latitudinal gradients in species diversity
TRANSCRIPT
Latitudinal Gradients in Species Diversity
Latitudinal Gradients in Species Diversity
Regional<—>Local <—> Point diversity
Four ways two systems can differ in diversity
Saturation with species, MacArthur’s foliage height diversity vs. birds
Latitudinal gradients in diversity, primary vs. secondary mechanisms
Time theories
Climatic stability and climatic predictability
Spatial heterogeneity
Productivity and stability of productivity
Competition —> specialization, narrow niches, higher diversity
Disturbance, intermediate disturbance hypothesis
Predation-induced diversity
Various Hypothetical Mechanisms for the Determination
of Species Diversity and Their Proposed Modes of Action __________________________________________________________________ Level Hypothesis or theory Mode of action__________________________________________________________________ Primary 1. Evolutionary time Degree of unsaturation with speciesPrimary 2. Ecological time Degree of unsaturation with speciesPrimary 3. Climatic stability Mean niche breadthPrimary 4. Climatic predictability Mean niche breadthPrimary or 5. Spatial heterogeneity Range of available resourcessecondarySecondary 6. Productivity Especially mean niche breadth, but
also range of available resourcesSecondary 7. Stability of primary Mean niche breadth and range of
production available resourcesTertiary 8. Competition Mean niche breadthPrimary, 9. Disturbance Degree of allowable niche overlapsecondary, and level of competitionor tertiaryTertiary 10. Predation Degree of allowable niche
overlap
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Intermediate Disturbance Hypothesis
Tree Species Diversity in
Tropical Rain Forests
Seed Predation Hypothesis
Nutrient Mosaic Hypothesis
Circular Networks Hypothesis
Disturbance Hypothesis
(Epiphyte Load Hypothesis)
Sea Otter (Enhydra lutris)
Amchitka ShemyaSea Otters 20-30 km2 only
vagrants
Kelp dense mats heavily
grazed
Sea Urchins 8/m2, 2-34mm 78/m2,
2-86mm
Chitons 1/m2 38/m2
Barnacles 5/m2
1215/m2
Mussels 4/m2 722/m2
Greenling abundant
scarce or absent
Harbor Seals 8/km l.5-2/km
Bald Eagles abundant
scarce or absent
Community Stability
Traditional Ecological Wisdom
Diversity begats stability
(Charles Elton)
More complex ecosystems with more
species have more checks and
balances
Types of Stability Point Attractors <——> Repellers Domains of Attraction, Multiple Stable States Local Stability <——> Global Stability Types of Stability 1. Persistence 2. Constancy = variability 3. Resistance = inertia 4. Resilience = elasticity (rate of return, Lyapunov stability) 5. Amplitude stability (Domain of attraction) 6. Cyclic stability, neutral stability, limit cycles, strange attractors 7. Trajectory stability
Types of Stability Point Attractors <——> Repellers Domains of Attraction, Multiple Stable States Local Stability <——> Global Stability Types of Stability Constancy = variability Resistance = inertia Resilience = elasticity (rate of return, Lyapunov stability) Amplitude stability (Domain of attraction) Cyclic stability, neutral stability, limit cycles, strange attractors Trajectory stability
Edward Lorenz
Strange Attractor
Generalized Lotka-Volterra Equations:
dNi/dt = Ni (bi + aij Nj)
Jacobian Matrix
Partial Derivatives ∂Ni / ∂Nj , ∂Nj / ∂Ni
Sensitivity of species i to changes in density of species jSensitivity of species j to changes in density of species i
Traditional Ecological Wisdom:Diversity begats Stability
MacArthur’s idea
Stability of an ecosystem should increase with both the number of different trophic links between species and with the equitability of energy flow up various food chains
Robert MacArthur
Robert May challenged conventional ecological thinking and asserted that
complex ecological systems
were likely to be less stable than simpler systems
May analyzed sets of randomly assembled Model Ecosystems. Jacobian matrices wereAssembled as follows: diagonal elements were defined as – 1. All other interaction terms were equally likely to be + or – (chosen from a uniform random distribution ranging from +1 to –1). Thus 25% of interactions were mutualisms, 25% were direct interspecific competitors and 50% were prey-predator or parasite-host interactions. Not known for any real ecological system!
May varied three aspects of community complexity:
1.Number of species (dimensionality of the Jacobian matrix)
2. Average absolute magnitude of elements (interaction strength)
3.Proportion of elements that were non-zero (connectedness)
Real communities are far from random in construction, but must obey various constraints.Can be no more than 5-7 trophic levels, food chain loops are disallowed, must be at least one producer in every ecosystem, etc.
Astronomically large numbers of random systems : for only 40 species, there are 10764 possible networksof which only about 10500 are biologically reasonable — realistic systems are so sparse that random sampling is unlikely to find them. For just a 20 species network, if one million hypothetical networks were generated on a computer every second for ten years, among the resulting 31.513 random systems produced, there is a 95% expectation of never encountering even one realistic ecological system!
Latitudinal gradients in species diversityTropical tree species diversitySeeding ringsNutrient mosaicCircular networksDisturbance (epiphyte loads)Connectance and number of speciesSea otters as keystone species, alternative stable statesTypes of stabilityConstancy = variabilityInertia = resistanceElasticity = resilience (Lyapunov stability)Amplitude (domain of attraction)Cyclic stability (neutral stability, limit cycles, strange attractors)Trajectory stability (succession)Traditional ecological wisdom: diversity begats stabilityMay’s challenge using random model systemsReal systems not constructed randomly