Neutral vs. niche theory: which one can explain spider diversity within temperate forest canopies?

Background/Question/Methods

There has been debate for more than two decades regarding whether niche and neutral model play a major role in structuring communities and determining local and regional biodiversity. The classical niche-based theory asserts that differing life-history tradeoffs between species for competition, colonization and adaptations to environmental heterogeneity can stabilize coexistence, whereas the neutral theory assumes that individuals are demographically identical regardless of the species and that stochastic processes are sufficient in maintaining species diversity. However, we lack a process-based knowledge of how community dynamics operate in different spatio-temporal dimensions and whether only one ecological dynamic consistently governs biodiversity patterns at the local level. European beech patches were selected according to three different age classes (old-growth, mature and young) within the 2,664-ha Würzburg University Forest, Germany. Using insecticidal knockdown fogging, I sampled arboreal spider communities from 324 beeches during 18 months spread over three years. 

Results/Conclusions

Here I show that both the neutral and the niche–based models operate side by side and are both needed to explain the dynamics of arboreal spider assemblages among different canopy strata. Although stochasticity (ecological drift) shapes the spider community in old-growth canopies (150+ years old), the assemblages in monoculturally mature (60 years old) and young forest stands (20 years old) are structured by deterministic processes (biotic interaction). Due to silviculture systems, which have brought major changes to natural forest ecosystems, old-growth tree crowns in even-aged plantation forests constitute isolated, rare and small islands containing discrete and non-equilibrium spider communities with greater diversity. The study of how this century-old forestry regime influences ecological processes and biodiversity patterns of the canopy fauna in temperate forests merits far more attention. To give broader significance to my findings, investigation of biodiversity mechanisms in highly isolated environments (e.g., soil communities in landscape mosaics within long-term intensively managed agricultural systems) would be necessary.