Print PageRapid developments in spatial spread modeling have allowed important insights into drivers of invasion dynamics. Key demographic processes include plant survival, growth and flowering; these are size dependent in many plant populations, but also vary among individuals of the same size. This individual variation, along with variation in dispersal caused by differences in e.g. seed release height, seed characteristics and wind speed, may be an important determinant of the spread rate of species through homogeneous landscapes. Here we develop spatial integral projection models (SIPMs) that model both demography and dispersal with continuous state variables. This allows the effect of variation in plant size and size-dependent vital rates to be studied at much higher resolution than is possible in discrete-stage spread models. Comparing spatial matrix models to SIPMs allows us to assess the importance of modeling individual variation with high resolution. As a case study we parameterized a SIPM with data on the invasive monocarpic thistle Carduus nutans in New Zealand.
Results/Conclusions
Spread rate (c*) estimates were 14% lower than for standard spatial matrix models, and stabilized with as few as 7 evenly distributed size classes. The SIPM allowed us to calculate spread rate elasticities over the range of plant sizes, showing the size range of seedlings that contributed most to c* through their survival, growth and reproduction. The annual transitions of these seedlings were also the most important ones for local population growth (λ). However, seedlings that reproduced within a year contributed relatively more to c* than to λ. In contrast, plants that grow over several years to reach a large size and produce many more seeds, contributed relatively more to λ than to c*. We show that matrix models pick up some of these details, while other details disappear within wide size classes. We conclude with a discussion of the relative benefits of the two types of spatial models.
