- Adaptation
- Generation times: shorter life cycles increase potential for evolution
- Greater phenotypic variation amongst a population [25]: greater phenotypic variation indicates greater genetic variation and thus higher probability that a beneficial gene already exists and can be passed down
- Acclimation (phenotypic plasticity)
- Behavioral Plasticity: an individual's ability to have different responses or actions to similar events to avoid particular stresses that would compromise their fitness
What is Tolerance?
Species have ranges of environmental conditions in which performance is maximized, and these parameters are set by certain thresholds of tolerance for the organisms. Ecological tolerance refers to the ability to withstand certain environmental conditions without a large decline in fitness. [33] Each species is tolerant to a specific set of conditions, and outside this tolerance window, species are at risk fo damage and even extinction. Tolerance can be evaluated on individual, species-wide, or whole ecosystem scales. It comes in many different forms, such as thermal (heat) tolerance and drought tolerance, all of which interact to determine resilience of the species to changing conditions.
What is a Tolerance Mechanism?
A tolerance mechanism is a tactic by which organisms of natural systems cope with particular challenges. These mechanisms act in combinations to create "tolerance" to these environmental challenges. Some are biological processes whereas others are societal, thus operating not only on different scales but through different pathways. [7] These mechanisms contribute to resilience of organisms to climate change and thus are primary determinants of the survival and success of organisms in the presence of current and future climate change.
Key Tolerance Mechanisms in Sonoran Desert
As the desert's temperatures increase to new levels and water availability not only decrease but become increasingly variable, organisms and populations that are the least negatively impacted will be those who exhibit the most effective tolerance mechanisms. Here, we will discuss some biological tolerance mechanisms that will be defining factors for the future of the Sonoran Desert's inhabitants who are primarily threatened by warmer temperatures and more severe droughts. It's important to recognize that the mechanisms not only work in combinations but also serve multiple purposes, causing many overlaps between categories.
Physiological Mechanisms
Description: In ecology, adaptation implies natural selection to increase resistance of a population. Species are adapted to a certain set of conditions at which they have the highest fitness. A few factors that determine adaptive potential are:
Examples of Some Adaptations in the Sonoran Desert:
-Increased water use efficiency: this allows plants to conserve water while assimilating carbon through photosynthesis, providing for metabolic and reproductive needs, although there may be trade-offs of decreased photosynthetic rates and stomatal conductances
-Succulents: water storage in above-ground organs allows organisms to withstand long dry periods
-CAM plants: these take up and store carbon dioxide during the day so they can close their stomata during the day, thus minimizing water loss caused by high temperatures.
-Larger body sizes: low surface area to volume ratio in many species, such as the desert javelina, creates lower thermal inertia as the body heats up slower than a species would with greater surface area. This allows many larger species unable to avoid high temperatures to tolerate them.
Description: This entails an individual's ability to change their physiology in response to conditions that same individual has experienced previously.[27]
-Eurythermality [25]: ability to maintainphysiological function over a wide temperature range
Examples:
-Plasticity in expression of certain enzymes (Hsps) [25]: Stress can induce expression of proteins near lethal temperatures that are critical to maintain or restore function following thermal perturbations. Ability to recruit certain proteins, such as molecular chaperones, minimizes costs or protein destruction to help maintain protein homeostasis. This cellular mechanism increases thermal tolerance of species.
Animals:
-Shedding of fur coats, feathers, or other warm outer layers: allows animals to decrease insulation and thus increase loss of internal heat to the environment to help maintain thermal homeostasis
-Dilation of blood vessels: increases radiation of internal heat to environment to help cool down animal
Plants:
-Turgor Loss Point: point at which leaf cells' loss of water and thus turgor pressure is sufficient to cause their walls to lose rigidity, causing the leaf to go limp [24]
When it reaches this point, the plant either closes it's stomata, which halts photosynthesis, or continues photosynthesis but risks extensive damage. Either way, conditions are unfavorable for growth and threaten survival of the plant. Plants with lower turgor loss points can maintain turgor, and thus rigidity, in drier conditions. This greater ability to maintain turgor pressure is a determinate of a plant's drought tolerance.
To lower turgor loss point, plants can make their cells saltier, allowing them to maintain their turgor pressure in times of reduced water availability. This salty sap confers increased drought tolerance of plants.
-Root system: plants develop more complex root system and higher ratio of root to shoot in order to optimize water collection and storage
Avoidance Mechanisms
Description/examples:
-Phenological plasticity:
Though warmer temperatures are seen to promote earlier spring but later phenological events in many climates, the primary determinant of phenology in the Sonoran Desert is probably water availability. [2]
Animals:
-Seek cool microclimate (shade, migration, burrowing, etc.): this can help species escape the scorching ground surface temperatures, which can reach to 70 degrees Celsius during the summer [35]
-Avoid activity during highest temperatures of daytime, at least during warm summer months (nocturnal animals) [35]
-Avoid certain foods: Consumption of high fat content produces excess heat that has to be lossed by evaporative cooling, which risks additional water loss. Consumption of protein requires more water consumption to help dilute toxic bodily wastes. [35] Thus, certain species limit consumption of these macromolecules in warmer, drier months and instead may consume foods with high water content, such as fruits
Plants:
-plasticity of resource allocation of flower versus seed/fruit production
-shedding of leaves during dry periods: this conserves water in the stems and minimizes water loss by leaf transpiration to prevent plants from dying during extreme droughts
Importance of Tolerance
Climate change is causing an increase in the frequency and severity of abiotic stresses. Organisms' tolerance levels will ultimately determine their abilities to withstand this climate change. In other words, organisms able to tolerate the forecasted increases in temperature and decreases in temperature may show no signs of decreases in fitness and could even have the chance to increase in abundance. Tolerance mechanisms allow organisms to cope with the changes in water availability and temperature that are the primary abiotic stresses that will affect the Sonoran Desert.
Tolerance of Increased Drought and Variation in Water Availability
Tolerance of Warmer Temperatures Throughout the Year
-Warm-adapted species actually appear to have less ability to acclimate to higher temperatures[27] and thus may live closer to upper thermal tolerance limits
Tolerance of Invasive Species and their Range Expansion
-Lots of data suggests that invasive species may have higher tolerance to climate change. This will undoubtedly have negative consequences on native species as they will be forced to compete with invasive species for the already scarce resources of the desert. Consider the THEORETICAL model below...
Analysis: Currently, the expansion of invasive species is limited mainly by below-freezing conditions in the winters. Thus, the current realized niche is a factor of the lack of tolerance for freezing conditions. Assuming the purple square represents the current Sonoran Desert climate, there are parameters limiting the range of the species. However, the fundamental niche, which represents the possible climatic conditions in which the species could survive, is determined by a species tolerance levels. Thus, since invasive species have tolerance to warm and dry conditions, there fundamental niche extends much beyond the area of their current realized niche.
As global climate change takes place, Sonoran Desert temperatures will get warmer and water will become scarcer, although there will probably be more variability in each of these conditions. Thus, the Sonoran climate will cover a slightly greater combination of abiotic variables, and will be shifted down and to the right in the graph. Based off the predicted fundamental niche of an invasive species, the species will now be able to tolerate all possible climatic combinations in the Sonoran Desert and thus may have the chance to spread.
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| Shift in Climate Variables Resulting in Species Range Expansion: A theoretical climate envelope model to depict species distributions and tolerance, by Ellen Campbell |
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