Food chain efficiency (FCE), the proportion of primary production converted to production of the top trophic level, can influence several ecosystem services as well as the biodiversity and productivity of each trophic level. AquaticFCEis affected by light and nutrient supply, largely via effects on primary producer stoichiometry that propagate to herbivores and then carnivores. Here, we test the hypothesis that the identity of the top carnivore mediatesFCEresponses to changes in light and nutrient supply.
We conducted a large‐scale, 6‐week mesocosm experiment in which we manipulated light and nutrient (nitrogen and phosphorus) supply and the identity of the carnivore in a 2 × 2 × 2 factorial design. We quantified the response ofFCEand the biomass and productivity of each trophic level (phytoplankton, zooplankton, and carnivore). We used an invertebrate,Chaoborus americanus, and a vertebrate, bluegill sunfish (Lepomis macrochirus), as the two carnivores in this study because of the large difference in phosphorus requirements between these taxa.
We predicted that bluegill would be more likely to experience P‐limitation due to higher P requirements, and hence thatFCEwould be lower in the bluegill treatments than in theChaoborustreatments. We also expected the interactive effect of light and nutrients to be stronger in the bluegill treatments. Within a carnivore treatment, we predicted highestFCEunder low light and high nutrient supply, as these conditions would produce high‐quality (low C:nutrient) algal resources. In contrast, if food quantity had a stronger effect on carnivore production than food quality, carnivore production would increase proportionally with primary production, thusFCEwould be similar across light and nutrient treatments.
Carnivore identity mediated the effects of light and nutrients onFCE, and as predictedFCEwas higher in food chains withChaoborusthan with bluegill. Also as predicted,FCEinChaoborustreatments was higher under low light. However,FCEin bluegill treatments was higher at high light supply, opposite to our predictions. In addition, bluegill production increased proportionally with primary production, whileChaoborusproduction was not correlated with primary production, suggesting that bluegill responded more strongly to food quantity than to food quality. These carnivore taxa differ in traits other than body stoichiometry, for example, feeding selectivity, which may have contributed to the observed differences inFCEbetween carnivores.
Comparison of our results with those from previous experiments showed thatFCEresponds similarly to light and nutrients in food chains withChaoborusand larval fish (gizzard shad: Clupeidae), but very differently in food chains with bluegill. These findings warrant further investigation into the mechanisms related to carnivore identity (e.g., developmental stage, feeding selectivity) underlying these responses, and highlight the importance of considering both top‐down and bottom‐up effects when evaluating food chain responses to changing light and nutrient conditions.
Ufanisi wa mlolongo wa chakula (FCE) ni sehemu ya uzalishaji wa msingi iliyobadilishwa kuwa uzalishaji wa kiwango cha juu cha trophic, ambayo inaweza kuathiri huduma kadhaa za ikolojia na pia utofauti wa kibiolojia na uzalishaji wa kila ngazi ya trophic. AquaticFCE huathiriwa na mwanga na ugavi wa virutubisho, hasa kupitia athari kwenye mtayarishaji wa msingi wa stoichiometry ambayo huenea kwa wanyama wanaokula mimea na kisha wanyama wanaokula nyama. Katika utafiti huu, tulijaribu nadharia kwamba utambulisho wa carnivore ya juu hupunguza majibu ya FCE kwa mabadiliko ya mwanga na ugavi wa virutubisho. Katika jaribio la mesocosmic, tulichunguza jinsi nyenzo za nishati, nitrojeni na fosforasi zinavyoweza kuathiri mwili wa mnyama katika muundo wa 2 × 2 × 2. Kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano. Kwa mfano, katika utafiti wa mwaka wa 2015, mnyama wa mviringo wa Chaoborus americanus na mnyama wa mviringo wa Blue-gilled sunfish (Lepomis macrochirus) walichukuliwa kama wanyama wawili wa nyama katika utafiti huu kwa sababu ya tofauti kubwa katika mahitaji ya fosforasi kati ya taxons hizi. Kwa hivyo, kwa sababu ya mahitaji ya juu ya P, FCE itakuwa chini katika matibabu ya bluegill kuliko katika matibabu ya Chaobor. Pia tulitarajia athari ya mwingiliano wa nuru na virutubisho kuwa na nguvu zaidi katika matibabu ya bluegill. Katika matibabu ya nyama, tulitabiri FCE ya juu zaidi chini ya mwanga mdogo na ugavi wa virutubisho vya juu, kwani hali hizi zitatokeza rasilimali za mwani za ubora wa juu (C: virutubisho vya chini). Kwa upande mwingine, ikiwa wingi wa chakula una athari kubwa kwa uzalishaji wa nyama kuliko ubora wa chakula, uzalishaji wa nyama utaongezeka kwa usawa na uzalishaji wa msingi, kwa hivyo FCE itakuwa sawa katika matibabu ya nuru na virutubisho. Uwezo wa nyama ulifanya athari za mwanga na virutubisho kwenye FCE, na kama ilivyotarajiwa, FCE ilikuwa ya juu katika minyororo ya chakula na Chaoborustans na bluegill. Pia kama ilivyotarajiwa, FCE katika Chaobor matibabu ilikuwa ya juu chini ya mwanga mdogo. Hata hivyo, FCE katika matibabu ya bluegill ilikuwa ya juu katika usambazaji wa mwanga wa juu, kinyume na utabiri wetu. Kwa kuongezea, uzalishaji wa bluegill uliongezeka kwa usawa na uzalishaji wa msingi, wakati uzalishaji wa Chaoborus haukuunganishwa na uzalishaji wa msingi, ikidokeza kwamba bluegill ilijibu kwa nguvu zaidi kwa wingi wa chakula kuliko ubora wa chakula. Taxoid hizi za nyama hutofautiana katika sifa nyingine mbali na stoichiometry ya mwili, kwa mfano, uteuzi wa kulisha, ambayo inaweza kuwa imesababisha tofauti zilizochunguzwa katika FCE kati ya wanyama wa nyama. "Ulinganisho wa matokeo yetu na yale ya majaribio ya awali ulionyesha kwamba ""FCE"" inajibu kwa mwanga na virutubisho katika minyororo ya chakula na samaki wa Chaobor na Clupeidae, lakini tofauti sana katika minyororo ya chakula na bluegill." Matokeo haya yanahitajika zaidi katika uchunguzi wa utaratibu unaohusiana na utambulisho wa nyama (kwa mfano, hatua ya maendeleo, uteuzi wa kulisha) chini ya majibu haya, na inasisitiza umuhimu wa kuzingatia athari za juu-chini na chini-juu wakati wa kutathmini majibu ya mlolongo wa chakula kwa mabadiliko ya hali ya nuru na virutubisho.