Climate change impacts on animal migration

Abstract

Commentary This is the first in a regular series of mini-review which highlight outstanding recently published papers that shed new light on biological responses to climate change. We chose migration as the topic for the first mini review because its global geographical scale makes migrating individuals particularly vulnerable to climate change, and at the same time, the process of migration has fundamental impacts on ecological processes and biodiversity. Movement is an integral part of the ecology of many animals, and it can affect individual fitness and popula-tion persistence by enabling foraging and predation, be-havioural interactions, and migration [1]. Migration, in particular, affects biodiversity at regional and global scales, and migratory animals affect ecosystem processes. Animals use predictable environmental cues for the timing and navigation of migration. A change in these cues will affect the phenology and extent of migration. Arrival date and hatching date are phenological markers in migrating birds, for example, that can be strongly af-fected by global warming. These dynamics have been in-corporated into a mathematical model recently [2]. Higher temperatures cause earlier appearance of the in-sect prey of hatchling birds, which exerts pressure on birds to breed earlier so that hatchling development co-incides with peak prey abundance. Advanced breeding is dependent on the arrival time of the adults at the breed-ing site, as well as the delay between arrival and the start of breeding. These traits can change synchronously or asynchronously, and a mismatch between prey abun-dance and hatching can cause population declines. The mathematical model [2] explores the dynamics of these interactions and their evolutionary trajectories, and it can explain patterns observed in European flycatchers. Conversely, departure from their non-breeding grounds in Africa also appears to occur earlier for at least some Palearctic migratory birds [3]. Hence, there is a global shift in departure and arrival times that affects migratory bird movement and local abundance as a result of cli-mate warming. Phenological shifts in migration of endothermic birds are linked to the abundance of their ectothermic prey. Although endotherms are also directly affected by changes in temperature, which affects their metabolic demands for thermoregulation, these direct effects are more pro-nounced in ectotherms. The body temperature and hence physiology of ectotherms such as invertebrates and fish is closely tied to environmental temperatures. Climate warming will therefore influence metabolism and other physiological processes directly in ectotherms, and this can have pronounced effects on movement and migration. The proximate mechanisms that enable movement are the physiological functions that provide energy to the muscles and the muscles themselves that transform chemical energy (ATP) to work. All physiological pro-cesses are influenced by temperature, to varying degrees and usually optimal physiological rates are achieved within a relatively narrow temperature range. At extreme low or high temperatures, cessation of physiological functions leads to mortality [4]. But even at more benign temperatures that nonetheless diverge from the optimal range, decreases in physiological performance increase ecological failure by impairing movement [5]. Hence, changing environmental temperatures, including changes resulting from anthropogenic global warming, impact migration and other ecological processes via the thermal sensitivity of physiological processes. A new theoretical model now suggests that in Chinook salmon, metabolic constraints exacerbate the effect of temperature on the metabolic costs of migration [6]. Salmon often migrate hundreds of kilometres from their natal riverine areas to the ocean and return to their natal areas for spawning. Salmon prefer relatively cool water, and increasing water temperatures compromise their cardiovascular and metabolic physiology [5]. If fish did not have to replenish metabolic substrates (in particular glycogen) during their migration, the effects of warm water could be mitigated by swimming through warm sections of river rapidly. However, the need to replenish