Nearly all species transition through different stages during their lives (e.g., juvenile to adult). The objective of this study is to introduce a technique for estimating the mean age at which such transitions occur. We present a new methodology that can be used for any species (though especially useful for long lived species) to quickly and accurately estimate transition age in a population. In the field, a researcher identifies the oldest members of the pre-transition stage (e.g. juveniles) and the youngest already transitioned members (e.g. adults). We employ order statistics to establish the foundation and confirm the validity of our model. We created a spreadsheet that gives instant results (http://onlinelibrary.wiley.com/doi/10.1002/env.2351/suppinfo). Then, to demonstrate the application of this method, eight independent datasets for a long-lived species (Carnegiea gigantea, Cactaceae) are used. Four datasets test the transition from non-reproductive juvenile to adult, and as a different example of a stage transition, four datasets are for the onset of branching (representing the last life stage with the greatest reproductive output in this species). We present results for our case studies including how climate influences age of transition and reproduction for this species.
Results/Conclusions
We introduce a new field method (published in Environmetrics) to estimate transition points in populations. We find that the oldest juveniles and youngest adults can be effectively and accurately used to estimate age (or other measure) of transition. A Microsoft Excel spreadsheet (link given above) instantly calculates the estimated population mean, the standard deviation, and their standard errors (SE). The statistical development of the approach confirms it is sound. The mean age of transition to reproductive adult in Carnegiea range from 52-106 years of age in the four populations. Demonstrating the effectiveness of a small dataset, the SE are less than 1 year in all four populations. The ages for the transition to the branched form ranged from 78-139 years (SE <1 year in all four also). Differences in transition ages across populations can be explained by climate differences across sites. We find that age estimates fluctuate little with changes in dataset size. This technique is robust and reliable for estimating transition ages in any plant or animal population. The ages (or other measurement unit) associated with these stages are fundamentally important for understanding life cycle stages, their duration, and for quantifying reproduction and establishing baseline data in populations.