The Importance of Niches for the Maintenance of Species Diversity

1. The Importance of Niches for the Maintenance of Species Diversity Levine & HilleRisLambers (2009) Nature 461:254.

2. How is ecological diversity maintained? Classic Theory Neutral Theory

3. Classic Theory Coexistence requires competitors to differ in their niches Stabilizes density-dependent dynamics Self-Regulation > Interspecific Competition

4. Challenged by the Neutral Theory Coexistence between competitors that are ecologically identical (no niche differences) Self-regulation is no more forceful than between-species competition Altering relative abundance doesn’t affect the total competitive inhibition an individual feels under neutrality

5. Levine & HilleRisLambersstudy explored theory & experimentation, and provided strong evidence that niche differences collectively stabilize serpentine annual plant diversity

6. No Niche Difference Self-Regulation = Interspecific Competition Species relative abundance doesn’t affect its per capita growth rate Variation between species reflects differences in fitness & competitive ability

7. Niche Differences Self-Regulation > Interspecific Competition Species relative abundance affects its per capita growth rate Coexistence occurs when stabilizing effects overcome species differences in overall competitive ability

8. No Niche Differences Population growth rates vary by several orders of magnitude between species (per capita population growth rate vs. year) Species Not Equivalent: 2 Orders of Magnitude in Geometric Mean The difference in magnitude is sufficient for rapid competitive exclusion

9. No Niche Differences Discrete-Time Dynamics: Competitive Exclusion Simulation initially involved an equal abundance of all competitors  each yr the demographic rate of either 2007 or 2008 was assigned (randomly)  population growth rates were calculated  the relative abundance of each species updated Predicted communities would became 99.9% Salvia in less than 20 yrs

10. Niche differences stabilize community dynamics 20 replicate communities initially sown with an equal fraction of the 10 competitors (by seed mass) 10 maintained niche differences while the other 10 were subjected to ‘niche-removal treatment’ Subjected to 2 yrs. of change in biodiversity & composition of communities

11. Niches Removed Shift in relative abundance After 2 years: Most common species were Salvia, Chorizanthe, & Plantago Salvia constituted ~60% of the community Rare Species constituted 8% of the Community

12. Control (Niche differences) Shift in relative abundance After 2 years: Diversity was 50% Greater Most common species were Salvia, Plantago& Vulpia Rare Species constituted 35% of the Community

13. Demographic Effects of Niche Differences Signature of Coexistence via Self-Regulation (seed production)

14. Summary • The per capita growth rate for species with niche differences (greater when they are rare & their competitors are common) vs. the per capita growth rate for species with no niche differences ( unchanged by species relative abundance) (Figure 1) • Null model predicted population growth rates would vary by several orders of magnitude between species resulting in rapid competitive exclusion (Figure 2)

15. Summary Cont’d After 2 generations of community change in the field, Diversity was over 50% greater in communities stabilized by niche differences relative to those exhibiting dynamics predicted by the null model (Figure 3) Demographic signature was demonstrated in an experiment manipulating species’ relative abundances (Figure 4)

16. Weakness Focused on small spatial and temporal scale used m2 plots Missing niche operations on larger spatial and temporal scales serpentine annual plants specialize on soil variation that occurs over tens of metres not all climatic conditions were experienced during the study

17. Strength Evaluating the collective importance of niche differences with regards to coexistence & biodiversity Findings provide strong empirical support for niche differences stabilizing species diversity