BIOB50H3F Ecology Biodiversity & Biogeography Fall 2024 Lecture 10 PDF

BIOB50H3F Ecology Fall 2024, Week 10 Biodiversity & Biogeography From Individuals to Populations to Communities and Ecosystems Lecture 3 Lectures 9-11 Individual...

BIOB50H3F Ecology Fall 2024, Week 10 Biodiversity & Biogeography From Individuals to Populations to Communities and Ecosystems Lecture 3 Lectures 9-11 Individual Community (an association of interacting populations of different species, living and Lectures 4-5 interacting in the same area) Population (group of individuals of same species, living and interacting with one another in a particular area) Ecosystem (a community of organisms plus their abiotic (physical) environment) Biodiversity How would you define “biodiversity”? The most widely accepted definition of biodiversity comes from the United Nations Environment Programme’s Convention on Biological Diversity, and includes: (i) the total number of species on Earth (global species richness) (ii) the evolutionary diversity represented by these species (iii) the diverse communities and ecosystems that these species build GLOBAL SPECIES RICHNESS Species Richness Documented Global Species Richness About 2 million species have been described in the scientific literature. But how many species are there? Species Richness Estimated Global Species Richness Estimates of global species richness range from ~5 million to several billions. Species Richness The Challenges of Estimating Global Species Richness 1) A historical reliance on physical and morphological characteristics often led to morphs of a single species being described as separate species. Species Richness The Challenges of Estimating Global Species Richness 2) A historical reliance on physical and morphological characteristics often led to individuals of different species being classified as morphs of the same species. Species Richness The Challenges of Estimating Global Species Richness 3) Biases exist with respect to which species are studied. Species Richness The Challenges of Estimating Global Species Richness 3) Biases exist with respect to which ecosystems are studied. Species Richness The Challenges of Estimating Global Species Richness 4) Many species are hard to find and researchers are limited by logistic, time, and funding constraints. Species Richness The Challenges of Estimating Global Species Richness 5) Many species are going extinct before they are described. cf. Lecture 5 DETERMINING SPECIES RICHNESS & COMMUNITY COMPOSITION Determining Species Richness & Community Composition Determining which species are present in a community can be challenging; recording all species in a community is typically impossible Determining Species Richness & Community Composition Sampling techniques for determining community composition are similar to those discussed in Lecture 4 for populations, such as sampling plots, line transects, and camera traps. Determining Species Richness & Community Composition Species-Abundance Relationships Rarity imposes challenges for sampling, as rare species are rarely sampled and may, thus, be missed when documenting community composition Determining Species Richness & Community Composition Species-Abundance Relationships In most communities, a few species account for most individuals; most species can be considered rare. Example: Species abundance distribution of Big Chico Example: Species abundance distribution of Creek, California Breitenbach stream, Germany Determining Species Richness & Community Composition Species-Abundance Relationships Rarity imposes substantial challenges on sampling. Efforts are limited by time, logistic, and financial constraints, so researchers need to determine how much sampling effort to expend in a given area. Given that we cannot sample every individual in a community, and given that we do not know the true state of a community, how do we know when our sampling efforts have captured a community’s species richness and composition adequately? Example: You sampled a community of mushrooms, taking 20 individuals. You found four species. - Is this an adequate representation of the community? - How likely is it that you have missed some species? - Should you keep sampling to obtain a better representation of the community? Determining Species Richness & Community Composition Species-Abundance Relationships Rarity imposes substantial challenges on sampling. Efforts are limited by time, logistic, and financial constraints, so researchers need to determine how much sampling effort to expend in a given area. Given that we cannot sample every individual in a community, and given that we do not know the true state of a community, how do we know when our sampling efforts have captured a community’s species richness and composition adequately? Species accumulation curves describe how the number Sample 2 of species that were identified increases with the number of individuals sampled. Sample 5 - Species accumulation Sample 3 curves are initially steep but then level off once most species have been identified (the more you sample, the less likely it is that you see something Sample 4 new). Leveling off of the species accumulation curve Sample 1 can indicate adequate representation of the community in the samples. Determining Species Richness & Community Composition Species-Area Relationships Similarly to how we are unable to sample every individual in a community, we are also unable to sample every area of an ecosystem. Hierarchical sampling designs, collecting data at multiple spatial scales at randomly chosen locations allows understanding how community composition might vary across different areas. Determining Species Richness & Community Composition Species-Area Relationships Similarly to how we are unable to sample every individual in a community, we are also unable to sample every area of an ecosystem. Hierarchical sampling designs, collecting data at multiple spatial scales at randomly chosen locations allows understanding how community composition might vary across different areas. Species-area relationships suggest that the number of species that are found will increase with the area that is sampled: steeply first, and then more slowly as the probability increases that sampled species have already been observed in previous areas. Determining Species Richness & Community Composition Species-Area Relationships Species-area relationships can typically be described using a power function, S = cAz or equivalently, its log-log transformation log S = log c + z log A where A = area S = number of species in the area c = avg. number of species per unit area z = slope of log-log transformed species- area relationship (informing on how quickly new species are accumulated when new areas are added) Example: Species-area relationships for bird species on islands in the East Indies and West Indies. Determining Species Richness & Community Composition Species-Area Relationships Species-area relationships can typically be described using a power function, S = cAz or equivalently, its log-log transformation log S = log c + z log A where A = area S = number of species in the area c = avg. number of species per unit area z = slope of log-log transformed species- area relationship (informing on how quickly new species are accumulated when new areas are added) Example: Species-area relationships for Great Britain QUANTIFYING DIVERSITY Quantifying Diversity Species Richness & Species Evenness Which of these communities would you consider to be more diverse? Two communities may have the same species richness (the number of species in a community), but differ in their composition. Species evenness describes how evenly individuals are spread among species by considering the relative abundances of each species compared to one another. In the above example, species richness is four in both communities, but community B is considered more diverse because of a higher species evenness. Quantifying Diversity The Shannon-Wiener Diversity Index The Shannon-Wiener Diversity Index is a commonly used summary metric for species diversity. It incorporates both species richness and species evenness. s H = − pi ln ( pi ) S = number of species in the community i =1 pi = proportion of individuals that belong to the ith species Properties of the Shannon-Wiener Diversity index: - For a community that contains exactly one species, H=0. - For a fixed species richness (S), increasing evenness increases H. - For a fixed evenness (e.g., pi = 1/S for all species i), increasing species richness (S) increases H. - A higher H indicates a higher diversity, accounting for both richness and evenness. - A higher H indicates a higher diversity, but it does not inform on whether this is because of high richness or evenness. Quantifying Diversity The Shannon-Wiener Diversity Index H = 0.589 H = 1.388 higher diversity (due to higher species evenness) Example: Two hypothetical mushroom communities Quantifying Diversity The Shannon-Wiener Diversity Index Example: Two hypothetical fish communities Quantifying Diversity Rank Abundance Curves The Shannon-Wiener Diversity Index can indicate which communities are more diverse than others (higher H). Rank abundance curves, which plot the proportional abundance of each species (pi) relative to the others in rank order, can be used to understand whether differences in diversity are due to differences in species richness, evenness, or both. The length of a rank abundance curve indicates species richness and its steepness measures evenness Quantifying Diversity Species Identity Summary metrics, such as the Shannon-Wiener Diversity Index, can be useful for summarizing and comparing key characteristics of communities. However, two communities could have identical species richness and evenness but completely different species compositions and functions. Quantifying Diversity Species Identity Summary metrics, such as the Shannon-Wiener Diversity Index, can be useful for summarizing and comparing key characteristics of communities. However, two communities could have identical species richness and evenness but completely different species compositions and functions. It matters which species are present in a community; simply tallying up species richness / evenness might key details. Quantifying Diversity Seven Forms of Rarity Quantifying Diversity Seven Forms of Rarity Quantifying Diversity How would you define “biodiversity”? The most widely accepted definition of biodiversity comes from the United Nations Environment Programme’s Convention on Biological Diversity, and includes: (i) the total number of species on Earth (global species richness) (ii) the evolutionary diversity represented by these species (iii) the diverse communities and ecosystems that these species build Quantifying Diversity Phylogenetic Diversity Phylogenetic trees graphically depict the evolutionary relationships among a set of species. The length of branches typically represent the time since evolutionary divergence from the previous node. Phylogenetic diversity (PD) can be measured by adding up all branch lengths within a phylogeny to create a single number that can be compared across locations and through time. A set of species that are evolutionarily very different from one another will have a higher PD score that a set of closely related species. Quantifying Diversity Phylogenetic Diversity Evaluating experimental data on the thermal sensitivity of the development of helminths (parasitic worms) for 87 species revealed that thermal sensitivities were systematically influenced by phylogenetic relationships: related species tended to have similar thermal sensitivities, indicating that some helminth taxa are inherently more affected by rising temperatures than others. Example: The effects of phylogeny on the thermal sensitivity of helminth development (from Phillips et al. 2021. Proc. Roy. Soc. B 289: 20211878). Quantifying Diversity Phylogenetic Diversity Human-linked prehistoric extinctions resulted in a loss of two billion years of evolutionary history among mammals; an additional loss of another 500 million years of evolutionary history is expected through recent extinctions. Example: Past and expected future losses of phylogenetic diversity in mammals through extinctions Quantifying Diversity How would you define “biodiversity”? The most widely accepted definition of biodiversity comes from the United Nations Environment Programme’s Convention on Biological Diversity, and includes: (i) the total number of species on Earth (global species richness) (ii) the evolutionary diversity represented by these species (iii) the diverse communities and ecosystems that these species build Quantifying Diversity Functional Diversity Functional traits are characteristics of species that describe their ecological roles in a community. Example: Functional traits of plants are often defined based on their morphology (e.g., woodiness, leaf characteristics, height) and on how they contribute to forest canopy structure. Quantifying Diversity Functional Diversity Functional traits are characteristics of species that describe their ecological roles in a community. Functional diversity dendrograms graphically depict the relationships among a set of species in terms of their functional traits. Adding up the length of all branches produces a metric that can be regarded as an estimate of a community’s functional diversity (FD), as measured through functional richness (the number of traits present in a community) and functional evenness (the proportion of individuals in each functional group) Quantifying Diversity Functional Diversity Example: Functional diversity loss in bird communities on Pacific Islands due to extinctions Quantifying Diversity Diversity across Ecosystems: α-, β-, and γ-Diversity Species richness and other metrics may vary across habitats within an ecosystem, with heterogeneous habitats holding more species than homogeneous habitats. Species richness across different habitats can be described using α-, β-, and γ-diversity γ-diversity: species richness across a region made up of smaller localities The average number of species α-diversity: the average number of species found within small, local-scale habitats in a region β-diversity: a measure of heterogeneity of community composition among localities. β-diversity can be defined in many ways; one of the most common definitions is β = γ / α Quantifying Diversity Diversity across Ecosystems: α-, β-, and γ-Diversity GLOBAL GEOGRAPHIC PATTERNS OF BIODIVERSITY Global Geographic Patterns of Biodiversity Tropical rainforests hold the largest biodiversity among terrestrial biomes; tropical coral reefs hold the largest biodiversity among aquatic zones Global Geographic Patterns of Biodiversity Latitudinal Gradients of Species Richness Global Geographic Patterns of Biodiversity Latitudinal Gradients of Phylogenetic Diversity Trends in phylogenetic diversity tend to be similar to those in species richness. Differences between the two (e.g., areas with comparatively low species richness but high phylogenetic diversity) can indicate hotspots of evolutionary diversity (e.g., due to long and unique evolutionary histories). Global Geographic Patterns of Biodiversity Latitudinal Gradients of Diversity: Hypotheses Many hypotheses have been proposed for explaining the observed latitudinal gradients in diversity, including differences between lower and higher latitudes in (A) diversification rate, (B) diversification time, and (C) primary productivity. Hypothesis 1: Species diversity is largest in the tropics because the tropics have a higher species diversification rate than higher-latitude areas: large, thermally stable areas result in larger population sizes and larger population ranges, which decreases each species’ extinction risk and increases the rates of speciation Global Geographic Patterns of Biodiversity Latitudinal Gradients of Diversity: Hypotheses Many hypotheses have been proposed for explaining the observed latitudinal gradients in diversity, including differences between lower and higher latitudes in (A) diversification rate, (B) diversification time, and (C) primary productivity. Hypothesis 2: Species diversity is larger in the tropics than in higher-latitude areas because the tropics have been climatically stable for longer periods of time, thus giving species more time to evolve. In fact, it has been hypothesized that many species that are found elsewhere also originated in the tropics and then dispersed from there. Global Geographic Patterns of Biodiversity Latitudinal Gradients of Diversity: Hypotheses Many hypotheses have been proposed for explaining the observed latitudinal gradients in diversity, including differences between lower and higher latitudes in (A) diversification rate, (B) diversification time, and (C) primary productivity. Hypothesis 3: Species diversity is larger in terrestrial tropical systems than in temperate ones, because terrestrial productivity is generally highest in the tropics: high productivity promotes higher carrying capacities and thus larger population sizes and decreased extinctions risks. REGIONAL GEOGRAPHIC PATTERNS OF BIODIVERSITY  THE EQUILIBRIUM THEORY OF ISLAND BIOGEOGRAPHY (Robert MacArthur & Edward O. Wilson, 1967) Robert MacArthur E. O. Wilson The Equilibrium Theory of Island Biogeography The Effects of Island Size and Distance to the Mainland on the Island’s Species Richness On which of these islands would you expect to find more species? The Equilibrium Theory of Island Biogeography The Effects of Island Size and Distance to the Mainland on the Island’s Species Richness The size of an island positively correlates with the number of species it harbors. Example: Bird species richness on islands off the coast of New Guinea The Equilibrium Theory of Island Biogeography The Effects of Island Size and Distance to the Mainland on the Island’s Species Richness On which of these islands would you expect to find more species? The Equilibrium Theory of Island Biogeography The Effects of Island Size and Distance to the Mainland on the Island’s Species Richness The distance between an island and the mainland negatively correlates with the island’s species richness. Example: Bird species richness on islands off the coast of New Guinea The Equilibrium Theory of Island Biogeography The Equilibrium Theory of Island Biogeography suggests that the number of species on an island depends on how immigration rates and extinction rates balance against one another. Rates of immigration: - The theory assumes that ecological timescales are much faster than evolutionary timescales, so that the rate of new species arrivals on an island is primarily driven by immigration from somewhere else, rather than by speciation on the island. - Species richness increases when a new species that is not present on the island arrives; the rate of increase at which new species arrive slows when the island’s species richness is high, as many arrivals are already present on the island - The remoteness of an island influences immigration rates: higher immigration rates for islands that are closer to a source of species, such as the mainland The Equilibrium Theory of Island Biogeography The Equilibrium Theory of Island Biogeography suggests that the number of species on an island depends on how immigration rates and extinction rates balance against one another. Rates of extinction: - Higher species richness results in higher extinction rates because (a) more species can go extinct when more species are present; and (b) higher species richness leads to increased competition for limited resources, leading to smaller population sizes and higher extinction probabilities for each species. - Island size influences extinction rates: higher extinction rates for smaller islands because increased resource limitation leads to smaller population sizes and higher extinction probabilities for each species. The Equilibrium Theory of Island Biogeography The Equilibrium Theory of Island Biogeography suggests that the number of species on an island depends on how immigration rates and extinction rates balance against one another. - The equilibrium number of species on an island occurs where immigration and extinction rates balance exactly. - The model predicts that species richness will be highest on large islands near to the mainland, and smallest on small islands far from the mainland - Turnover rates (the rates at which newly colonizing species replace newly extinct species) are predicted to be largest on small islands near the mainland, and smallest on large islands far from the mainland The Equilibrium Theory of Island Biogeography The Equilibrium Theory of Island Biogeography has been tested in many ecosystems. Predictions generally match observed patterns. Example: Bird species richness on islands off Example: Recolonization of islands following the the coast of New Guinea eruption of the Krakatoa volcano The Equilibrium Theory of Island Biogeography The Equilibrium Theory of Island Biogeography has also been tested experimentally, most notably through a large-scale experiment by Simberloff and Wilson (1969/70) on small mangrove islands off the coats of Florida. The Equilibrium Theory of Island Biogeography The Equilibrium Theory of Island Biogeography also applies to island-like habitats.  Sample Short Answer Question (a) Using the Equilibrium Theory of Island Biogeography, explain why a large island that is near to the mainland will likely have a larger number of species present than a small island that is farther away from the mainland. (b) Will a small island that is near to the mainland always have a larger species richness than a large island that is far from the mainland? Why / why not?  ASSIGNED READINGS: Chapters 11 & 12.6 Please note: Quiz 10 covers lectures 10 and 11, and is therefore due two weeks from today (Nov 26).