Longer and Less Overlapping Food Webs in Anthropogenically Disturbed Marine Ecosystems: Confirmations from the Past

Abstract

The human exploitation of marine resources is characterised by the preferential removal of the largest species. Although this is expected to modify the structure of food webs, we have a relatively poor understanding of the potential consequences of such alteration. Here, we take advantage of a collection of ancient consumer tissues, using stable isotope analysis and SIBER to assess changes in the structure of coastal marine food webs in the South-western Atlantic through the second half of the Holocene as a result of the sequential exploitation of marine resources by hunter-gatherers, western sealers and modern fishermen. Samples were collected from shell middens and museums. Shells of both modern and archaeological intertidal herbivorous molluscs were used to reconstruct changes in the stable isotopic baseline, while modern and archaeological bones of the South American sea lion Otaria flavescens, South American fur seal Arctocephalus australis and Magellanic penguin Spheniscus magellanicus were used to analyse changes in the structure of the community of top predators. We found that ancient food webs were shorter, more redundant and more overlapping than current ones, both in northern-central Patagonia and southern Patagonia. These surprising results may be best explained by the huge impact of western sealing on pinnipeds during the fur trade period, rather than the impact of fishing on fish populations. As a consequence, the populations of pinnipeds at the end of the sealing period were likely well below the ecosystem's carrying capacity, which resulted in a release of intraspecific competition and a shift towards larger and higher trophic level prey. This in turn led to longer and less overlapping food webs.

Back to the future? Late Holocene marine food web structure in a warm climatic phase as a predictor of trophodynamics in a warmer South‐Western Atlantic Ocean

“…The declining levels of nutrient load in the Beagle Channel during the Late Holocene revealed by the declining values of δ 15 N in the mollusc shells (Saporiti, Bearhop, et al, ; this study) are the likely reason for such change in the relative contribution of phytoplankton and macroalgae to the C pool. Furthermore, debris from terrestrial vegetation is currently the most 13 C‐depleted source of organic matter in the Beagle Channel (Riccialdelli et al, ) and supply of terrestrial debris to coastal waters might be higher in the past due to a milder and wetter climate and hence a higher freshwater run‐off (Borromei et al, ; Kilian & Lamy, ; Ponce, Borromei, Menounos, & Rabassa, ; Tonello, Mancini, & Seppä, ).…”

“…The N baseline may shift throughout time due to changes in N fixation and denitrification and may result into dramatic changes in δ 15 N values of consumers even if trophic position has remained unmodified (Brugam et al, ; Robson et al, ; Saporiti, Bearhop, et al, ). The mollusc shells values reported here match the declining pattern since the Middle Holocene reported by Saporiti, Bearhop, et al () for the Beagle Channel and support the hypothesis that marine primary productivity was higher between 1,278 and 1,013 cal years BP than currently.…”

Isotopic niche partitioning between two apex predators over time

“…Similar patterns of isotopic niche segregation between the two species were also reported for the second half of the Holocene in northern Patagonia and Tierra del Fuego (Saporiti et al . ). Thus, ancient and historical South American fur seals foraged more pelagically (lower δ 13 C values) and at a lower trophic level (lower δ 15 N values) than contemporary sympatric South American sea lions everywhere in the Southwestern Atlantic.…”

“…), and stable isotope ratios of zooarchaeological bone material suggest that the same was also true during the second half of the Holocene (Saporiti et al . ). Nevertheless, the contribution of demersal prey to the diet of South American fur seals in northern Patagonia is negligible but they represent 16–37% of the biomass consumed by fur seals in areas influenced by the Río de la Plata estuary (Uruguay‐Argentina) (Naya, Arim & Vargas ; Franco‐Trecu et al .…”

“…; Saporiti et al . ; Zenteno et al . ) in order to first look at changes over time in resource partitioning between South American fur seals and sea lions and, second, to ascertain whether current resource partitioning in the Río de la Plata estuary and adjacent Atlantic Ocean is the result of recent anthropogenic forcing caused by two types of changes: changes in the relative abundance of South American fur seals and sea lions; and changes in the availability of pelagic and demersal prey.…”