Spatial and functional determinants of long-term fecundity in serotinous shrub communities

Abstract

Understanding the dynamics of biological communities is a central aim of ecological research. Contemporary environmental change reinforced this interest: in order to predict how communities will react to environmental change, we have to understand the processes driving their dynamics. Ultimately, the dynamics of a community depends on the reproduction, mortality and dispersal of its component individuals, and on how these demographic processes are altered by environmental factors and biotic interactions. A general understanding of biological communities is unlikely to arise from a species-specific approach that attempts to quantify all pairwise interactions between species. Instead, it seems promising to pursue a trait-based research program that quantifies how variation in the performance of species and individuals is shaped by the interplay of functional traits, biotic interactions and environmental factors. In this thesis, I investigated how functional plant traits determine plant-plant, plant-pollinator and plant-herbivore interactions in space and time, and how these spatiotemporal interactions affect the long-term fecundity of plants. In the South African Fynbos biome (a global biodiversity hotspot), I studied a species-rich, ecologically and economically important group of woody plants (genus Protea) and its interactions with pollinators and seed predators. The objectives of this thesis were: (1) to combine plant traits and high-resolution maps of Protea communities in order to quantify the landscapes of nectar sugar and seed crops that plant communities provide for pollinators and seed predators, (2) to examine how sugar landscapes shape pollinator behaviour, and how pollinator behaviour and pollinator-mediated interactions between plants affect the reproduction of Protea individuals, (3) to study how the spatial structure of plant communities and seed crop landscapes determine direct and predator-mediated interactions between plants, and (4) to understand how the interplay of these biotic interactions shapes the dynamics of plant communities. I addressed these objectives by analysing spatially-explicit data and high-resolution maps from 27 sites of 4 ha each that contained 129,750 plants of 22 Protea species. The results show that Protea plants and their pollinators interact on several spatial and temporal scales, and that these interactions are shaped by sugar landscapes. Within plants, inflorescences compete for pollination. At a neighbourhood scale, Protea reproduction benefits from nectar sugar of conspecific neighbours but not from heterospecific neighbour sugar. Seed set also increases with the amount of nectar sugar at the scale of entire study sites. This corresponds with the finding that the abundance and the visitation rates of key bird pollinators strongly depend on phenological variation of site-scale sugar amounts. Nectar sugar furthermore influences the strength of interactions between Protea species and bird pollinators: Protea species that provide nectar of high sugar concentration depend more strongly on bird pollinators to reproduce. When foraging in sugar landscapes, these bird pollinators show both temporal specialization on single plant species and a preference for common plant species. In addition to these pollinator-mediated interactions, the long-term fecundity of Protea individuals is reduced through both competition and apparent competition mediated by seed predators. Competition is stronger between conspecifics than between heterospecifics, whereas apparent competition shows no such differentiation. The intensity of competition between plants depends on their size and the intensity of apparent competition between plants depends on their seed crops. Moreover, competition has a stronger effect on plant fecundity than apparent competition. These findings have interesting implications for understanding the dynamics of Protea communities and the maintenance of plant diversity in the Fynbos biome. The positive interspecific density-dependence resulting from pollinator-mediated interactions causes community-level Allee effects that may lead to extinction cascades. My analyses also imply that competition stabilizes the coexistence of Protea species (because intraspecific competition is stronger than interspecific competition), whereas apparent competition via seed predators does not have such a stabilizing effect. In summary, this study highlights the benefits of ‘community demography’, the demographic study of multiple interacting species. Community demographic studies have the potential to identify general determinants of biotic interactions that act across species and communities. In this thesis, I identified nectar sugar and seed crops as interaction currencies that determine how multiple plant species interact through shared pollinators and seed predators. In megadiverse systems such as Fynbos, such generalizations are urgently needed to understand and forecast community dynamics. The analysis of community dynamics with respect to such interaction currencies provides an alternative to the classical species-specific approach in community ecology.

Räumliche und Zeitliche Interaktionen zwischen Pflanzen, Pflanzen und ihren Bestäubern und Räubern spielen eine Schlüsselrolle in der Dynamik von terrestrischen Gemeinschaften. Doch in der Ökologie fehlt weitgehend ein quantitatives Verständnis der Merkmale und Prozesse, die sowohl räumliche und zeitliche Interaktionen einschließen. Deshalb untersuchte ich in dieser Doktorarbeit, wie funktionale Pflanzenmerkmale Pflanzen-Pflanzen-Interaktionen und Pflanzen-Tier-Interaktionen in Raum und Zeit bestimmen, und wie diese räumlich-zeitliche Interaktionen die Pflanzenreproduktion beeinflussen. Ich studierte eine artenreiche, ökologisch und ökonomisch wichtige Gruppe von südafrikanischen Büschen (Gattung Protea) mit ihren wichtigsten Bestäubern (Cape Sugarbird, Promerops cafer and sunbird, Anthobaphes violacea) in der Cape Floristic Region (CFR). Durch die Kombination von Experimenten und Beobachtungen in verschiedenen Protea Gesellschaften die mit hoher Auflösung kartiert wurden, untersuchte ich wie die räumliche und zeitliche Verteilung von Protea-Ressourcen („Ressourcen-Landschaften") die Verhaltensweisen von Bestäubern prägt und wie sich Nachbarpflanzen auf die Verhaltensweisen von Bestäubern auswirken und im Umkehrschluss wie sich die Bestäuber auf die Reproduktion von Protea Individuen auswirkt. Darüber hinaus studierte ich, wie Ressourcen-Landschaften die Konkurrenz und die Prädationsrate beeinflusst und wie sich die Kombination von beiden auf die Dynamik der Protea-Gesellschaften auswirkt. Um dies zu tun, habe ich Pflanzenmerkmale durch raumzeitliche Interaktionen aus der Perspektive der beiden Pflanzen und Tiere quantifiziert und analysierte Interaktionen zwischen Protea Individuen (insgesamt 129750 Protea Positionen mit 22 Arten in 27 verschiedene Pflanzengesellschaften). Die Ergebnisse dieser Studie tragen zum Verständnis von demographischen Prozessen und Gesellschaftsdynamiken bei und zeigen die Auswirkungen auf die Erhaltung der biologischen Vielfalt in diesem „Biodiversitäts-Hotspot“.