Fish life histories in a warming climate: a mechanistic basis of change and a community context

Abstract

Body size dependent interactions structure food webs, and these are changing with climate warming. We cannot yet predict how warming affects many aspects of life history evolution and species ecology, despite a longstanding interest in the structuring effects of temperature and body size in food webs. This is in part due to not recognizing the temperature dependence two aspects, namely that 1) withinspecies differences govern species interactions and 2) processes of adaptation depend on body size. In this thesis, I assess how body size dependent effects of temperature govern such interactions and processes using theoretical models of individual growth and reproduction. First, I study effects of warming on the energy allocation trade-off between somatic growth and energy reserves and find that warming favours allocation to reserves and reproduction through increasing importance of early life history processes. Specifically, failing to adapt to warming winters compromises viability of population through juvenile mortality. Second, I study how effects of warming on consumer-resource systems depend on energy allocation strategies. Here, energy allocation can modulate temperature dependent competition for food between stages, but competition mediated by diet is the main determinant of effects of warming. Last, I show how effects of warming affect the feedback mechanism of stage dependent competition and predation on interacting species and thus prevent adults from cultivating a low competition environment for their young. I conclude that linking underlying individual body size dependent physiological responses to warming to effects in population and communities provides novel mechanistic understanding of adaptation and food web processes. While these mechanistic predictions form a basis for, and require, empirical tests, I propose that diversity and function of aquatic food webs are at stake.

Larger but younger fish when growth outpaces mortality in heated ecosystem

“…While e.g., benthic invertebrate density was not affected by the initial warming of the heated area ( Sandstrom et al, 1995 ), warming-induced mortality may have led to higher benthic prey availability per capita for the studied perch. Conversely, altered growth and body sizes can lead to changes in size-specific mortality, such as predation or starvation, both of which are expected to change with warming ( Thunell, 2023 ). In conclusion, individual-level patterns such as the TSR can only be used to predict changes in the population-level size structure in limited cases, as it does not concern changes in abundance-at-size via mortality.…”

Larger but younger fish when growth outpaces mortality in heated ecosystem

“…While e.g., benthic invertebrate density was not affected by the initial warming of the heated area (Sandström et al, 1995), warming-induced mortality may have led to higher benthic prey availability per capita for the studied perch. Conversely, altered growth and body sizes can lead to changes in size-specific mortality, such as predation or starvation, both which are expected to change with warming (Thunell, 2023). In conclusion, individuallevel patterns such as the TSR may be of limited use for predicting changes on the populationlevel size structure as it does not concern changes in abundance-at-size via mortality.…”