Food subsidies usually present an easy (“ready-made”) and abundant resource that is normally not accessible or available to organisms. Sources of food subsidies in ecosystems are varied and may be from human activities, for example carcasses discarded by game hunters that are fed on by scavengers [1, 2], rubbish dumps that are fed on by various species of predators (dingoes, coyotes and red foxes) [3]; or from natural processes, such as allochthonous dissolved and particulate matter that subsidise lake ecosystems and benefit both benthic and pelagic communities [4], marine carrion and detritus washed on shore providing a trophic base for terrestrial consumers [5], stranded kelp on beaches provides a food source for invertebrate shredders and juvenile fish [6]. Food subsidies in natural ecosystems have the potential to modify ecosystem dynamics [2] causing the decline or the loss of essential goods and services, threatening the world’s life support system [2].
Predictable Anthropogenic Food Subsidies (PAFS) [2] modify ecosystems by altering consumer-resource relationships, simultaneously benefiting several different trophic levels and changing food web structure [1–3, 7]. PAFS have the capacity to influence directly individuals’ fitness, population dynamics and community composition and interactions, resulting in substantial ecosystem modification, with effects pervading adjacent ecosystems [2–3, 5]. In cases where natural food resources are scarce, PAFS may be necessary to supplement energy requirements of some endangered species such as European vultures [8, 9]. Predictability in space and time of subsidised food supplies makes this food resource easier to access compared with many natural food sources [10, 11]. Predictability of food supply may influence, for example, a scavenger’s foraging behaviour and distribution (in time and space), factors important in the survival of both the predator and prey. In cases of environmental stress, predictability plays an important role in the survival of the scavengers [2] and studies have shown the importance of PAFS in improving demographic parameters for the viability of some endangered species [12, 13]. On the other hand, PAFS can have detrimental effects for example, negative density-dependant effects on fecundity of scavenger species [14]. In some cases, scavengers may increase in number and become over-abundant, modifying ecosystems through changes in food webs [2]. PAFS can also cause an ecological trap, where species face increased predation risk from opportunistic carnivores attracted to the feeding stations [15]. Finally, PAFS are considered to have potential as a powerful management tool in conservation and social issues [2], for example, where food is provided voluntarily to improve survival of endangered species, although there are risks associated with these animals then becoming dependent on these food sources [16].
The potential impacts of PAFS on ecosystems globally has led to the development of new environmental policies aimed at curbing the production or, ultimately, banning of PAFS. However, some policy decisions can influence or even contradict biodiversity management and conservation efforts [13]. In Europe, for example, sanitary regulations have drastically reduced carrion available to vultures [7]. The effects of sanitary policies on the demographic parameters of long-lived species such as some endangered European vulture species, was evident during the outbreak of Bovine Spongiform Encephalopathy (BSE) [13, 17]. The vulture population declined due to food scarcity, with the situation worsening in Spain when the policy implementation coincided with the deployment of wind farms within the foraging range of the starved vultures [18].
In marine ecosystems, discards from fisheries represent a major source of PAFS with over 7 million tonnes of catch discarded annually world-wide [19]. The first comprehensive report on by-catch and discards in world fisheries was published in 1983 [20]. Historically, discards were acknowledged as a component of fisheries before the Common Era [21], but have only been recognized as a management problem since the beginning of the 20th Century [22]. Given this long history, the animals in many ecosystems have adapted to this super-abundant source of energy for multiple generations. The sudden removal of such an abundant food source has the potential for marked impacts on a broad range of organisms across the entire food web, including seabirds [23], dolphins [24], sharks [25, 26], amphipods, isopods, cephalopods, ophiuroids, fish [27,28], hermit crab, starfish, whelk and crabs [29, 30]. Internationally, environmental policies are moving towards the banning of discards (e.g. the European Union (EU) Common Fisheries Policy proposed ban on discards, [23, 31, 32]) as part of the global push for greater environmental sustainability of fisheries [23, 31]. The implementation of the discard ban in the EU might still lead to perverse incentives and changes in fishing practices that have not been anticipated and there might be some unknown consequences where the fisheries do not recover but scavengers still suffer from food reduction. However, it is not known how a sudden or even gradual reduction in the availability of PAFS would affect food webs that have long been exposed to the ready food sources. Reducing or banning PAFS could do more harm than benefit to an ecosystem through cascading effects in the food web. In the event of a ban, those species that were previously dependent on PAFS may revert to their original diet, causing a population crash in that prey from sudden increases in predation pressure. An understanding of the effects of reductions in PAFS will assist in making informed decisions on the management of PAFS and conservation of PAFS-dependant species and help to move fisheries to a more sustainable basis.
Even though studies on the impacts of PAFS in ecosystems have been done [2, 5, 33], very few studies have modelled the role of PAFS in marine ecosystems [34–36]. Ecosystem modelling provides an approach to studying the effects of PAFS at very large spatial scales that incorporate natural processes like environmental variability and human activities such as fishing, agriculture etc. Through ecosystem modelling, it is possible to examine the effects of PAFS on ecosystem dynamics and improve our understanding of ecological roles of food and food webs in ecosystems.
In this paper, we explore the consequences of the removal of PAFS from a marine ecosystem under two scenarios, a gradual removal (over 20 years) compared with an abrupt ban on their release into the system. These two scenarios model real world situations where, first, legislation may be introduced to bring in bans on discards slowly over many years (e.g. green zones as part of Marine Park zonation [37]) versus an immediate ban on the release of discards into a region (e.g. the European Union Common Fisheries Policy [23, 31]). The following hypotheses were tested (after [2]):
- Reduction or loss of PAFS will modify trophic levels and food webs through effects on foraging by opportunistic species. Opportunistic species directly exploit PAFS and also consume species at lower trophic levels that also scavenge on PAFS.
- Reduction or loss of PAFS will increase the resilience of opportunistic species to food shortages. In cases of food scarcity, PAFS can provide supplementary energy requirements of opportunistic species, thus making them resilient to food shortages.
- Reduction or loss of PAFS will modify predator–prey interactions through shifts in prey consumption. Reduction of PAFS is expected to increase consumption of prey as food resources decrease.
