Hugo Saiz

Changes in semi-arid plant species associations along a livestock grazing gradient.

In semi-arid ecosystems, vegetation is heterogeneously distributed, with plant species often associating in patches. These associations between species are not constant, but depend on the particular response of each species to environmental factors. Here, we investigated how plant species associations change in response to livestock grazing in a semi-arid ecosystem, Cabo de Gata-Níjar Natural Park in South East Spain. We established linear point-intercept transects at four sites with different grazing intensity, and recorded all species at each point. We investigated plant associations by comparing the number of times that each pair of species occurred at the same spatial point (co-occurrences), with the expected number of times based on species abundances. We also assessed associations for each shrub and grass species by considering all their pairs of associations and for the whole plant community by considering all pairs of associations on each site. At all sites, the plant community had a negative pattern of association, with fewer co-occurrences than expected. Negative association in the plant community increased at maximum grazing intensity. Most species associated as expected, particularly grass species, and positive associations were most important at intermediate grazing intensities. No species changed its type of association along the grazing gradient. We conclude that in the present plant community, grazing-resistant species compete among themselves and segregate in space. Some shrub species act as refuges for grazing-sensitive species that benefit from being spatially associated with shrub species, particularly at intermediate grazing intensities where positive associations were highest. At high grazing intensity, these shrubs can no longer persist and positive associations decrease due to the disappearance of refuges. Spatial associations between plant species and their response to grazing help identify the factors that organize plant communities, and may contribute to improving management of semi-arid ecosystems.

Relationships between plant spatial patterns, water infiltration capacity, and plant community composition in semi-arid mediterranean ecosystems along stress gradients

Water redistribution from bare soil to vegetation patches is a key feature of semi-arid ecosystems, and is responsible for their patchy vegetation patterns. The magnitude of water redistribution depends on the properties of the bare soil (which determine the amount of water run-off) and the capacity of vegetation patches to trap water run-on. We examined the relationships between plant spatial patterns, water infiltration into bare soil, and plant community composition in semi-arid sites with different hydro-physical properties (silty and gypseous soils) in NE Spain. We also studied the effect of two stressors, aridity and grazing, on water infiltration and plant spatial patterns. Our results indicate a negative correlation of bare soil sorptivity (the capacity to absorb water by capillarity) and vegetation aggregation. There was a strong positive correlation between perennial grass cover and the spatial aggregation of vegetation, but aggregation was not associated with positive associations of different plant types. The aggregation of vegetation was positively correlated with species richness and the overall extent of vegetation cover. Grazing reduced water infiltration into silty soils, which are prone to compaction. In contrast, soil crust affected the hydrology of gypseous soils, especially in the most arid sites, where grazing increased infiltration, reducing surface sealing due to breaking of the soil crust. Together, our results suggest that biotic and abiotic factors affect the hydro-physical properties of soils in the semi-arid ecosystems of NE Spain, which is linked to the plant communities through the spatial distribution of plants.

Positive and Negative Feedbacks and Free-Scale Pattern Distribution in Rural-Population Dynamics

Depopulation of rural areas is a widespread phenomenon that has occurred in most industrialized countries, and has contributed significantly to a reduction in the productivity of agro-ecological resources. In this study, we identified the main trends in the dynamics of rural populations in the Central Pyrenees in the 20th C and early 21st C, and used density independent and density dependent models and identified the main factors that have influenced the dynamics. In addition, we investigated the change in the power law distribution of population size in those periods. Populations exhibited density-dependent positive feedback between 1960 and 2010, and a long-term positive correlation between agricultural activity and population size, which has resulted in a free-scale population distribution that has been disrupted by the collapse of the traditional agricultural society and by emigration to the industrialized cities. We concluded that complex socio-ecological systems that have strong feedback mechanisms can contribute to disruptive population collapses, which can be identified by changes in the pattern of population distribution.

Plant–plant interactions as a mechanism structuring plant diversity in a Mediterranean semi‐arid ecosystem

Abstract Plant–plant interactions are among the fundamental processes that shape structure and functioning of arid and semi‐arid plant communities. Despite the large amount of studies that have assessed the relationship between plant–plant interactions (i.e., facilitation and competition) and diversity, often researchers forget a third kind of interaction, known as allelopathy. We examined the effect of plant–plant interactions of three dominant species: the perennial grass Lygeum spartum, the allelopathic dwarf shrub Artemisia herba‐alba, and the nurse shrub Salsola vermiculata, on plant diversity and species composition in a semi‐arid ecosystem in NE Spain. Specifically, we quantified the interaction outcome (IO) based on species co‐occurrence, we analyzed diversity by calculation of the individual species–area relationship (ISAR), and compositional changes by calculation of the Chao‐Jaccard similarity index. We found that S. vermiculata had more positive IO values than L. spartum, and A. herba‐alba had values between them. Lygeum spartum and A. herba‐alba acted as diversity repellers, whereas S. vermiculata acted as a diversity accumulator. As aridity increased, A. herba‐alba transitioned from diversity repeller to neutral and S. vermiculata transitioned from neutral to diversity accumulator, while L. spartum remained as diversity repeller. Artemisia herba‐alba had more perennial grass species in its local neighborhood than expected by the null model, suggesting some tolerance of this group to its “chemical neighbor”. Consequently, species that coexist with A. herba‐alba were very similar among different A. herba‐alba individuals. Our findings highlight the role of the nurse shrub S. vermiculata as ecosystem engineer, creating and maintaining patches of diversity, as well as the complex mechanism that an allelopathic plant may have on diversity and species assemblage. Further research is needed to determine the relative importance of allelopathy and competition in the overall interference of allelopathic plants.

Effect of Stipa tenacissima L. on the structure of plant co-occurrence networks in a semi-arid community

Network approaches can increase our understanding of both changes in ecosystems and the role that individual species play in such changes. In ecology, networks have been applied mainly to the study of food webs and mutualistic interactions, with few studies on plant communities. This study used a network approach to examine a semi-arid plant community along a Stipa tenacissima abundance gradient at two locations in SE Spain: (1) an open shrub land where S. tenacissima is a highly competitive species, and (2) an alpha steppe where S. tenacissima forms the end stable successional community. In alpha steppe, the influence of slope was also examined. We detected that S. tenacissima influenced the network structuring process, and that network organization changed along the gradient. In open shrub land, when S. tenacissima became abundant, it dominated the community and other species disappeared. This resulted in a reduction of the number of links that S. tenacissima established. At the alpha-steppe, S. tenacissima coexists with other species, developing more links as it becomes more abundant. On gentle slope zones of alpha steppe, S. tenacissima is more competitive and becomes dominant for high abundance values, reducing its links with other species. The organization of networks varied similarly in both locations. When plant species reduce their abundance and number, links are more heterogeneously distributed in networks. This leads to a concentration of most of the links around a few species, particularly S. tenacissima, which is the most abundant in this case. We conclude that, in order to study plant communities, it is convenient to consider the properties of individual components together with the interaction between them.

Evidence of structural balance in spatial ecological networks

of these networks presents some difficulties over the analysis of networks with only one type of interactions, being necessary the use of new methodologies or theoretical frameworks (like the use of multilayer networks, Mucha et al. 2010, Boccaletti et al. 2014, Kivelä et al. 2014). Networks which include positive and negative links are called signed networks and have been mostly considered theoretically (Harary et al. 1953, Zaslavsky 1982, Traag and Bruggeman 2009) and used to study social networks (Leskovec et al. 2010, Szell and Thurner 2010, Szell et al. 2010, Facchetti et al. 2011), while being ignored in other contexts such as ecological systems. Signed networks exhibit a property called structural balance, which is based on how nodes organize in subgroups within the network (Cartwright and Harary 1956). A signed network is said to be balanced if it can be partitioned into groups of nodes in such a way that 1) every pair of connected nodes within the same group share a positive link, and 2) links between nodes within different groups have a negative sign (Doreian and Mrvar 2009). Structural balance is associated with the resilience of social networks because it prevents the appearance of conflicts that might disrupt the system (Cartwright and Harary 1956). However, real networks rarely organize in a perfectly balanced way (i.e. some links do not fulfill the criterion for structural balance), and the deviation from perfect balance is called ‘frustration’ Ecography 40: 733–741, 2017 doi: 10.1111/ecog.02561

The structure of plant spatial association networks is linked to plant diversity in global drylands

1. Despite commonly used to unveil the complex structure of interactions within ecological communities and their value to assess their resilience against external disturbances, network analyses have seldom been applied in plant communities. We evaluated how plant-plant spatial association networks vary in global drylands, and assessed whether network structure was related to plant diversity in these ecosystems. 2. We surveyed 185 dryland ecosystems from all continents except Antarctica and built networks using the local spatial association between all the perennial plants species present in the communities studied. Then, for each network we calculated four descriptors of network structure (link density, link weight mean and heterogeneity, and structural balance), and evaluated their significance with null models. Finally, we used structural equation models to evaluate how abiotic factors (including geography, topography, climate and soil conditions) and network descriptors influenced plant species richness and evenness. 3. Plant networks were highly variable worldwide, but at most study sites (72%) presented common structures such as a higher link density than expected. We also find evidence of the presence of high structural balance in the networks studied. Moreover, all network descriptors considered had a positive and significant effect on plant diversity, and on species richness in particular. Synthesis. Our results constitute the first empirical evidence showing the existence of common network architectures structuring dryland plant communities at the global scale, and suggest a relationship between the structure of spatial networks and plant diversity. They also highlight the importance of system-level approaches to explain the diversity and structure of interactions in plant communities, two major drivers of terrestrial ecosystem functioning.

Structure of Stockmen Collaboration Networks Under Two Contrasting Touristic Regimes in the Spanish Central Pyrenees

ABSTRACT Ecosystem management is a difficult task because it must conciliate the ecological, economic, and social dimensions of socioecological systems. In those systems, the action of any single component can have an effect on the others and result in a critical impact on the organization of the entire system. This study examined the collaboration networks among stockmen within two traditionally agropastoral regions in the Spanish Central Pyrenees, which in the past 30 yr included touristic activities: one under the influence of a national park and centered on ecotourism and the other in a region where there are ski resorts and local stockmen have turned to snow tourism. Our hypotheses were that economic regime affects the structure of the networks, and the type of collaboration (e.g., for economic reasons) influences the collaborations among stockmen. We built stockmen collaboration networks by connecting breeders within the same pastoral partnerships and calculated the importance of collaborations (links density), the occurrence of collaborative subgroups (network modularity), and the existence of collaborations between stockmen in different regions (Krackhardt Ratio). In addition, we identified the distribution of links among types of pastoral partnerships. The network under the influence of the National Park presented higher link density and modularity than did the network influenced by ski resorts, where the presence of nonlocal stockmen is higher. Furthermore, economic partnerships played a major role connecting stockmen. In the study area, differences in the collaboration networks between the two regions suggest that changes in the economic trend in the past 30 yr has influenced the collaborative structure of the stockmen. We discuss possible reasons behind these differences and propose some recommendations that could help to strengthen the collaborative bounds between stockmen in the area.

Integrando escalas y métodos LTER para comprender la dinámica global de un espacio protegido de montaña: el Parque Nacional de Ordesa y Monte Perdido

Bonache, J., de Mingo-Sancho, G., Serrada, J., Amengual, P., Perales, J., Martinez, R., Rodado, S., Albornos, E. 2016. Long-term monitoring and evaluation in Spanish National Parks Network. Ecosistemas 25(1): 31-48. Doi.: 10.7818/ECOS.2016.25-1.05 The Spanish National Parks Network, consisting of a selection of natural spaces containing a representative sample of the main existing natural systems in Spain, is a very appropriate setting for research and long-term monitoring, in which the rules protects and promotes the improvement of knowledge and its application to management. In this regard, the National Parks Agency develops, in collaboration with the Autonomous Communities and with specific Scientific advise, a Research Program and a Monitoring and Evaluation Plan of the National Parks Network, latter having programs of ecological, sociological monitoring and functional. The article briefly explains the various monitoring initiatives underway, with examples of outcomes in each: mapping of natural systems, productivity monitoring through remote sensing, monitoring plant health, common bird monitoring, studies of social perception etc. Future prospects and areas for improvement of long-term monitoring in the National Parks Network are explained taking into account the synergies with the LTER Network, which is useful for monitoring in National Parks, especially concerning the application of comparable protocols and information management.

Synchronization unveils the organization of ecological networks with positive and negative interactions

Network science has helped to understand the organization principles of the interactions among the constituents of large complex systems. However, recently, the high resolution of the data sets collected has allowed to capture the different types of interactions coexisting within the same system. A particularly important example is that of systems with positive and negative interactions, a usual feature appearing in social, neural, and ecological systems. The interplay of links of opposite sign presents natural difficulties for generalizing typical concepts and tools applied to unsigned networks and, moreover, poses some questions intrinsic to the signed nature of the network, such as how are negative interactions balanced by positive ones so to allow the coexistence and survival of competitors/foes within the same system? Here, we show that synchronization phenomenon is an ideal benchmark for uncovering such balance and, as a byproduct, to assess which nodes play a critical role in the overall organization of the system. We illustrate our findings with the analysis of synthetic and real ecological networks in which facilitation and competitive interactions coexist.