Abstract:
Much of New Zealand flora is reliant on bird visitation for pollination, and the majority of this
visitation is done by three native bird species: the tūı̄,, korimako and tauhou. Ecologists have
already found evidence of pollen limitation and inbreeding depression in several plant species,
presumably due to habitat fragmentation and reduced bird populations. In addition, while alien
plants may act as competitors, attracting pollinators away from native plants and increasing
rates of heterospecific pollen deposition, it is hypothesized that at low abundance, alien plants
may also generate facilitative effects for nearby natives by attracting more pollinators to a region.
The balance of these positive and negative effects in the New Zealand context is unknown. In
service of understanding these factors, we develop a stochastic, empirically informed, mechanistic
individual-based model of bird foraging behaviour on a local scale to track pollen transfer.
We then use the individual-based model to parameterize a Lotka-Volterra competition model to
estimate bird populations densities in different habitat types. Variations on the parameterized
Lotka–Volterra model, including patch-diffusion and non-autonomous parameters, further illuminate
how bird populations might be affected by the presence of invasive species in complex
habitat scenarios. Finally, we re-simulate the individual-based model using the bird population
densities derived from the Lotka–Volterra model. Our findings indicate that the predicted increase
in abundance of birds due to invasive plant species does not necessarily compensate for
negative effects within a habitat or meaningfully improve pollen outcomes for plants in adjacent
habitats. However, these results are dependent on the exact pollen limitations and needs of plant
species.
