Habitat fragmentation and connectivity : Spatial and temporal characteristics of the colonization process in plants

The connectivity between habitat patches or between populations indicates the potential for transfer of genetic material between habitat patches or populations. In plants, genetic material is usually transferred by dispersal of seeds or pollen. A sufficient level of connectivity is essential for regional species survival. Firstly, because it prevents inbreeding and loss of genetic material and adaptability. Secondly, because it is required for re-colonization of habitat patches in which populations have gone extinct, colonization of new habitat patches, migration, and range expansion. Connectivity is dependent on the dispersal ability of an organism and the spatial configuration of habitat patches or populations of that organism. Habitat fragmentation affects the spatial distribution of habitat patches or populations, and thus connectivity. The effect of habitat fragmentation on connectivity is relatively well-studied in animals, but not in plants; partly, because dispersal of plants is extremely difficult to assess. This study quantifies the effect of habitat fragmentation on connectivity for four plant species with different dispersal abilities (Centaurea jacea, Cirsium dissectum, Hypochaeris radicata and Succisa pratensis). The species main dispersal mechanism is seed dispersal by wind. Their habitat consists of nutrient-poor, wet to moist grasslands. The dispersal ability of the plant species was simulated with a mechanistic seed dispersal model. Simulation allows assessment of long-distance dispersal under different conditions, while measurement of long-distance dispersal is extremely difficult. Tests of simulation results against field data show that the model is accurate and gives more realistic results than previously developed models. Similarities and differences between the model and previous models are discussed. Simulations show that wind dispersal distances are not as long as commonly assumed; long-distance dispersal over >100 m is extremely rare in the current small populations, even for species with plumed seeds. The spatial habitat configuration in an 11x10 km study area in The Netherlands was mapped for a time series spanning the 20th century. The time series shows the rate and extent of habitat fragmentation. During the 20th century, more than 99.9 % of the grassland habitat was lost and minimum distances between the grassland patches increased from several meters to several hundreds of meters. The largest part of these effects occurred already during the first half of the 20th century. Habitat fragmentation greatly reduced the connectivity of the studied grassland habitat during the 20th century. This was mainly caused by the increasing distances between the habitat patches, but also by the decreasing sizes of the habitat patches and populations. Smaller populations produce fewer seeds and thus the probability that some of these seeds disperse over the long distances that separate habitat patches becomes even lower. Paradoxically, habitat fragmentation reduced the connectivity for plant species with plumed seeds with high long-distance dispersal ability more than for plant species with seeds with lower long-distance dispersal ability. This is because before fragmentation the connectivity was higher for the former plant species, but now populations of all species have become isolated regarding seed dispersal by wind. Suggestions for improvement of connectivity between habitat patches and between populations are given