16 Temporal Patterns of Biodiversity: Ecological to Geological Scale
Dr. Felix Bast
1. Learning outcomes
1.1 To learn about an overview of temporal patterns of global biodiversity
1.2 To learn about the pace of macroevolution
1.3 To learn about ecological factors affecting temporal changes, including predation, disturbance regime, species introduction and species coexistence
1.4 To learn about geological factors affecting temporal changes including mass extinctions, Global climate, Milankovitch cycles, Plate Tectonics, Vicariance, Glacial Maxima and Biogeography
1.5 To learn about HIPPO
2. Concept map
3. Description
3.1. Introduction
It is estimated that 99.9 percent of species ever lived on this planet have gone extinct. Of course, almost the entire extinction was natural ‘background’, not human mediated. In fact, our own species came to existence only very recently in the deep geological time. Global biodiversity changes both spatially as well as temporally and an understanding about such changes would enable us to know various contributory factors, to predict the future and to derive in effective conservation measures.
While there are many levels of biodiversity from macromolecules, genes and cells to species, community and biomes, there seem to be some consensus among scientists that the basic unit is species, and thus the term biodiversity directly means the diversity of species; both in the number (species richness) and diversity (species evenness). Change in number of species historically, the temporal change, can be studied by quantifying species richness of fossil biota at various geological strata. As the rock layers are formed and deposited one above other, the oldest is usually the bottom-most (if these layers are not disturbed by igneous intrusions or tectonic reworking). Number of species of fossils at each of these named and time-calibrated rock layers would correspond to the level of global biodiversity when these species existed (or when these rock layers were formed).
Temporal change in global biodiversity has no obvious patterns; it increases and decreases in response to a wide varieties of factors. A graph of such a temporal change is analogous to a stock market index; both goes up and down in response to a complex, intertwined network of factors. The rate of biodiversity change depends on two principal factors, rate of speciation and rate of extinction, therefore all the factors that contribute to these two principal factors indirectly drive the global biodiversity change. As rate of speciation increases, so as the biodiversity (i.e., directly proportional); while the rate of extinction is inversely proportional to the biodiversity.
Consider an indirect factor, genetic drift. This evolutionary process is one of the principal mechanisms through which microevolution (change in allele frequencies within a population over time) works, the other mechanism is Darwinian natural selection. Genetic drift is random sampling of alleles (gene variants), and is especially prominent when the population size is low. The effect of genetic drift towards speciation is positive; it increases variation between populations, while decreases variation within the populations, and both of these are good for these populations to evolve into separate species. Therefore, genetic drift indirectly increases the biodiversity, as it increases the speciation. As mentioned, genetic drift depends upon the population size (or effective population size to be exact, effective population size is the number of fertile individuals of a population), and drift becomes more rigorous when population size is low. This is especially important after catastrophic events when the population drastically decreases; for example, a major asteroid impact, or ‘snowball earth’ when much of the earth’s surface is covered with massive ice sheets. Such events that drastically reduce the population is called population bottleneck. As drift increases speciation during population bottlenecks, an increase in biodiversity is expected after these major catastrophes. Of course, after second snowball earth, a major ‘explosive’ increase in global biodiversity happened approximately 540 million years ago, the so called Cambrian explosion. After each major extinction events, global biodiversity spikes and a number of new species are populated on the planet earth.
Let us consider yet another factor, resource limitation. During the periods of global glaciation when much of the planet earth is covered under ice sheets, resources for the sustenance of life become extremely scarce, and this drives many species go extinct. Resource limitation thus increases rate of extinction, and therefore decreases biodiversity.
On a lesser scale, global biodiversity is also affected by immigration (incoming) and emigration (outgoing) of species, together called migration. However, migration causes changes in spatial patterns of biodiversity, not its temporal patterns.
The major evolutionary changes that occurs at or above species level over deep geologic time period is known as macroevolution. Principal process in macroevolution is speciation, the formation of new species. As global biodiversity is directly proportional to the rate of speciation, the pace of macroevolution is one of the principal factors driving the temporal change in global biodiversity.
Studying the evolution of morphological traits at specific taxa of fossils, palaeontologists could trace various patterns of the pace of macroevolution. One such a pattern is directed selection (gradualism), when one form gradually changes to the next form through the process of natural selection. Natural selection is the survival and reproduction of traits that confers fitness of individuals, as these traits make the individuals well adapted to the environment. In this mode, evolutionary change (or rate of mutations) occurs at a constant and even pace. This causes slow, incremental changes in species, and is depicted in a smoothed tree-like fashion.
Yet another mode of macroevolution as evident in a number of paleontological studies is what is called as random walk. Random walk, as in the walk of a drunkard (predicting the way how drunkard would walk mathematically is indeed called Random Walk Theory) means traits randomly changing between two contrasting forms. For example, pelvic girdle size of a particular fish lineage; over the course of a million years, the girdle size sometimes increases, sometimes decreases without any clear pattern or directionality. The mean pelvic girdle size would remain same. Random walks are not directional and is usually caused by genetic drift.
The third mode of macroevolution is punctuated equilibrium, a now famous theory proposed by palaeontologists Niles Eldredge and Stephen Jay Gould. In this mode, long periods of little or no evolutionary changes of traits, the ‘stasis,’ follows periods of rapid phenotypical changes over a short period, the ‘saltations.’ In punctuated equilibria, long periods of evolutionary stasis is punctuated by short periods of evolutionary saltations. The punctuated equilibrium is typically represented in a tree-like fashion as in Darwinian directed evolution, however, the branches are not smooth but strait lines, and rapid diversification, the speciation, represented as perpendicular straight-line branches. The period of stasis where virtually no morphological changes happen could also be due to random walks. In fossil records, diversification of lineages (i.e., speciation) usually happen over relatively a very short period (narrow rock layer). This rapid diversification (saltation) often occurs immediately after a major catastrophe, during the periods of population bottlenecks. Therefore, in this mode, the major evolutionary force that drive the speciation is random genetic drift that occurs during catastrophes, not Darwinian natural selection.
Comparative prevalence of these three modes in various fossil lineages had been studied in detail, and conclusion is that the Darwinian directed selection is extremely rare; occurs less than 5% of traits. The remaining 95% of traits were divided equally between random walks and punctuated equilibria. Directed Darwinian selection was also found to be rare in a number of protein coding genes, where the prevailing mode was neutral evolution. Japanese evolutionary biologist Motoo Kimura found that most of the mutations that occurs in nature are synonymous (i.e., mutations that do not change the encoded amino acid), does not confer evolutionary fitness to the individuals, and thus, they are evolutionarily neutral Synonymous mutations are fixed in a population through the stochastic event of Genetic drift. These ‘neutral evolution’ at molecular level (within microevolution) can be compared with either stasis, or random walks at macroevolutionary level; neutral evolution has no directionality. Among the minority of non-synonymous mutations (mutations that change the encoded amino acid) that Kimura observed, he found that a vast majority of these mutations are ‘purifying’ in nature; i.e., these mutations are selected by nature to get rid of deleterious mutations (this is called purifying selection). Positive, Darwinian Selection (fixation of advantageous mutations, directional selection) are a rarity in DNA data; as in the case of rarity of directional selection a rarity in the case of macroevolution in fossil lineages as discussed earlier. Taking together, Darwinian natural selection is now thought to be rather a minor force to shape the evolution; most of the speciation happens after major catastrophes, and are results of random genetic drift.
As explained in the last section, temporal changes in the biodiversity is largely driven by rapid speciation immediately after major catastrophic events, the so called saltations, as well as by extinctions. A number of ecological factors contribute in speciation as well as extinction, and therefore these factors affects the biodiversity change.
Predation is one of the well-studied ecological factor, and is now considered to be a crucial factor behind Cambrian explosion. Just prior to this event, during Ediacaran period, biodiversity was comparatively scarce. At the Cambrian, however, a huge spike in marine biodiversity happened, and almost all known animal phyla of today emerged. A remarkable characteristic of several Cambrian fossils is the predatory marks on it, which are non-existent in the Ediacaran fossils. Predation increases intra-population competition within prey species, which lead to a higher diversification. As prey species gets better and better at avoiding the predator (by swimming at higher speeds, for instance) through natural selection, so as predators. This competition between predators and prays through natural selection is called ‘evolutionary arms race’, and is thought to be a major driver for the diversification of lineages.
Species introductions dramatically change the ecological niche of habitats. Introduced ‘exotic’ species are often more evolutionarily fit; they outcompete the local organisms and often have no predators. Eventually these exotic species could drive the local endemic species to extinction. Species introduction also enable the introduced species to establish itself in a resource-rich habitat. This would lead founder effects and associated peripatric speciation within the exotic species lineage, thus driving the biodiversity up. In one study, a lizard species Anolis sagrei was experimentally introduced to fourteen islands in Bahama. Within next decade, each of these lineages had several unique morphological trait evolutions, the first step toward speciation. Here, selection along with adaptation to divergent habitats is the principal driving force behind the evolutionary change. Consequences of species introductions are often tremendous affecting the entire habitat. Introduction by deep-rooted desert trees dramatically reduce the river flow, and affect almost the entire downstream riverine habitat. Invasive deep-rooted star thistles make the habitat arid by consuming large amount of water. Species invasions also result in disturbance regimes (widespread floral replacement). For example, when a grass species that are prone to fire invaded Hawaii, it increased the frequency of fire, and affected the local populations of trees and shrubs.
Several studies have revealed that a higher biodiversity corresponds to a higher ecosystem function; the more species richness, more will be the health and resilience of the ecosystem. A related concept is portfolio effect; species occupying different trophic levels coexisting at a particular habitat make the biodiversity more resilient from disturbances. If this species coexistence is disturbed, for example through a species introduction, the health of the entire ecosystem is affected that might lead to species extinctions.
Ecology also plays a key role in at least two modes of speciation as discussed in speciation module; parapatry and sympatry. In parapatry, population occupying at different ecological niche diversify, gets reproductively isolated and evolve into separate species. In the sympatry, speciation happen within the range of parental population through differential resource utilization (as in cichlid fishes) or through disruptive selection. A wellknown example of sympatric speciation is apple maggot flies Rhagoletis pulmonella. The original host had been hawthorns. In 1864, these flies adopted to utilize new resource, the host apple. In 1960, yet another host adaptation to cherry happened. All these fly populations differentially adapted to these contrasting resources evolved into different ‘races’ partly due to differences in phenology of hosts leading to the reproductive isolation of flies. Phenology is the study of periodic plant and animal life cycle events and how these are influenced by seasonal and inter-annual variations in climate, as well as habitat factors (such as elevation). Several recent studies have confirmed that selection caused by shifts in ecology, as well as through species invasion plays a crucial role in speciation.
Geology and biogeography plays a crucial role in the process of speciation and extinction, and, therefore, influence temporal patterns of global biodiversity. Life on earth originated approximately 4 billion years ago, immediately after the formation of oceans. In this origin of life, various geological processes played its role as a crucible of life. As discussed in speciation module, reproductive isolation caused by geographical barrier (vicariance) is the major driver of allopatric speciation-the most common speciation mode. While there are no barrier involved, biogeography-mediated reproductive isolation plays major roles in peripatric speciation as well; founder effects for instance. Together, allopatry and peripatry are two of the most common speciation modes and both are under the influence of biogeography.
Several types of reproductive barriers are the result of geologic processes; continental collision leading to the orogenic uplift resulted in massive mountain ranges such as Himalayas. Formation of glaciers, rivers, lakes, land bridges (isthmus), etc., have ramifications on reproductive isolation of species. Hot springs and hydrothermal vents, the product of geological processes, create unique habitats for the evolution of various chemoautrotrophic and thermophilic lineages. Earthquakes, volcanic eruptions, landslides, among other such geological processes might create effective reproductive barriers, change species distribution patterns, or drive species to extinction. For example, a massive asteroid impact at Yucatán Peninsula in Mexico created not only Chicxulub crater, but it also drove a number of species to extinction, including dinosaurs (Cretaceous-Paleogene extinction event). Plate tectonics have changed routes of dispersal of several marine as well as terrestrial species. For example, collision of Indian plate with Eurasian plate approximately 45 million years ago resulted in the introduction of frogs, caecilians and turtle species from Africa to Eurasia, while reverse introduction of anguid lizards, iguanid lizards and boid snakes from Eurasia into India. After the formation of isthmus of panama, invasion of ungulate animals from North America to South America quickly replaced the native ungulate species, and this caused extinction of several lineages. In addition, the role of geology in the development of several landscapes and biomes are unequivocal; geology of the underlying rock influences the type of soil and substratum for the development of biota.
Global climate is an important determinant for the temporal changes in biodiversity, and is being influenced by a large number of factors, perhaps the most prominent one being Milankovit ch cycles, three astronomical attributes causing systematic changes in the distance and orientation of Earth relative to the Sun. Precession is a change in the orientation of the rotational axis of a rotating body. Earth’s precession happens once in 26,000 years. Axial tilt, also known as obliquity of the ecliptic, is the angle between an object’s rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. Earth’s axial tilt changes once in 41000 years. The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptical orbit, 1 is a parabolic escape orbit, and greater than 1 is a hyperbola. Earth’s orbital eccentricity changes once in 100,000 years. These three factors are thought to account for glacial cycles, and thereby the global biodiversity.
- Summary
4.1. Two principal factors controlling temporal changes in global biodiversity are rate of speciation and rate of extinction
4.2. Rates of speciation and extinctions are controlled by complex, intertwined network of factors comprising of both biotic as well as abiotic elements.
4.3. The pace of macroevolution is a determinant of biodiversity change; three principal modes are directed Darwinian evolution, random walk and punctuated equilibrium. Combined modes of random walk and punctuated equilibrium are responsible for approximately 95% of trait evolution.
4.4. Evidence from DNA data also corroborate that most of the speciation happens immediately after population bottleneck situations caused by major catastrophes.
4.5. A number of ecological factors contribute in temporal changes in global biodiversity, including predation, disturbance regime, species introduction and species coexistence. Examples of ecology-mediated speciation modes are parapatry and sympatry.
4.6. A number of geological and biogeographical factors also contribute in temporal changes in global biodiversity. The origin of life on planet earth was driven by geologic processes. Global climate is principally determined by three components of Milankovitch cycles. A number of geologic processes create effective reproductive isolation (vicariance) resulting in allopatric speciation. Plate tectonics results in species introductions en masse, leading to profound changes in biodiversity of both the plates.
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Further e-resources and learn more
- YouTube videos:
- https://www.youtube.com/watch?v=E56_hAHKMsU
- https://www.youtube.com/watch?v=VsIf8eBj4vY
- Gaston, K. J. (2000). Global patterns in biodiversity. Nature, 405(6783), 220.
- Ricklefs, R. E. (2004). A comprehensive framework for global patterns in biodiversity. Ecology letters, 7(1), 1-15.
- Kier, G., Mutke, J., Dinerstein, E., Ricketts, T. H., Küper, W., Kreft, H., & Barthlott, W. (2005). Global patterns of plant diversity and floristic knowledge. Journal of Biogeography, 32(7), 1107-1116.