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Contrasting consequences of species removal from higher and lower trophic levels in marine ecosystems

Geer, JD van de (2011) Contrasting consequences of species removal from higher and lower trophic levels in marine ecosystems. Bachelor's Thesis, Biology.

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Industrial fishing can have a profound impact on food webs present in the system by altering the balance between species. The change in abundance of species and competitive interactions can sometimes have further consequences than the species directly affected. The effects depend, among other things, on the trophic position of the target species. In this report I compare food web consequences between removal of upper level predators and lower level filter feeding bivalves, which have radically different functions in their respective food webs. Effects of removing predators from a food web depend on the predator diversity and niche overlap in prey consumption. When predator diversity is high, there is a greater chance that the loss of a specific predator can be overcome when other predator species feed on the same prey. If predators show very distinct niches, food webs may be far stronger affected, since their specific prey can flourish. It can also be so that predators don’t have a strong effect on their prey, which would produce very small effects in the food web. Lastly, an omnivorous predator can alter its diet when their regular prey becomes scarce. This predator would then switch to the lower trophic levels, stabilizing the food web by controlling the trophic cascades. Interestingly, in some cases predator removal led to higher predation pressure. This is thought to result from the lowered competition between predators. Removal of filter feeding bivalves has very different consequences. Studies show that bivalve communities can sustain great stress, where a large drop in population size does not necessarily have to mean the end for the species in that area. However, when repopulation is made impossible, such as storms and dredging removing the finer silt needed for younger individuals, the population drop can be quite significant. When the direct predators find less prey, they look around for similar bivalves, competing directly with species that are otherwise not competed with. Where removal of higher trophic levels results in varying new balances in the food web, removal of lower trophic levels often leads to severe damage to an ecosystem. These effects don’t limit themselves to the food web, but also impact the physical properties of the system. Removal of bivalves can severely affect the ability of the system to hold together sediment, for example. Fisheries must take into consideration that continued fishing pressure will result in weaker and weaker ecosystems, where every subsequent action will have a higher chance of completely disrupting it.

Item Type: Thesis (Bachelor's Thesis)
Degree programme: Biology
Thesis type: Bachelor's Thesis
Language: English
Date Deposited: 15 Feb 2018 07:46
Last Modified: 15 Feb 2018 07:46

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