A thermophile is an organism that tolerates high temperature. A poikilotherm is an organism whose internal temperature varies, often in response to the external temperature. An ectotherm is an organism whose body temperature is determined by the external environment. A homeotherm is an organism whose body temperature is regulated within narrow limits. An endotherm is an organism whose internal temperature is maintained by metabolic activity.
All organisms interact with the physical environment in two important ways. They obtain physical resources, the inorganic materials or energy they require for existence. Physical (or abiotic) factors are physical conditions that affect growth and survival. In reality, the line between physical resources and physical factors is indistinct. The law of tolerance states that there are upper and lower limits to the physical factors an organism can tolerate. Species differ in the factors that limit their growth and in their range of tolerance. Furthermore, the deleterious effects that occur outside the range of tolerance depend on the specific physical factor. Homeostasis is the ability to maintain physiological systems within certain limits across a range of external conditions. Consequently, there is strong selection for mechanisms that ensure that the tolerance limits match the abiotic conditions the organism typically faces. The adaptive process is complicated by the fact that many of the physical challenges faced by species are not constant. In general, it is much simpler to adapt to a harsh but constant environment than to one that is constantly changing. Moreover, unpredictable changes pose a greater challenge than predictable ones. The adaptive responses to the physical environment fall into two broad categories: 1) avoidance of harsh physical conditions or 2) adaptations that match the organism’s tolerance limits to the physical conditions it faces. The principle of allocation states that adaptations to one challenge may preclude or reduce adaptations to others.
Avoidance of harsh conditions entails two possible strategies: the organism can enter an inactive, resistant state until conditions improve, or it can move somewhere where conditions are more favorable. Torpor is a state of decreased physiological function during periods of harsh conditions. Hibernation is an extended form of torpor. Aestivation refers to periods of torpor or hibernation during the summer to avoid heat and water stress. Obligate hibernators must enter hibernation each year. Facultative hibernators do not have to become torpid but do so during harsh conditions and can arouse quickly by external stimuli. Migration is the seasonal movement from one region to another and back. Like hibernation, it may be obligate or facultative. This strategy is advantageous when the abiotic challenges are seasonal and thus predictable or there are appropriate cues that foretell a change in conditions. Migration and hibernation are strategies that avoid harsh conditions that occur over large spatial or temporal scales. However, sometimes the harsh conditions are local or only last a short while. In this case, avoidance may be a simple as moving about to a more favorable spot in the local environment. Behavioral thermoregulation are behaviors that allow the organism to seek and use external factors to change their internal temperature.
The physiological adaptations to difficult abiotic conditions fall into two broad categories: 1) some species’ physiology tolerates a wide range of internal conditions and 2) some species employ mechanisms that counteract the external challenge so that the internal conditions remain within narrow limits. Among the many physical challenge species face, two are nearly universal in their importance: temperature and water affect many species in some way. Tolerance limits can be shaped by natural selection. In general, poikilotherms tolerate greater variation in their internal temperature than homeotherms. Acclimation is the process in which an individual physiologically adjusts to challenging abiotic conditions. Most species can tolerate more difficult conditions if they are exposed to them gradually. In most species, populations that encounter greater variation in their abiotic environment acclimate more readily than other populations of the same species whose environment is constant. Heat shock proteins and cold-acclimation proteins protect organisms from sudden increases and decreases in temperature. There are five physical processes for heat gain or loss (metabolism, conduction, radiation, convection, and evaporation). The morphological, physiological, or behavioral adaptations to thermal stress are limited to these five mechanisms. Evaporation is the only means of lowering the temperature of a body if it is warmer than its environment. The body temperatures at which the metabolic rate increases to maintain a constant internal temperature are known as lower and upper critical temperatures. The thermal neutral zone is the range of temperatures between these upper and lower bounds, where no additional metabolic activity is required to maintain a constant internal temperature. Countercurrent heat exchanges are specialized morphological adaptations of the blood vessels that raise or lower the internal temperature relative to the environment. Plants make use of avoidance strategies, including seed design, dormancy, and dispersal mechanisms. The vast majority of plants are ectothermic. However, a few thermogenic species are able to raise their temperature above the ambient air temperature. Thermogenic plants produce heat by increased mitochondrial energy production. Many biochemical pathways require the input of water to function. The movement of water is governed by the water potential, an energy gradient between two systems caused the their relative water and solute concentrations. Water flows from high energy (high water potential) to low energy (low water potential). Three processes determine the total water potential – osmosis, pressure potential, and matric pressure. Plant cells develop pressure potential known as turgor potential. The water balance of any organism is determined by the value of the water potential between the organism and its environment. If the water potential of the organism is higher than its surroundings, it will tend to lose water by osmosis. An osmoconformer is an organism that allows its internal water balance and solute concentration to vary with the external conditions. An osmoregulator is an organism that maintains its internal water balance and solute concentration within narrow limits. The primary environmental dichotomy driving water potential in aquatic species is the osmotic difference between fresh and salt water. The fundamental physiological challenge of terrestrial organisms is water loss. Oxidative metabolism produces metabolic water that can be used by the organism. The amount of water held in the soil is its field capacity. The permanent wilting point occurs when the water potential of the soil falls so low that water cannot be extracted by the roots. The C4 pathway is a biochemical adaptation found in some species that allows the plant to take in CO2 while limiting evaporative losses through the stomata. Too much water also poses physiological problems for plants. Flooding affects the plant’s oxygen status. Adventitious roots are roots produced at or above soil surface.
—June 2021
Lance Jones 