Why climate change will invariably alter selection pressures on phenology

Abstract

The seasonal timing of lifecycle events is closely linked to individual fitness and hence, maladaptation in phenological traits may impact population dynamics. However, few studies have analysed whether and why climate change will alter selection pressures and hence possibly induce maladaptation in phenology. To fill this gap, we here use a theoretical modelling approach. In our models, the phenologies of consumer and resource are (potentially) environmentally sensitive and depend on two different but correlated environmental variables. Fitness of the consumer depends on the phenological match with the resource. Because we explicitly model the dependence of the phenologies on environmental variables, we can test how differential (heterogeneous) versus equal (homogeneous) rates of change in the environmental variables affect selection on consumer phenology. As expected, under heterogeneous change, phenotypic plasticity is insufficient and thus selection on consumer phenology arises. However, even homogeneous change leads to directional selection on consumer phenology. This is because the consumer reaction norm has historically evolved to be flatter than the resource reaction norm, owing to time lags and imperfect cue reliability. Climate change will therefore lead to increased selection on consumer phenology across a broad range of situations.

Keywords: climate change; phenology; phenotypic plasticity; reaction norm; selection.

Figure 1.

Schematic of resource and consumer reaction norms using the great tit–caterpillar system as an example. (a) Great tit breeding time and caterpillar phenology respond to temperatures E₁ (shaded light grey) and E₂ (shaded darker grey), respectively. E₁ is measured over an earlier and shorter period (A–B, hence showing a larger variation as indicated by the wider distribution), whereas E₂ is measured over a longer period (A–C). (b) Linear relationships between great tit breeding time and E₁ (solid line, slope = β_P) and between caterpillar phenology and E₂ (dashed line, ). The thick solid line indicates the population-average reaction norm and the thin grey lines indicate individual-specific reaction norms. The optimal reaction norm slope with respect to E₁ (dotted line) is equal to the resource reaction norm slope multiplied by the slope of E₂ versus E₁ . Ignoring the existing mismatch between bird and caterpillar phenology (see Discussion), the initial difference in elevation between the reaction norm elevations, τ*, is set to approximately 30 days, so that food demands and availability coincide.

Figure 2.

(a) Selection on reaction norm elevation and (b) slope. Absolute standardized selection differentials (average annual values, calculated over a 100 year period) are plotted against the rates of increases in consumer temperatures (E₁) and resource temperatures (E₂). Note that in the figure only the strength of the selection is indicated but not the direction. Above the ‘ridge of no selection’ selection favoured earlier phenology (i.e. a lower intercept) and a steeper reaction norm while below this ridge selection favoured later phenology and a shallower slope. Note also that in the area below the ridge of no selection (i.e. with selection for later phenology relative to that expressed) the optimal consumer phenology would still advance driven by a plastic response to the increase in E₁; in other words, selection would oppose the direction of the plastic component of trait change. The increases in resource and consumer temperatures varied from no increase to an increase of 0.06°C per year. Most combinations of differential increases in E₁ and E₂ lead to directional selection on both the elevation and the slope of the reaction norm.