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Amphibians are sensitive indicators of environmental conditions and show measurable responses, such …


Biology Articles » Paleobiology » Paleoecology » Temporal response of the tiger salamander (Ambystoma tigrinum) to 3,000 years of climatic variation » Discussion

Discussion
- Temporal response of the tiger salamander (Ambystoma tigrinum) to 3,000 years of climatic variation

An assumption made in interpreting the fossil record is that the specimens are representative of the sample population. Another caveat is that the common characteristics of the population also will occur in the fossil record if there are no effective sampling biases [21]. With this in mind, we expect changes in characteristics of the fossil specimens to represent changes in the sample population. Much of the fossil record for A. tigrinum through the last 3000 years indicates relatively conservative changes in population trends in morphotype, but nevertheless paedomorphic and metamorphic individuals are found throughout the deposit. Because there were no absolute shifts to a single life history strategy (i.e. paedomorphosis) within the sampled population over a relatively long ecological time period, it is likely that the two distinct life history strategies were maintained during short-term climatic fluctuations of the late Holocene. Thus, we conclude according to this case study that although change in frequency of paedomorphosis in a population occurs in relation to different environmental conditions over one generation, on the time scale of 100 to 1000s of years, if there is short-term climate variability little overall change would be expected in a population, even with coarsely distinguished climatic events.

Our results provide an opportunity to determine how the local population of A. tigrinum responded to the largest climatic anomaly in the Yellowstone region over the last 3000 years, the Medieval Warm Period (MWP) [39]. The MWP in Yellowstone is characterized by a warm and dry climate [40] and occurred approximately 1150 to 650 ybp [39]. These conditions suggest low pond permanence, or rapid drying, which could lead to shallower ponds that provide less protection from predators. The several ponds in the vicinity of Lamar Cave are all fishless glacial kettles with approximately the same water depth. Total A. tigrinum fossil abundance over the last 3,000 years shows a marked increase during Interval C, which corresponds to the MWP. Due to the deposition of this fossil assembly, fossil abundances reflect an overall increase in salamander abundance, an increase in successful predation and collection, or a combination of both factors. However, prior study on taphonomic biases potentially at work in this cave accumulation find no evidence for a change in predators, a change in collection radius, or an adjustment in collection vectors [21].

The periods of higher effective moisture may provide deeper, more permanent ponds that would offer greater protection from aerial predators, but we cannot conclude that such a sampling bias is the only factor affecting fossil abundance. The significant difference in BSI between paedomorphic and terrestrial adults during interval C, along with the increase in abundance, is evidence for warm climatic conditions that allows indeterminate growth of a terrestrial ectotherm to continue. This agrees with expected changes in A. tigrinum populations during the warming based on modern studies [41,42].

Our results also demonstrate that a population of A. tigrinum can show changes in average body size without changes in the percentage of different morphologies. In the oldest intervals, E and D, paedomorphs appear to have been smaller (based on BSI) than in other intervals, while the percent paedomorphic remained relatively constant. This suggests a favorable terrestrial environment where metamorphosing individuals continue to grow following the transition from the aquatic environment. This terrestrial growth will be most facilitated by warm conditions and abundant food. The available climate records show these relatively long time periods as variable, with periods of high and low effective moisture [43-45].

There are a variety of confounding factors that complicate our interpretations. First, although the excavation was remarkably rich and the number of identified salamander specimens quite large, our conservative use of MNI reduced our sample size. Our use of time intervals (A-E), while justified based on our own 14C chronology, exacerbated time-averaging [20]. In addition to impacts on the fossils, the intervals create a potential time averaging error for the climatic record. In fact, several intervals span more than one particular climatic period, while certain major climatic periods span more than one interval. Thus, although the fossil record does not indicate significant changes in certain traits, there may be larger variation in those traits that simply is not recorded in the fossil record or that is masked by the effect of time averaging. Also, the overall influence of the abiotic environment may have the most significant effect on Ambystoma metamorphosis, yet we cannot dismiss the role of certain biotic elements. Specifically, density-dependent effects are correlated with body size and metamorphosis, which may skew our interpretations. Larval density will impact resource competition, with higher competition driving metamorphosis. Patterns in larval individuals have not been documented here because of small sample sizes. Finally, likelihood of paedomorphosis may have a significant genetic control, which we do not discount here. Our results support the maintenance of alternate, but coexisting, life history strategies suited to selection pressures in variable environments.


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