Ecological systems in space: From islands to contiguous landscapes

Licentiatavhandling, 2021

Dispersal and interaction networks underlie many ecological systems, often dictating spatio-temporal dynamics as well as emergent patterns. Dispersal has implications for establishment for distinct subpopulations of individual species as well as for the emergence of universal spatial patterns and varied community level structures. Such implications and the search for mechanisms that drive them are the central themes of this thesis. The accompanying manuscripts discuss two very different approaches of analysing ecological systems.

The less complex of the two systems consists of sites along a coastal landscape that harbours the seagrass Halodule uninervis. The task is to partition the entire population into subpopulations that have very little dispersal across them. We use an algorithm to aggregates sites that are more strongly connected in the same subpopulation for a range of dispersal scenarios. These scenarios are useful since the dispersal processes and life-spans of the seagrass are not well-understood.

In subsequent work, the idea of complexity is more precise. We analyse ecosystems whose complexity is captured by the strength of interactions between species and the fraction of interaction links realized out of all possible ones. On one hand we study how ecological communities on islands are assembled from a large pool of species that can also immigrate from the mainland source, and if it can provide mechanisms to explain the scaling of species richness with island areas – namely the species-area relationship. A drastically different setting is large contiguous landscapes.

We posit that species interactions vary across space, which helps characterize habitat patches that would be connected by high dispersal of species. This simplified picture provides a range of situations to understand how habitat heterogeneity affects species richness.

An overarching theme in investigating these complex ecological systems is to first analyze the fully random case without assuming any structure. This null case admits description in terms of a few statistics that greatly simplify the study of system level properties and processes. One can then ask if there are parameter regimes where these properties break down. The difficulty in finding such regimes might indicate universal properties but even the sudden disappearance might unravel phase transitions or unexplored new properties.