Effects of environmental predictability on life history traits and population dynamics. Insights from a three-year population experiment on the common lizard (Zootoca vivipara)

Abstract

[eng] A central theme in ecology and evolution is to understand whether and how species respond to environmental changes. The amount of studies focusing on how species might cope with climate change has been steeply growing, given the importance of the actual climate change. At present, it is widely accepted that changes in mean environmental conditions can affect individual performance, life history strategies and population dynamics. While most climate change scenarios forecast a change in average climatic conditions, they as well predict a decrease in environmental predictability. However, evidence for the effects of environmental predictability is scarce and the few existing studies were mainly theoretical or generated extreme events, which are by themselves negative. Therefore, robust evidence for the predicted negative effects is needed and experiments should test the impact of environmental predictability on life-history traits and population dynamics. For this reason, this doctoral thesis aims at providing robust evidence for the effects of environmental predictability on life-history traits, population dynamics, and species responses. We experimentally tested whether and how changes in the predictability of precipitation affected life history traits and population dynamics of the common lizard Zootoca vivipara, a highly hygrophile species. We simulated more and less predictable precipitation in 12 semi-natural populations. Populations were maintained during one year and the experiment was repeated in two subsequent years. This experimental design allows to understand the generality of the responses and it allows testing whether differences in average climatic conditions may affect differences induced by precipitation predictability. Several parameters were measured to unravel effects of environmental predictability on individual performance and to allow for complex population dynamic models. Decreased environmental predictability, let to reduced growth of yearlings and reduced body condition of female juveniles, but no effects on body condition, growth and survival of adults. This suggests that different age classes and sexes might exhibit different sensitivities to environmental predictability. Less predictable environments negatively affected ornamental coloration of adult males, which shows that less predictable environments are costly. This also suggests that ornamental coloration may act as an early signal of changes in environmental conditions. The results also show that individuals exhibit compensatory strategies to prevent detrimental effects on fitness. The observed differential investment in adult growth, body condition and male ornamental coloration may have allowed to compensate negative effects of decreased precipitation predictability, since no significant differences in reproductive parameters between environmental predictability treatments were found at the end of the experiment. Environmental predictability did not significantly affect the individual key life history traits (i.e. survival and reproduction) and suggesting that compensatory strategies may have prevented major loses in fitness and effects on population dynamics. Nevertheless, the here applied stage-structured matrix model showed that less predictable environments led to a significant decrease of the population growth rate. This suggests that in order to assess the potential effect of climate change on species persistence, complex population dynamics modelling is required. This doctoral thesis is one of the first experimental studies demonstrating whether and how species might respond to changes in environmental predictability. The obtained results underpin predictions from theoretic models, that decreased environmental predictability might negatively affect species responses. Results also suggest that decreased environmental predictability may seriously reduce population persistence and increase species extinction risk. Since species, which are highly susceptible to changes in abiotic conditions, might respond similarly to changes in environmental predictability, the forecasted decrease in environmental predictability might aggravate the actual trends of species decline and extinction. This shows that environmental predictability is a very important non-neglectable determinant of population dynamics, which highlights the necessity to consider its effects in studies forecasting the consequences of climate change.

[cat] Entendre com les espècies fan front als canvis ambientals es un tema principal en ecologia I evolució. En el context del present canvi climàtic, la majoria d’escenaris futurs prediuen canvis en les condicions ambientals mitges però també pronostiquen canvis en la predictibilitat ambiental. La majoria d’estudis que expliquen com les espècies poden respondre al canvi climàtic es centren en canvis en la condicions ambientals mitjanes. Els pocs estudis sobre els efectes de la predicibilitat ambiental són teòrics o generen situacions extremes i pronostiquen efectes negatius en ambients menys predictibles. L’objectiu principal d’aquesta tesis es aportar informació robusta dels efectes de la predictibilitat ambiental en les històries de vida, la dinàmica de poblacions i la resposta de les espècies. S’ha provat experimentalment com canvis en la predictibilitat de la precipitació afecten a la sargantana vivípara Zootoca vivípara, una espècia altament hidròfila En 12 poblacions semi-naturals es van simular ambients més i menys predictibles i es van mesurar diferents paràmetres per determinar els efectes de la predictibilitat ambiental en els individus i per realitzar models complexes de dinàmica de poblacions. Els resultats obtinguts suggereixen que diferents classes d’edat i sexes presenten diferents sensibilitats a la predictibilitat ambiental i que els individus poden compensar efectes negatius d’ambients menys predictibles evitant efectes majors en la eficàcia biològica (“fitness”). Els efectes en la coloració ornamental suggereixen que pot actuar com a indicador precoç de canvis en l’ambient. Finalment, anàlisis complexes de dinàmica de poblacions mostren un decreixement de les poblacions en ambients menys predictibles, indicant la importància d’aquests models per avaluar els efectes potencials del canvi climàtic. Aquesta tesis es un dels primers estudis experimentals que demostra com les espècies poden respondre a canvis en la predictibilitat ambiental. Els resultats obtinguts recolzen les prediccions teòriques apuntant que ambients menys predictibles poden afectar de forma negativa les espècies. També, indiquen que la persistència de les espècies es podria veure afectada en ambients menys predictibles. Es demostra que la predictibilitat ambiental es determinant per la dinàmica de poblacions i se l’hauria de considerar per predir les conseqüències del canvi climàtic.