Descriptor 4: Food Webs
“All elements of the marine food webs, to the extent that they are known, occur at normal abundance and diversity and levels capable of ensuring the long-term abundance of the species and the retention of their full reproductive capacity”
What are food webs?
Food webs are networks of feeding interactions between consumers and their food (or predators and prey). This descriptor addresses the functional aspects of marine food webs, especially the rates of energy transfer within the system and levels of productivity in key components, and ecosystem structure in terms of size and abundance of individuals.
Figure 1. Illustration of a marine ecosystem.
Source: University of Waikato, NZ
All components of food webs are to be considered in the descriptor. However in practical terms, this descriptor is operational by including only those food web components that can effectively be sampled by robust methods of monitoring. The objective is the same as the one for fish and shellfish stocks (which are just one element of the food web – see Descriptor 3): ensure the long-term abundance of the species and the retention of their full reproductive capacity. In the case of Descriptor 4, the scope is much broader, including both exploited and non-exploited species.
Assessing the status of food webs is likely to include:
- Biological groups with fast turnover rates (e.g. phytoplankton, zooplankton, bacteria) that respond quickly to system change;
- Groups that are targeted by fisheries,
- Habitat-defining groups; and
- Charismatic or sensitive groups, which are often found at the top of the food web.
Why should we pay attention to marine food webs?
Healthy and abundant food webs are crucial to ensure the survival of species. The best way to measure the functioning of an ecosystem in terms of food webs is by measuring the ratios of production at different trophic levels, the productivity (production per unit biomass) of key species or groups and trophic relationships (i.e. relationships between species that have the same predators and prey in a food web).
By measuring energy flows, we can determine possible changes, for example low abundance/productivity in specific components of the food web or changes in predator-prey relationships. Changes in the size structure of individuals in the ecosystem (e.g. fish communities) or the relative abundance of the different species are also indicators of changes taking place in the food web.
The interactions between species in a food web are complex and constantly changing, making it difficult to identify one condition that represents “good” environmental status. Changes in the relative abundance of species in an ecosystem will affect interactions in several parts of a food web, and may have an adverse effect on the food web status. There is a need for better scientific understanding of the consequences on the ecosystem of such changes, as well as the value that society should attribute to marine food webs. It is important to bear in mind that all marine food webs have already been affected by humans.
What are the main pressures on marine food webs?
Fishing is the human activity which places the greatest pressures on fish stocks and consequently has direct effects on food webs. Fishing is usually size-selective so larger individuals generally suffer greater rates of mortality. Therefore exploited fish populations and communities contain relatively fewer large fish and the average size of these individuals is reduced. This may in turn have an indirect impact on their prey populations. Fishing intensity may be too high on specific components such as small pelagic fish (e.g. herring, sardine, anchovy, etc.), leading to dramatic effects on the food web as these species are, typically, important prey for many other species, including sea mammals. Conversely, a removal of a predatory fish component can result in completely different effects. The abundance (and distribution) of carefully selected populations can help describe food web status and/or levels of human perturbation.
In addition, there are numerous indirect effects of fishing, such as changes in abundance/productivity, or theeffects of destructive fishing practices leading to a deterioration of habitat (e.g. sea floor). Food web components are also subject to environmental and climate variation and other natural drivers, which sometimes makes the precise attribution of cause and effect difficult.
Pollution of the marine environment, by chemical substances for instance or by marine litter, is another direct pressure on marine food webs. Chemical substances, which accumulate in marine organisms – even the tiniest organisms present at the bottom of the food chain – will eventually find their way to the top of the food chain and contaminate top predators, such as large fish and marine mammals. The effects of contaminants on fish and marine mammals are described in Descriptor 8.
What can be done?
Fishing and pollution are the two main pressures on elements of the marine food webs. Ensuring normal abundance and diversity of the elements of marine food webs involves therefore taking actions to control fishing activities and levels of pollution in the marine environment. Actions related to the control of fishing activities are described in Descriptor 3, while the EU legislation and policies adopted to control the levels of contaminants and marine litter in the marine environment are described in Descriptor 8, 9 and 10.
In parallel to these actions, monitoring the status and the trends of marine food webs is essential in order to collect information needed to assess the current situation and implement the necessary preventive measures.
A rule of thumb when dealing with ecosystem functioning and structure is that they should be assessed using a variety of tools or indicators. This is because marine ecosystems are generally complex – even for relatively simple ecosystems such as the Arctic – and the available tools cannot capture all of this complexity.
Figure 2. Schematic illustration of a food web, produced with the Ecopath ecosystem modelling software. Source: Ecopath
One possible approach to monitor progress toward good environmental status is ecosystem modelling for which there are various tools and approaches available. A steady-state ecosystem model is depicted in Figure 2, showing the biomass (size of boxes) and the trophic relationships (connecting lines).
Energy flows, productivity, and biomass ratios can easily be estimated from these. Trophic levels of each functional group and of fisheries catches are easily calculated from such a model. More importantly, this model can be made dynamic simulating change over time.
Figure 3. Application of the Large Fish Indicator (LFI) to the Celtic Sea.
Source: ICES WGECO 2011
The boxes depict trophic functional groups, which may include one or more species, and the connecting lines depict predator-prey relationships.
Other, simpler, approaches can be used as indicators, such as the proportion of large fish from samples taken in surveys of European regional seas. This is related to the effects of fishing on the size structure of fish communities, where the proportion of larger individuals is seen to diminish over time as a result of exploitation.
The Large Fish Indicator (LFI) is the proportion of large fish (by weight) measured from survey samples, using various cut-off sizes (40cm, 45cm, 50cm, respectively from top to bottom). Note, on Figure 3, the clear decreasing trend starting in the 1990s, which is assumed to be an effect of fishing. Top