Ocea 130/230 Spring 2004 Ocean Processes and Ecology
Biodiversity and the Paradox of the Plankton
Miller, Chapter 9 Optional Reading: Valiela, Chapter 10 Lalli and Parsons, pp. 5-6, Chapter 8 (parts…read critically) Summerhayes & Thorpe, Diversity chapter
I. Defining Diversity
"If today is a typical day on planet Earth, we will lose 116 square miles of rainforest, or about an acre a second. We will lose another 72 square miles to encroaching deserts, as a result of human mismanagement and overpopulation. We will lose 40 to 100 species, and no one knows whether the number is 40 or 100. Today the human population will increase by 250,000. And today we will add 2,700 tons of chlorofluorocarbons to the atmosphere and 15 million tons of carbon. Tonight the Earth will be a little hotter, its waters more acidic, and the fabric of life more threadbare." David Orr (1991) “What is Education For?” http://www.context.org/ICLIB/IC27/Orr.htm.
• The above quote is somewhat typical of the pro-environmental movement, which suggests that the world is going to hell in a handbasket, and it all started relatively recently. The purpose of our discussion is to put these ideas into the context of biological oceanography, and more importantly, to critically think about whether we should believe this or not.
• To begin, we must agree on what “biodiversity” means: - Biodiversity: refers to the broadly diverse forms into which organisms have evolved and is considered at three levels:
Genetic diversity: Variation in genes enabling organisms to evolve and adapt to new conditions.
Species diversity: The number, types, and distribution of species within an ecosystem.
Ecosystem diversity: The variety of habitats and communities of different species that interact in a complex web of interdependent relationships. • Next, we must consider how we can categorically and rationally measure diversity (how do we know, for example, that the coral reefs are more diverse than the open ocean?) • It’s generally considered that we must take into account both species richness and also abundance Ocea 130/230 Spring 2004 Ocean Processes and Ecology - So, for example, if we found a patch of ocean with 100 species of phytoplankton, but there was only 1 individual of each species for 99 of them, and 1 million individuals of the last species, versus another patch of ocean that contained 20 species, equally abundant, which would be more diverse? - There are numerous mathematical models for determining diversity. The one thing they all have in common is that (except for some recent ones based on information theory), they are empirically derived, and not based on any a priori knowledge about how diversity or evolution works. - A simple equation that captures both components was proposed by Ramon Margalef (1951):
R = (S – 1) / logN
R = Richness (diversity) S = Species number N = total number of individuals
• In general, species richness increases with increased sample size (area, etc.) up to a certain point. This is because the larger the area, the more likely you’re sampling different ecosystems • No one really knows how many species exist, nor how fast they evolve, or how fast they go extinct…but a widely accepted value for the abundance is 6-30 million.
II. Evolutionary Time Scales
• Going backward in time, we know that, starting about 6 billion years ago, there has been a more or less steady increase in species on our planet • This is determined primarily by looking for marine fossils, because marine organisms are more likely to be preserved, and there is likely a strong bias towards organisms that are well preserved in the sediments • The evolutionary record is also biased by the availability of rock…marine sediments are destroyed at continental plate boundaries, so we don’t have a great record of the past…but it’s the only thing we’ve got • Evolutionarily, species richness is punctuated by the occasional massive loss of species, typically due to a worldwide disruption (meteor impact, volcanic eruption, glacial/interglacial changes, etc.) • A conflicting viewpoint of how this works is given by the theory of Punctuated Equilibrium, proposed by Gould and Eldredge (1972). In this theory, evolution is non-existent for long periods of time, and is then “jump-started” by some change that causes a rapid change (in geological time) to the species. A non-technical review of this can be found at http://www.sciam.com/explorations/072196explorations.html, with links to more details.
III. Factors Affecting Diversity Ocea 130/230 Spring 2004 Ocean Processes and Ecology • A number of proposed reasons for species diversity have been proposed, and are summarized in Valiela:
1) The Time Hypothesis - More time means more diversity…tropics vs. temperate, for example 2) The Spatial Heterogeneity Hypothesis - More ecological niches mean more species 3) The Competition Hypothesis - More biological competition means more specialization 4) The Environmental Stability Hypothesis - More stability allows more diversity, due to adaptation 5) The Productivity Hypothesis - More productivity means more species—easier to survive 6) The Predation Hypothesis - Lowers competition, increases speciation due to less selection
• Several of these were combined by Sanders (1968) to form the Time-Stability Hypothesis: - Based on studies of benthic marine habitats - States that stressful, shallow habitats are less diverse than environmentally constant, benign, abyssal habitats - Combines time, stability, competition, etc. into one model
• This is essentially the working theory of biodiversity that we’re most familiar with.
A. Terrestrial vs. Marine Diversity
• Marine ecosystems are much more diverse at the level of phyla, with 28 marine phyla and 11 terrestrial phyla • At the species level, though, the terrestrial environment has more numerically: - There are about 1500 know species of copepod (the most of any marine group), but about 1000x more species of beetles on land - There are about 4000 species of marine phytoplankton, and approximately 250,000 terrestrial plants • How do we explain this? • Time---the oceans have existed longer, which would explain why there are more phyla than on land • Homogeneity---there are many more ecological niches in the terrestrial environment than the oceans • We’re looking at several different types of diversity at once, so it’s not an equal comparison
B. Speciation by Isolation
• Speciation often involves the isolation of a continuous population into sub- populations, that over time become separate species. This can occur because of: Ocea 130/230 Spring 2004 Ocean Processes and Ecology • Spatial Isolation (different islands in a chain, etc.) • Temporal Isolation (not from the same geological time period) • Cultural Isolation (refusing to mate with another group) • On land, this is fairly easy to do—there are many ways to isolate one population from another • In the oceans, spatial separation is extremely difficult. Except for the permanent thermocline, there are essentially no barriers horizontally or vertically between populations • Over geological time, ocean basins have opened/closed depending on the position of the continents, ice flows, etc. This is the most prevalent way for isolation to have occurred in the oceans. • Because of the lack of isolation, there are numerous cosmopolitan species, which are species that have a global distribution
C. Habitat Selection
Horizontal • We expect that regions (spatial) with more variability would have a greater diversity of habitats for the formation of distinct species • Moving onshore-offshore, we find more species in deep water (more ecological niches), with a rapid change at the shelf-break. Why? • Inshore of the shelfbreak, conditions are essentially uniform…shallow, dominated by physical dispersion of energy, uniform distribution of benthic habitat • If we keep moving inshore, though, and expand our region to include different kinds of shoreline (more diverse habitats), we see a linear increase in speciation
Latitudinal • There’s a well-known decrease in biodiversity moving from the equator towards the poles (with more diversity in the tropics) • This is thought to occur because of: - Tropics are older (glaciation) - Tropics are more stable - Tropics have less seasonality, meaning longer reproductive time - generally more productivity at lower latitudes
Vertical • In the oceans, biomass increases logarithmically with depth (as does light, which fuels primary production) •
Disrupted Communities • In any habitat, disruption will often cause an increase in species diversity, but continued disruption eventually causes a decline • This can be caused by removal of competition, removal of predators, etc. Ocea 130/230 Spring 2004 Ocean Processes and Ecology • When a region is “polluted”, the productivity and biomass often go up, but the species abundance drops, leaving behind a handful of species adapted to high nutrient loads (for example)
D. Sampling Issues
• When looking at species diversity, it is always important to examine the sampling methodology. - Increasing the sample size will automatically increase the abundance of species that are not uniformly distributed - Increasing the spatial sample size will also increase the chance of includng additional habitats - Number of species will often go up as sampling methods improve
• There’s also the question of molecular diversity versus morphometric diversity: - In the oceans, diversity may be better characterized by the molecular sequence of DNA/RNA than by other characteristics - Prochlorococcus exhibits at least two different ecotypes, which are essentially identical except one is found deeper in the water column than the other
• Given everything we know about biodiversity and what shapes it, a problem arose which was coined “The Paradox of the Plankton” by G. Evelyn Hutchinson (1961). Simply stated, he could not find enough limitations in the ocean to support the number of species observed • He based this on the Competitive Exclusion Principle (which was formulated in 1904). This concept says that if two species are absolutely the same (i.e. they are competing for the same habitat, nutrients, etc.) then over time one of them must out- compete the other • In the oceans, we know that light, nutrients, and temperature are the 3 most important factors controlling productivity. The problem arises when we try to account for the incredible array of phytoplankton, based solely on the number of niches available in a fairly homogenous ocean