Climate change and animal migration.

• Author: Moore, Thomas T.

1. INTRODUCTION II. Climate Change Effects on Animal Migration A. Habitat Alteration B. Changes in Resource A variability C. Increases in Disturbance D. Changes in Phenology E. Migration Cessation F. Spatial Variability in Climate Change Effects III. Conclusion I. INTRODUCTION

   Migrations are recurrent movements between habitats that often take advantage of seasonally productive habitat during the breeding season. (1) Migrations span a range of movements from long, cross-continental movements to comparatively short movements of a few hundred meters. Animal migrations are essential to the healthy function of many ecosystems by facilitating dispersal of plant species, cycling resources between locations, and allowing life to persist in some of the most extreme locations on earth. (2) Conditions in areas such as savannah, boreal forest, and alpine regions can be extremely harsh for much of the year, but do have brief periods of high resource productivity. (3) Through migrations, species can take advantage of these productive periods while escaping the harsh periods. (4) However, climate change may alter the dynamics of the habitats and processes on which migration depends. Due to their interactions with their environment, migrating species can be especially sensitive to climate change. (5) While there has been much work on the effect of climate change on biological systems in general, (6) few studies have attempted to gather that information and apply it to how climate change will affect migrations.

   Climate change is predicted to have many varied impacts on species around the world. (7) In this Introduction, I will discuss how climate change is predicted to impact biological systems in general. In the following Parts, I will provide more detail on how those impacts are most likely to be significant to animal migrations. Finally, in the Conclusion I will describe what scientific research and management techniques are needed to confront these effects.

   The most well-known impact of climate change is an increase in temperature. (8) The temperature mean and variability are predicted to rise, so the average temperature will be hotter while temperature extremes also become more severe. (9) In addition, precipitation patterns are calculated to change around the world, with much more rainfall in some regions while other areas will receive much less. (10) Projecting exactly how precipitation patterns will change in the future is exceptionally difficult, with similar global circulation models (11) predicting different results. (12) Through the interaction of temperature and precipitation changes, water availability is predicted to decrease--even if it rains more--because evapotranspiration (13) will increase with increased temperatures. (14) In addition, glaciers and alpine snow pack will melt at accelerated rates, decreasing the sources of water that have sustained mountain streams and lower elevation rivers for thousands of years. (15)

   The spatial variability (16) of climate change impacts will be large. This is one reason why climate change is so difficult to manage for and predict. Some areas will become drier, others will become wetter. (17) Some areas will become much hotter, while others may change only a little. However, evidence shows that colder regions (i.e., montane and poleward regions) have been most strongly affected by climate change and will likely continue to be. (18)

   The result of all these impacts will be a likely reduction in primary productivity (19) and the creation of "novel ecosystems" unlike anything in recorded history. (20) Diets will shift as the quality and type of food is altered. There will be other changes in community interactions as species composition and habitat types are transformed in reaction to climate change.

2. CLIMATE CHANGE EFFECTS ON ANIMAL MIGRATION

   Climate change will affect all species, and migratory species will be confronted with many of the same climate change driven issues that nonmigratory species must confront. However, due to their reliance on migration, migratory species may feel the effects of climate change in novel ways. When biotic or abiotic (21) cues are altered by climate change in a way that organisms cannot correctly interpret or adapt to, migrations will be disrupted. As a corollary, since migratory species track seasonally available resources, (22) if climate changes in such a way that resources--such as food or water--become reliably available throughout the year in one location, a species might cease migration and remain with the resources. In the following Parts, I will describe some of the ways migrants' biotic and abiotic cues are being altered by climate change.

   1. Habitat Alteration

      Climate change is predicted to alter habitat around the world. (23) Significant changes in habitat, either along the travel corridor or at the destinations, could disrupt migration and lead to a decline in the migrant population. For example, migratory shorebirds that use coastal wetlands as stopover sites rely on these sites to rest and eat, gathering energy for their continued journey. (24) Rising ocean levels may inundate these areas, increasing the distance shorebirds must travel between stopover sites. (25) Stopover sites are critical to shorebird migration success, but coastal wetlands are predicted to decrease in the future by as much as seventy percent in some areas due to climate change. (26) If stopover sites are lost, shorebirds may not have enough energy to travel to another available site.

      As water availability decreases, migration may become more dangerous for terrestrial migrants as well. Amphibians, such as spadefoot toads and crested newts, move between pools of water to reach breeding and nonbreeding habitat, relying on these movements to complete their life cycle. (27) As precipitation decreases, these bodies of water may decrease in size and number, making the necessary movement farther. (28) Amphibians cannot withstand extended periods without water, so there may be a point at which amphibian populations can no longer reach the next body of water, increasing death along the route or stopping movement completely. However, as mentioned earlier, future precipitation patterns are not clear, so there may also be areas where increasing precipitation creates larger or more numerous bodies of water. Whether these pools will be located within the range of a migration depends on the location of the population.

   2. Changes in Resource Availability

      As habitat is modified by climate change, habitat productivity will also change. For example, climate change may disrupt the growing season by changing temperature or precipitation beyond acceptable ranges for a particular species. This could reduce productivity and cause a decrease in available resources, such as the food and shelter necessary for survival. One of the most important factors allowing species to migrate long distances is the availability of resource-rich areas to provide enough energy for completing the migration. (29)

      Conversely, some migratory species may benefit from climate change induced habitat modification. For example, high latitude ecosystems, such as boreal forests, are predicted to become more productive, at least temporarily. (30) As the temperature rises, the growing season may be extended in these colder regions allowing for greater productivity. Generally, however, the greater productivity may eventually be constrained by limits in other soil nutrients, such as nitrogen (31) and increases in disturbance that accompany climate change may also limit increases in productivity. (32)

   3. Increases in Disturbance

      If climate change proceeds as predicted, disturbances such as fires, drought, and floods will increase in number and magnitude, leading to further habitat alteration. (33) Long distance migrants, because of the long distances traveled, will therefore have a greater chance to come into contact with areas of fire, drought, or flood. These natural disturbances could also disrupt migrations by disturbing the landscape, which could cause a loss of shelter for migrants or a loss of the habitat productivity that migrants depend on for successful migration. However, new flooded areas may provide additional stopover sites for wetland and aquatic species and fires could provide a rich insect resource.

      In addition, biological disturbances such as invasive species, pests, and pathogens are also predicted to increase as a result of climate change. (34) Areas that were previously inhospitable may become newly accessible to pests, predators, and pathogens due to changes in climate. Pests, such as the pine beetle, have already dramatically expanded their range...