The response of species living in lowlands and on plains tends to be more variable and complex than those living in mountains. While some species may only need to make minor range adjustments, researchers estimate that some African taxa may need to move 500 km —maybe even 1,000 km—to keep up with climate shifts.
For species, such as Tanzania’s savannah birds that have already shifted their distributions by 200–300 km, adapting seems relatively easy thanks to their mobility and largely intact ecosystems. Unfortunately, the rate of climate change will likely outpace the ability for most species to adapt.
For example, nearly 62% species that live on mountain peaks are vulnerable to climate change because they may have nowhere else to go as the world heats up and 37% of species are facing extinction if climate forecasts hold true, Species living in Southern Africa’s Miombo Woodlands are even more vulnerable, where as many as 90% of amphibians, 86% of birds, and 80% of mammals face extirpation.
Species of tropical lowland forests and deserts are also highly vulnerable to shifting climates. Many tropical species have narrow tolerances for temperature and rainfall variation, while desert specialists may be at the limits of their physiological heat and desiccation tolerances.
Consequently, even small changes in the climate of these two ecosystems may have major effects on reproduction, species distributions, and hence ecosystem composition. One species already impacted is the nocturnal aardvark (Orycteropus afer, LC): a study in Southern Africa’s Kalahari Desert found over 80% mortality rates in this species during recent summers.
The high levels of mortality in this species were attributed to above-average temperatures, which subjected the animals to heat stress, leading to behavioral disruptions, declining body conditions, and eventually starvation. The impact of climate change on the aardvark is of concern because it is an ecosystem engineer: their burrows provide denning and refuge sites for multiple other species.
Deserts, with their extreme temperatures and scarce and unpredictable rainfall, are among the most inhospitable environments on the planet. To survive and breed in arid regions, organisms must minimize their energy and water requirements, and avoid exposure to potentially lethal temperatures.
Birds are generally small and diurnal, and are therefore among the groups of animals most vulnerable to even small increases in air temperatures associated with climate change.
Studies of the effects of temperature on arid-zone birds can thus be highly informative in terms of identifying new conservation challenges posed by global warming, developing mitigation measures, and understanding the management interventions that may become necessary during the 21st Century.
Daytime temperatures in many deserts regularly exceed avian body temperature, creating conditions under which birds can avoid lethal heat stroke only by dissipating heat via evaporation. But rapid rates of evaporation increase
the risk of birds becoming lethally dehydrated.
Desert birds thus face life-or-death decisions between avoiding hyperthermia by evaporative cooling versus avoiding lethal dehydration by minimizing water losses.
Mass mortality events occasionally take place during extreme heat waves when air temperatures exceed birds’ physiological tolerance limits. In Australia, for example, there are both historic and contemporary accounts of die-offs sometimes involving millions of birds.
As Earth heats up under climate change, the risk of such die-offs in desert birds is expected to increase dramatically for the deserts of Australia and North America during the 21st Century.
Africa’s arid regions are also experiencing significant temperature increases which are predicted to continue over the next several decades. Under these conditions, the impact of air temperature on avian physiology can be mediated by behavior. Birds employ a trio of behavioral adjustments to manage heat load and keep their body temperatures within safe limits.
These include shade-seeking, reducing activity to minimize metabolic heat production, and gaping the beak (panting, sometimes accompanied by gular flutter) to facilitate respiratory evaporative cooling. Although these behaviors can buffer birds against the physiological costs of high temperature, they carry subtle but important costs of their own, notably via their impact on birds’ ability to forage.
For desert birds, foraging is critically important for maintaining both energy and water balance, as most species obtain all their water from food. Reduced activity almost inevitably means reduced food intake via impacts on time
available for foraging. Seeking shade also carries costs: for some species, returns on foraging effort in shaded locations are significantly lower than in the sun.
Finally, respiratory evaporative cooling can severely restrict the ability of actively-foraging birds to acquire food due to mechanical constraints on simultaneously gaping the bill and using it for prey capture and handling.
Under climate change, the implications of these behavioral trade-offs between foraging and thermoregulation are non-trivial. Inability to balance water and energy budgets means birds progressively lose body condition during heat waves.
Compromised foraging also affects birds’ capacity to provision offspring, resulting in reduced nest success and/or
smaller, lighter fledglings which may struggle to survive and recruit into the breeding population.
Successfully balancing the trade-offs between foraging and thermoregulation, and between hyperthermia and dehydration, is the secret to success for birds in hot places.
As the climate warms, achieving this balance will become ever more challenging. Sublethal behavioural costs of keeping cool kick in at temperatures cooler than those promoting mass mortalities. In some parts of the world, such as Southern Africa, the loss of birds from desert ecosystems may therefore occur through the insidious whittling away of fitness and weakening of populations before we even witness the dramatic die-off events for
which Australia is already infamous.
An additional concern for lowland ecosystems is that climate change will likely lead to the creation of novel (i.e. hotter) ecosystems unlike any others currently on Earth. These changes will lead to biotic attrition. The gradual
impoverishment of biological communities of lowland ecosystems as species either go extinct or move away while tracking their climatic envelopes.
What is not clear is how the niches left open by the net loss of species, and newly created niches in the novel ecosystems, will be filled. The most likely scenario is that more tolerant, generalist species will fill the empty niches. However, with the inevitable loss of some species, combined with the decoupling of important biological interactions, some functions, and services associated with lowland ecosystems are likely to eventually collapse.
It is important to note that tropical lowland forests and deserts are by no means the only ecosystems vulnerable to biotic attrition. For example, researchers have found that even mild warming would expose the Ethiopian
Highlands to biotic attrition.