Introduction
Dwindling numbers of large marine predators are one of the most pervasive signatures of ecosystem overfishing (Jackson et al. Reference Jackson, Kirby, Berger, Bjorndal, Botsford, Bourque and Bradbury2001), and the removal of top predators often spreads through food webs as trophic cascades (Frank et al. Reference Frank, Petrie, Choi and Leggett2005; Mumby et al. Reference Mumby, Dahlgren, Harborne, Kappel, Micheli, Brumbaugh and Holmes2006) and competitor release (Laws Reference Laws1977; Aalto and Baskett Reference Aalto and Baskett2013; Surma et al. Reference Surma, Pakhomov and Pitcher2014). The southwestern Atlantic Ocean is no exception, and the sequential development of industrial whaling, sealing, and fishing largely reduced the populations of many marine mammals and predatory fishes between the eighteenth and twentieth centuries (Vales et al. Reference Vales, Cardona, Loizaga, García and Crespo2020 and references therein). In parallel, most coastal predators in the southwestern Atlantic Ocean shifted their diets following the decimation of their own populations, which resulted in a significant increase in their trophic positions and a reduction in the degree of individual trophic specialization (Drago et al. Reference Drago, Crespo, Aguilar, Cardona, García, Dans and Goodall2009, Reference Drago, Cardona, Franco-Trecu, Crespo, Vales, Borella, Zenteno, Gonzáles and Inchausti2017; Zenteno et al. Reference Zenteno, Borella, Gómez Otero, Piana, Belardi, Borrero, Saporiti, Cardona and Crespo2015; Vales et al. Reference Vales, Cardona, Zangrando, Borella, Saporiti, Goodall, Oliveira and Crespo2017; Bas et al. Reference Bas, Briz i Godino, Álvarez, Vales, Crespo and Cardona2019, Reference Bas, Salemme, Green, Santiago, Speller, Álvarez, Briz i Godino and Cardona2020b).
In contrast to marine mammals and large fishes, seabirds have not been intensely exploited by humans in the southwestern Atlantic Ocean, although they were consumed regularly by hunter-fisher-gatherer people inhabiting the region since the middle Holocene (Tivoli and Zangrando Reference Tivoli and Zangrando2011; Borella and Cruz Reference Borella and Cruz2012; Zangrando and Tivoli Reference Zangrando and Tivoli2015), and European sailors and settlers hunted them for oil and collected their eggs (Armstrong Reference Armstrong1994; Cruz et al. Reference Cruz, Astete, Nauto and Borrero2010; Grosso Reference Grosso2016 and references therein).
Magellanic penguins (Spheniscus magellanicus) and Imperial shags (Leucocarbo atriceps) are currently the most abundant coastal seabirds off Patagonia and nest all the way from latitude 42°S to latitude 54°S (Frere et al. Reference Frere, Quintana and Gandini2005; Schiavini et al. Reference Schiavini, Yorio, Gandini, Raya Rey and Boersma2005). Magellanic penguins have dramatically increased both their population size and geographic range during the twentieth century (Boersma et al. Reference Boersma, Stokes and Yorio1990; Bouzat et al. Reference Bouzat, Walker and Boersma2009; Raya Rey et al. Reference Raya Rey, Rosciano, Liljesthröm, Samaniego and Schiavini2014). Information about the Imperial shags is scarcer and does not reveal any consistent trend in the region, although most colonies increased since the 1990s (Frere et al. Reference Frere, Quintana and Gandini2005; Raya Rey et al. Reference Raya Rey, Rosciano, Liljesthröm, Samaniego and Schiavini2014).
Magellanic penguins foraging off Patagonia feed mainly on small pelagic fish and juvenile Argentine hake, as well as squid and crustaceans (Frere et al. Reference Frere, Gandini and Lichtschein1996; Scolaro et al. Reference Scolaro, Wilson, Laurenti, Kierspel, Gallelli and Upton1999; Schiavini et al. Reference Schiavini, Yorio, Gandini, Raya Rey and Boersma2005; Scioscia et al. Reference Scioscia, Raya Rey, Samaniego, Florentín and Schiavini2014). This pattern is reversed in Tierra del Fuego, where pelagic crustaceans prevail in the diet of Magellanic penguins, although this has been a recent change (Dodino et al. Reference Dodino, Riccialdelli, Polito, Pütz and Raya Rey2020). In contrast, Imperial shags feed primarily on benthic fishes, although small pelagic fishes and cephalopods are also consumed (Gosztonyi and Kuba Reference Gosztonyi and Kuba1998; Punta et al. Reference Punta, Yorio and Herrera2003; Harris et al. Reference Harris, Quintana, Ciancio, Riccialdelli and Raya Rey2016).
Interestingly, the expansion of Magellanic penguins in Atlantic Patagonia has paralleled the decline of otariids and hake (Boersma et al. Reference Boersma, Stokes and Yorio1990; Koen-Alonso and Yodzis Reference Koen-Alonso and Yodzis2005; Vales et al. Reference Vales, Cardona, García, Zenteno and Crespo2015), and all three are preying largely on small pelagic fishes and squid (Angelescu and Prensky Reference Angelescu and Prensky1987; Koen Alonso et al. Reference Koen Alonso, Crespo, Pedraza, García and Coscarella2000; Baylis et al. Reference Baylis, Arnould and Staniland2014; Vales et al. Reference Vales, Cardona, García, Zenteno and Crespo2015). Predator decimation has resulted in the increase in biomass of small pelagic fishes and squid (Koen-Alonso and Yodzis Reference Koen-Alonso and Yodzis2005; Sánchez et al. Reference Sánchez, Navarro and Rozycki2012), thus suggesting that the expansion of Magellanic penguins might have resulted from a reduction of competition and increased food availability (Boersma et al. Reference Boersma, Stokes and Yorio1990).
Stable isotope analysis is a valuable technique to reconstruct historical changes in the diet of predators over long periods, because the stable isotope ratios in their tissues integrate those of their prey (Bearhop et al. Reference Bearhop, Adams, Waldron, Fuller and MacLeod2004), although turnover rates vary across tissues and integrate dietary information at different temporal scales (Bearhop et al. Reference Bearhop, Adams, Waldron, Fuller and MacLeod2004). Bone has a slow turnover rate, and stable isotope ratios in the bone organic matrix average the isotopic signatures of prey during several years (Tieszen et al. Reference Tieszen, Boutton, Tesdahl and Slade1983; Hobson and Clark Reference Hobson and Clark1992) and hence offer a proxy for individual variability comparable to repeated measurements of other tissues (Cardona et al. Reference Cardona, Martins, Uterga and Marco2017). This is particularly useful in birds, as diets may vary largely during breeding and non-breeding seasons (Hobson and Clark Reference Hobson and Clark1992; Silva et al. Reference Silva, Saporiti, Vales, Tavares, Gandini, Crespo and Cardona2014).
This study aims to test the hypothesis that Magellanic penguins and Imperial shags currently have increased their trophic position compared with their conspecifics living during the middle and late Holocene. To do this, we compare the stable isotope ratios of nitrogen in the bone tissue of modern Magellanic penguins and Imperial shags with those of ancient conspecifics recovered from archaeological sites throughout Atlantic Patagonia and the Beagle Channel coasts. Furthermore, we compare resource partitioning between ancient King penguins (Aptenodytes patagonicus) and Magellanic penguins from the late Holocene of southern Patagonia with that of their modern conspecifics in the nearby Malvinas/Falkland Islands, as they are not sympatric anymore on the mainland.
Materials and Methods
Study Area and Sample Collection
Both modern and archaeological samples were collected along the Argentine coast and grouped in three regions: northern Patagonia (Río Negro and Chubut Provinces), southern Patagonia (Santa Cruz Province and the Atlantic coast of Tierra del Fuego region), and the Beagle Channel (Tierra del Fuego Province). These three regions present different oceanographic features and distinct isotopic baselines (Saporiti et al. Reference Saporiti, Bearhop, Vales, Silva, Zenteno, Tavares, Crespo and Cardona2015). Additionally, with the purpose of comparing stable isotope ratios of ancient seabirds (this study) with those in their modern counterparts (Cherel et al. Reference Cherel, Pütz and Hobson2002; Weiss et al. Reference Weiss, Furness, McGill, Strange, Masello and Quillfeldt2009) in southern Patagonia, a fourth area was defined: the Malvinas/Falkland Islands (Fig. 1).
Figure 1. Map of the locations under study in Argentina (South America) showing the archaeological sites and sampled species. Dashed squares show the four large areas: northern Patagonia (A), southern Patagonia (B), Beagle Channel (C), and Malvinas/Falkland Islands (D). Black dots denote the acronym of each archaeological site (see Supplementary Table S1) with an extension showing the species sampled there (black and white animals). Gray dots denote the modern sampling locations with an extension showing the species sampled there (grayscale animals).
Bone samples of ancient seabirds (Magellanic penguins, King penguins, and unidentified cormorants/shags) and shells of ancient limpets (Nacella magellanica), Chilean mussels (Mytilus chilensis), and ribbed mussels (Aulacomya atra) were collected from archaeological sites dating back to the middle and late Holocene (Fig. 1 and Supplementary Table 1, respectively). Mollusk shells were used to reconstruct the isotopic baseline for each region and period. The stable isotope ratios for a few ancient fish species were obtained from the literature (Supplementary Table 1).
In order to avoid pseudoreplication, bones from the neurocranium (fishes) or long limb bones (birds) with the same laterality were used for both ancient and modern specimens. The skeletal elements analyzed for each species varied across archaeological sites, because of uneven occurrence, but intra-individual variability in the stable isotope ratios of skeletal elements of fish and birds is usually much smaller than interindividual variability (Bas and Cardona Reference Bas and Cardona2018; Hyland et al. Reference Hyland, Scott, Routledge and Szpak2021).
Stable Isotope Analysis
Modern samples were stored in a freezer at −20°C until analysis. Soft tissues were removed from the seabird bone and mollusk shells, rinsed with water, and allowed to dry at room temperature. Fishes were thawed at room temperature, boiled between 5 and 10 minutes, and dissected to remove the selected bones. Shells and bones were latter dried in a stove at 60°C for 24 hours.
Once dry, each sample was ground to fine powder using a mortar and pestle. Powdered shell samples were first demineralized by soaking in 1 N HCl until no more CO2 was released (Saporiti et al. Reference Saporiti, Bala, Gómez Otero, Crespo, Piana, Aguilar and Cardona2014a,Reference Saporiti, Bearhop, Silva, Vales, Zenteno, Crespo, Aguilar and Cardonab), rinsed with distilled water for 24 hours, and dried again for 24 hours at 50°C, and then lipids were removed through sequential rinses with a 2:1 chloroform:methanol solution until the solution was transparent (Folch et al. Reference Folch, Lees and Stanley1957). Samples were then dried again for 24 hours at 50°C, and 0.5 mg of each sample was weighed into a tin cup. Lipids were removed from dry, powdered bone samples as described above, but they were demineralized with 0.5 N HCl (Newsome et al. Reference Newsome, Etnier, Aurioles-Gamboa and Koch2006; Bas and Cardona Reference Bas and Cardona2018). Then, samples were dried again for 24 hours at 50°C, and 0.3 mg of each sample was weighed into a tin cup. Acidification has no significant effect on the δ15N value of mollusk shells (Carmichael et al. Reference Carmichael, Hattenrath, Valiela and Michener2008) or bone collagen (Tuross et al. Reference Tuross, Fogel and Hare1988). Tin cups were combusted at 900°C and analyzed in a continuous-flow isotope ratio mass spectrometer (Flash 1112 IRMS Delta C Series EA, Thermo Finnigan).
Abundance of stable isotopes is expressed using the δ notation, where the relative variations of stable isotope ratios are expressed as per mil (‰) deviations from a predefined reference scale: atmospheric nitrogen for δ15N. Stable isotopic reference materials of known 15N/14N ratios, as given by the International Atomic Energy Agency (Vienna, Austria), were used for calibration. Isotopic reference materials were employed to recalibrate the system once every 12 samples and were analyzed in order to compensate for any measurement drift over time. The raw data were recalculated taking into account a linear regression previously calculated for isotopic reference materials (Skrzypek Reference Skrzypek2013).
Statistical Analysis
The stable isotope ratios of modern and ancient organisms cannot be compared directly, because the isotopic baseline may vary temporally (Casey and Post Reference Casey and Post2011). Nonetheless, the proteins that make up the organic matrix of mollusk shells are preserved and offer suitable material to reconstruct the changes in the isotopic baseline (Casey and Post Reference Casey and Post2011; Drago et al. Reference Drago, Cardona, Franco-Trecu, Crespo, Vales, Borella, Zenteno, Gonzáles and Inchausti2017; Misarti et al. Reference Misarti, Gier, Finney, Barnes and McCarthy2017). First, the offset between the average δ15N of ancient limpets and mussels and that of their modern conspecifics was calculated when differences were statistically significant, and that amount was later subtracted from the δ15N values of ancient vertebrate samples to allow comparison with modern values (called the “correction factor”; Table 1).
Table 1. Results of general linear model (GLM) with two fixed factors (species and period) performed to assess the temporal variation of the δ15N values in shells and, when necessary, compensate for any isotopic baseline shift between the periods considered. N is sample size; δ15N (‰) is reported as mean ± SD. Correction factor (CF) was calculated by difference between mean isotope values of mollusks of modern and ancient samples. *Statistically significant differences (p < 0.05) between ancient and modern samples. †Stable isotope data from Bas et al. (Reference Bas, Salemme, Green, Santiago, Speller, Álvarez, Briz i Godino and Cardona2020b). ‡Stable isotope data from Bas et al. (Reference Bas, Briz i Godino, Álvarez, Vales, Crespo and Cardona2019).
Second, after correction for any baseline shift according to molluscan stable isotope ratios for each area and period, ancient and modern values of δ15N of Magellanic penguins and cormorants/shags were compared (see “Results”). It should also be noted that the published δ15N values of modern grenadier and eelpout were obtained from demineralized and delipided bone (Zangrando et al. Reference Zangrando, Riccialdelli, Kochi, Nye and Tessone2016) and muscle samples (Riccialdelli et al. Reference Riccialdelli, Newsome, Fogel and Fernández2017), respectively. According to Ankjærø et al. (Reference Ankjærø, Christensen and Grønkjær2012), δ15N values from muscle in fishes did not differ from those to bone collagen, and hence we used δ15N values from muscle of eelpout to compare it with the stable isotope ratios of bones from ancient conspecifics. In addition, published δ15N values of modern King and Magellanic penguins from the Malvinas/Falkland Islands were obtained from feathers (Weiss et al. Reference Weiss, Furness, McGill, Strange, Masello and Quillfeldt2009) and blood cell samples (Cherel et al. Reference Cherel, Pütz and Hobson2002), respectively. Therefore, the δ15N values of blood cells from Magellanic penguins were converted to those expected for feathers according to the offset between these two tissues of King penguins to allow comparison (Cherel et al. Reference Cherel, Hobson, Bailleul and Groscolas2005a,Reference Cherel, Hobson and Hassanib).
Third, the trophic position of each predator (TPp) was calculated as:
$${\rm T}{\rm P}_{\rm p} = [ {( {\delta^{15}N_p-\delta^{15}N_m} ) /3} ] + 2$$where δ15Np is the δ15N average values of each predator; δ15Nm is the δ15N average value of mollusks; “3” corresponds to the trophic discrimination factor; and mussels and limpets were considered herbivores at TP = 2 (Caut et al. Reference Caut, Angulo and Courchamp2009). Then, ancient and modern values for the trophic position (TP) of Magellanic penguins and cormorants/shags were compared.
All ancient and modern δ15N and TP values were compared independently using general linear models (GLM) as run in IBM SPSS Statistics (v. 23.0.0.2 for Mac), with two fixed factors (species and period) for invertebrates and one fixed factor (period) for the seabird species, unless otherwise stated. Then, Tukey's (HSD) post hoc tests were run to assess the temporal variation of the δ15N and TP values in each area. The Bonferroni correction was used to adjust α levels per test depending on compared periods per area, respectively.
Results
The δ15N values of all groups (species × period) were normally distributed and fulfilled the homoscedasticity requirement. The shells of ancient mollusks were always enriched in 15N compared with those of their modern conspecifics, and differences in the δ15N values of mollusks of all periods were statistically significant (Table 1, Supplementary Fig. 1, Supplementary Table 3), thus revealing the existence of a parallel drop in the δ15N baseline in the three regions during the past 2000 years. Accordingly, the stable isotope ratios from ancient penguins and unidentified cormorants/shags were transformed using the correction factors to allow comparison with those of modern conspecifics from the same region (Tables 1, 2).
Table 2. Archaeological and modern samples used in the current study. Superscript letters denote statistically significant differences (p < 0.05) between archaeological and modern samples; however, the Bonferroni correction was used to adjust the α levels per test depending on the number of periods compared. N is sample size; δ15N, δ15Ncorr (‰), and trophic position (TP) are reported as mean ± SD. Cormorants/shags are reported as Phalacrocorax/Leucocarbo spp., because ancient cormorants are unidentified cormorant/shag species. *Stable isotope data from Bas et al. (Reference Bas, Briz i Godino, Álvarez, Vales, Crespo and Cardona2019).
The δ15N values and the mean trophic position of modern Magellanic penguins were significantly higher than those of their ancient conspecifics in the three regions (Table 2, Fig. 2). In addition, statistically significant differences were also observed between ancient samples from different periods in the Beagle Channel (Table 2, Fig. 2). Likewise, the δ15N values and the mean trophic position of modern Imperial shags were significantly higher than those of ancient unidentified cormorants/shags in southern Patagonia and the Beagle Channel (Table 2, Fig. 2). Ancient Magellanic and King penguins from southern Patagonia differed in their average δ15N and hence foraged at a different trophic position (Student's t-test; Table 3), but the difference was not as large as currently exists in the Malvinas/Falkland Islands.
Figure 2. δ15N values (mean ± SD) of ancient and modern Magellanic penguins and cormorants/shags for each large area. Black circles: modern samples; white circles: ancient samples. See Supplementary Table 1 for acronyms.
Table 3. Archaeological and modern samples from Magellanic and King penguins used in the current study. Superscript letters denote statistically significant differences (p < 0.05) between contemporary samples. N is sample size; δ15N, δ15Ncorr (‰), and trophic position (TP) are reported as mean ± SD. *Stable isotope ratios of modern specimens from Weiss et al. (Reference Weiss, Furness, McGill, Strange, Masello and Quillfeldt2009). †Stable isotope ratios of modern specimens from Cherel et al. (Reference Cherel, Pütz and Hobson2002).
We lack bone samples from ancient fishes from northern Patagonia, but the δ15N values of ancient Magellanic penguins were only 4‰ higher than those of contemporary ribbed mussels (Fig. 3A), thus suggesting a lower trophic position than that of their modern conspecifics. Currently, the δ15N values of Magellanic penguins from northern Patagonia are similar to those of banded cusk eel, pink cusk eel, and cod icefish; 1‰ higher than those of hake; 3‰ higher than those of Argentine anchovy; and 6‰ higher than the average value of limpets and ribbed mussels (Fig. 3B).
Figure 3. δ15N values of invertebrates, pelagic fishes, benthic fishes, and Magellanic penguins for (A) Faro San Matías (Sondeo 2) and Bajada de los escadores 2 (3004–2197 cal yr BP) and (B) modern from northern Patagonia. Arithmetic mean and standard deviation (mean ± SD) are shown for each species. Key: squares, invertebrates (INV); diamonds, pelagic fishes (PF); triangles, benthic fishes (BF); circles, air-breathing predators (ABP). See Supplementary Table 1 for acronyms.
The δ15N values of ancient unidentified cormorants/shags from southern Patagonia were highly variable, but they were always in between those of predatory fishes such as hake, eelpout, snoek, and pink cusk eel and only 3‰–4‰ higher than the average of limpets and mussels (Fig. 4A–C). However, ancient Magellanic penguins from southern Patagonia were always extremely depleted in 15N, and their δ15N values were so low that they did not differ from those of contemporary limpets in Teis XI, Cabo Vírgenes, and Margen Sur (Fig. 4A–C), thus suggesting that they were not foraging locally most of that time. The same was true for ancient King penguins (Fig. 4B). Current Magellanic penguins and Imperial shags from southern Patagonia have similar δ15N values and higher values than those of fishes and mollusks: 1‰ above those of eelpout; 3‰ higher than those of hake, pink cusk eel, and Patagonian blenny; 6‰ above those of Patagonian grenadier; and 7‰ higher than the average of limpets and mussels (Fig. 4D).
Figure 4. δ15N values of invertebrates, pelagic fishes, benthic fishes, and seabirds for (A) Río Chico 1 and La Arcillosa 2 (6585–5776 cal yr BP); (B) Cabo Vírgenes 20 and Margen Sur (1131–885 cal yr BP); (C) Teis XI (442 cal yr BP); and (D) modern from the southern Patagonia. Arithmetic mean and standard deviation (mean ± SD) are shown for each species. Key: squares, invertebrates (INV); diamonds, pelagic fishes (PF); triangles, benthic fishes (BF); circles, air-breathing predators (ABP). See Supplementary Table 1 for acronyms.
Finally, the δ15N values of ancient Magellanic penguins from the Beagle Channel were only 3‰–5‰ higher than those of mollusks (Fig. 5A–C), and ancient unidentified cormorants/shags had extremely low values of δ15N (Fig. 5B). Conversely, both species of modern seabirds have δ15N values similar to those of eelpout and cod icefish and 8‰ higher than the average of limpets and mussels (Fig. 5D).
Figure 5. δ15N values of invertebrates, pelagic fishes, benthic fishes, and seabirds for (A) Imiwaia I (M/K) and Mischiúen (F) (6723–5014 cal yr BP); (B) Lanashuaia II, Mischiúen (C) and Shamakush I (D) (1278–772 cal yr BP); (C) Shamakush X (E) and Kaiawoteha III (K) (487 and 545 cal yr BP, respectively); and (D) modern from the Beagle Channel (Tierra del Fuego). Arithmetic mean and standard deviation (mean ± SD) are shown for each species. Key: squares, invertebrates (INV); diamonds, pelagic fishes (PF); triangles, benthic fishes (BF); circles, air-breathing predators (ABP). See Supplementary Table 1 for acronyms.
Discussion
The protocol used here would allow obtaining unbiased δ13C and δ15N values for bone collagen, as they are measured using the standard procedure for this type of sample (Newsome et al. Reference Newsome, Etnier, Aurioles-Gamboa and Koch2006; Guiry et al. Reference Guiry, Szpak and Richards2016; Bas and Cardona Reference Bas and Cardona2018; Guiry and Hunt Reference Guiry and Hunt2020). DeNiro (Reference DeNiro1985) reported carbon to nitrogen (C:N) atomic ratios of bone collagen to range from 2.9 to 3.6, and this has been the standard requirement for decades both in ecology and archaeology. Most of the samples analyzed here satisfied this requirement (Supplementary Table 2), but recently Guiry and Szpak (Reference Guiry and Szpak2020, Reference Guiry and Szpak2021) have reported a much narrower acceptable range (3.0–3.3). Accordingly, many samples in this study with C:N atomic ratios ranging from 3.4 to 3.6, may still contain some traces of lipid or humic acid, in modern and ancient samples, respectively, and hence might yield slightly biased δ13C values. For this reason, we discuss here only their δ15N values, as neither lipids nor humic acid contain nitrogen, and hence collagen is the only source of nitrogen in acidified bone samples (Bas and Cardona et al. Reference Bas and Cardona2018; Bas et al. Reference Bas, García, Crespo and Cardona2020a; Guiry and Hunt Reference Guiry and Hunt2020; Guiry and Szpak Reference Guiry and Szpak2020, Reference Guiry and Szpak2021). It should also be noted that the organic matrix of mollusk shells is a mixture of proteins and chitin, a polysaccharide containing nitrogen (Furuhashi et al. Reference Furuhashi, Schwarzinger, Miksik, Smrz and Beran2009). As a result, the C:N ratio of the organic matrix of mollusk shells including equal amounts of protein and chitin is close to 5.5 and hence differs from that expected for collagen.
The results reported here reveal major changes in the δ15N of Magellanic penguins and cormorants/shags in the southwestern Atlantic Ocean since the middle Holocene. Certainly, sample size for some species and archaeological sites is small, but differences between ancient and modern conspecifics are so huge and consistent across areas, particularly for Magellanic penguins, that we believe that our conclusions are robust. It should be noted that male and female modern Magellanic penguins do not differ in their average δ15N values (Scioscia et al. Reference Scioscia, Raya Rey, Samaniego, Florentín and Schiavini2014; Silva et al. Reference Silva, Saporiti, Vales, Tavares, Gandini, Crespo and Cardona2014; Barrionuevo et al. Reference Barrionuevo, Ciancio, Steinfurth and Frere2020; Rosciano et al. Reference Rosciano, Polito and Raya Rey2020; Dodino et al. Reference Dodino, Lois, Riccialdelli, Polito, Pütz and Raya Rey2021), and the comparison of δ15N in ancient and modern Magellanic penguins is therefore unlikely to be affected by biased sex ratios. Furthermore, the use of stable isotope ratios from ancient and modern mollusk shells allowed us to account for temporal changes in the isotopic baseline (Casey and Post Reference Casey and Post2011; Misarti et al. Reference Misarti, Gier, Finney, Barnes and McCarthy2017; Bas et al. Reference Bas, Briz i Godino, Álvarez, Vales, Crespo and Cardona2019) and compare the stable isotope ratios of ancient and modern seabirds, although selecting the most suitable isotopic baseline for migratory species is challenging. The use of compound-specific isotopic analyses of individual amino acids offers an alternative approach to identify the relative contribution of changes in diet or baseline to the variability of δ15N in consumers (Lorrain et al. Reference Lorrain, Graham, Ménard, Popp, Bouillon, van Breuguel and Cherel2009) but requires a much larger amount of protein (3 mg), which limits the technique to bulky samples.
The analysis of mollusk shells confirmed a drop in the δ15N baseline of the three regions considered during the past 2000 years. Such a pattern has already been suggested by Saporiti et al. (Reference Saporiti, Bala, Gómez Otero, Crespo, Piana, Aguilar and Cardona2014a) with a more limited data set and was likely caused by a decrease in the intensity of vertical mixing and coastal upwelling (Somes et al. Reference Somes, Schmittner, Galbraith, Lehmann, Altabet, Montoya, Letelier, Mix, Bourbonnais and Eby2010). If so, the primary productivity of the coastal ecosystems of the southwestern Atlantic Ocean south to latitude 40°S is currently lower than during most of the late Holocene (Saporiti et al. Reference Saporiti, Bala, Gómez Otero, Crespo, Piana, Aguilar and Cardona2014a). Furthermore, the coastal areas of the southwestern Atlantic Ocean currently support smaller populations of top predators due to sequential sealing and fishing since the late eighteenth century (Vales et al. Reference Vales, Cardona, Loizaga, García and Crespo2020 and references therein). Both processes might have operated synergistically to modify the diet of Magellanic penguins and cormorants/shags and might contribute to the changes in δ15N reported here.
The optimal foraging theory predicts an increase in the trophic position of predators far below carrying capacity as they experience higher per capita food availability. This is because predators close to satiation select prey to maximize net energy intake, whereas hungry predators close to carrying capacity are less selective and capture prey according to their encounter rates (Schoener Reference Schoener1971; Pulliam Reference Pulliam1974; Stephens and Krebs Reference Stephens and Krebs1986). On the other hand, prey density in aquatic ecosystems decreases with body size (Blanchard et al. Reference Blanchard, Heneghan, Everett, Trebilco and Richardson2017), whereas the trophic position of prey is positively correlated with body size (Jennings Reference Jennings, Belgrano, Scharler, Dunne and Ulanowicz2005). From here follows that aquatic carnivores close to carrying capacity rely largely on small, highly abundant prey with a low trophic level. However, in a scenario in which the populations of predators decrease as a result of harvesting, individuals can shift to scarcer but more rewarding prey with a larger body size and a higher trophic position, due to a relaxation of intraspecific competition. This mechanism may explain the increase in the δ15N values of Magellanic penguins and cormorants/shags following competitor release as a result of the sequential exploitation of otariids and hake since the late eighteenth century.
Currently, the diet of Magellanic penguins is dominated by fish over most of its range (Scolaro et al. Reference Scolaro, Wilson, Laurenti, Kierspel, Gallelli and Upton1999; Clausen and Pütz Reference Clausen and Pütz2002; Schiavini et al. Reference Schiavini, Yorio, Gandini, Raya Rey and Boersma2005; Weiss et al. Reference Weiss, Furness, McGill, Strange, Masello and Quillfeldt2009; Scioscia et al. Reference Scioscia, Raya Rey, Samaniego, Florentín and Schiavini2014), although the squat lobster (Munida gregaria) is also a major component of the diet of Magellanic penguins in the Malvinas/Falkland Islands (Thompson Reference Thompson1993; Pütz et al. Reference Pütz, Ingham, Smith and Croxall2001; Clausen and Pütz Reference Clausen and Pütz2002) and very recently has become their staple diet in the Beagle Channel (Dodino et al. Reference Dodino, Riccialdelli, Polito, Pütz and Raya Rey2020). Squat lobsters have two morphs, and the specimens of the pelagic one are highly depleted in 15N compared with both benthic and pelagic fishes (Ciancio et al. Reference Ciancio, Pascual, Botto, Amaya-Santi, O'Neal, Riva Rossi and Iribarne2008; Drago et al. Reference Drago, Crespo, Aguilar, Cardona, García, Dans and Goodall2009; Dodino et al. Reference Dodino, Riccialdelli, Polito, Pütz and Raya Rey2020). Accordingly, the low δ15N values of ancient Magellanic penguins and unidentified cormorants/shags might have resulted from increased consumption of pelagic squat lobsters during the late Holocene compared with modern conspecifics.
Squat lobsters have a much lower energy density than small pelagic fishes (Thompson Reference Thompson1993; Ciancio et al. Reference Ciancio, Pascual and Beauchamp2007) and hence the former are expected to be consumed by Magellanic penguins only when the latter are scarce. This prediction fits the expectations of the competitive release hypothesis, because large populations of piscivorous fur seals, sea lions, and hake might have reduced dramatically the availability of small fishes for seabirds during the late Holocene. However, this is not the only possible explanation for the pattern reported here because of the changes in primary productivity reported earlier. Since 2012, the abundance of pelagic squat lobsters has increased markedly in the Beagle Channel, as a result of increased primary productivity (Diez et al. Reference Diez, Cabreira, Madirolas and Lovrich2016), which in turn has triggered a major dietary shift in Magellanic penguins (Dodino et al. Reference Dodino, Riccialdelli, Polito, Pütz and Raya Rey2020). It is worth noting that the samples of Magellanic penguins from the Beagle Channel analyzed here were collected in 2010, before the increase in the population of pelagic squat lobsters. The trophic position of the Magellanic penguins from the Beagle Channel reported here is similar to that reported by Dodino et al. (Reference Dodino, Riccialdelli, Polito, Pütz and Raya Rey2020) for penguins sampled in 2009, but much higher than those sampled from 2013 to 2017 and reported to rely largely on pelagic squat lobster (Dodino et al. Reference Dodino, Riccialdelli, Polito, Pütz and Raya Rey2020).
The abundance of pelagic squat lobsters is highly dependent on the intensity of vertical mixing and the level of primary productivity (Diez et al. Reference Diez, Cabreira, Madirolas and Lovrich2016), and the high δ15N values of ancient limpets and mussels reported here suggest a more intense coastal upwelling and vertical mixing during the second half of the Holocene (Somes et al. Reference Somes, Schmittner, Galbraith, Lehmann, Altabet, Montoya, Letelier, Mix, Bourbonnais and Eby2010; Saporiti et al. Reference Saporiti, Bala, Gómez Otero, Crespo, Piana, Aguilar and Cardona2014a). If so, the abundance of pelagic squat lobsters was perhaps much higher in the past, independent of the abundance of small fishes, and possibly due to more favorable environmental conditions. In this scenario, the lower trophic position of ancient Magellanic penguins and unidentified cormorants/shags might be just the result of a higher availability of pelagic squat lobsters resulting from a bottom-up processes.
The migratory behavior of Magellanic penguins (Stokes et al. Reference Stokes, Boersma and Davis1998; Pütz et al. Reference Pütz, Schiavini, Raya Rey and Lüth2007; Barrionuevo et al. Reference Barrionuevo, Ciancio, Steinfurth and Frere2020; Dodino et al. Reference Dodino, Lois, Riccialdelli, Polito, Pütz and Raya Rey2021) and the low turnover of bones (Tieszen et al. Reference Tieszen, Boutton, Tesdahl and Slade1983; Hobson and Clark Reference Hobson and Clark1992) add additional complexity to interpreting the temporal changes of δ15N values in these species. Currently, Magellanic penguins nesting in northern Patagonia overwinter in southern Brazil (Stokes et al. Reference Stokes, Boersma and Davis1998), whereas those nesting in southern Patagonia and the Beagle Channel overwinter at much higher latitudes, with just a small fraction of the penguins from southern Patagonia reaching southern Brazil in winter (Pütz et al. Reference Pütz, Schiavini, Raya Rey and Lüth2007; Barrionuevo et al. Reference Barrionuevo, Ciancio, Steinfurth and Frere2020; Dodino et al. Reference Dodino, Lois, Riccialdelli, Polito, Pütz and Raya Rey2021). The baseline of δ15N values shifts latitudinally along the Atlantic coast of South America, from Brazil to Tierra del Fuego (Saporiti et al. Reference Saporiti, Bearhop, Vales, Silva, Zenteno, Tavares, Crespo and Cardona2015; Vélez-Rubio et al. Reference Vélez-Rubio, Cardona, López-Mendilaharsu, Martinez Souza, Carranza, Campos, González-Paredes and Tomás2018), and hence the δ15N values of penguin bone collagen are influenced not only by diet but also by the foraging grounds exploited. This might also explain the differences observed in the δ15N values of ancient and modern Magellanic penguins, if migratory patterns changed throughout time. This is certainly the case for the ancient Magellanic penguins from southern Patagonia, as they were extremely depleted in 15N compared with coastal contemporary species from the same area (Fig. 4A–C), which resulted in an unrealistically low trophic position. They were likely vagrants that had foraged elsewhere for most of the time recorded in their bones and were captured by hunter-fisher-gatherer people when they approached the coast. Interestingly, the δ15N values of ancient Magellanic penguins from Margen Sur (885–838 cal yr BP) were only 2‰ above those of almost contemporary King penguins from Cabo Virgenes 20 (1131 cal yr BP), whereas the offset between the two species in the Malvinas/Falkland Islands is currently 4.4‰ (Fig. 4B, Table 3). Modern myctophids, which are the main prey for King penguins (Cherel et al. Reference Cherel, Verdon and Ridoux1993, Reference Cherel, Ridoux and Rodhouse1996; Moore et al. Reference Moore, Robertson and Wienecke1998), are highly depleted in 15N compared with small pelagic fishes (Ciancio et al. Reference Ciancio, Pascual, Botto, Amaya-Santi, O'Neal, Riva Rossi and Iribarne2008), thus suggesting that the low δ15N values of ancient Magellanic penguins from southern Patagonia and their unrealistically low trophic positions might be because of an intense use of offshore foraging grounds. This is not true for modern Magellanic penguins from southern Patagonia, whose δ15N values fit well with those of coastal fish species (Fig. 4D).
In contrast to Magellanic penguins, the cormorants and shags inhabiting the southwestern Atlantic Ocean are sedentary, and temporal changes in their δ15N values are therefore likely the result of dietary shifts. Furthermore, local mollusks offer a reliable proxy to set the baseline for the calculation of the trophic positions of cormorants and shags. Interestingly, cormorants/shags from southern Patagonia and the Beagle Channel exhibited the same temporal pattern as Magellanic penguins, with a recent increase in both δ15N values and trophic position. It should be noted, however, that the ancient unidentified cormorants/shags from the Beagle Channel (Lanshuaia II archaeological site) were extremely depleted in 15N compared with their contemporary marine potential prey. This divergence was interpreted by Bas et al. (Reference Bas, Briz i Godino, Álvarez, Vales, Crespo and Cardona2019) as an evidence of consumption of catadromous galaxid fishes, an abundant resource at that time in estuarine ecosystems of Tierra del Fuego, which are highly depleted in 15N compared with marine fishes. Differences in the feeding habits between Magellanic penguins and unidentified cormorants/shags during the Holocene were already reported in the Beagle Channel by Kochi et al. (Reference Kochi, Pérez, Tessone, Ugan, Tafuri, Nye, Tivoli and Zangrando2018), although the δ15N values reported for unidentified cormorants/shags were not so low.
Although decreasing primary productivity and changes in the migratory patterns of Magellanic penguins may explain the changes in the δ15N values reported here, only competitor release simultaneously explains the changes in the distribution of Magellanic penguins observed during the twentieth century and the changes in the δ15N values of both Magellanic penguins and Imperial shags. Historical data suggest that Magellanic penguins in the southwestern Atlantic Ocean have been moving northward since the early twentieth century, when they started nesting along the northern continental coast of Patagonia (Boersma et al. Reference Boersma, Stokes and Yorio1990; Boersma Reference Boersma2008; Bouzat et al. Reference Bouzat, Walker and Boersma2009; Cruz et al. Reference Cruz, Astete, Nauto and Borrero2010). Genetic data also supported the hypothesis that Magellanic penguins have experienced a recent expansion during a favorable period (Bouzat et al. Reference Bouzat, Walker and Boersma2009). This expansion suggests that ecological conditions in this region during the second half of the twentieth century were better for this species than at the beginning of the last century (Boersma et al. Reference Boersma, Stokes and Yorio1990), which is not explained by declining productivity. However, competitor release resulting from the overharvesting of South American fur seals and sea lions and Argentine hake between the late eighteenth and the twentieth centuries (Lloris et al. Reference Lloris, Matallanas and Oliver2005; Crespo et al. Reference Crespo, Schiavini, García, Franco-Trecu, Goodall, Rodríguez, Morgante and De Oliveira2015; Romero et al. Reference Romero, Grandi, Koen-Alonso, Svendsen, Reinaldo, García, Dans, González and Crespo2017) might have contributed to improving the habitat conditions for Magellanic penguins and simultaneously increased the consumption of fish revealed by increased δ15N values. This is because the populations of those competitors, feeding on small pelagic fishes and squid (Scolaro et al. Reference Scolaro, Wilson, Laurenti, Kierspel, Gallelli and Upton1999; Punta et al. Reference Punta, Yorio and Herrera2003; Schiavini et al. Reference Schiavini, Yorio, Gandini, Raya Rey and Boersma2005; Baylis et al. Reference Baylis, Arnould and Staniland2014; Scioscia et al. Reference Scioscia, Raya Rey, Samaniego, Florentín and Schiavini2014), are currently well below original numbers, despite the legal protection of pinnipeds (Crespo et al. Reference Crespo, Schiavini, García, Franco-Trecu, Goodall, Rodríguez, Morgante and De Oliveira2015; Romero et al. Reference Romero, Grandi, Koen-Alonso, Svendsen, Reinaldo, García, Dans, González and Crespo2017).
Conclusion
The trophic ecology of modern Magellanic penguins and Imperial shags in the southwestern Atlantic Ocean differs from that of conspecifics living in the same region during the late Holocene, as revealed by a recent increase in δ15N values. Three, non–mutually exclusive processes, namely competitor release, reduced primary productivity, and changes in migratory patterns between isotopically dissimilar regions, may explain that increase. Although disentangling the relative contribution of all those processes is not possible at this time, the results reported here demonstrate that the ecology of Magellanic penguins and Imperial shags has undergone major changes since the late Holocene, and competitor release remains as a plausible hypothesis.
Acknowledgments
We are very grateful to L. Silva for her assistance with sample processing and to M. Álvarez and J. Gómez Otero who kindly provided us with archaeological samples from Teis XI and Playa Las Lisas 2, respectively. P. Rubio helped us with isotopic analyses at Centres Científics i Tecnològics de la Universitat de Barcelona (Barcelona, Spain). I. Briz i Godino is member of the “María Zambrano” program at the University of Barcelona. This study was funded by project no. 309765 from Fundació Bosch i Gimpera. All biological samples included in this paper were obtained, transported, and analyzed following the legal terms and conditions of the Argentine government.
Data Availability Statement
The data used to support the findings of this article are available in the article and in its online Supplementary Material, which is available on Dryad at: https://doi.org/10.5061/dryad.dbrv15f3k.
Open access
