11.3.3 Drivers
Beyond these constraints, natural selection shapes life histories into suites of correlated traits, often remarkably convergent among alpine organisms facing the same
environmental challenges. Life-history theory predicts that components of the
environment, such as resources, predation, herbivory, competition, disease or
physical stresses, favour different combinations of life histories and yield general
patterns in their variation. Altogether, these factors can be grouped in two major
extrinsic drivers of variation, associated broadly with resource availability and
disturbance, respectively. Abundant resources, such as food and light, and competitive environments foster fast growth, short life cycles and high levels of
reproduction, thus a ‘fast’ strategy (Clark and Clark 1992; Ghalambor and Martin
2001). This ‘fast’ strategy is also favoured when a disturbance, for instance, disease
and predation, increases juvenile extrinsic mortality (Franco and Silvertown 1996).
The investment in parental care reflects responses to perceived risks and environmental stresses. High juvenile (extrinsic) mortality or reduced recruitment may
yield greater allocation in offspring quality instead of quantity to enhance (intrinsic)
juvenile survival and recruitment (Clutton-Brock 1991; Armstrong and Westoby
1993). Life-history theory also predicts that where environmental conditions are not
constant across years, individuals should favour a bet-hedging strategy. This
strategy involves a reduction in annual breeding performance to reduce the probability of investing too much in reproduction during poor years, and an increase in
self-maintenance so that reproduction can be attempted over multiple years (Stearns
1976).
As we detailed above, mortality risk (especially for juveniles) associated with
disease, competition and predation tends to decline, while those associated with
abiotic stress or a paucity of resources increases in alpine environments. These
factors are expected to tilt the life-history continuum towards slower life cycles and
enhanced offspring quality vs. quantity, thus a ‘slow’ strategy. Moreover, the
environmental variability of alpine regions, consisting both of predictable (seasonality) and less predictable components (e.g. between-year variability), should
also favour the evolution of bet-hedging strategies and longer lifespan. This combination of longevity and limited reproductive effort reduces the deleterious effects
of environmental stochasticity on population growth and persistence.
In the next section, we present an assessment of the above predictions, reviewing
literature that measured responses of life histories along elevation clines in major
taxonomic groups, as summarised in Table 11.1.
11 Life-History Responses to the Altitudinal Gradient
259
Beyond these constraints, natural selection shapes life histories into suites of correlated traits, often remarkably convergent among alpine organisms facing the same
environmental challenges. Life-history theory predicts that components of the
environment, such as resources, predation, herbivory, competition, disease or
physical stresses, favour different combinations of life histories and yield general
patterns in their variation. Altogether, these factors can be grouped in two major
extrinsic drivers of variation, associated broadly with resource availability and
disturbance, respectively. Abundant resources, such as food and light, and competitive environments foster fast growth, short life cycles and high levels of
reproduction, thus a ‘fast’ strategy (Clark and Clark 1992; Ghalambor and Martin
2001). This ‘fast’ strategy is also favoured when a disturbance, for instance, disease
and predation, increases juvenile extrinsic mortality (Franco and Silvertown 1996).
The investment in parental care reflects responses to perceived risks and environmental stresses. High juvenile (extrinsic) mortality or reduced recruitment may
yield greater allocation in offspring quality instead of quantity to enhance (intrinsic)
juvenile survival and recruitment (Clutton-Brock 1991; Armstrong and Westoby
1993). Life-history theory also predicts that where environmental conditions are not
constant across years, individuals should favour a bet-hedging strategy. This
strategy involves a reduction in annual breeding performance to reduce the probability of investing too much in reproduction during poor years, and an increase in
self-maintenance so that reproduction can be attempted over multiple years (Stearns
1976).
As we detailed above, mortality risk (especially for juveniles) associated with
disease, competition and predation tends to decline, while those associated with
abiotic stress or a paucity of resources increases in alpine environments. These
factors are expected to tilt the life-history continuum towards slower life cycles and
enhanced offspring quality vs. quantity, thus a ‘slow’ strategy. Moreover, the
environmental variability of alpine regions, consisting both of predictable (seasonality) and less predictable components (e.g. between-year variability), should
also favour the evolution of bet-hedging strategies and longer lifespan. This combination of longevity and limited reproductive effort reduces the deleterious effects
of environmental stochasticity on population growth and persistence.
In the next section, we present an assessment of the above predictions, reviewing
literature that measured responses of life histories along elevation clines in major
taxonomic groups, as summarised in Table 11.1.
11 Life-History Responses to the Altitudinal Gradient
259