Ecosystem is a distinct space on earth where community of living organism including flora and fauna interact with the abiotic elements like air, water and soil as a system. The interaction between the biotic and abiotic components of the ecosystem is linked together through energy flows and nutrient cycles.
Biologists define biodiversity as the totality of species, genes and ecosystems of a region. This encompasses genetic diversity, species diversity and the ecosystem diversity. Traditionally, the term was used to communicate the species richness of an area. The latter definition of biodiversity is used in this essay.
The word stability is defined in detail in the relevant section. In broad terms, ecosystem stability is the capability of the ecosystem to return to its equilibrium state after a disruption.
The essay tries to identify the ecosystem function, which should be the indicator to demonstrate the stability of the ecosystem. This document also studies the effect of various biodiversity factors, which can modify the stability of ecosystem.
Framework for the Ecological Functions of Biodiversity
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Organisms in an ecosystem 'service' themselves and carry out certain other functions, which benefit other organisms of the community. These functions are the ecological functions of biodiversity. These ecological functions are classified into four categories depending on the complexity of the services provided by these organisms.
Complexity level 1: Functions performed primarily by Individual Organism
Energy fixed in the process of photosynthesis or chemosynthesis is called Primary Production. In photosynthesis sunlight is used as the source of energy and is fixed as carbohydrates, proteins fats and other organic compounds needed by all herbivores. In Chemosynthesis, oxidation or reduction of chemical compounds like that of sulpher is used for energy fixation.
Variety of primary producers existing in an ecosystem has a direct influence in the diversity of herbivores, carnivores and detritivores in the ecosystem. Symbiotic associations among organisms can sometime increase Primary production.
Oxygen production is one important function of biodiversity. The primary producers produce oxygen as a byproduct of photosynthesis.
Oxygen is required for respiration of aerobic biota and decomposition by detritivores. Oxygen is also required for Earth's protective ozone shield.
Sequestering of Carbon Dioxide
Many organisms in the ecosystem does the function of removal of carbon dioxide from the earth's atmosphere. They also help in removal of carbon dioxide from soil and water by process such as precipitation of calcium salts or amassing of organic deposits.
Marine life like charophytes use carbon dioxide to make limestone plates; protozoans absorb dissolved carbon dioxide to create coral reefs; trees, shrubs and herbs use atmospheric carbon dioxide during the process of photosynthesis.
Herbivorous animals eat primary producers. Herbivory is the function of Herbivorous animals. Herbivorous organisms serve the function of feeding carnivorous animals and thus maintain the food chain and food web of the ecosystem.
Carnivorous animals eat other animals. Carnivory is the function of Carnivorous animals. The carnivorous organisms functions as the balancing force to regulate the population of the herbivorous organisms in an ecosystem.
The trophic structures of an ecosystem would be simplified significantly if there are fewer carnivorous animals.
Control of Erosion
Erosion Control is a function provided by plants in a terrestrial ecosystem. The root growth of plants prevents soil erosion. Accumulation of litter and organic remnants of the plants helps in retaining and recycling nutrients. Consequently, the rate of soil build up will be faster than the erosion rate and thus control erosion. Aquatic plants play a vital role in the control of erosion in estuaries, riverine and marine ecosystem.
Complexity level 2: Functions involving interactions among low numbers of different kinds of organisms
Herbivores, Carnivores and Parasites deliver the function of population moderation in an ecosystem. Human species is the only species that could develop methods to control disease, parasites and predators selectively. This control measures has resulted in the elimination of natural method of population moderation for those selected species.
Dispersal and Migration
Dispersal/ Migration is an ecological function of organisms. The tendency of an organism to move away from its birth site is called 'natal dispersal' and from its breeding site is called breeding dispersal. The seeds and spores normally undergo passive dispersal using agents such as wind, birds, animals etc. This process of dispersal facilitates the development of new adaptive variants, which could re-colonize and restore, an already destroyed or highly modified natural ecosystem.
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Migration is a mass directional movement of large numbers of organisms from one location to another. This is a tactical response to overcrowding in an unpredictable environment, which affect the survival. Migration can provide ecological links between different habitats.
Symbiosis is a close and long-term, interaction between different species. These interractions may be mutually beneficial and the degree of benefit for each species is varied. Symbiosis is an ecological function of biodiversity.
Pollination is one area where symbiosis is very evident. The pollinating agents like insects and birds pollinate different kinds of flowering plants. The plants respond to this by evolving different flowering-time sequence, floral fragrances and markings. The reduction in any one of the symbiotic species can reduce the possibility of new symbiosis and new life forms.
Fragmentation of habitat, pollution and usage of pesticides affect symbiosis. This has resulted in extinction of many species of organisms in the ecosystem.
Complexity level 3: Functions involving interactions among large numbers of different organisms
Bioturbation is the activity of reworking of soils and sediments by flora and fauna of the habitat. This is a ecological function essential to maintain the quality of the abiotic ecosystem.
Bioturbation affects the texture of sediments, bioirrigation, displacement of abiotic particles and microorganisms. The growth of roots, tunneling by worms, burrowing by animals, ingestion and defecation of sediment grains, infilling of abandoned dwellings displace sediment grains and modify the sediment composition.
Climate moderation is an ecological function of biodiversity. Climate affects the biodiversity of an ecosystem and the biodiversity affects the climate of the ecosystem.
Macroclimate of a region is an abiotic factor, which affects the biodiversity of the region. The flora and fauna of a desert habitat is quite different from that of the tropical rain forests. This is especially true before establishing the vegetation in the region. Once the vegetation is established, the ecosystem will start influencing the climate. The transpiration from forest canopies and other vegetation can increase atmospheric humidity which can influence the rainfall.
Plants have an important effect on the microclimate of an ecosystem. Trees and shrubs affect the shade and humidity, which can affect the other biodiversity of the area.
Decomposition is an essential function to ensure natural recycling of residues of life. Decomposition requires oxygen. Primary detritivorous organisms consume free organic molecules and metabolize to create nutritive blocks, which are required by primary producers in a habitat. Secondary detritivorous organisms digest all kind of plant and animal tissues both dead and alive.
Mineralizing bacteria has the capability to metabolize toxic organic compounds and return harmless mineral matter. They play an important role in detoxifying soils, waters of the ecosystem.
Creation and Maintenance of Ecosystem Structures
Creating and maintaining a Ecosystem structure is a function of biodiversity. The evolution of different phyla into multicellular structures was derived from this ecological function. A tree can accommodate many other organisms like nesting birds, wood-boring insects and other parasites.
Communication is a profound function of biodiversity. The individual organisms of the same species and organisms of different species have diverse methods to communicate. These communication methods are essential for food gathering, courtship, herding, migration, flocking, escaping from carnivores and parenting. Methods used by organisms for communication could be sight, sound, smell, taste, radar, sonar, electric currents etc.
The communication significantly improves with the evolution of biodiversity. Many specialized flowers reflect ultraviolet light, which can be seen only by some pollinating insects.
Complexity level 4: Ecosystem Functions and Process
Food Webs and Chains -Trophic Structure
Trophic structure is the movement of energy through organisms in a community. This is an important ecological function, which enables species to utilize alternate trophic pathways, and shift from one to another depending on availability and thus meet the energy requirement. The species with multiple links in the food web has better survival rate than those with limited links. As the biodiversity evolves, the trophic structure will become complex and highly interlinked.
Nutrient Cycling is the movement and exchange of inorganic and organic matter back into the production of living organisms. The process of nutrient recycling is regulated through food webs pathways, which decompose matter into mineral nutrients. This is an ecological function, which describes the transport of nutrients such as phosphorus, nitrogen, potassium and trace elements through living tissue and the abiotic part of ecosystem. Nutrient cycling is a complex process, which affects the existence of ecosystem and the population of species in it.
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Stability is the factor that ensures the movement of ecosphere at a steady rate with no drastic change from its equilibrium. The stability is a very complex function with correlation factor mapping to all other ecological functions in various degrees.
Redundancy is an important factor that provides long term resilience to the ecosystem. Similarly the 'Generalist Behavior' of the species in the ecosystem reduces the risk of instability. Other important factor, which promotes stability, is the existence of Keystone species, which has higher influence over the entire biodiversity. The real long term stability can be evaluated by the ecosystem's capability for ecological succession.
The function of stability in an ecosystem is further elaborated in next section.
Harmony is the ecological function of biodiversity. Harmony in nature is pervasive, diverse and persistent. This exists at all levels necessary for the maintenance of the ecosystem. Harmony means the peaceful coexistence of different elements of the ecosystem like biotic and abiotic; flora and fauna; herbivores and carnivores etc. Another aspect of harmony is the inherent capacity for many life forms to live together within a larger ecosystem, and to make linkages, symbioses and co-adaptations.
Ecological stability is the measure of its immunity from perturbation. Ecologists define perturbation or disturbance as "any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment" (White and Pickett 1985:7).
Terminologies used to define Stability
The definition for the Stability of an ecosystem is currently not very precise. However the most accurate details for stability is provided by Orions (1974) where he has suggested that stability may mean many different things: the absence of change ('constancy'), the length of survival ('persistence'), resistance to perturbation ('inertia'), speed of return after perturbation ('elasticity'), the displacement from which return is possible ('amplitude'), the degree of oscillation ('cyclic stability'), and the tendency to move towards a similar end point ('trajectory stability').
The scientists over a period of time have used the above terminologies interchangeably and hence there is no common understanding. This essay uses the below mentioned definitions for clarity.
In the above terminologies, the persistence and elasticity have units in time (t) while Inertia and amplitude are expressed as functions of perturbation (x).
If Persistence is TP, Inertia is i(x), Elasticity is TE, and Amplitude is a(x).
When a perturbation is applied on an ecosystem, the ecosystem resist the disturbance by Inertia i(x) for a persistence period TP upto which the ecosystem can survive. If the cause for the disturbance can be absorbed or outdone, the system will persist and this is called resilience of the ecosystem.
Similarly when a perturbation is lesser than the amplitude a(x), then ecosystem tries to get back to its previously established stable state within an Elasticity period of TE. This is called Stability of the ecosystem. (Kolding, J. 1997. Diversity, Disturbance and Dubious Dogma.)
The ecological stability is a complex function, which has direct and indirect relationships with all the other ecosystem functions and other abiotic factors. For Example, the stability of the ecosystem has a relationship with other complex functions like trophic structure and nutrient cycling. These sub functions are further related to many other ecological functions like Primary Production, Herbivory, Carnivory, detritivory, Symbiosis etc. Similarly factors like natural abiotic perturbations such as wildfire, earthquake etc OR Anthropogenic perturbations like Oil-spill, building of dams etc. also affects the stability of the ecosystem.
Effort of scientists to model an ecosystem was not very successful because of the complexity of the model. The parameters involved in the model are dynamic and has multiple linkages to other parameters. Scientists attempted Lyapunov stability model for non-linear systems to model the ecosystem. However the results of the model is still under investigation with conflicting views about its utility.
The ecosystem modeling is a time consuming and resource hungry exercise and model can vary between different ecosystems. The primary task of stability modeling is to find the correlation of biodiversity and their different ecosystem function with different reasons of perturbation. These factors can then be integrated into a model. The effectiveness of the model need to be further tweaked through studies and investigations. Scientists believe that best method to test the applicability of this stability model is by using it to analyze ecological destructions. The projects like Biosphere 2 can contribute in developing the model.
Scientist believe that higher biodiversity in an ecosystem provides better stability. Department of Environmental Science and Policy, California in 1999 conducted a study on the effect of biodiversity on the ecosystem function.
In this study, the correlation of biodiversity and Ecosystem functions were qualitatively assessed to identify to one of the types of graphs shown below.
The study concluded that "Although 19 of 20 experimental tests identified a generally positive relationship between species number and ecosystem function, only 3 identified a type A response curve."
Thus it can be concluded that the stability of the ecosystem and biodiversity are generally maintaining a Type B graph. This means that stability of the ecosystem does not improve much once the biodiversity has achieved the saturation level.
However, the study assessed the linkages between stability and biodiversity through indirect means. The assumption of the scientists that higher ecosystem function means better stability is yet to be proved. Further the study did not use perturbations to measure the elasticity and amplitude of the ecosystem, which are more direct means to assess stability.
The species richness in the ecosystem beyond saturation point is the redundancy in biodiversity. This redundancy in biodiversity is an important factor, which helps stability of an ecosystem to recover from the effects of perturbation. This is theoretically demonstrated using Reliability Engineering principles by University of Minnesota.
If a perturbation affects population of a species in the biodiversity, the equivalent redundant species will take its role and keep the population of dependent species unaffected. This provides improved elasticity to the ecosystem.
Keystone species are the important species of an ecosystem whose absence is detrimental to the persistence of other species. Paine (1969) was the first researcher to coin the term "keystone species". According to him, keystone species are species of high trophic status whose activities exert a disproportionate influence on the pattern of species diversity in a community. The above definition did not help ecosystem managers and conservationist to identify the real keystone species. Hence keystone species was redefined as a strongly interacting species whose top-down effect on species diversity and competition is large relative to its biomass dominance within a functional group.
Keystone species support the stability of the ecosystem mainly in three ways.
Keystone predators maintain the population of lower level competitors and thus allow other species to survive.
The typical case of key stone predator controlling other low level competitors is that of the sea otter of West Coast of North America. Sea otters kept the population of sea urchins in check. In 18th and 19th centuries, sea otters were hunted to the point of near extinction. This resulted in the population explosion of sea urchin. Sea urchins decimated kelp beds, which was critical for spawning fish. This caused reduction in fish population and affected the fishing community of the region. Later an international treaty was signed to protect sea otters. Consequently, in many regions where sea otter population could recover, fish catch improved.
Keystone symbiotists provide resources for other organisms.
For example, in Western Australia, a tree, Acorn Banksia is the only source of nectar for honeyeaters in certain period of year. Loss of this species of tree would cause population collapse for honeyeaters (pollinators) and consequently the pollination of other plants would be affected. Thus the implication of losing this keystone mutualist species has profound effect on the ecosystem.
Keystone Engineers shape the environment for other organisms.
In the African savanna, the tree shade prevents growth of grass. Grass is a primary source of energy for herbivores. Elephants help in destroying trees and make room for grassland to develop. If this keystone engineer is removed from the ecosystem, within a few years, the populations of herbivores like antelopes, zebras, and deers would shrink.
A generalist species are species, which can survive in a wide variety of environmental conditions and varied diet. For Example, raccoon, which is found in North and Central America, is adoptable to wide range of environmental condition. It is omnivorous and eats berries, eggs, insects and small animals.
On the contrary, specialist species can thrive only in a limited range of environmental condition and can consume restricted diet. Koala, a monophagous organism, eats only eucalyptus leaves, is a typical example for specialist species.
Generalists have higher threshold to survive a perturbation and hence higher persistence. Thus an ecosystem with a predominantly generalist biodiversity is more stable than a specialist biodiversity.
Scientists believe that the toxins like allelochemicals, tannins, oxalates, terpenoids, alkaloids contained in the diet of many specialist-herbivores are higher than what other animals can afford to consume. In the absence of these specialists, population of toxic biodiversity cannot be kept in check. This would result in the reduction of population of other organisms and thus affect the stability of ecosystem indirectly. Thus the specialists also play a vital role in overall stability. However, their own existence itself is precariously balanced by their special requirements.
Ecological Succession is defined as a continuous directional change in an ecosystem over a longer time frame, which involves change in the composition and organization of the ecological community.
During this period depending on the genetic traits, the population of certain species may diminish or flourish. In certain cases, depending on the type of perturbations, new variants of species may evolve which is more adaptable to the changes. Scientists believe that ecological succession bring in stability.
The growth of hardwood trees like oak within the red pine forest is a typical example of ecological succession. The seedlings of hardwood trees are shade-tolerant while the pine tree seedlings need sun to thrive. Hence growth of pine tree seedlings in the shaded forest floor is difficult while the Oak seedlings would flourish. Over a period of time, oak would replace the red pine forest and would support a stable ecosystem for a longer time.
Extinction of certain variants and evolution of new variants are symptoms of a vibrant ecosystem. Though these ecosystems may appear unstable over a short duration, these structures would last longer and hence stable in the real sense.
Stability of Natural Ecosystem and Artificial Ecosystem
All the above-mentioned factors affecting the stability of Ecosystem are more applicable to a natural ecosystem. In an artificial ecosystem, human beings modify the ecosystem and play a more vital role in regulating the ecosystem functions by a process of feedbacks and control action.
Human is the only species, which could consistently maintain and increase the population over longer period of time. This is attributed to his ability to analyze the cause for perturbation and take necessary control measure. But mostly his response does not consider the effect of his control action to the biodiversity in the ecosystem. This leads to higher instability in the system.
Indiscriminate usage of pesticides to protect agriculture is a classic example of such activity. The pesticide while eliminating the pests would kill its natural enemies and would increase pest resistance. Pesticide also contaminates unintended water source and land when they are allowed run off fields or sprayed aerially which would destroy other biodiversity and thus reduce stability.
Biodiversity plays a vital role in maintaining stability in a natural ecosystem. However, to prove the statement quantitatively, lot more research have to take place. Developing an ecosystem model and perfecting the model through analysis on a continuous basis will be the step to understand this issue more accurately.
The short term and long term study on the stability is needed to understand the impact of biodiversity on the ecosystem. The changes in the ecosystem exhibited during the ecological succession process can be misunderstood for instability. In fact, the ecological succession process improves the stability of the ecosystem. This raises the question whether the extinction of species happening now is a part of ecological succession.
With the burgeoning of human population, more part of ecosystem may require large-scale modification and artificial control measures to support the trophic structure. This has to be carried out responsibly without impairing the stability of the ecosystem.