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Ecosystem trophic foundations: Lindeman exonerata
Abstract
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... The seasonality of lower trophic levels due to environmental conditions is expected to affect higher trophic levels in the Humboldt system. To better understand the Humboldt system trophodynamics, we derive equations for trophic transfer efficiency and food chain length based on Ulanowicz (1995). It allows us to disentangle their roles in variations of the food chain efficiency and identify a dominant contribution of trophic transfer efficiency. ...
... To investigate the disproportionate energy transfer, we attribute variations of food chain efficiency (FCE) to variations of trophic transfer efficiency (TTE) and food chain length (FCL). We calculate TTE and FCL based on the amount of energy transferred between trophic levels (TL), applying the analytical formulations in Ulanowicz (1995). This approach accounts for the fact that food webs typically represent "webs" where predators graze on multiple prey types and converts them into "chains." ...
... We define FCL as the highest trophic position, mesozooplankton in our model. Following Ulanowicz (1995), we define a trophic transformation matrix T (Equation 1) that allows the mapping of net production of the plankton compartments of the model food web to a chain. The rows of T represent trophic levels, and the columns are different plankton groups, that is, small and large phytoplankton and then small and large zooplankton. ...
Plain Language Summary
The Humboldt Upwelling System is a fishery‐important region. A common assumption is that a certain amount of phytoplankton supports a proportional amount of fish. However, we find that a small seasonal change in phytoplankton can trigger a larger variation in zooplankton. This implies that one may underestimate changes in fish solely based on phytoplankton. Using ecosystem model simulations, we investigate why changes of phytoplankton are not proportionally reflected in zooplankton. The portion of phytoplankton that ends up in zooplankton is controlled by the changing depth of the surface ocean “mixed layer.” The “mixed layer” traps both the phytoplankton and zooplankton in a limited amount of space. When the “mixed layer” is shallow, zooplankton can feed more efficiently on phytoplankton as both are compressed in a comparatively smaller space. We conclude that in the Humboldt System, and other “food‐rich” regions, feeding efficiently, determined by the “mixed layer,” is more important than how much food is available.
... Summary statistics built into Ecopath were used to describe the properties of our model and compare them to the properties of other Antarctic food web models. Ecopath calculates the trophic level for each functional group and calculates energy flow through aggregated trophic levels using methods cited in Lindeman (1942) and Ulanowicz (1995). We used the 'Lindeman spine' (Lindeman, 1942) to describe energy flow from one aggregated trophic level to another, and used transfer efficiencies to describe the proportion of the flow consumed or exported at the next trophic level. ...
Annual phytoplankton blooms on the northern region of the Kerguelen Plateau fuel a productive food web that supports highly valuable commercial fisheries for Patagonian toothfish (Dissostichus eleginoides) and mackerel icefish (Champsocephalus gunnari). The food web on the plateau is understudied in comparison to other regions of the Southern Ocean. Major linkages and energy pathways have not been explored, and the combined effects of fishing and a changing climate on the ecosystem are largely unknown. Single species studies on the plateau have shown that the combined effects of climate change and fisheries are impacting populations, however, it is unclear how these impacts translate to the ecosystem. We extended an existing Ecopath model to describe food web dynamics on the plateau and investigate food web interactions with the fishery. Results from our model highlight, for the first time, the properties of the food web, major energy pathways and energy transfers between trophic levels. Energy transfer from detritus was most efficient at the lowest trophic level while energy from primary production was more efficient at higher trophic levels. Consumption and respiration were high in our system, most likely due to the inclusion of bacteria and microzooplankton. Killer whales, cephalopods and myctophids were key functional groups for energy transfer in the system. These groups were relatively data poor, suggesting a useful focus area for future updates to the model. Patagonian toothfish and mackerel icefish were heavily consumed in the food web, however, the inclusion of fisheries catches and by-catch had little to no impact on food web dynamics.
... We obtained the largest fraction of chain-like responses when considering all zooplankton species as a single trophic level and all phytoplankton species as another trophic level, suggesting that consistent chain dynamics may emerge as a complexity reduction of communities into few interacting levels composed by multiple species or groups of species (Ulanowicz, 1995). ...
Food chains present an interconnected set of properties. Some empirically accessible, such as the biomass distribution across trophic levels, others more elusive, such as cascading responses to perturbations.
Intuitive links connect these structural and dynamical properties. Yet, do more abundant predators always entail stronger trophic cascades?
Using a combination of theory and data from 31 pelagic experiments, we explore whether static features carry direct information about dynamical responses. We identify three regimes, where upward, downward or both cascades shape the biomass distribution. These regimes depend on what we call trophic dissipation, i.e. the balance between consumer mortality and primary productivity modulated by species interactions. Within-ecosystem causality translates into across-ecosystem patterns if trophic dissipation varies little among systems, as we show in experimental data. Our approach can reveal direct causal links between trophic cascades and biomass distributions, and thus guide predictions from empirical patterns.
... TL aggregation combines energy from different functional groups into several TLs (expressed as an integer) to simplify a complicated food web (Ulanowicz, 1995). Aggregated TL1 comprises primary producers (phytoplankton in this study) and detritus. ...
To assess the impact of offshore wind farms (OWFs) on the structure and energy flow of coastal ecosystems, Ecopath models of the Jiangsu coastal ecosystem (JCE) based on biological field data collected before and after the establishment of the Rudong OWFs in 2007 and 2015, respectively, were constructed and compared. The results indicated that after OWF construction, detritus, phytoplankton, zooplankton, anchovies, and some benthic fish were positively impacted. The increased primary production and detritus resulted in the increased food supply for zooplankton, which made it possible for planktivorous species (particularly anchovies) to be fed. Consequently, the biomass and production of some benthic fish increased, which indicates a potential reef effect. Other groups with decreased biomasses and productions may have been negatively impacted by the OWFs. Herbivory flows dominated the pre- and post-construction JCE despite their low transfer efficiencies; however, the proportion of detritivory flow increased after OWF construction, and this was especially prominent at high trophic levels. The post-construction JCE was immature with lower system connectivity, trophic flow utilisation, and transfer efficiency. However, the ecosystem tended to develop towards higher maturity with higher energy throughput, ecosystem activity, and recycling capability.
Efforts to model marine food-webs are generally undertaken by small teams working separately on specific regions (<10⁶ km²) and making independent decisions about how to deal with data gaps and uncertainties. Differences in these largely arbitrary decisions (which we call ‘model personality’) can potentially obscure true differences between regional food-webs or lead to spurious differences. Here we explore the influence of model personality on a comparison of four Southern Ocean regional food-web models. We construct alternative model versions which sequentially remove aspects of personality (alternative model ‘currencies’, schemes for aggregating organisms into functional groups, and energetic parameter values). These alternative versions preserve regional differences in biomass and feeding relationships. Variation in a set of model metrics that are insensitive to absolute biomass and production identifies multiple regional contrasts, a subset of which are robust to differences in model personality. These contrasts imply real differences in ecosystem structure which, in conjunction with differences in primary production and consumer biomass (spanning two and four orders of magnitude respectively), underpin differences in function. Existing regional models are therefore a useful resource for comparing ecosystem structure, function and response to change if comparative studies assess and report the influence of model personality.
The biomass distribution across trophic levels (biomass pyramid) and cascading responses to perturbations (trophic cascades) are archetypal representatives of the interconnected set of static and dynamical properties of food chains. A vast literature has explored their respective ecological drivers, sometimes generating correlations between them. Here we instead reveal a fundamental connection: both pyramids and cascades reflect the dynamical sensitivity of the food chain to changes in species intrinsic rates. We deduce a direct relationship between cascades and pyramids, modulated by what we call trophic dissipation – a synthetic concept that encodes the contribution of top‐down propagation of consumer losses in the biomass pyramid. Predictable across‐ecosystem patterns emerge when systems are in similar regimes of trophic dissipation. Data from 31 aquatic mesocosm experiments demonstrate how our approach can reveal the causal mechanisms linking trophic cascades and biomass distributions, thus providing a road map to deduce reliable predictions from empirical patterns.
ABSTRACT
There is substantial evidence that climate warming affects terrestrial and marine ecosystems. In addition to the geographic shifts of marine species and communities, comprehensive mesocosm experiments provide insight in the behavior of species and simplified ecosystems under climate warming conditions. Food web dynamics and stability has been topical in contemporary ecology, and while these aspects receive considerable attention, few studies have quantitatively examined the impact of climate warming on complex marine ecosystems and their food webs. Here we examine the response of a large marine ecosystem, the Sylt-Rømø Bight in the northern German Wadden Sea, to warmer temperatures by means of ecological network analysis (ENA). Three quantitative network models (of 67 model compartments) were constructed for each of 4 non-consecutive years (1995, 2007, 2011, 2013). A base-line model at the mean annual ambient temperature, and models at +3oC and +5oC above the annual mean were constructed for each year (a total of 12 models) and assessed by ENA protocols. Results show i.a. an intensification of detrital production and consumption, substantial increase in the total system throughput (TSTP), decline in consumption of phytoplankton and macrophytes, an increase in the total overhead, an increase in community respiration, and an increase in the system’s P/B and R/B ratios. The mean relative ascendency declines by 2.35% and 2.28% from the base models in the +3oC and +5oC networks models respectively. It is clear from the suite of system metrics and ratios that the ecosystem becomes less organized, more dissipative and shifts towards detritus based food webs at higher temperatures.
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