Leonardo A. Saravia

Modelling periphyton dynamics in running water

The short-term biomass dynamics of periphyton communities in running water is characterized by deep variations. This temporal variability is mainly produced by changes in running water velocity. The shear force caused by the friction of water over the surface of the periphyton produces a removal of biomass and creates open sites for colonization. Running water also brings new suspended algae that can establish on open sites. An increase in the velocity of the water can also improve the renewal of nutrients in depleted areas and the elimination of waste products, producing higher reproductive rates. In this paper, we have developed a model of periphyton biomass dynamics taking the water velocity and nutrient concentration as external driving variables influencing immigration, removal and reproductive rate of the algae. We fitted the model to field data encompassing high and low water velocities and different seasons. We have qualitatively compared the parameters obtained from different situations with the expected ones based on bibliographic information. The model has shown a good fit to field data and parameters were similar to expected ones, giving evidence that the model provides a good description for the processes that dominate the periphyton dynamics in running waters.

Multifractal spatial patterns and diversity in an ecological succession.

We analyzed the relationship between biodiversity and spatial biomass heterogeneity along an ecological succession developed in the laboratory. Periphyton (attached microalgae) biomass spatial patterns at several successional stages were obtained using digital image analysis and at the same time we estimated the species composition and abundance. We show that the spatial pattern was self-similar and as the community developed in an homogeneous environment the pattern is self-organized. To characterize it we estimated the multifractal spectrum of generalized dimensions Dq. Using Dq we analyze the existence of cycles of heterogeneity during succession and the use of the information dimension D1 as an index of successional stage. We did not find cycles but the values of D1 showed an increasing trend as the succession developed and the biomass was higher. D1 was also negatively correlated with Shannons diversity. Several studies have found this relationship in different ecosystems but here we prove that the community self-organizes and generates its own spatial heterogeneity influencing diversity. If this is confirmed with more experimental and theoretical evidence D1 could be used as an index, easily calculated from remote sensing data, to detect high or low diversity areas.

A photographic method for estimating chlorophyll in periphyton on artificial substrata

The collection of time-series of periphyton biomass is a difficult task due to the destructive nature of the standard methods. A non-destructive method based on photography and digitalization, for the estimation of Chla of periphyton colonizing artificial substrata is presented. The standard spectrophotometric method was used to obtain a calibration curve. The relative errors of the proposed method were similar to those of other published methods. The photographic method should be used when a large quantity of samples from the same community is needed and a high precision on the individual measurement is not required.

Ecological Network assembly: how the regional meta web influence local food webs

Local food webs can be studied as the realisation of a sequence of colonising and extinction events, where a regional pool of species — called the metaweb— acts as a source for new species. Food webs are thus the result of assembly processes that are influenced by migration, habitat filtering, stochastic factors, and dynamical constraints. Therefore, we expect their structure to reflect the action of these influences. We compared the structure of empirical local food webs to (1) a metaweb, (2) randomly-constructed webs, and (3) webs resulting from an assembly model. The assembly model had no population dynamics but simply required that consumer species have at least one prey present in the local web. We compared global properties, network sub-structures—motifs— and topological roles that are node-level properties. We hypothesised that the structure of empirical food webs should differ from other webs in a way that reflected dynamical stability and other local constraints. Three data-sets were used: (1) the marine Antarctic metaweb, built using a dietary database; (2) the Weddell Sea local food web; and (3) the Potter Cove local food web. Contrary to our expectation, we found that, while most network global properties of empirical webs were different from random webs, there were almost no differences between empirical webs and those resulting from the assembly model. Further, while empirical webs showed different motif representations compared to the assembly model, these were not motifs associated with increased stability. Species’ topological roles showed differences between the metaweb and local food webs that were not explained by the assembly model, suggesting that species in empirical webs are selected by habitat or dispersal limitations. Our results suggest that there is not a strong dynamical restriction upon food web structure that operates at local scales. Instead, the structure of local webs is inherited from the metaweb with modifications imposed by local habitats. Recently, it has been found in competitive and mutualistic networks that structures that are often attributed as causes or consequences of ecological stability are probably a by-product of the assembly processes (i.e. spandrels). We extended these results to trophic networks suggesting that this could be a more general phenomenon.

Power laws and critical fragmentation in global forests

Aim Forests provide critical habitat for many species, essential ecosystem services, and are coupled to atmospheric dynamics through exchanges of energy, water and gases. One of the most important changes produced in the biosphere is the replacement of forest areas with human dominated landscapes. This usually leads to fragmentation, altering the sizes of patches, the structure and function of the forest. Here we studied the distribution and dynamics of forest patch sizes at a global level, examining signals of a critical transition from an unfragmented to a fragmented state. Location Forest across continents and big islands. Methods We used MODIS vegetation continuous field to estimate the forest patches at a global level and defined wide regions of connected forest across continents and big islands. We search for critical phase transitions, where the system state of the forest changes suddenly at a critical point; this implies an abrupt change in connectivity that causes a increased fragmentation level. We studied the distribution of forest patch sizes and the dynamics of the largest patch over the last fourteen years. The conditions that indicate that a region is near a critical fragmentation threshold are related to patch size distribution and temporal fluctuations of the largest patch. Results We found that most regions, except the Eurasian mainland, followed a power-law distribution. Only the tropical forest of Africa and South America met the criteria to be near a critical fragmentation threshold. Conclusion This implies that human pressures and climate forcings might trigger undesired effects of fragmentation, such as species loss and degradation of ecosystems services, in these regions. The simple criteria proposed here could be used as early warning to estimate the distance to a fragmentation threshold in forest around the globe, and provide a guide to direct conservation efforts at a continental level.The replacement of forest areas with human-dominated landscapes usually leads to fragmentation, altering the structure and function of the forest. Here we studied the dynamics of forest patch sizes at a global level, examining signals of a critical transition from an unfragmented to a fragmented state, using the MODIS vegetation continuous field. We defined wide regions of connected forest across continents and big islands, and combined five criteria, including the distribution of patch sizes and the fluctuations of the largest patch over the last sixteen years, to evaluate the closeness of each region to a fragmentation threshold. Regions with the highest deforestation rates---South America, Southeast Asia, Africa---all met these criteria and may thus be near a critical fragmentation threshold. This implies that if current forest loss rates are maintained, wide continental areas could suddenly fragment, triggering extensive species loss and degradation of ecosystems services.

Effects of macroalgae loss in an Antarctic marine food web: applying extinction thresholds to food web studies

Antarctica is seriously affected by climate change, particularly at the Western Antarctic Peninsula (WAP) where a rapid regional warming is observed. Potter Cove is a WAP fjord at Shetland Islands that constitutes a biodiversity hotspot where over the last years, Potter Cove annual air temperatures averages increased by 0.66 °C, coastal glaciers declined, and suspended particulate matter increased due to ice melting. Macroalgae are the main energy source for all consumers and detritivores of Potter Cove. Some effects of climate change favor pioneer macroalgae species that exploit new ice-free areas and can also decline rates of photosynthesis and intensify competition between species due to the increase of suspended particulate matter. In this study, we evaluated possible consequences of climate change at Potter Cove food web by simulating the extinction of macroalgae and detritus using a topological approach with thresholds of extinction. Thresholds represent the minimum number of incoming links necessary for species’ survival. When we simulated the extinctions of macroalgae species at random, a threshold of extinction beyond 50% was necessary to obtain a significant number of secondary extinctions, while with a 75% threshold a real collapse of the food web occurred. Our results indicate that Potter Cove food web is relative robust to macroalgae extinction. This is dramatically different from what has been found in other food webs, where the reduction of 10% in prey intake caused a disproportionate increase of secondary extinctions. Robustness of the Potter Cove food web was mediated by omnivory and redundancy, which had an important relevance in this food web. When we eliminated larger-biomass species more secondary extinctions occurred, a similar response was observed when more connected species were deleted, yet there was no correlation between species of larger-biomass and high-degree. This similarity could be explained because both criteria involved key species that produced an emerging effect on the food web. In this way, large-biomass and high-degree species could be acting as source for species with few trophic interactions or low redundancy. Based on this work, we expect the Potter Cove food web to be robust to changes in macroalgae species caused by climate change until a high threshold of stress is reached, and then negative effects are expected to spread through the entire food web leading to its collapse.

Architecture of marine food webs: To be or not be a ‘small-world’

The search for general properties in network structure has been a central issue for food web studies in recent years. One such property is the small-world topology that combines a high clustering and a small distance between nodes of the network. This property may increase food web resilience but make them more sensitive to the extinction of connected species. Food web theory has been developed principally from freshwater and terrestrial ecosystems, largely omitting marine habitats. If theory needs to be modified to accommodate observations from marine ecosystems, based on major differences in several topological characteristics is still on debate. Here we investigated if the small-world topology is a common structural pattern in marine food webs. We developed a novel, simple and statistically rigorous method to examine the largest set of complex marine food webs to date. More than half of the analyzed marine networks exhibited a similar or lower characteristic path length than the random expectation, whereas 39% of the webs presented a significantly higher clustering than its random counterpart. Our method proved that 5 out of 28 networks fulfilled both features of the small-world topology: short path length and high clustering. This work represents the first rigorous analysis of the small-world topology and its associated features in high-quality marine networks. We conclude that such topology is a structural pattern that is not maximized in marine food webs; thus it is probably not an effective model to study robustness, stability and feasibility of marine ecosystems.