The memory of spatial patterns – Using information from spatial pattern to detect changes in the abundance of tropical tree species

Background/Question/Methods

The current spatial pattern of a population is the result of previous individual birth, death and dispersal events. Previous research has shown how locally restricted seed dispersal often leads to stronger aggregation in less abundant populations than it does in more abundant populations. In contrast, little is known about how changes in the local abundance of a species may affect the spatial distribution of individuals. If the level of aggregation within a species depends to some extent on the abundance of the species, then changes in abundance should lead to subsequent changes in aggregation. However, an overall change of spatial pattern relies on many individual birth and death events and a surplus of deaths or births may have short-term effects on aggregation that are opposite to the long-term change predicted by the change in abundance. The change in aggregation may therefore lag behind the change in abundance, and consequently the current aggregation may hold information about recent population dynamics.

Results/Conclusions

Using an individual-based simulation model with local dispersal of seeds, we show that on average, recently growing populations should be more aggregated than shrinking populations of the same current local abundance. We test this hypothesis using spatial data on individuals from the long-term tropical rain forest plot at Barro Colorado Island. We find support for this relationship in canopy trees but not in understory and shrub species. On this basis we argue that current spatial aggregation is an important characteristic that contains information on recent changes in local abundance, and may be applied to taxonomic groups where dispersal is limited and within-species aggregation is observed.