Changes to communities and structure
Marine communities are complex and there are many interconnections between species and with the habitat in which they reside. A community is simply a group of different species which are found living together in a particular environment or habitat, in other words, the ‘living’ component of an ecosystem. The fragile balance between each interconnected relationship within the community can be altered by even one small change, particularly if an important or 'keystone' species is impacted upon.
Tasmanian kelp forests have shown signs of receding for the last 30 years, and with them the entire communities based around them are changing. Like corals on the Great Barrier Reef, the kelp forests support a myriad of other species and are considered foundation or keystone species, creating habitat for many different creatures. Communities supported by the kelp are also lost when the kelp forests recede. While there are a number of external pressures on kelp such as marine pests (eg. sea urchins) and pollution, increases in water temperature associated with global climate change is also a suspected cause of kelp decline as kelp requires cold nutrient-rich waters to survive.
Cascading effects along the food chain can often occur. A food chain is used to explain particular relationships between species, a list of who eats who, the predators and the prey. This list, comprised of species that share a similar ecosystem or habitat, describes the trophic level of species. The trophic level of the species details where they sit in this chain, they may be a producer (plant) or consumer (animal), or if they are a consumer they may eat plants (herbivore), other animals (carnivore) or a bit of both (omnivore).
An example of a simplified Tasmanian marine food chain may be:
Algae – Sea Urchin – Rock Lobster – Octopus
There is evidence to suggest that octopus are increasing in numbers in Tasmania, which may impact lobster populations, which may then reduce predation on sea urchins. . .and so it goes on. The sea urchin, however, is a voracious consumer of algae or seaweed and can devastate kelp forest, an important habitat for rock lobster. This is made more complex by the fact the sea urchin in the above food chain, Centrostephanus rogersii, is an invasive warm-water species which has expanded its range or distribution from NSW to eastern Tasmania, which is likely to be a function of climate change. Another example of the cascading effects of changing climatic conditions on a Tasmania food chain may be:
Phytoplankton – Krill – Jack Mackerel
Simply put there is evidence to suggest that the strong La nina event in 1988/89 which caused an increase in water temperatures, set off a chain of events causing a decline in phytoplankton, then a decline in krill, and then a decline in Jack Mackerel, which caused the fishery to collapse in Tasmania.
New species moving into existing communities may provide a new food source for existing predators or compete directly with existing species. Sea urchins, for example, directly compete for food with native abalone, which also graze on algae. However, urchins are superior competitors and experiments have shown that an increase in urchin destiny causes an increase in abalone mortality.
Below, an example of the complex and interacting impacts of climate change on a simplified
Tasmanian marine community (Images: TAFI)
Introduced Marine Pests
Introduced marine pests are species that do not naturally occur in our waters and have been introduced through human activities. Most marine pests already in Australia have been introduced by vessels exchanging ballast water or biofouling (i.e. attached to the hull of vessels). In 2001, mandatory ballast water management requirements for international shipping were introduced to reduce the likelihood of marine pest introductions. Measures to manage international biofouling are currently being developed.
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While some introduced species do not survive, others increase rapidly to form self-supporting populations (also known as feral or naturalised populations) that can become a threat to human health, fisheries production and/or the natural marine environment. They often cause a drastic reduction in the abundance of native species. For example, sampling in the Derwent Estuary showed the five most dominant benthic invertebrates to be five introduced species, including the well known marine pest, the northern Pacific Seastar (Asterias amurensis).
Northern Pacific Seastar (Asterias amurensis) (Image: CSIRO)
Want to know more about pest species in our oceans? Check out these links:
The DPIPWE website is useful to find out more information on marine pests or to report a marine pest sighting:
Tasmanian State of the Environment Report, marine pests and diseases chapter for a summary of Tasmania marine pest issues.
Sources and acknowledgments:
A big thankyou to Zoe Doubleday (TAFI) and Al Morton (DPIPWE) for their contribution to the 'changes to community structure 'section of the Redmap site.
Strain, EMA and Johnson, CR (2009). Competition between an invasive urchin and commercially fished abalone: effect on body condition, reproduction and survivorship. Marine Ecology Progress Series 377: 169-182.
Edyvane, KS (2003). Conservation, monitoring, and recovery of threatened giant kelp (Macrocystis pyrifera) beds in Tasmania. Final report to Environment Australia. Hobart, Department of Primary Industries, Water and Environment.
Ling, SD, Johnson, CR, Ridgway, K, Hobday, AJ and Haddon, M (2009). Climate-driven range extension of a sea urchin: Inferring future trends by analysis of recent population dynamics. Global Change Biology 15: 719-731
Harris, GP, Griffiths, FB and Clementson, LA (1992). Climate and the fisheries off Tasmania - interactions of physics, food chains and fish. South African Journal of Marine Science 12: 585-597.
Harris, GP, Griffiths, FB, Clementson, LA, Lyne, V and Van Der Doe, H (1991). Seasonal and interannual variability in physical processes, nutrient cycling and the structure of the food chain in Tasmanian shelf waters. Journal of Plankton Research 13 Suppl.
