There has been an increase in the number of evidence that shows how climate change is linked with plants and animals' phenology. To give one of the myriads of example, over the past 11 years, the active growing season of plants has advanced by approximately 8 days in the northern latitudes. Such changes in phenology can also result in changes in the reproductive cycle in animals. In, England, 20 out of 65 bird species that were analysed, had significantly increased their laying dates over the last 25 years.
Constant changes in the timing of breeding at the population level might, in principle, result from individual factors or a cumulation of factors. In seasonal environments, at least in birds, there are often directional changes for early breeding and additive genetic breeding time. These changes might be caused due to changes in gene frequency because of natural selection within populations. However, they might also be caused due to changes in gene frequency due to the northward migration of individuals adapted to breed relatively earlier in the year. One more cause maybe changes due to phenotypic plasticity. Phenotypic plasticity occurs when the expression of genotypes is environmentally dependent.
A study searched for all possible climatic variation effects during 16 years in a population of collared flycatchers breeding on Gotland, Sweden. The study aimed to find whether any parameter of reproductive performance had changed in the study population over this period; if there were any fluctuations, can these fluctuations be related to the NAO-index; and whether analyses based on cross-sectional and longitudinal data imply a similar role for climatic fluctuations in determining breeding phenology. If the third condition is met, it would positively suggest that the observed changes were due to phenotypic plasticity rather than microevolutionary processes.
Collared Flycatcher; Source: eBird
The NAO (North Atlantic Oscillation) is a natural, large scale atmospheric fluctuation between the subtropical and the subpolar North Atlantic region. The NAO index's positive values indicated warmer and wetter winters in this region, whereas a negative value incites cooler and drier winters. The NAO index has increased significantly in the past 20-30 years, correlated with warmer winters. AN increase in spring temperatures in this region is directly related to the rise in the NAO index. A shift towards earlier breeding associated with warm springs has been documented in numerous animals species.
In long-distance migrants, a direct relationship has been found between winter NAO- index and breeding timing. Even though there is considerable distance between northwestern Europe and the collared flycatcher's wintering quarters in central Africa, the same cannot be said for the climatic variation. Studies show that the divergence between the export of dust from the Sahara and the Sahel regions in Africa is related to NAO-index variations, suggesting that there could be corrections between winter NAO-index and climate on the wintering quarters of the flycatchers. Even if the flycatchers don't proportionally experience climatic variation correlated with the winter NAO-index, the effect of the winter NAO can be shared by the NAO's impact on the phenology of the food plants, which are the primary source of food for their young (caterpillar larvae).
This study found that the laying date was significantly affected by the large scale climatic fluctuations, measured by the winter NAO-index over the study period. When the study used collard flycatchers that had experienced at least two different NAO-index values, it was found that these birds have changed their breeding performance (laying date and clutch size) in response to changes in the NAO index. This suggests that the cross-sectional data reflects phenotypic plasticity in these variables. Evidence supporting phenotypic plasticity is that the absence of consistent change in laying date with time, in the presence of correlation between laying and environmental factor implies that this pattern would not suggest gradual evolutionary changes.
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R. Przybylo, B. Sheldon and J. Merila, "Climatic effects on breeding and morphology: evidence for phenotypic plasticity", Journal of Animal Ecology, vol. 69, no. 3, pp. 395-403, 2000. Available: 10.1046/j.1365-2656.2000.00401.x.