Flow network indices signal a directional change in ecosystems: Evidence from a small mountain lake (Lake Santo, northern Italy)

Empirical evidence of the theoretically expected trends of ecosystem development is scarce so far. In this research, we used long-term empirical data about the plankton community of a small mountain lake (Lake Santo, northern Apennines, Italy) to reconstruct its developmental trajectory during a period comprised between early 1970 s and 2010 s. We exploited these data to build yearly ecological networks and from their configuration of energy flows we computed network information indices. The trends of these indices enlighten about the developmental trajectory of this ecosystem during the period covered by the data set. In particular, they indicate that Lake Santo evolved in the direction of increasing stability at the expense of efficiency in energy transfer. We compared these results with current hypotheses about the directionality of ecosystem development, which are rooted in ecosystem theory, and discussed the possibility that, counter to some theoretical models of ecosystem development, Lake Santo followed an unimpeded direction of development rather than a trajectory typical of an ecosystem under stress. Finally, the long-term trends of flow network indices provided insights about the health status of the ecosystem

Flow network indices signal a directional change in ecosystems: Evidence from a small mountain lake (Lake Santo, northern Italy)

Highlights: • Flow network indices signal directional changes during ecosystem development. • They showed linear trends in the evolution of a mountain lake. • The lake evolved to increase stability at the expense of efficiency in energy transfer. • Trends of the indices challenge current hypotheses about the directionality of ecosystem development. • Flow network indices from long term field data can help assessing ecosystem health. Empirical evidence of the theoretically expected trends of ecosystem development is scarce so far. In this research, we used long-term empirical data about the plankton community of a small mountain lake (Lake Santo, northern Apennines, Italy) to reconstruct its developmental trajectory during a period comprised between early 1970 s and 2010 s. We exploited these data to build yearly ecological networks and from their configuration of energy flows we computed network information indices. The trends of these indices enlighten about the developmental trajectory of this ecosystem during the period covered by the data set. In particular, they indicate that Lake Santo evolved in the direction of increasing stability at the expense of efficiency in energy transfer. We compared these results with current hypotheses about the directionality of ecosystem development, which are rooted in ecosystem theory, and discussed the possibility that, counter to some theoretical models of ecosystem development, Lake Santo followed an unimpeded direction of development rather than a trajectory typical of an ecosystem under stress. Finally, the long-term trends of flow network indices provided insights about the health status of the ecosystem

Ciclo di alcuni radionuclidi in un ambiente lagunare stressato

Dottorato di ricerca in ecologia. 7. ciclo. Relatore I. Ferrari. Correlatori C. Triulzi e P. Viaroli. Coordinatore P. MenozziConsiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7, Rome; Biblioteca Nazionale Centrale - P.za Cavalleggeri, 1, Florence / CNR - Consiglio Nazionale delle RichercheSIGLEITItal

How interaction strength affects the role of functional and redundant connections in food webs

Links in food webs can be classified as functional, which affect robustness, and redundant, which seem not to do so. We explore whether this topologically based distinction may still be informative when interaction strength is considered. In 81 weighted food webs we identified functional and redundant links by exploiting the method of generalized dominators, and tested whether the two groups could be patterned according to link strength. Overall, redundant connections are weaker than functional links. However this pattern does not hold for every food web. Also, the difference in strength between functional and redundant links seems to depend on food web topology. In fact the systems in which redundant connections are significantly weaker than functional ones show greater connectance than systems in which the difference in strength between the two groups is not significant. We observed that redundant connections are fundamental in the formation of omnivory modules that have been described as being crucial for the stability/persistence of food webs. Although stability can be promoted by the specific arrangement of links magnitude within these modules, it does not necessarily imply that redundant links have to be weaker than functional connections at the whole web scale

Ecological Succession Investigated Through Food-Web Flow Networks

Succession is the orderly process of ecosystem development which involves changes in species composition and community processes over time (Odum, 1969;Ulanowicz, 1986). Patterns to be ascribed to an ideal sequence for this process have been searched for. Odum provided a list of features that would accompany ecosystem evolution. His view was criticized following observational studies that showed that changes in structural and functional characteristics often did not conform to the proposed generalizations. To investigate the consistency of trends expected in ecosystems during succession we investigated the developmental trajectories of a lacustrine ecosystem using a network approach. Data about species and dietary habits that were collected throughout 6 years of intense sampling activity were used to build up the food web of the lake: links and their magnitude. The topology and intensity of the links were converted into indices of growth and development according to Ulanowicz (1986). These indices were used as proxies for some of the Odum’s attributes of ecosystem development. Furthermore, some of the classical paramters, those for which a network counterpart could not be found, were calculated (i.e. Production, Biomass, P/B, Shannon index) to be used as benchmark for comparison. Trends observed for network indices seem to confirm some of the expectations but not all. The overall picture that emerges in this study reveals that for certain indices the Odum’s scenario seems to hold but further long term studies will be necessary to confirm predicted generalizations

Carbon flows in the ecosystems Lake Santo and Lake Scuro (northern Apennines, Italy)

System level indices from ecosystem network analysis (ENA) were calculated for the carbon flow networks representing Lake Santo and Lake Scuro ecosystems. Long-term data were used to construct the time series of networks representing carbon flows in the two ecosystems (Lake Santo: 1973, 1991, 2001, 2003 and 2007-2009; Lake Scuro: 1986, 1989-1994, 1998, 2003 and 2007-2009). The following indices were calculated: H - diversity of flows; AMI - average mutual information; HC - residual diversity; DC - development capacity; A - ascendency; O - overhead; TST - total system throughput; P - primary productivity (of phytoplankton); TOM - total organic matter; IR - flow redundancy; IC - internal connectance (data in file ScottiM_2019_lakes-indices.zip). All flows in the networks constructed to model carbon circulation in Lake Santo and Lake Scuro are reported. The time series includes 7 networks in the case of Lake Santo (1973, 1991, 2001, 2003 and 2007-2009) and 12 networks for Lake Scuro (1986, 1989-1994, 1998, 2003 and 2007-2009). Lake Santo is composed of 26 compartments (23 living and 3 non-living) while Lake Scuro has 22 compartments (19 living and 3 non-living). Four types of flows are summarized: (i) input from outside the system (i.e. photosynthesis or carbon flows due to leaves or other organic materials that contribute to the standing stocks of water DOC and water POC); (ii) export of still usable carbon outside system's boundaries (i.e. carbon burial into the sediment from benthic POC); (iii) respiration/dissipation; and (iv) inter-compartmental exchanges among compartments composing the system (i.e. flows from prey/resources to predators/consumers). All flows are in grams of carbon per cubic meter per year. Table columns represent: (i) donor compartments; (ii) recipient compartments; (iii) short explanation of the processes represented; (iv) strength of the flows (i.e. amount of carbon transferred) during the various years of the time series (i.e. 7 years for Lake Santo and 12 years for Lake Scuro) (data in file ScottiM_2019_lakes-dataset.zip)