Differential species responses to environmental conditions can contribute to species coexistence and emergent patterns of diversity. In many communities, these differences can be associated with resource use, dispersal ability, and tolerance of local environmental conditions. Specialist species tend to be better adapted to local conditions, but more likely to be populations in decline. In contrast, generalist species tend to occupy many habitats, have larger distributions, and are less prone to extinction. Further, the differences among specialists and generalists have been observed to lead to species coexistence at local and regional spatial scales. The purpose of this study was to test differences betwen habitat specialists and generalists in response to environmental conditions. While most studies focus on one habitat, this study evaluated species that occupy the aquatic and terrestrial phases of temporary ponds. A fully factorial experiment that included hydroperiod length, nutrients, and leaf litter was conducted using mesocosms lined with California vernal pool soil. Aquatic invertebrate and terrestrial plant species densities and richness were measured in response to treatments and species were categorized by traits. MANOVA and NMDS were used to assess specialist and generalist patterns of species coexistence and diversity.
Specialists and generalists differentially responded to treatments during the aquatic (invertebrates) and terrestrial (plants) phases. Hydro-regime and litter were the primary treatments affecting the species groups. Specialist invertebrates were negatively affected by shorter hydroperiods and thatch presence, whereas generalists were less affected by hydroperiod and positively affected by thatch presence. Once the pools desiccated, specialist plants were only negatively affected by thatch presence. Nutrients had indirect effects by increasing algal mats which then reduced specialist plants. A hydroperiod x thatch interaction affected generalist species whereby thatch had positive effects during the long hydroperiod treatments. Ultimately, species diversity was most influenced by specialist species being more sensitive to disturbances (increased thatch and short hydroperiod). Interestingly, specialists during both the aquatic and terrestrial phases exhibited similar responses resulting in similar patterns of diversity in response to treatments. Many of the specialist species are also vernal pool endemics and therefore management of these species should consider their sensitivity to changes in the environment. Lastly, these results highlight the important differences between specialists and generalists based on their traits and habitat conditions for patterns of coexistence and diversity.