ECOLOGY

ECOLOGY

The intermediate Disturbance Hypothesis

The intermediate Disturbance Hypothesis is one of the most controversial ecological presumptions. This is following its assumptions that species diversity maximizes when the ecological experiences neither too frequent nor seldom disturbance (Connell, 1978).  Therefore, at low intensity of disturbances, numerous competitive organisms will push to eliminate subordinate organisms from the ecosystem hence; causing the extinction of some organisms while others thrive.  In high levels of disturbance such as wild fire and human activities such as deforestation, all species face the risk of extinction. Hence, IDH theory believes in the moderate level of disturbance, which shall ensure that both selected opportunistic-selected and competitive K- selected species coexist (Connell, 1978).

This hypothesis assumes that when there are high or low levels of disturbance, many species might risk extinction. Hence, to avert the risk of ecological extinction and create harmony between organisms, there should be moderate levels of ecological disturbance. The ability for organism to coexist guarantees the continuality in the ecosystem where both completive and opportunistic organisms give each other room for survival (Connell, 1978). K-selected species tend to develop resilient and are competitive since these organisms occupy large resources and dominates stable ecosystem over a lengthy duration. On the other hand, opportunistic r-selected occupy open areas rapidly; areas cleared by any form disturbance recently. Therefore, areas that experience disturbance occasionally provides both competitive and opportunistic species survival merits in the same area.

 

 Literature Review  

Intermediate disturbance hypothesis states that some organism stands to hinder the development and survival of other inferior species if there is no disturbance. There are various forms of disturbances that may affect the diversity of an ecosystem. Some of the common disturbance includes insects, fire, floods, size, intensity and wind. The theory of intermediate disturbance hypothesis originated from Phillip Grime.  In 1973, Grime proposed the intermediate disturbance hypothesis. Later in 1975, Henry Horn echoed the same propositions, but it was until 1978 that Joseph Connell published the idea of IDH. Connell published the IDH theory in his book; “The diversity in tropical and coral reef”. Connell explained that the theory of intermediate disturbance hypothesis anticipated that ecosystems thrive under intermediate levels of disturbance. According to Connell, a high diversity ecosystem diversity of coral reef and tropical trees is achievable only in a non-equilibrium condition.  Although Grime did not publish a book about his IDH theory, most of his ideas are present in Connell book. The two scholars believed that if the non-equilibrium state between the tropical trees and the coral reef remains undisturbed, there would be a decline in the ecological equilibrium diversity (Connell, 1978). However, this may not occur if there is a climate change that supports the survival of one species. It is imperative to note that in such an ecological relationship between tropical trees and the coral reef reached at the equilibrium state, one of these species would have a higher niche diversification. Nevertheless, following the inconsiderate disturbance that affects the tropical trees and the coral reef, there can be no equilibrium state in their ecosystem. Connell proposed that if there were high levels of disturbance in the tropical trees and the coral reef ecosystem, the ecosystem would eliminate organism that lack swift re-colonization. On the other had, too low levels of disturbance would cause the low variety through competition. For example, too much disturbance such as strong winds and heavy rains affects negatively both the tropical trees and the coral reef; winds may cause tree breakage while storm affects the coral reef (Connell, 1978).

The concept species diversity in a non-equilibrium state is a complex mechanism that facilitates the coexistence of species in an ecosystem. Another test to validate the intermediate disturbance hypothesis is Wayne Sousa’s legendary test performed on the intertidal zones. Sousa conducted his test on IDH. In 1979, Souse conducted an IDH test on rocky foreshore zones; he found that intermediate disturbance rates created patches. These patches were different compared to those disturbed by either high or low levels of disturbances (Sousa, 1979). Sousa examined the effect of beach waves on foreshore organisms. He determined that the size of the organism and the force required in moving species from one place to the other applied in the IDH. This is because some small rocks registered more movement while large rocks seldom move; when the rocks rolled over to the attached organisms, there organisms encountered damages or displaced (Sousa, 1979).  According to Sousa, the measurement on the size of the rocks on the intertidal caused either low, or high intermediate disturbance intensity.  Sousa acknowledged that opportunist organism occupied the small rocks; some of these species includes barnacles and the sea lettuce. On the large rocks, there was low species diversity where in a competitive dominate the red algae occupied much area (Sousa, 1979).

Another intermediate disturbance hypothesis test to ascertain Connell’s theory is the test conducted by Jane Lubchenco in 1978 (Padisák, et al, 2011).  In her test, Lubchenco studied the outcome of the herbivorous snail on microalgae diversity.  This study took place on the coast of England on the both low and high surfaced of intertidal rocky zone. In her test, Lubchenco determined that algal diversity varied from pool to pool. This is because some pool had only the green algae while other pools accommodated more than 10 algae species (Padisák, et al, 2011).  Another observation noted by Lubchenco was the difference in the snail density from pool to pool. According to her findings, snails preferred the green algae and not the other species (Sousa, 1979). The ecosystem relationship between the snail and algae diversity had a different outcome on exposed boulders in low intertidal. The waves reduced the snail’s diversity, but their food preference remained the same. However, the intense wave disturbance restricted competition of the green algae. On the other side, at low snail density, numerous algae coexisted and some grew on each other. This increased the algae diversity gave the snail easy preferred food.

The test on physical system and the floodplain soils is proves the principles of Intermediate disturbance hypothesis. The study conducted by Rayburg, Melissa and Thompson validates the intermediate disturbance in increasing diversity ecosystem. In flood-disturbed ecosystem, intermediate disturbance proposition apply to aquatic and terrestrial vegetations (Carson & Schnitzer, 2008).  However, there is conflicting outcomes in some fish, macro-vertebrates or amphibians. In this test, the rationale of providing the validity of the intermediate disturbance hypothesis regardless to the form of disturbance is the notion that the frequency of intermediate disturbance encouraged diversity in an ecosystem.  This can be through averting the competitive segregation by the prevailing species; dominate species can lead to the uncommonly disturbed locations (Carson & Schnitzer, 2008).   More so, the intermediate disturbance hypothesis provides a valid principle since it facilitate greater diversity. This diversity is greater than the diversity experimented in sites with high frequency of disturbance where only species adaptive to the disturbance flourishes.

Over the year, some scholars have objected the intermediate disturbance hypothesis claiming that it is not scientifically recognized (Carson & Schnitzer, 2008).  One of the studies that do not support the intermediate hypothesis is the study conducted by Schwilk, Keeley, and Bond explaining that the intermediate disturbance hypothesis fails to explain fire and the diversity in fynbos. Fynbos is a scrubland in South Africa that encounter fire disturbance.  In fact, fire is the only disturbance form that affects this area.  According to the intermediate disturbance hypothesis, places sites that encounter intermediate frequency of distance should have high diversity while sites with low intermediate disturbance encounters very low or high disturbances (Barnes & Barnes, 1990).However, in the case of Fynbos, the intermediate disturbance does not provide the highest diversity like the hypothesis state. In the case of fire and the Fynbos ecosystem, there is no relationship with the frequent of disturbance and the ecosystem diversity.  Species in the fynbos area showed contradictory results that did not support the IDH theory. Species diversity was high at the least fire interval of 40 years. On the other hand, the species were low at the moderate area with 15 to 20 years interval fire (Barnes & Barnes, 1990).

In 1995, Collins conducted test analysis on the intermediate diversity disturbance and the preliminary floristic composition-decoupling source and effect. In his experiment, he used Tall grass plains and fire to find out how fire would influence the floristic composition.  However, upon his findings he found that neither high nor low fire diversity supported the intermediate disturbance hypothesis   (Collins, et al 1995). The study of tall grassland plains and their response to fire showed that both the study did not support the IDH theory or predictions. Instead of species reducing, there was vast increase in the glass species and the results that IDH between fire and tall glasses does not depend on each other. The reaction and the response of the tall trees dos not use the IDH principle. Instead, the response of the tall grass and the fire takes a different preposition (Collins, et al 1995).

According to Denslow in “The disturbance-mediated co-existence of species”, intermediate disturbance hypothesis is not scientifically correct because organisms adapts to both high, and low levels of disturbances in their ecosystems. For example, some species develop means of adapting to fire. For instance, species like ruderal plants have adopted ways of surviving in both high and low fire disturbance.  Therefore, the ability for some species to develop techniques of adapting to the various intensities of disturbances in their diversity shows that Connell’s theory of intermediate hypothesis is indeed not accurate.  It does not provide sufficient prove to supports that intermediate disturbances affects the diversity in a given ecosystem.

The intermediate disturbance hypothesis faces opposition in the tree diversity in the tropical rain forest.  This is because it is hard to predict how the environment will change in seasons to make judgment on the intermediate disturbance diversity.  This deficiency in produce absolute prediction shows that IDH gives impartial results (Carson & Schnitzer, 2008).  Such hypothesis that cannot support its assumptions is questionable. Another study that opposes to the intermediate disturbance hypothesis is the research about defensive damselfishes (Barnes & Barnes, 1990). According to “Oceanography and marine biology”, territorial damselfishes don not conform to the principles of intermediate disturbance hypothesis.  This is because the grazing intensity may be greater compared to the outside territories.  Thus, depending on the territorial difference, it is hard to predict which organism will have the competitive or opportunist advantage over the others.  For instance, the Gulf of California damselfish sustains an approximately a pure position of an algae genre in its territory. The relationship in the algae species in the Gulf of California damselfish is a case of disturbance dependent and not intermediate disturbance (Barnes & Barnes, 1990).  Another critique, about the intermediate disturbance hypothesis is that most authors only focused the calculating the outline of higher diversity at IDH, and availed little complement information explaining the probable mechanism of diversity and disturbance (Barnes & Barnes, 1990) This means that the hypothesis only offer a fraction of research findings about diversity and disturbance.

In conclusion, intermediate disturbance hypothesis is an ecological theory with conflicting principle; some scholars support its propositions while other refutes IDH validity.  In my opinion, intermediate disturbance hypothesis is a valid ecological hypothesis because the moderate level of disturbance in an ecosystem helps to provide balance diversity in any given ecosystem. For instance, intermediate disturbance ensures that tropical tress and tehe coral reef establishes a diversity ecosystem that supports the survival of the two species (Connell, 1978).   If the disturbance increases or reduces, both the tropical forest trees, and the coral reef encounters unstable ecosystem to coexist.   Too high and too low disturbance will pave way for either of the species to try to colonize the other in search for survival. The intensity of the disturbance is a significant factor in validating this hypothesis. In Sousa’s test of intermediate disturbance hypothesis, he notes that the intensity of the waves moved smaller rocks often while the large boulders seldom moved. In this test, the size of the boulder and the intensity of the intertidal waves describe the IDH theory (Barnes & Barnes, 1990).

The growing consensus in the validity of intermediate disturbance hypothesis is the ability to measure the disturbance intensity (Wagner, 2008). For instance, it is hard to measure  the disturbance level in categorizing whether it is too high , or low to achieve a high diversity in that ecosystem.  Scholars like Denslow challenges this hypothesis arguing that over time species develop means of adapting to the various forms of disturbances in their ecosystem. This adaptive ability to disturbances shows that species can coexist in any ecosystem if they have adaptive techniques (Golley, 1977).

Denslow’s disturbance meditated in the co-existence of organism theory is one of the developed hypotheses that seek to challenge the validity of IDH. This is because there are organisms that find surviving means amidst any level of disturbance. According to Denslow disturbance meditated proves that some organisms have devised ways of living with some form of disturbances in their ecosystem.  Another ecological disturbance hypothesis is the Clements ecological succession hypothesis (Golley, 1977). This disturbance hypothesis states that an ecological society undergoes more or less systematic predictable changes following preliminary colonization, or disturbance in a new environment (Golley, 1977).This hypothesis believes in succession after a disturbance in creating a new habitat.  For example, a fire disturbance can cause an ecological sequence where grass, and tress starts to grow after the fire disturbance.  The initial encounter with a disturbance in a grassland or forest leads to the introduction of a new environment.  This disturbance theory of ecology shows that although an ecosystem may encounter disturbance, there is succession after the disturbance and a new ecosystem develops (Golley, 1977).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Reference:

Barnes, H., & Barnes, M. (1990). Oceanography and marine biology: An annual review. Aberdeen: Aberdeen University Press.

Carson, W., & Schnitzer, S. (2008). Tropical forest community ecology. Oxford: Blackwell.

 

Collins, S.L., Glenn, S.M. & Gibson, D.J. (1995). Experimental analysis of intermediate disturbance and initial floristic composition – decoupling cause and effect. Ecology, 76, 486492.

Golley, F. B. (1977). Ecological succession. Stroudsburg, Pa: Dowden, Hutchinson & Ross.

Lindenmayer, D. B., & Burgman, M. A. (2005). Practical conservation biology. Collingwood, Vic: CSIRO Publishing.

Padisák, J., Reynolds, C. S., & Sommer, U. (2011). Intermediate disturbance hypothesis in phytoplankton ecology. Dordrecht: Springer.

Sousa, W.P. (1979b). Experimental investigations of disturbance and ecological succession in a rocky intertidal community. Ecol. Monogr., 49, 227254.

Wagner, L. N. (2008). Urbanization: 21st century issues and challenges. New York: Nova Science Publishers.

Connell, J.H. (1978). “Diversity in tropical rain forests and coral reefs”. Science 199

volume 1302–1310.