What can be done, being a bird, to adapt to a climate that is inexorably warming? Three solutions: fleeing under milder skies, adapting locally or disappearing. The best-known mechanism is the so-called « habitat tracking »: the bird follows the thermal envelope to which it is adapted, going up in latitude or altitude. Another solution is to adapt to rising temperatures if warming is neither too fast nor too high. But the safest mechanism is to adapt through microevolution, i.e. by modifying the genetic structure of the population. When, for some reason, none of these mechanisms is possible, extinction is inevitable. Species living in extreme environments (such as the tundra or high mountains) are particularly vulnerable to extinction because the movement of their habitats is constrained by geographical limits. Some categories of birds, particularly long-distance migrants frequenting many habitats during their travel, are hardly hit by the new constraints they face.
Climate change, which is no longer disputable, is one of the six components of what is called « global change« , the other components being habitat alteration, chemical inputs, overexploitation of natural resources, and biological invasions (Figure 1). Their main characteristics, which earned them the title « global », is that they concern the whole planet; most of their effects being felt all over the world (see Biodiversity is not a luxury but a necessity). Climate change includes six phenomena: carbon dioxide concentrations, rising temperatures, rising global sea level, melting polar packs ice, expanding drylands and deserts, and an increase in extreme events (heat waves, floods, storms and cyclones). These upheavals are the symptoms ofanthropocenization [1] of the planet and have serious consequences on many aspects of the life histories of organisms, at all scales of space, from local to global [2].
The pace of change is so rapid and abrupt that we are faced with a new set of « natural experiments » that illustrate the challenges encountered by populations of organisms facing completely new environments. These new living conditions may lead organisms to extinction, but they can also offer them new opportunities depending on how they respond (or not) to these changes (see The adaptation of life to environmental constraints). Birds have been subject of a large number of studies on their vulnerability and responses to global warming. They are expected to be particularly affected [3] to the point that each degree Celsius increase in global temperature could lead to the extinction of 100 to 500 species [4].
3. What are birds’ responses to global warming?
Birds provide two primary kinds of responses to global warming. The first is a phenotypic response that occurs instantly in response to temperature variation in the bird’s behavior (see The adaptation of life to environmental constraints). Each life history trait expresses itself within a “window of phenotypic plasticity,” or the so-called “reaction norm” (see Focus Acclimation or adaptation?) that enables the body to react instantly to environmental changes. This type of response is made possible by this. The second, which is far more challenging to prove, is an adaptation to novel selection regimes. Subsequently, it is an issue of microevolutionary adaptation, and consequently, genetic nature, in reaction to the fresh environmental selection pressures. A phenotypic response occurs instantly, but a microevolutionary response takes generations to manifest because it goes through a selective directional screening process on the offspring of genetic variants that offer a beneficial variation for the relevant trait, such as laying date or migration departure.
2. Temperature, a key factor in physiology and ecology
The metabolic profile of an organism governs all physiological and ecological processes at the level of individuals, populations, and ecosystems, according to the metabolic theory of ecology [5]. When body masses and metabolic rates are expressed in logarithms, this macroecological constant, which is applicable to both macro- and microorganisms, varies with the organism’s size in a linear relationship. Because birds are temperature sensitive, it is possible to determine a thermal tolerance threshold for each species, which determines the thermal envelope to which it is adapted (Figure 2). Any breach of this barrier has a domino effect on a range of physiological, morphological, and demographic characteristics related to metabolism. The origin of the biogeographic distribution of organisms is these effects.
The reactions of living things to climate change demonstrate a cascade of effects from one degree of life resolution to the next:
- Temperature has an impact on an individual’s size, growth, and ability to reproduce, all of which affect fitness;
- Individual responses then have an impact on population-level demographics, density, and diversity, as well as phenotypic and genetic diversity.
- Afterwards, at the level of species assemblages, the dynamics, composition, structure, energy production, and specific diversity of the population are all impacted by the performance at the species level.
- Ultimately, the functioning of the ecosystem is determined by what occurs at the population level.
There are numerous examples of how vulnerable organisms are to temperature variations, especially in birds. These include when the organism reaches and then exceeds the thermal tolerance threshold, when ecosystem productivity is reduced, or when interactions between species are disrupted, leading to the disarray of food webs.
3.2. Climate warming and bird migration: new challenges
The steady decline of trans-Saharan migratory species, which has been occurring for the last twenty years at a rate of about 1% per year, is caused by several constraints, such as deteriorating conditions for migration and wintering habitats that fragment, desertify, and transform, as well as the worsening of metabolic constraints linked to crossing an expanding Sahara [12].
The intricate phenomenon of migration (Figure 7) is controlled by highly developed endocrine, physiological, and behavioral adaptations. It is accomplished in accordance with strict timing guidelines and requires the presence of resources at different points along the journey that enable birds to periodically replenish their energy stores at stopover locations. The delicate balance that evolution has established between these various factors is being upset by climate change.
The seventeen species of warblers that breed in Europe—some are trans-Saharan migrants, some are partial migrants, and some are sedentary—make the family a good model for researching these issues. Breeding areas are predicted to shift northward by an average of three degrees depending on different warming scenarios, according to a simulation of how the various species will react to global warming. 8 to 4. 4 degrees latitude, which will result in an increase of 400 to 600 kilometers in the distance that highly migratory species, like the garden warbler (Sylvia borin), must travel because wintering habitats are not predicted to change. The increase in the distances to be covered has been calculated to require an increase in the bird’s net mass of approximately 9%, which corresponds to the excess energy reserves that must be stored in the form of fat for this journey. We are aware that a tiny sparrow, weighing roughly fifteen grams, needs roughly three 5 grams of fat to cover 1000 km. Such an additional expense suggests the difficult task of obtaining more food during migration stopovers and being able to store it as fuel in the body, which is likely one of the reasons for the observed population declines. If genetic responses to new selection pressures are slower than needed to keep up with the rate at which temperatures are rising, it’s also possible that the process of adapting to the new circumstances thrust upon migrants is not happening quickly enough [13].
In fact, it is anticipated that migratory behavior will change due to the directional selection pressures imposed by the present warming since it is a genetically determined and inheritable trait. A group of German scientists has skillfully shown through experimentation that migratory behavior can emerge or vanish as a result of selection pressures placed on backcrosses in aviaries in the partially migratory blackcap warbler Sylvia atricapilla (Figure 8). In three generations, a small experimental population that was partially migratory at the start of the experiment became totally migratory, and in six generations, it became totally sedentary [14].
Many partially migratory species are expected to become residents under current selection pressures, as is already the case, while many trans-Saharan migratory species will need to adapt their migratory behavior to the new demands posed by the deterioration of their migration conditions. Throughout the history of birds, there have been enormous fluctuations in the intensity of migratory behavior. This was particularly true during the Pleistocene, when alternating glacial and interglacial periods drastically changed the timing and mode of migration.
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