1. The latitudinal diversity gradient (LDG), the pattern of increasing biodiversity from the poles to the tropics, has been recognized for > 200
Years. Although many hypotheses have been proposed to explain the LDG each hypothesis is confounded with one or more of the others. The explanations
for the LDG can be categorized broadly into four groups: those proposing spatial mechanisms like the mid-domain effect, climatic/energy mechanisms, historical and evolutionary mechanisms, biotic mechanisms. Despite its long history and the identification of potential mechanisms, no broad consensus on the causes of the LDG has emerged.

3. Higher evolutionary rates in the tropics have been attributed to higher ambient temperatures, higher mutation rates, shorter generation time and/or faster physiological processes

4. Broad theories and conceptual frameworks to explain diversity patterns
Niche models
Neutral models
Island biogeography theory
Metacommunity models

5. Niche-based models of diversity
Niches provide a means for species to accumulate
Assumption is that each species has a niche
More niches = more species

6. Neutral-based models of diversity
No niche specialization
All individuals of a species considered equivalent
Many species can occupy the same niche
Diversity is a function of who occupies a niche first
Dispersal and basic demographics (birth, death) are key variables

7. Niche-based models - species sorting based on match between species life history traits and availability of its niche
Neutral models - ecological properties of species are identical. Random shuffling of species shapes composition
Pure neutral-assembly models
Pure niche-assembly models

9. Niche versus neutrality
Both perspectives valid
They differ in how they view species redundancy
Niche model: unique species, less redundancy
Neutral model: equivalent species, greater redundancy

10. Niche versus neutrality
Their explanatory value varies depending upon scale extent and resolution

11. Niche versus neutrality
Their explanatory value varies depending upon habitat and organism type
Neutral theory works well in the tropics for rainforest tree species, the location where the theory was developed.
The overall value of neutral theory rests in its use a null model, from which departures from it are to be expected but will vary according to circumstance and scale

12. Island Biogeography Theory
In 1967 Robert H. MacArthur and Edward O. Wilson published The Equilibrium Theory of Island Biogeography (ETIB)
Marked turn toward quantification in ecology
Tested in the Florida Keys by Simberloff and Wilson
Still undergoing refinement today
MacArthur and Wilson published a general mathematical theory to explain the regulation of species richness on islands. Their theory was based on the argument that island biotas eventually reach a
dynamic equilibrium between processes that add species, particularly by immigration counterbalanced by processes that cause local extinction of species
Specifically, the model at the core of their theory predicts that the rates of these two key processes are determined by geographical context, represented in the first instance by island area and isolation

14. Revisions to original ETIB
Recognized now that equilibrium is seldom attained
Original ETIB did not address:
Habitat heterogeneity
Species interactions
Multiple dispersal directions
Geologic change of oceanic islands

16. Contributions of island biogeography theory: island dynamics applied to conservation
Diversity of fragmented islands of forest in Costa Rica can be framed in terms of ‘island’ size and separating distances among islands

17. SLOSS (Single large versus several small) debate
Insights about the design and management of conservation areas came out of island biogeographic theory. The SLOSS debate is the conversation over the best strategy for designing reserve areas

19. Metacommunity models of diversity
Communities linked by dispersal of potentially interacting species. Diversity measured locally within a community or regionally across all communities.
Mechanisms are more biotic and ecological than in island biogeography

20. Metacommunity models of diversity
Localized environmental heterogeneities which favor certain species through niche processes
Source sink dynamics: net flow of individuals constrains outcomes of species interactions
Competition - colonization tradeoffs: expressed in relation to disturbances
Rescue effects: local extinction of a poor competitor is prevented by immigration.
Neutrality: species abundances are due in part to random processes and dispersal limitation

21. What are the relationships between diversity and the stability of ecosystem function?
A very complex question, one that is very simplified in this lecture
High diversity is stable
High diversity promotes aggregate stability but low population stability
High diversity is not necessary for stability in ecosystem function

22. High diversity promotes stability in ecosystem function

23. High diversity promotes aggregate stability but low population stability
Aggregate stability – measure of variability of ecosystem property like biomass or net primary productivity
More species = lots of ways to redistribute recovery from disturbance or enviromental fluctuation = more aggregate stability
However, populations of species fluctuate to achieve aggregate stability.
Cedar Creek, MN

25. High diversity is not directly necessary for stability in ecosystem function
High diversities are mechanism to derive stability
Stability may not require high diversity, but is derived from it.
Some lower diversity system may actually be very stable
Example: salt marsh

26. Salt marsh

27. Fire-reinforcing longleaf pine stability domain
Fire-resisting longleaf pine stability domain 27

28. Oligotrophic stability domain
Eutrophic stability domain

29. Woody shrub-dominated stability domain
Grassland stability domain
Woody shrub-dominated stability domain

30. High diversity in this sense is unstable – there is high population turnover in the range of conditions associated with bistability. This turnover may, or may not, flip to one of two stability domains, or continue as a transient state with high turnover.

31. Cross-scale resiliency hypothesis:
The spatial and temporal structure of diversity generates resilience, not just its presence
Resilience is analogous to stability
Niche overlap will be limited within a single spatial or temporal scale because of competition

32. Cross-scale resiliency hypothesis
Species interact with their niche at different scales to minimize competition in the CSR model

33. Shortfalls that beset broad understanding of biodiversity – what is it we need to know?