19 Species Diversity in Communities Chapter 19 Species

19 Species Diversity in Communities

Chapter 19 Species Diversity in Communities CONCEPT 19. 1 Species diversity differs among communities due to variation in regional species pools, abiotic conditions, and species interactions. CONCEPT 19. 2 Resource partitioning is theorized to reduce competition and increase species diversity.

Chapter 19 Species Diversity in Communities CONCEPT 19. 3 Processes such as disturbance, stress, predation, and positive interactions can mediate resource availability, thus promoting species coexistence and species diversity. CONCEPT 19. 4 Many experiments show that species diversity is positively related to community function.

Powered by Prairies? A Case Study Dwindling supplies of fossil fuels has led to development of biofuels—liquid or gas fuels from plant material (biomass). In the United States, ethanol is made from corn, and biodiesel is made from soybeans.

Powered by Prairies? A Case Study Ideally, biofuels are carbon neutral— the amount of CO 2 produced by burning them is equal to the amount taken up by the plants from which they are made. They are a nearly limitless renewable resource, as long as the crops can be grown.

Powered by Prairies? A Case Study Biofuels have many downsides as well. Growing corn and soybeans for biofuels competes for land water that could be used for growing food. Fossil fuels, in the form of fertilizers and pesticides and used for farm work, are required to grow these crops.

Powered by Prairies? A Case Study Another option is to use non-edible plants (or parts), such as corn stalks, straw, or waste wood, to make biofuels. Biofuel crops could be grown on degraded land that is no longer suitable for high-yield food crops.

Powered by Prairies? A Case Study David Tillman has studied prairie plant species diversity in abandoned agricultural land at Cedar Creek, Minnesota. He has shown that plots with more species produce more biomass than plots with few species.

CONCEPT 19. 1 Species diversity differs among communities due to variation in regional species pools, abiotic conditions, and species interactions.

Concept 19. 1 Community Membership in a community depends on: 1. Regional species pools and dispersal ability 2. Abiotic conditions 3. Species interactions These factors act as “filters, ” which exclude or include species in particular communities.

Figure 19. 4 Community Membership: A Series of Filters

Concept 19. 1 Community Membership 1. The regional species pool provides an upper limit on the number and types of species that can be present in a community. The importance of dispersal can be seen in cases of non-native species invasions.

Concept 19. 1 Community Membership Humans have greatly expanded regional species pools by serving as vectors of dispersal. Example: Aquatic species travel around the world in ballast water carried by ships. Ships are now larger and faster, so transocean trips take less time—species are more likely to survive.

Concept 19. 1 Community Membership Zebra mussels (Dreissena polymorpha), arrived in the Great Lakes in ballast water in the late 1980 s and have had major impacts on aquatic communities. The comb jelly Mnemiopsis leidyi was also introduced via ballast water, into the Black Sea.

Figure 19. 5 Humans Are Vectors for Invasive Species

Concept 19. 1 Community Membership 2. Abiotic conditions: A species may be able to get to a community but be unable to tolerate the abiotic conditions. For example, a lake might not support organisms that require fast-flowing water.

Concept 19. 1 Community Membership 3. Species interactions: Coexistence with other species is also required for community membership. Other species may be required for growth, reproduction, or survival. Species may be excluded by competition, predation, parasitism, or disease.

Concept 19. 1 Community Membership Some non-native species do not become part of the new community. Biotic resistance occurs when interactions with the native species exclude the invader. Example: Native herbivores can reduce the spread of non-native plants.

CONCEPT 19. 2 Resource partitioning is theorized to reduce competition and increase species diversity.

Concept 19. 2 Resource Partitioning Resource partitioning: Competing species are more likely to coexist if they use resources in different ways. We can think of each type of resource as varying along a “resource spectrum, ” representing different nutrients, prey sizes, habitat types, etc.

Concept 19. 2 Resource Partitioning In a simple model, each species’ resource use falls on a spectrum of available resources. Figure 19. 7 A Resource Partitioning

Concept 19. 2 Resource Partitioning Assumption: The greater the overlap of resource use, the more competition between species. The less overlap, the more specialized species have become, and the less strongly they compete.

Concept 19. 2 Resource Partitioning If species have a high degree of specialization, it can result in less competition and high species richness. More species can be “packed” into a community if overlap is small. Or, if the resource spectrum is broad, a diversity of resources would be available for use by a wide variety of species.

Figure 19. 7 Resource Partitioning

CONCEPT 19. 3 Processes such as disturbance, stress, predation, and positive interactions can mediate resource availability, thus promoting species coexistence and species diversity.

Concept 19. 3 Resource Mediation and Coexistence If disturbance, stress, or predation keeps the dominant competitor from reaching carrying capacity, competitive exclusion cannot occur, and coexistence will be maintained.

Figure 19. 12 The Outcome of Competition under Constant and Variable Conditions

Concept 19. 3 Resource Mediation and Coexistence G. E. Hutchinson considered the idea in his paper “The Paradox of the Plankton” (1961). Lake phytoplankton communities have very high diversity (30– 40 species), all using the same limited resources, in a homogeneous environment.

Figure 19. 13 Paradox of the Plankton

Concept 19. 3 Resource Mediation and Coexistence His explanation was that conditions in the lake changed seasonally, which kept any one species from outcompeting the others. As long as conditions changed before competitively superior species reached carrying capacity, coexistence would be possible.

Concept 19. 3 Resource Mediation and Coexistence Intermediate disturbance hypothesis, first proposed by Connell (1978): Species diversity will be greatest at intermediate levels of disturbance. At low levels of disturbance, competition regulates diversity. At high disturbance levels, many species cannot survive.

Figure 19. 14 The Intermediate Disturbance Hypothesis

Concept 19. 3 Resource Mediation and Coexistence There have been many tests of this hypothesis. Sousa studied communities on intertidal boulders in southern California that were overturned by waves. Small boulders were overturned frequently (disturbance), large boulders were overturned less often.

Concept 19. 3 Resource Mediation and Coexistence After two years: Most small boulders had one species living on them (frequent disturbance). Most large boulders had two species (rare disturbance). Intermediate sized boulders had four to seven species.

Figure 19. 15 A Test of the Intermediate Disturbance Hypothesis

Concept 19. 3 Resource Mediation and Coexistence Huston (1979) added competitive displacement to the intermediate disturbance model: The best competitor uses the limiting resources, reducing the weaker competitor’s population growth to the point of extinction.

Concept 19. 3 Resource Mediation and Coexistence Hacker and Gaines (1997) incorporated positive interactions into the intermediate disturbance hypothesis. Evidence suggests that positive interactions are more common under relatively high levels of disturbance, stress, or predation.

Concept 19. 3 Resource Mediation and Coexistence At low levels of disturbance, competition reduces diversity. At intermediate levels, species involved in positive interactions are released from competition and can increase diversity. At high levels, positive interactions are common and help to increase diversity.

Figure 19. 17 Positive Interactions and Species Diversity

Concept 19. 3 Resource Mediation and Coexistence The intermediate disturbance hypothesis considers disturbance and predation to be similar—a dominant competitor is killed or damaged, creating opportunities for other species. Menge and Sutherland (1987) argue that because predation is a biological interaction, it should be considered separately.

Concept 19. 3 Resource Mediation and Coexistence Their model predicts that predation is most important when environmental stress is low. As stress increases, importance of predation decreases, and competition increases in importance. At high stress levels, neither are important.

Figure 19. 19 The Menge–Sutherland Model

Concept 19. 3 Resource Mediation and Coexistence The above theories assume an underlying competitive hierarchy. What if species have equivalent interaction strengths? Lottery models and neutral models emphasize the role of chance in maintaining species diversity.

Concept 19. 3 Resource Mediation and Coexistence In lottery models, all species have equal chances of obtaining resources that were made available by disturbances, and this allows coexistence. Species must have similar interaction strengths and growth rates and be able to respond quickly to disturbances that free up resources.

Concept 19. 3 Resource Mediation and Coexistence The lottery model may be most relevant in very diverse communities where many species overlap in their resource requirements. Its relevance decreases in communities in which species have large disparities in interaction strength.

CONCEPT 19. 4 Many experiments show that species diversity is positively related to community function.

Concept 19. 4 The Consequences of Diversity A central idea in ecology is that species diversity can control community functions, such as plant productivity, soil fertility, water quality, etc.

Concept 19. 4 The Consequences of Diversity Many community functions also provide valuable services to humans: • Food and fuel production • Water purification • O 2 and CO 2 exchange • Protection from catastrophic events, such as floods

Concept 19. 4 The Consequences of Diversity The Millennium Ecosystem Assessment (2005) predicts that if current losses of species diversity continue, human populations will be severely affected by the loss of services those species provide.

Concept 19. 4 The Consequences of Diversity The Diversity–Stability Theory A long-standing idea in ecology is that species richness is positively related to community stability— The tendency of a community to remain the same in structure and function, or to return after a disturbance.

Concept 19. 4 The Consequences of Diversity In the experimental plots at Cedar Creek, Tilman and Downing (1994) showed that plots with higher species richness had better drought resistance than plots with lower species richness. High diversity plots lost less plant biomass during the drought.

Figure 19. 21 Species Diversity and Community Function (Part 1)

Concept 19. 4 The Consequences of Diversity In another experiment, they set up plots with different numbers of species, but the same number of individuals (equal density). Plant productivity (biomass) increased and nitrogen was more efficiently used as species richness increased, up to a threshold.

Figure 19. 21 Species Diversity and Community Function (Part 2)

Concept 19. 4 The Consequences of Diversity Four hypotheses have been proposed to explain the relationship between species diversity and community function. Two variables in all the hypotheses: • Degree of overlap in ecological function of species • Variation in strength of the ecological functions of species

Figure 19. 22 Hypotheses on Species Richness and Community Function (Part 1)

Concept 19. 4 The Consequences of Diversity 1. Complementarity hypothesis: As species richness increases, there will be a linear increase in community function. Each species added has an equal effect.

Figure 19. 22 Hypotheses on Species Richness and Community Function (Part 2)

Concept 19. 4 The Consequences of Diversity 2. Redundancy hypothesis: The functional contribution of additional species reaches a threshold. As more species are added, there is overlap in their function, or redundancy among species. This model best fits the results of Tillman et al.

Figure 19. 22 Hypotheses on Species Richness and Community Function (Part 3)

Concept 19. 4 The Consequences of Diversity 3. Driver and passenger hypothesis: Strength of ecological function varies greatly. “Driver” species have a large effect; “passenger” species have a minimal effect. Addition of driver and passenger species will have unequal effects on community function.

Figure 19. 22 Hypotheses on Species Richness and Community Function (Part 4)

Concept 19. 4 The Consequences of Diversity 4. A variation on the driver and passenger hypothesis: It assumes there could be overlap between driver and passenger functions.

Figure 19. 22 Hypotheses on Species Richness and Community Function (Part 5)

Concept 19. 4 The Consequences of Diversity Experiments to test these hypotheses will be logistically challenging. They can tell us something about how communities work. They may be able to tell us what the future holds for communities that are both losing species by extinction and gaining species by invasions.

A Case Study Revisited: Powered by Prairies? Tilman et al. (2006) showed that highdiversity plots produced nearly 238% more biomass per input of energy than single-species plots. They then compared three types of biomass—soybeans, corn, and lowinput, high-diversity (LIHD) biomass from their prairie plots.

A Case Study Revisited: Powered by Prairies? Biodiesel, ethanol, and synfuel (synthetic gasoline) can be made from these crops. Synfuel from LIHD prairie biomass had the highest net energy balance (amount of biofuel produced minus the amount of fossil fuels used to produce it).

Figure 19. 23 Biofuel Comparisons

A Case Study Revisited: Powered by Prairies? Energy inputs were lower for LIHD crops because they are perennial plants and require little water, fertilizer, or pesticides. LIHD crops had a very high yield of biomass due to diversity effects; and all of the aboveground plant material can be used.

Figure 19. 24 Environmental Effects of Biofuels (Part 1)

Figure 19. 24 Environmental Effects of Biofuels (Part 2)

A Case Study Revisited: Powered by Prairies? Prairie plants also take up and store more CO 2 than corn and soybeans. LIHD plots sequestered 160% more CO 2 in roots and soil than single-species prairie plots. Greenhouse gas emission reductions were 6 to 16 times greater for LIHD fuels than for corn ethanol or soybean biodiesel.

Figure 19. 24 Environmental Effects of Biofuels (Part 3)

Connections in Nature: Barriers to Biofuels: The Plant Cell Wall Conundrum Biofuels vary in the biomass and energy required to make them. Biodiesel is easily produced from oils such as soybean oil, but growing the crops can increase soil erosion, requires large amounts of water, and competes with food crops.

Connections in Nature: Barriers to Biofuels: The Plant Cell Wall Conundrum Ethanol is commonly made from corn that is fermented and distilled. The energy costs associated with growing the corn and producing the ethanol are high, so there is only a slight energy gain in ethanol production.

Connections in Nature: Barriers to Biofuels: The Plant Cell Wall Conundrum It also competes with food crops. An acre of corn produces about 440 gallons of ethanol. This is 4– 5 months of driving for the average individual in the United States. The same amount of corn could feed one person for 20– 27 years.