Visualisation of a marine food web. Illustration by Sebastian Dahlström
The ongoing loss of global biodiversity is probably old news to many people, but it remains a pressing issue. A simple google search reveals an endless stream of news articles and research papers documenting the decline of populations and species in habitats ranging from terrestrial to aquatic, tropical to polar. The primary drivers of this catastrophe have been direct human activities including overexploitation and habitat degradation, but these are likely soon to be eclipsed by the looming and varied effects of climate change, as already-beleaguered species find environmental conditions becoming less and less hospitable.
The big question is, what does the loss of organisms actually mean at a wider scale? How does the extinction of a single species, say an obscure deep-sea fish or planktonic shrimp (as a marine ecologist I feel obliged to use such ‘fishy’ examples), affect the functioning of regional ecosystems? To investigate this means diving into the world of species interactions.
The most obvious interaction between species is that of predators feeding on prey, and it’s this type of trophic relationship that I am most interested in. Everybody’s got to eat and, in fact, while nature can often appear chaotic and unorganised, this shared need provides structure to natural systems. In turn, this structure drives not only the functioning of ecosystems, but also their ability to cope with perturbations, as I’ll explain later on.
Take any group of species within a habitat and it is possible to organise them into who eats whom, thereby revealing the structure of the ‘food web’. This sounds pretty simple, but of course the difficulty is in knowing what each species consumes. For large animals in terrestrial environments, it might be enough to observe feeding behaviour, but what about organisms that we can’t always directly observe? This is where it gets a bit less glamorous. The most common method for identifying feeding interactions is to sift through stomach contents, identifying and documenting the different species within, as well as often the proportion of the contents that they each make up. This painstaking (and often smelly) method is repeated for as many different species as possible. Molecular methods including DNA identification and stable-isotope analyses can provide further detail to help distinguish less obvious species such as tiny marine plankton that otherwise would be hard to identify visually.
The result of these efforts is an understanding of the different pathways of energy flow through the ecosystem, from tiny phytoplankton in the ocean through various secondary consumers (e.g. zooplankton and small forage fish) all the way up to large marine mammals, seabirds and even us humans. We can then use our new knowledge of the structure of the food web to investigate how population reductions or extinctions of different organisms will propagate through the ecosystem, and which other species might be most affected. For example, simulated reductions in the biomass of target fishery species allow managers to check what knock-on consequences harvesting will have on other dependent species like protected seabirds or cetaceans. Knowing the range of prey and predators that different species have can reveal information as to their importance for maintaining energy flow within the food web, which is useful for targeting conservation actions.
One aspect of food web theory that I have found really interesting is the concept that, in some food webs, species may be organised into ‘compartments’, or groupings of species that interact more often with other species in their group than with species in other groups. A simple example of this could be species on the sea floor versus in mid-water – the majority of feeding links are restricted to either habitat, though a few bottom-feeding fish and invertebrates provide links between them. Any effect of the loss of a species within one compartment (say the sea floor) is most likely to be restricted to that habitat, with minimal effect (i.e. secondary extinctions) in the wider (midwater) food web. A compartmented structure therefore stabilises ecosystems in the face of environmental and/or ecological perturbations.
That was a bit of a whirlwind tour of food web research, and barely scratches the surface, but I hope it has emphasised the importance of considering each species in the context of its interactions with the wider ecosystem. My PhD focuses on food webs within the Southern Ocean, surrounding the Antarctic continent. Despite its image as ‘pristine’, the Southern Ocean has a history of human overexploitation (for example the hunting of the great whales), and is also increasingly being altered by climate change, with warming waters and melting sea ice. There will doubtless be changes in the community composition of Antarctic marine biota in consequence, and I aim to identify how the structure of different regional food webs may determine their vulnerability to such changes, and which species may be pivotal in driving ecosystem stability.
