second clutch sizes on CWMA. Because variance estimates for clutch size on WR3 were similar between the first and second clutches, we assumed the same for CWMA. We calculated the mean proportion and variance of double-clutching females for CWMA using the same method as that used for WR3.
We defined hatch rate as the proportion of hatchlings that successfully emerged from the nest hatchlings divided by estimated clutch size. Successful eggs were easily distinguished from those that failed to hatch by appearance; white, papery eggs with a slit were assumed to be successful whereas dark brown, shriveled eggs with no evidence of an opening were considered failed.
Variation in vital-rate sensitivity between populations of Texas horned lizards
Larry Davis, Alumni Advisory Board Chair
LSA TINKER 2002 PDF