SNAMP Pub #35: Life-history tradeoffs and reproductive cycles in Spotted Owls
Article Title: Life-history tradeoffs and reproductive cycles in Spotted Owls
Authors: Ricka E. Stoelting, R. J. Gutiérrez, William L. Kendall, and M. Zachariah Peery
- Researchers modelled costs of reproduction in the California Spotted Owl (Strix occidentalis occidentalis) to assess whether costs of reproduction in the previous year affect reproduction and survival in the current year and, if so, whether these costs interact with climatic conditions and/ or age class of individual owls to create oscillations currently observed in annual reproductive output of Spotted Owls.
- Results indicate that reproduction in the previous year negatively impacts reproduction in the current year with evidence of higher costs to females during the dry phase of the El Niño–Southern Oscillation, but that reproduction in the previous year does not have a statistically significant effect on survival.
- Despite evidence of costs, cost magnitudes were not large enough to explain observed natural cycles in reproductive output of Spotted Owls; thus, the authors suggest that observed cycles in the reproductive output of Spotted Owls are related to as-yet-unmeasured, regionally occurring fluctuations in environmental conditions and/or prey resources.
Evolutionary ecologists have shown that reproductive costs have the potential to influence population dynamics. Spotted Owl populations show regionally synchronized, highly variable fluctuations in annual reproductive output. Such fluctuations have been correlated with weather conditions, but the observed biennial nature of these cycles has not been explained. Highly variable reproduction can lead to decreased population growth rates over time. Thus, examination of what drives these cycles is important. An examination of reproductive costs and potential mediating environmental and individual factors allows managers to better understand and predict population response of Spotted Owls under variable environmental conditions.
Researchers utilized a 20-year mark-recapture dataset of California Spotted Owls from the central Sierra Nevada to determine whether reproductive costs contributed to biennial cycles in population-level reproductive output and/ or affected yearly survival of individually-marked owls. Reproductive status was determined by the presence of juveniles in the owl’s territory, the owl’s behavior (e.g., attendance at a nest), and the owl’s response to live mice.
Researchers developed and tested five hypotheses about costs of current reproduction on future reproduction and survival in Spotted Owls. Hypothesis 1 stated that owls which bred in the previous year would be less likely to breed in the current year (e.g., a cost of previous year’s reproduction on current reproduction/ survival); Hypothesis 2 was similar but predicted that costs would be higher in sub-adults than adults (e.g., an age-mediated cost); Hypothesis 3 stated that costs would be higher following breeding in a ‘‘poor’’ year than breeding in a ‘‘good’’ year (where “good” versus “bad” conditions – in for example, prey availability – were indexed by mean reproductive output of the owl population in a given year); Hypothesis 4 stated that cost of reproduction would be influenced by climatic conditions, in this case, the mean El Niño-Southern Oscillation Index (SOI) from August-November in the current year (e.g., a weather-mediated cost); and Hypothesis 5 mirrored Hypothesis 4 in that a cost of reproduction existed but was mediated by SOI from August-November of the previous year (e.g., a weather-mediated cost with lag effect).
After determining whether costs of reproduction were present, researchers used a simulation-based population model to assess whether the observed magnitude of these costs could generate biennial cycles in reproductive output and whether these patterns could be detected in the presence of environmental variability.
Logistic regression analysis and multistate mark–recapture models detected a cost of current reproduction on future reproduction, revealing that breeding reduced the likelihood of reproducing in the subsequent year by as much as 38% (e.g., support for Hypothesis 1). Models also indicated an interaction of this cost with climatic conditions such that females experienced higher costs after dry La Niña winters than after wet El Niño winters (e.g., support for Hypothesis 4). In addition, models indicated that breeding was positively related to the age class of females, with sub-adults being less likely to breed than adults in the current year regardless of their previous year’s breeding state. No cost was statistically evidenced on survival.
In simulation models, researchers found that the cost of breeding on reproduction in consecutive years could result in biennial patterns in reproductive output depending on the strength of the cost and the modeled environmental conditions. Cycles could be maintained under high cost scenarios when periodically kick-started by extreme “bad” environmental conditions (e.g., years in which no owls bred). However, these cycles were negligible when costs were modeled at the levels detected within the study population and, at all cost levels, quickly attenuated over time without strong recurring environmental perturbations.
Thus, though costs of reproduction on future reproduction were detected, the magnitude of such costs was not high enough to explain observed biennial cycles in reproductive output of Spotted Owls. The researchers conclude that determining the impact of reproductive costs on population dynamics is complex and likely varies with a variety of intrinsic and extrinsic factors affecting natural populations, including abundance of prey, quality of individual owls and their territories, as well as fluctuating climatic conditions.
- It is likely that reproductive cycles in Spotted Owls are related more to unknown (i.e. unmeasured) fluctuations in resources or large-scale environmental processes than to life-history tradeoffs.
- Weather conditions related to the El Niño–Southern Oscillation typically vary on three-year to seven-year timescales and thus do not seem likely to generate biennial cycles in reproductive output.
- More detailed studies of spatial and temporal variation in prey abundances, climatic processes, and other factors are needed to understand the specific mechanisms responsible for reproductive cycles in Spotted Owls.
Stoelting, Ricka E., R. J. Gutiérrez, William L. Kendall, and M. Zachariah Peery. 2014. Life-history tradeoffs and reproductive cycles in Spotted Owls, in The Auk 132(1):46-64. 2014.
The full paper is available at: http://www.bioone.org/doi/full/10.1642/AUK-14-98.1
For more information about the SNAMP project and the California spotted owl team, please visit: http://snamp.cnr.berkeley.edu/teams/owl .