Tolerance's Law: Understanding Ecosystem Resilience And Diversity

Shelford's Law of Tolerance, developed by zoologist Victor Ernest Shelford in 1911, states that an organism's success is determined by a complex set of conditions, including environmental factors such as temperature, physical conditions, and climate. This law, also known as the theory of tolerance, posits that each organism has specific minimum, maximum, and optimum environmental requirements for survival and reproduction. The law has implications for understanding the distribution of species within ecosystems, as an organism's tolerance for various factors will influence where it is found. For example, plants have varying tolerances for light exposure, and while they require some light, too much can be detrimental, causing physiological stress or even death due to water loss. Assessing ecosystem tolerance levels is crucial for development planning, especially in regions like Shif Island in Iran, where industrial development can impact soil and water quality, agriculture, and the natural ecosystem.

Species distribution

The distribution of a species is influenced by its range of tolerance, which refers to the environmental conditions within which a species can survive and reproduce. This includes factors such as temperature, humidity, pH, salinity, and resource availability. Species are best adapted to a specific set of environmental conditions, and these conditions determine where the species can thrive and establish populations.

For example, a species of fish with a narrow temperature tolerance range will be limited in its distribution to areas where water temperatures fall within this range. If the temperature increases beyond the upper limit, the fish may experience stress or even death, and if it drops below the lower limit, the fish may struggle to find food or reproduce. Thus, changes in environmental conditions within or outside the range of tolerance can lead to shifts in the distribution of species.

The law of tolerance, also known as Shelford's law, developed by zoologist Victor Ernest Shelford in 1911, elaborates on this concept. It states that each species has a minimum, maximum, and optimum range of environmental factors that determine its success. These factors can be abiotic, such as temperature, physical conditions, and climate, or biotic, such as competition, predation, and mutualism. The fundamental niche of a species is determined by its abiotic tolerance ranges, while the realized niche considers the influence of biotic interactions on the species' distribution.

The preferences and needs for certain environmental conditions greatly influence the distribution of species. For example, the sugar maple tree's distribution is largely influenced by temperature and precipitation. It cannot tolerate average monthly temperatures above 24-27°C or winter temperatures below -18°C, and its western limit is determined by dryness. Thus, the sugar maple's distribution is confined to areas within these tolerance ranges.

Abiotic factors

Shelford's Law of Tolerance, developed by zoologist Victor Ernest Shelford in 1911, states that an organism's success is based on a complex set of conditions. Each organism has a certain minimum, maximum, and optimum range of environmental factors that determine its success. These factors include abiotic factors, which are the non-living parts of an ecosystem that shape its environment.

In a terrestrial ecosystem, abiotic factors include temperature, light, and water. For example, the Yak lives in cold regions with low temperatures. The presence of long fur makes it possible for the Yak to survive in such conditions. Similarly, the characteristic movement of the sidewinder (Crotalus cerastes) is an adaptation that allows it to navigate the sandy deserts where it is found.

In a marine ecosystem, abiotic factors include salinity and ocean currents. For example, the eggs and larvae of blue crabs require higher salinity than adults. Additionally, factors such as rainfall, sand, and water are essential in desert and rainforest ecosystems, respectively.

The atmosphere, hydrosphere, and lithosphere all contain abiotic factors that contribute to the continuity of life on Earth by supporting the survival and reproduction of organisms. For instance, plants require sunlight for photosynthesis, and both plants and animals need water to survive.

The pH of the environment is another abiotic factor that affects living creatures. An increase in acidity due to rising carbon dioxide levels has negatively impacted marine life, such as coral and snails, as their protective shells dissolve.

Mutual species

Shelford's Law of Tolerance, developed by zoologist Victor Ernest Shelford in 1911, states that an organism's success is based on a complex set of conditions. Each organism has a specific range of environmental factors that determine its success. These factors include abiotic features such as temperature, physical conditions, and climate.

• Pistol Shrimp and Gobies: Pistol shrimp are burrowers that dig holes in the sandy seafloor, which they sometimes share with gobies. The shrimp benefit from the relationship by receiving tactile and chemical cues from the goby, indicating when to hide from predators or venture out. The goby also benefits by using these cues to spot potential threats and seek cover.
• Aphids and Ants: Aphids are small insects that secrete honeydew, a sugary liquid that ants feed on by 'milking' the aphids with their antennae. In return, ants protect aphids from predators and parasites, and some even move aphid eggs and nymphs underground to their nests, ensuring a constant supply of honeydew.
• Woolly Bats and Pitcher Plants: Woolly bats roost in Nepenthes hemsleyana, a tropical pitcher plant found in Borneo. The bat benefits by having a safe place to rest, while the plant gains nutrients from the bat's guano (faeces).
• Oxpeckers and Large Mammals: Oxpeckers are birds that feed on parasites such as ticks and blood-sucking flies found on large grazing mammals like wildebeest, rhinos, and zebras. While the birds get an easy meal, they may also help control the parasite load on the mammals. Additionally, oxpeckers have been observed warning their hosts of impending danger, such as helping rhinos evade humans.
• Clownfish and Anemones: Clownfish, also known as anemonefish, are immune to the stings of anemones, allowing them to nestle safely in their tentacles. In return, clownfish keep the anemones free of parasites and provide nutrients through their faeces, which may stimulate the growth of beneficial symbiotic algae. The movement of clownfish also helps circulate water and oxygenate the anemone.
• Honeyguides and Humans: Greater honeyguides lead humans to bee nests by emitting a demanding call. Once the humans locate the nest, they subdue the bees and harvest the honey, leaving behind beeswax, eggs, and larvae for the honeyguides to feed on.
• Senita Cactus and Senita Moth: The female senita moth collects pollen on its specialised abdominal scales and transfers it from flower to flower, acting as the only nocturnal pollinator of the senita cactus. While the cactus benefits from increased pollination, the moth lays its eggs on the cactus flowers, providing food for its larvae.

These examples illustrate the diverse and complex ways in which mutual species facilitate each other's presence and success within their respective ecosystems, showcasing the applicability of Shelford's Law of Tolerance.

Competitor species

Shelford's Law of Tolerance, developed by zoologist Victor Ernest Shelford in 1911, states that an organism's success is determined by a complex set of conditions, including environmental factors like temperature, physical conditions, and climate. This law also applies to competitor species within an ecosystem, which can restrict a species from a community by competing for resources such as food, space, and breeding sites.

Acclimation

The degree of acclimation is dictated by an organism's phenotypic plasticity, or its ability to change certain traits. For example, the ability of a microbial population to adapt and evolve in response to changed environmental conditions can be exploited for bioremediation purposes. Acclimation can also be seen in the ability of plants, such as maple trees, irises, and tomatoes, to survive freezing temperatures if the temperature is gradually lowered over a period of days or weeks. This same drop in temperature, if it occurred suddenly, could kill these plants.

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