“An ecological cascade effect is a series of secondary extinctions that is triggered by the primary extinction of a key species in an ecosystem”. (http://en.wikipedia.org/wiki/Cascade_effect_(ecology))
Recently, the removal of apex predators from food chains in certain areas of the world has had a devastating effect on the ecosystem from which it derives, one of the most common predators removed being the shark. This is caused by factors such as over fishing, which has resulted in 11 species of great sharks being greatly reduced in the last 35 years (Ransom A Myers et al., 2007). Sharks are very sensitive to over-exploitation as they have evolved as the top predator and have not had the selection pressures on them to develop protection against predation by other species (e.g humans). This is shown for example in their reproductive habits, great sharks do not mature until a late age and do not reproduce often and when they do, large numbers of offspring are not produced as a defence against mortality, because there are few (if any) predators around that will target them. Now that humans have began to target sharks, particularly for their fins, reproduction rate of these great sharks can not exceed the rate of removal by humans, meaning that their numbers are declining.
Removal of the top predator results in growth of the populations of species in the trophic level below, because the Shark, or apex predator is not present to control the population sizes. This, in turn, causes more feeding activity of the growing population (e.g elasmobranchs such as rays), causing the species in the trophic level below that to decline (e.g scallops). Once the declining population have been wiped out, then the numbers of the growing population may decline as they run out of food and starve, or move to different areas. However, this is a very simplified example and in fact it is much more complicated due to the interweaving of different food webs, some species feeding on different trophic levels and so on.
The UNC (University North Carolina) have been running shark surveys off North Carolina since 1972 and they have indicated that there has been a steep decline in great shark populations.
These charts show how 6 species of Shark have reduced in their numbers in North Carolina Bay. The next set of charts shows how the abundance of elasmobranchs has increased in correspondence with the decline of great Sharks, due to less predation.
(Images: Ransom et al.2007)
8 species, including rays, smaller sharks and skates have increased from 1970 to 1990, just as shark populations have decreased in parallel. From this data an assumption could be made that because of the increase in elasmobranch abundance, the number of scallops (that these species feed on) would be reduced.
As seen in the final graph, Bay Scallop numbers have declined from 1970 to 1990, which proves this study of North Carolina Bay a good example of the cascade effect. As the great Shark (Apex predator) numbers decline, ray, skate and smaller shark (elasmobranch) numbers increase due to less predation, effectively allowing the elasmobranchs to become the top predator, which then causes the decline of Bay Scallops. Therefore, the removal of the top predator causes a large change in the community structure, although this can vary between species and ecosystem.
Such examples where the ecosystem might not change as drastically as the example above include where another large predator may take the place of the shark.
Also, many sharks feed on a variety of prey meaning that the shark doesn’t have a specific effect on just one species.
The example given above represents a top-down model, where the predator controls the organisation of the community, also known as the trophic cascade model.
Another example is a lake system with four trophic levels, the model predicts with the removal of the top carnivore, the primary carnivore numbers will increase, in turn, reducing the number of herbivores, increasing the phytoplankton abundance, and decreasing the levels of mineral nutrients. The effects show a positive/ negative change in trophic level structure as it moves down.
Using the same example, biomanipulation can be used instead of adding chemical treatments to prevent algal blooms and eutrophication. If the numbers of fish in the lake are high, then the abundance of zoo plankton is most likely to be low, this may cause an algal bloom or high instances of algae in the lake, causing hypoxia(Reece et al., 2011)(“the condition in which dissolved oxygen is below the level necessary to sustain most animal life- generally defined by dissolved oxygen levels below 2mg/l [miligrams/liter] (or ppm [parts per million])”) (http://toxics.usgs.gov/definitions/hypoxia.html).
To rectify this situation, fish can be removed from the lake, allowing the abundance of zoo plankton to increase naturally, this will decrease the number of algae present and allow oxygen levels to be restored. (Reece et al. 2011)
It has been seen that top-down impacts are caused by predation, this can be influenced by factors such as body size in that a top predator such as a shark can prey on organisms which are varied in size due to its feeding behaviour, in particular being able to tear up prey into bite-size chunks. However other species do not have the ability to do this and prey size is determined by the size of the predator and therefore removal of this organism would have a different effect on the particular community structure, as it would when a shark is removed from a different community. (Z. MaciejGliwicz, 2002)
Bottom-up impacts are quite obviously influenced from the bottom, up. Whereas predation is the influential factor in top-down impacts, it is food availability which influences bottom-up impacts.
In another example (reece et al. 2011), presence or absence of mineral nutrients controls plant numbers, which controls herbivore numbers, which then controls predator numbers. To change the community structure of a bottom-up community, nutrients need to be added to increase the biomass of the lower trophic levels, which will then increase the biomass of the subsequent trophic levels.
In conclusion, the cascade effect comes in two forms, bottom-up and top-down. Top-down is influenced by predation and relies on factors such as feeding behaviour and body-size of apex predators to show the typical positive/ negative changes in trophic structure when moving downwards. Bottom-up causes change in the community structure when food availability is changed. Less food available will reduce the numbers of the species and abundance in the trophic levels which feed on other species in the lowest trophic levels which have become less productive.
An increase in food availability will propagate an increase in the numbers of organisms in the above trophic levels.
Restoring community structure can be done by altering either the numbers of top predator in the top-down effect to prevent eutrophication, or adding/ removing mineral nutrients in the bottom- up effect to increase biomass of the community.
Cascade effect (ecology) (2012) http://en.wikipedia.org/wiki/Cascade_effect_(ecology)
Ransom A Myers, Julia K. Baum, Travis D. Shepherd, Sean P. Powers, Charles H. Peterson.CascadingEffects of the Loss of Apex Predatory Sharks from a Coastal Ocean. Science 30 March 2007:
Vol. 315 no. 5820 pp. 1846-1850, DOI: 10.1126/science.1138657 http://www.sciencemag.org/content/315/5820/1846.short
Jane B. Reece, Lisa A. Urry, Micheal L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson. Campbell Biology (ninth edition, 2011) Pearson Education, Inc. Page 1252, Bottom-up and Top-down controls.
Hypoxia http://toxics.usgs.gov/definitions/hypoxia.html. UGCS, Committee on Environment and Natural Resources, 2000.
Z. MaciejGliwicz, On the different nature of top-down and bottom-up effects in pelagic food webs.
Freshwater Biology. Volume 47, Issue 12, Article first published online: 19 Nov 2002. http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2427.2002.00990.x/pdf