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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

High diversity promotes stability in ecosystem function

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

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

Salt marsh

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

Oligotrophic stability domain Eutrophic stability domain

Woody shrub-dominated stability domain Grassland stability domain Woody shrub-dominated stability domain

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.

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

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

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