The concept of food chains and webs is widely discussed, but can be frequently poorly understood. Complex interactions between marine organisms and their environment are what produce the resources utilised by coastal communities around Ireland, and the rest of the world. The basis of any food web, including the marine, is that of a primary producer. These organisms are the lowest level of the chain and provide all other levels with the energy required for survival. Photosynthesising phytoplankton (e.g. Diatoms, Coccolithophores, Dinoflagellates, and Cyanobacteria) utilise the sun’s energy to grow and reproduce and act as a vital food source for the next level of the web.
Other primary producers include larger algae and marine plant life. The next level is made up of the smallest floating animals: zooplankton. These organisms can be single celled or multicellular, such as amoeboids and cillates, and are eaten by larger zooplankton, like copepods and larval forms of mussels and jellyfish, small fish, and marine invertebrates.
Larger fish (e.g. herring [Clupea harengus]), jellyfish, squid, krill, and larger plankton feeders like baleen whales make up the third level of the web, which in turn are fed upon by the top predators. These top predators include, seabirds, marine mammals, and large predatory fish (e.g. Albacore tuna [Thunnus alalunga]). Finally, come human beings. Humans, as previously discussed, are what are posing the biggest threat to marine biodiversity. As human activity in coastal areas has increased, globally there have been dramatic reductions in marine biodiversity (Duffy, 2003). Reducing the populations of lower level organisms have knock on reductions to the higher levels, through depravation of food sources. Conversely, reducing the number of predators will cause an increase in numbers of the lower levels which will result in a “boom” of production in the lower levels. On a long term basis, this rapid proliferation eventually creates a depletion of resources leading to competition and population decline of the lower levels until the web itself collapses and ceases to exist. On a commercial level, the proliforation of smaller species due to lack of predation, may result in fisheries increasing total catches of smaller fishes, which in turn can can lead to overfishing. This process is known as ‘fishing-down-the-web’ and was first demonstrated by Daniel Pauly (Pauly et. al 1998).
Bottom trawling and dredging pose a most serious threat to the marine environment. Resuspended particulate matter prevents photosynthesis from occurring by blocking light. Without primary production the food web cannot continue to function. Even detritivores cannot survive once other organisms are removed. This delicate balance is further tipped by several traits of the creatures within the web. Small population size, small geographic range, slow growth and reproduction rates, and specialised ecological habitats are all natural limiting factors, which are placed under further strain by human activity (Pimm et al., 1988; Lawton, 1995; Didham et al., 1998; Purvis et al., 2000). It is for these reasons that certain areas and species come under legislative protection through the establishments of Special Protected Areas (SPAs), Special Areas of Conservation (SACs), and Marine Protected Areas (MPAs) and Reserves.
Duffy, J. E. (2003). Biodiversity Loss, Trophic Skew and Ecosystem Functioning. Ecology Letters, 6(8), 680–687.
Didham, R.K., Lawton, J.H., Hammond, P.M. & Eggleton, P. (1998). Trophic structure stability and extinction dynamics of beetles (Coleoptera) in tropical forest fragments. Philosophical Transactions of the Royal Society of London: Biological Sciences, 353, 437–451.
Lawton, J.H. (1995). Population dynamic principles. In: Extinction Rates (Eds Lawton, J.H. & May, R.M.). Oxford University Press, Oxford, pp. 147–163.
Pauly, D., Christensen, J., Dalsgaard, J., Froese, R. and Torres Jr., F. (1998) Fishing Down Marine Food Webs
Pimm, S. L., Jones, H. L., & Diamond, J. (1988) On the Risk of Extinction. Published by : The University of Chicago Press for The American Society of Naturalists. The American Naturalist, 132(6), 757–785.
Purvis, A., Gittleman, J.L., Cowlishaw, G. & Mace, G.M. (2000). Predicting extinction risk in declining species. Proceedings of the Royal Society of London: Biological Sciences, 267, 1947–1952.
WWW1 – Header Image
WWW2 – Simplified marine food web
WWW3 – Various Types of Zooplankton
WWW4- Fishing down the web.