In tropical forest ecology, it is common to recognise broad ecological zones, but few people look at specialisation along more narrow ecological gradients within these zones. It’s usually easy enough to distinguish between forests and open areas, or between evergreen and (semi-)deciduous forests, but finer subdivisions are uncommon. Most plant ecologists focus on trees, and tree diversity often functions at scales that are far coarser than environmental factors. A small wet area in an upland forest may have distinctly different soil, but may occupy the area of only one or a handful of tree crowns. Factors like dispersal limitation might prevent wetland specialists from arriving at the site frequently enough to catch it at the right time when it is available for colonisation – after all, no matter how much better a competitor it might be as an adult, it’s almost impossible for a tree seedling to grow up and displace an adult tree. When you have this sort of fine-grain heterogeneity, these “mass effects” are likely to outweigh ecological factors.
One of the few types of habitat heterogeneity that is readily apparent is the effect of small streams. Streams create areas of saturated soil and allow more light in – distinct habitats which tend to have distinct riparian vegetation. Drucker and co-authors predicted that understorey herbs are likely to show a more readily discernible pattern than trees, since herbs are more sensitive to changes in light and soil moisture than are canopy trees.
They were able to detect a difference in the understorey herb composition up to 100 m from the stream. They didn’t find a distinct assemblage of riparian herbaceous species except in a narrow band along the stream, but they were able to detect a gradient of change in the species composition away from the streams. From a methodological viewpoint this study was interesting – attempts to classify the sites into groups did not work well, but gradient analysis – an idea which has been around since Whittaker – was able to detect a change away from the stream.
The other interesting thing they found had to do with diversity. They noted that previous studies had found that streams contributed to beta diversity, but not alpha diversity. In other words, streams contributed to the overall species diversity because they added additional habitats. New habitats means new species, so overall species diversity increases. Drucker and colleagues found that streams also increased alpha diversity – they found more species per plot in the riparian zone. One of the interesting questions in community ecology is the way in which diversity is “constructed” – is it driven by changes in the number of species in a local area, or is it the product of having many habitats (each with their own species complement) in an area? In this case, along small streams in Reserva Ducke in Brazil, the answer to the question is “both”.
Drucker, D.P., Costa, F.R., Magnusson, W.E. (2008). How wide is the riparian zone of small streams in tropical forests? A test with terrestrial herbs. Journal of Tropical Ecology, 24(1), 65-74. DOI: 10.1017/S0266467407004701