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The Assembly and Disassembly of Ecological Networks in a Changing World

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The Assembly and Disassembly of Ecological Networks in a Changing World The assembly and disassembly of ecological networks in a changing world Memoria presentada por: Miguel Lurgi Rivera para optar al grado de Doctor Con el visto bueno de: Director: Dr. Jose´ M. Montoya Teran´ Instituto de Ciencias del Mar / CSIC Centre de Recerca Ecologica i Aplicacions Forestals Tutor: Bernat Claramunt Lopez´ Centre de Recerca Ecologica i Aplicacions Forestals Departament de Biologia Animal, Biologia Vegetal i Ecologia / Universitat Autonoma de Barcelona Programa de Doctorado en Ecologia Terrestre Centre de Recerca Ecologica i Aplicacions Forestals Universitat Autonoma de Barcelona Bellaterra, diciembre de 2013 The assembly and disassembly of ecological networks in a changing world Ph.D. Thesis Miguel Lurgi Rivera Bellaterra, diciembre de 2013 Abstract The assembly, structuring and functioning of natural communities, composed of many species forming complex networks of ecological interactions, has puzzled ecologists for many gener- ations. Early ecological research determined that community size and complexity (measured as connectivity in the network of ecological interactions) limit community stability, and hence impose constraints to communities to become indefinitely complex or speciose. Community assembly and stability research uncovered the fact that food web architecture is the key to community stability and persistence. Scientists thus started to focus on the un- derstanding of complex networks of interactions between species, and it was soon realised that species population dynamics are influenced by biotic interactions within the overall network. Moreover, certain features observed in the structure of ecological networks are responsible for the maintenance of stability and species persistence in different kinds of ecological communi- ties. The next step in ecological networks research is to incorporate several interaction types into a broader ecological scenario. This will further our knowledge in community structure and stability. Global change is affecting all ecosystems across the globe, having profound impacts over the delicate balance of nature. It has already caused an unprecedented number of extinctions, and the consequent damage to ecosystem structure and functioning has prompted many to suggest that we are currently witnessing the sixth mass extinction in the history of the Earth. The main big challenge for ecological research that lies ahead is to understand and predict how different components of global change are affecting and will likely affect complex ecosystems. In this thesis I tackle this challenge following an integrative empirical-theoretical approxi- mation exploring the effects of global change -climatic warming, biodiversity loss and species invasion- on multispecies communities. In addition, I investigate what makes ecological com- munities stable through their assembly, and how this stability may be affected by global change. Specifically, I employed a combination of empirical results review and data analysis, a novel conceptual framework for the analysis of relationships between different dimensions of stability, theoretical models grounded on realistic food web structure and ordinary differential equations to simulate populations dynamics, and individual-based spatially explicit models with a mixture of ecological interaction types in order to gain predictive insights on the effects of different components of global change on natural communities and several factors behind the stability of these assemblages of species. Some of my key findings are: (1) Species range shifts triggered by climate change are gen- erating novel communities. These are characterized by consistent novel patterns where body size distributions within the food webs are getting shifted towards smaller sizes, specialised interactions are getting lost, and interaction strengths are getting stronger in general, with fur- ther consequences for community dynamics. (2) Different dimensions of ecological stability are correlated in non-trivial ways. Biodiversity loss leads to a decoupling of the correlations previously observed between stability measures. This leads to highly unpredictable dynamics of ecological communities after major disturbances. (3) When focusing on biological invasions I find that food web structure is a strong determinant of invasion success. Less connected, more modular, and more heterogeneous communities in terms of diet breadth are more robust to bi- ological invasions. Invasions make communities more connected and less modular in general, rendering them even more fragile to invasions. Species traits of the invasive species, such as body size and the ability to capture prey, are also strong determinants of invasion success. (4) Finally, mutualistic interactions increase both temporal stability and spatial stability, by keep- ing spatial aggregation more constant. Distributions of interaction strengths across the entire ecological network are shifted towards lower values as mutualism increases. Resumen El ensamblaje, estructuracion´ y funcionamiento de las comunidades naturales, compuestas de muchas especies que forman redes complejas de interacciones ecologicas,´ ha desconcertado a los ecologos´ durante muchas generaciones. Investigacion´ ecologica´ pionera determino´ que el tamano˜ de la comunidad y su complejidad (medida como la conectividad en la red de in- teracciones ecologicas)´ limita la estabilidad de la comunidad, y por lo tanto impiden que las comunidades sean indefinidamente complejas o ricas en especies. Investigacion´ sobre el ensamblaje y la estabilidad en comunidades naturales ha descubierto que la arquitectura de la red trofica´ es la clave para la estabilidad de la comunidad y su persis- tencia. As´, los cient´ficos comenzaron a centrarse en la comprension´ de las complejas redes de interacciones entre especies, y pronto se dieron cuenta de que la dinamica´ de las poblaciones de las comunidades naturales se rigen por la estructura de estas redes. Por otra parte, ciertas caracter´sticas observadas en la estructura de las redes ecologicas´ son responsables del manten- imiento de la estabilidad en diferentes tipos de comunidades ecologicas.´ El siguiente paso en la investigacion´ de las redes ecologicas´ es incorporar varios tipos de interaccion´ en un escenario ecologico´ mas´ amplio. Esto incrementara´ el conocimiento de la estructura y la estabilidad de la comunidad. El cambio global esta´ afectando los ecosistemas de todo el mundo, con profundos impactos sobre el delicado equilibrio de la naturaleza. Ya ha causado un numero´ sin precedentes de extinciones, y el consiguiente dano˜ en la estructura y funcionamiento del ecosistema ha llevado a muchos a sugerir que en estos momentos estamos presenciando la sexta extincion´ masiva en la historia de la Tierra. El principal desaf´o para la investigacion´ ecologica´ que tenemos por delante es entender y predecir como´ el cambio global esta´ afectando, y es probable que afecte en el futuro, los ecosistemas complejos. En esta tesis enfrento este desaf´o utilizando una aproximacion´ emp´rico-teorica´ integradora para explorar los efectos del cambio global -cambio climatico,´ perdida´ de biodiversidad e in- vasion´ de especies- en comunidades compuestas por multiples´ especies. Adicionalmente, in- vestigo que´ hace que las comunidades ecologicas´ sean estables durante su ensamblaje, y como´ esta estabilidad puede verse afectada por el cambio global. En concreto, he empleado una combinacion´ de revision´ de resultados y analisis´ de datos emp´ricos, un novedoso marco conceptual para el analisis´ de las relaciones entre diferentes di- mensiones de la estabilidad ecologica,´ modelos teoricos´ fundamentados en redes troficas´ con estructuras realista y ecuaciones diferenciales ordinarias para simular la dinamica´ de las pobla- ciones, y modelos espacialmente expl´citos basados en el individuo con una mezcla de tipos de interacciones ecologicas;´ con el fin de obtener una vision´ predictiva de los efectos de los diferentes componentes del cambio global sobre las comunidades naturales y sobre los factores que explican la estabilidad de estos conjuntos de especies. Algunos de mis principales hallazgos son: (1) cambios de distribuciones de especies provo- cados por el cambio climatico´ estan´ generando comunidades nuevas. Estas ultimas´ se carac- terizan por nuevos patrones en que las distribuciones de tamano˜ corporal dentro de las redes troficas´ se estan´ desplazando hacia tamanos˜ mas´ pequenos,˜ las interacciones especialistas se estan´ perdiendo, y las fuerzas de interaccion´ son cada vez mas´ fuertes en general, con conse- cuencias importantes para la dinamica´ de la comunidad. (2) Las diferentes dimensiones de la estabilidad ecologica´ se correlacionan de manera no trivial. La perdida´ de biodiversidad lleva a un desacoplamiento de estas correlaciones. Esto conduce a dinamicas´ altamente impredeci- bles en comunidades ecologicas´ sujetas a perturbaciones. (3) Enfocandonos´ en las invasiones biologicas´ vemos que la estructura de la red trofica´ es un factor determinante para exito´ de la invasion.´ Comunidades menos conectadas, mas´ modulares, y mas´ heterogeneas´ en terminos´ de amplitud de la dieta de las especies que las componen son mas´ robustas a las invasiones biologicas.´ Las invasiones hacen a las comunidades mas´ conectadas y menos modulares en general, lo que las hace aun´ mas´ fragiles´ a las invasiones. Algunos rasgos de las especies invasoras, como su tamano˜ corporal y su capacidad de capturar la presa, tambien´ son fuertes
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