Understanding the processes limiting the natural distributions of species is central to ecology and its application to land management. Most species are limited to a fraction of their potential distribution by abiotic or biotic conditions along local gradients. The environment serves as the first filter to species’ occurrence, and although abiotic factors strongly influence every species’ distribution, biotic factors can also limit or exclude species from habitats that are otherwise suitable. The concept that a species may occupy only a portion of its potential range, and that the habitat occupied may vary by site or region, is essential to understand when species distribution information is used in land management and regulation. The US Army Corps of Engineers (USACE), for example, uses species occurrence as an indication of ecological site conditions and for subsequent use in wetland delineation. Wetland plant indicator status and its application to wetland delineation are heavily debated among delineators, as classification is based on presumptions of ecological site preferences and not on landscape-level frequency data.
Our research in southeast Alaska addressed this gap in knowledge. We used a local hydrologic gradient to identify the main drivers of three local tree species: Pinus contorta ssp. contorta, Picea sitchensis, and Tsuga heterophylla. We found that species tolerances did not conform to presumed preferences for site conditions. Species that could be successful across the entire hydrologic gradient were constrained by competition from co-occurring tree species. Across this gradient, local topography and associated hydrologic conditions act as the first filter to species occurrence. Pinus contorta was then constrained by competitive exclusion. The other tree species were able to occur across the gradient, but biomass was limited by hydrologic stress and nutrient limitations. We found that species’ success varied across a hierarchy of spatial scales and that plant indicator status does not accurately capture the main limitations on species occurrence and abundance.
Check out my recent Ecological Society of America talk on this subject Bisbing_ESA2014_reduced.