The eco-evolutionary dynamics of mutualistic networks: from pattern of emergence to stability

Thesis (PhD)--Stellenbosch University, 2016.ENGLISH ABSTRACT: Mutualistic interactions, such as the interactions between flowering plants and their pollinators are ubiquitous in nature. A community in which members are involved in multiple mutualistic interactions forms a mutualistic network. The structure of such a network exhibits well-organized pattern, suggesting that complex ecological and evolutionary processes underlie the assembly of mutualistic networks. Although significant development has recently been achieved in uncovering the structural patterns of mutualistic networks, understanding ecological and evolutionary mechanisms that contribute to the emergence of these patterns and the overall ecological and evolutionary stability of mutualistic communities stays a challenge. Addressing this challenge is the aim of this thesis. We develop a mathematical and simulation model of the ecological dynamics of population densities and the evolutionary dynamics of functional traits of a mutualistic community to deal with the problem. Based on the fact that ecological and evolutionary mechanisms are interdependent, we use the framework of adaptive dynamics. We found that the wellorganized structural patterns observed in mutualistic networks such as a high nestedness and high modularity can emerge when interactions are trait-dependent and the accessibility to intra and cross trophic resources controlled. Moreover, we found that mutualism plays a determinant role in sustaining evolutionary stability and the productivity of the community. However, the evolutionary trajectories of functional traits in a mutualistic community can experience bistability and bifurcation when mutualistic interactions are highly specialized. In this case, mutualism often creates opportunity empty niche for invasion. Biological invasions targeting a specific peak of empty niche could lead to a strong directional selection in the community. In addition, an introduced species that has a trait different from those of native species and a level of interaction specialization similar to that of the native species is more likely to invade. The structure of the recipient network, such as its nestedness or modularity, is not a primary determinant of invasibility compared to other measurements of network stability such as robustness, resilience and disruptiveness. Consequently, we argue that the interplay of ecological and evolutionary processes through trait-mediated interactions can shed light on important questions in mutualistic networks.AFRIKAANSE OPSOMMING: Mutualistiese interaksies, soos byvoorbeeld die interaksie tussen blomplante en hul bestuiwers, is alomteenwoordig in die natuur. 'n Gemeenskap waarvan die lede aan veelvuldige mutualistiese interaksies deelneem vorm 'n mutualistiese netwerk. Die struktuur van so ʼn netwerk vertoon ʼn goed georganiseerde patroon, wat daarop dui dat komplekse ekologiese en evolusionêre prosesse die samestelling van mutualistiese netwerke aandryf. Hoewel daar onlangs aansienlike vordering gemaak is in die ontdekking van strukturele patrone van mutualistiese netwerke, is dit steeds ʼn uitdaging om die ekologiese en evolusionêre meganismes wat bydra tot hierdie patrone en die algehele stabiliteit van die ekologiese en evolusionêre stabiliteit van mutualistiese gemeenskappe te verstaan. Hierdie uitdaging is dus die doel van hierdie tesis. Om hierdie probleem op te los, ontwikkel ons ʼn wiskundige en simulasie model vir die ekologiese dinamika van populasie digtheid en die evolusionêre dinamika van funksionele eienskappe van ʼn mutualistiese gemeenskap. Aangesien ekologiese en evolusionêre meganismes onafhanklik is van mekaar, het ons die raamwerk van aanpasbare dinamika gebruik. Ons het gevind dat die goedgeorganiseerde strukturele patrone wat in die mutualistiese netwerke waargeneem word, soos byvoorbeeld ʼn hoё genestheid en sterk kompartementalisering, kan na vore kom wanneer interaksies eienskap-afhanklik is en die bekombaarheid tot intra en kruis trofiese hulpbronne gekontroleer word. Verder het ons ook gevind dat mutualisme ʼn belangrike rol speel in die volhoubaarheid van evolusionêre stabiliteit en die produktiwiteit van ‘n gemeenskap. Maar, die evolusionêre trajek van funksionele eienskappe in ʼn mutualistiese gemeenskap kan bistabiliteit en bi-furkasie ondervind as die mutualistiese interaksies te gespesialiseerd is. In so ʼn geval kan mutualisme ʼn geleentheid skep vir indringerspesies in ʼn leë nis. Biologiese indringing wat ʼn sekere gedeelte van die leë nis teiken kan tot sterk direksionele seleksie in die gemeenskap lei. Verder, ʼn indringerspesie wat ander eienskappe het as die inheem sespesies en ʼn vlak van interaksie spesialisasie ewe aan die inheem sespesie is meer geneig om die gemeenskap te betree. Die struktuur van die ontvanger gemeenskap, soos die nes en kompartement, is nie ʼn primêre determinant van indringerheid nie as daar na ander mates gekyk word, soos byvoorbeeld robuustheid, veerkragtigheid en versteurbaarheid. Gevolglik argumenteer ons dat die interaksie tussen ekologiese en evolusionêre meganismes deur middel van eienskapbemiddeling, lig kan werp op belangrike vrae in mutalistiese netwerke