|Garbage dump in India.|
Human food waste (aka subsidies) may come from a variety of human activities, with the three most prominent being crop residuals (remnants of harvest remaining on fields), waste dumps, and fishery discards (by-catch thrown overboard). Each of these forms of subsidy occurs globally and large numbers of species rely partially or completely on them for food. For example, dumps are global in distribution, and contain enough edible waste to attract 20-30% of all mammal and bird in a region (particularly omnivorous and carnivorous species). Crop residues usually attract herbivorous or granivorous species (particularly birds), while fisheries’ waste alter marine ecosystems. Eight percent of all catch (~7 million tonnes!) is simply released back into the ocean, and this supplements species across the food web, including half of all seabirds.
Food waste from human activities may not seem so terrible – after all, they are predictably available, easy to access, fast to forage, and can lead to increased condition and fertility among species that take advantage of them. For example, seabirds foraging among fishing boats for by-catch take advantage of the predictability of boat (and food) appearances and as a result have decreased foraging time and areas, higher individual fitness and reproductive success, and ultimately increased population growth. But the authors suggest that these benefits must be considered in a more complicated web of interactions. After all, human food subsidies tend to be much more predictable than natural sources of food and quickly have large effects spanning from individuals, communities, ecosystems, and evolutionary pressures.
Individuals often, though not always, experience positive effects from subsidies – increased biomass, fertility, and survival, accompanied by changes in dispersal and ranges. If food waste draws in high densities of individuals, it may be associated with greater disease occurrence, or draw in predators attracted to easy pickings. Populations also often respond positively to food subsidies, and become larger and more stable as food waste availability increases. But this boon for one species can cascade through the food web, and have large negative effects in communities and ecosystems. For example, yellow-legged gulls are found around dumps and fishing trawlers, taking advantage of the quantities of food available there, and as a result have increased greatly in population. The downside is that in turn these larger populations increase predation pressure on other vulnerable seabird species. Seabirds in particular can create complicated interactions between human food waste and far-flung ecosystems, connecting as they do both terrestrial and marine systems, moving nutrients, pollution, and calories between systems and through trophic levels.
|Snow goose exclosure in northern Canada. |
Only the small green rectangle has avoided goose grazing.
Finally, and of most concern, food waste subsidies can alter the selective pressures a population faces. Species that become reliant on dumps or fields for food may experience changes in selective pressures, leading to selection for traits necessary to exploit these subsidies, and a loss of genotypic/phenotypic variation from the population. Changes in selective pressure change with the situation, of course. In the case of Yellowstone, the dump closure and loss of food source seemed to have large effects on traits important for sexual attractiveness in males, suggesting potential effects on reproductive success. In the best known (and my favourite) example of the selective effect of human food waste, dogs eventually were domesticated from wolf ancestors. Of course dumps can also relax selective pressures, if they allow individuals in poor condition (juveniles, the elderly) to successfully feed and reproduce.
Though food waste subsidies are clearly important and can have wide ranging effects, it is worth noting that the effect and importance of food subsidies is context-dependent. Studies seem to indicate that effects are greatest when food is low naturally or habitat quality is poor; in high quality systems, food waste may only be used by juveniles or individuals in poor condition. Unfortunately, as humans degrade natural habitats, subsidies are only likely to increase in importance as a food source for species. The extent and effects of human food waste are yet another legacy of the global alterations the human species has made. Unfortunately, like so many of the changes we have made, the issues are complex and transcend political and regional boundaries. Practices in one system or nation are tied to effects in another nation, and this complexity can make it difficult to monitor and measure the effects of subsidies as thoroughly as is necessary. This review from Oro et al. certainly makes a case for why our garbage needs to receive more attention.
|One example of the ecosystem wide effects of subsidies: here, fisheries inputs.|