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Invasive species

Gademann, Tamara (2019) Invasive species. Colloquium, Marine Biology.

Tamara_Gademann_Colloquiu_10.2019_Unstable syste_Kelp forests in Tasmania.pdf

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Kelp forest is a threaten habitat, due to climate change, but also because of human impact, i.e. pollution and overfishing. Extirpation of predators, leads into cascade effect: The herbivore population increases due to a leaking population of predators. Subsequently the primary producers decline, since they are food for the herbivore/primary consumers. In the case of kelp forests, sea urchin are those primary consumers, that increase, when their predators (secondary consumers) decline. The sea urchins feed on kelp (primary producers), which leads into a widespread kelp deforestation. In this colloquium I will give two examples of cascade effects of the kelp forests ecosystems. The first example deals with the sea otter situation at the West Coast of the US, the second with the current situation in Tasmania. USA: From 1700 until the early 1900 sea otters have been hunted for their fur, but sea otters are the secondary consumers of the kelp ecosystem, the predators of sea urchins. Due to the radical decreased otter population, the sea urchin population increased, and the kelp forest was deforested. Studies show that areas where otters are existing, sea urchin populations are controlled. Whereas in an area where the sea otter is absent, the sea urchin population is further increased (Carter et al., 2007). Studies show furthermore, that the absence of sea otters lead into a leak of other predator species, for example harbor seals decrease with the loss of the sea otters (Simenstad et al., 1978). Sea urchins increase means that kelp decreases, which means that the habitat for other species as fish decreases, too. And no fish means no food for a predator as the harbor seal (Simenstad et al., 1978). In 2006, Reisewitz et al. showed that the abundance of sea otters and kelp is combined: When sea otters are abundant, kelp and diversity is abundant, too. When sea otters are absent, sea urchins are abundant but nothing else. The second example is the current situation of the kelp forests of Tasmania. Since the marine waters around Tasmania are not part of a marine reserve, kelp forests are victims of overfishing. Which is the same situation as at the US West Coast. The secondary consumers in this kelp forest are probably rock lobsters, but they were fished a lot, why Tasmania also shows a cascade effect of fishing down the food web. During the last years a climate driven range extension of sea urchins was monitored, which leads into an increase of the sea urchin population of C. rodgersii. This increase succeed into a decrease of kelp and diversity. Currently observation and research just started in Tasmania, and scientists want to find out if rock lobsters as for instance the species Jasus edwardsii, is a natural predator of sea urchins. Because the topic just came up little is already known about it and results are not clear yet. But it is to say, that lobsters do feed on sea urchins. Open questions are still: Is the sea urchin the preferred food of the rock lobster? What is the actual size a lobster needs to feed on sea urchins, or does it matter? Does temperature play a role? How much energy does a lobster need to spend when feeding on a sea urchin; what is the energetic cost-benefit relation? Anyways, scientists also describe the climate driven range extension of sea urchins a lot, which indicates that sea urchins are an invasive species in Tasmanian reefs. In that case both examples are not comparable anymore.

Item Type: Thesis (Colloquium)
Supervisor name: Boer, M.K. de
Degree programme: Marine Biology
Thesis type: Colloquium
Language: English
Date Deposited: 07 Oct 2019
Last Modified: 08 Oct 2019 09:18

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