55 Communities and Ecosystems 55 Communities and Ecosystems

55 Communities and Ecosystems

55 Communities and Ecosystems • Introduction • Communities: Loose Assemblages of Species • Process and Pattern in Communities and Ecosystems • Disturbance and Community Structure • Dispersal, Extinction, and Community Structure

55 Introduction • The species that live together in a particular area constitute an ecological community. • Each species interacts in unique ways with other species in its community and with its physical environment. • The species that form an ecological community, together with the physical environment, constitute an ecosystem.

55 Communities: Loose Assemblages of Species • Ecological communities contain many species that interact with one another. The species and interactions may change over time. • In 1926, Henry Gleason argued that communities were loose assemblages of species, with each one distributed individualistically according to its unique interactions with the physical environment. • In 1936 Frederick Clements argued that plant communities were tightly integrated “superorganisms. ”

55 Communities: Loose Assemblages of Species • Detailed studies of plants generally supported Gleason’s view. • Studies of mountain vegetation in Oregon showed that different combinations of plant species are found at different locations. • Species enter and drop out of communities independently over environmental gradients. • However, where environmental conditions change abruptly (e. g. , at the edges of lakes and streams), the ranges of many species may terminate abruptly.

Figure 55. 1 Plant Distributions along an Environmental Gradient

55 Communities: Loose Assemblages of Species • Ecologists ask the following questions: § What patterns exist in ecological communities and ecosystems? § How does the physical environment influence those patterns? § What are the relative roles of historical accident and current interactions? § How does evolution influence the assemblage of species that live together?

55 Communities: Loose Assemblages of Species • A few interactions may determine the features of a community. • Although hundreds of species live in oak forests in the eastern U. S. , the ecological interactions are dominated by the oak trees, white-footed mice, gypsy moths, and white-tailed deer. • Mice and deer survive well during years of heavy acorn production. • Gypsy moth larvae eat oak leaves and pupate on the trunks, where they may be eaten by mice; every few years the moths become extremely abundant.

55 Communities: Loose Assemblages of Species • To test the hypothesis that the mice generate fluctuations in the gypsy moth populations, ecologists did an experiment during a year when mice populations were high and gypsy moth populations were low. • By removing mice from experimental plots, they determined that the mice were preventing development of the gypsy moths by eating all the pupae from the tree trunks.

55 Communities: Loose Assemblages of Species • In another experiment, acorns were added to experimental plots during a year of low acorn production. • Mouse populations became much more dense on plots with added acorns than on control plots. • The research found that mice control gypsy moth populations, allowing oak trees to recover from defoliation by the moth and produce large crops of acorns. When mouse populations are reduced by a low acorn crop, moth populations rise, resulting in the defoliation of the oak trees.

Figure 55. 2 Interactions within Communities Control Populations

55 Process and Pattern in Communities and Ecosystems • Organisms need energy inputs, water, and minerals for metabolism and growth. • The sun is the source of energy, either directly or indirectly, for almost all organisms. • Fossil fuels, such as coal, oil, and natural gas, are stored solar energy. • About 5% of solar energy is captured by photosynthesis. • The remaining energy is either radiated back into the atmosphere as heat or consumed by the evaporation of water.

55 Process and Pattern in Communities and Ecosystems • The rate at which plants assimilate energy is called gross primary productivity. • Plants use some of this energy for their own metabolism; the rest is stored or used for growth or reproduction. The accumulated energy is called primary production. • The energy available to organisms that eat plants is called net primary production; gross primary production minus the energy used by the plants. • The energy content of an organism’s net production—its growth plus reproduction—is available to other organisms that consume it.

Figure 55. 3 Energy Flow through an Ecosystem

55 Process and Pattern in Communities and Ecosystems • The distribution of primary production worldwide reflects the distribution of land masses, temperature, and moisture. • Tropical areas with high temperatures and adequate water all year are most productive. • In lower-latitude and mid-latitude deserts, primary production is low because plants are limited by lack of moisture. • At higher latitudes with adequate moisture, low temperatures during much of the year limit production.

Figure 55. 4 Primary Production in Different Ecosystems (Part 1)

Figure 55. 4 Primary Production in Different Ecosystems (Part 2)

Figure 55. 4 Primary Production in Different Ecosystems (Part 3)

Figure 55. 5 Net Primary Production of Terrestrial Ecosystems

55 Process and Pattern in Communities and Ecosystems • Production in aquatic systems is limited by light, nutrients, and temperature. • Primary productivity influences two other community characteristics: species richness and food web structure.

55 Process and Pattern in Communities and Ecosystems • The species richness of an ecological community is correlated with gross primary productivity—to a point. • Species richness often increases with productivity at first, but then decreases. • One hypothesis to explain the decrease postulates that interspecific competition becomes more intense with higher productivity, resulting in competitive exclusion. • This hypothesis is supported by a long-term experiment in England, in which species richness of plants in unfertilized plots has remained unchanged, and species richness in fertilized plots has declined.

Figure 55. 6 Local Species Richness Peaks at Intermediate Productivity

55 Process and Pattern in Communities and Ecosystems • Organisms in a community can be categorized into trophic levels depending on how they get their food. • Photosynthesizers (autotrophs) are the primary producers; they get energy from sunlight and produce the organic molecules that other organisms (heterotrophs) consume. • Organisms that eat plants are called herbivores or primary consumers. • Secondary consumers eat herbivores. Those that eat secondary consumers are tertiary consumers, and so on.

55 Process and Pattern in Communities and Ecosystems • Detritivores or decomposers consume dead organisms. • Organisms that eat foods from primary producers and another trophic level are omnivores. • A sequence of linkages in which a plant is eaten by an herbivore, and so on, is called a food chain. • Food chains are usually interconnected to make a food web, because most species eat or are eaten by more than one species.

Figure 55. 7 Food Web of Isle Royale National Park

55 Process and Pattern in Communities and Ecosystems • Most communities have only three to five trophic levels. • Only a portion of energy captured at one trophic level is available to organisms at the next higher level. Energy pyramids show energy decreases as it flows from lower to higher trophic levels. • A biomass pyramid illustrates the amount of biomass available at a given time for organisms at the next trophic level. • The shapes of the pyramids depend on the dominant organisms and how they allocate their energy.

Figure 55. 8 Pyramids of Biomass and Energy

55 Process and Pattern in Communities and Ecosystems • In most terrestrial systems, the primary producer level contains a large biomass. • However, trees store much of their energy in difficult-to-digest wood, whereas much of the primary net production of grasslands is consumed. • Thus the herbivore level has a relatively larger biomass in grasslands than in forests.

55 Process and Pattern in Communities and Ecosystems • Most aquatic communities have dominant primary producers that are bacteria and protists. • These have such high rates of cell division that a small biomass can feed a much larger biomass of herbivores. • This pattern can produce an inverted biomass pyramid, even though the energy pyramid for the same ecosystem has the typical shape.

55 Process and Pattern in Communities and Ecosystems • Much of the energy in biomass is consumed by detritivores. • Detritivores, such as bacteria, fungi, worms, mites, and insects, transform the remains and waste products of organisms into CO 2, water, and minerals. • Continued ecosystem productivity depends on rapid decomposition of detritus.

55 Process and Pattern in Communities and Ecosystems • Does species richness influence ecosystem productivity? • Ecologists hypothesized the following: § Species richness might enhance productivity because a richer mixture of species should result in a more complete use of resources. § If the environment changes, a species-rich system is more likely to contain species already adapted to the new conditions. § A species-rich ecosystem should be more stable —it should change less over time in terms of both productivity and species composition.

55 Process and Pattern in Communities and Ecosystems • To test this, ecologists planted grasses in plots with various mixtures of species. • In 11 years of measurements, the plots with more species had greater biomass (greater net primary productivity) and varied less from one year to another. • However, population densities of individual species varied independently of the plot’s species richness, because different species performed better during drought and during wet years. • Although species richness and productivity were positively correlated, this could have resulted if only one or a few species had exerted very strong influences on the ecosystem.

Figure 55. 9 Species Richness Enhances Community Productivity and Stability (Part 1)

Figure 55. 9 Species Richness Enhances Community Productivity and Stability (Part 2)

55 Process and Pattern in Communities and Ecosystems • Species whose influences on ecosystems are greater than would be expected on the basis of their abundance are called keystone species. • They may influence both the species richness of communities and the flow of energy and materials through ecosystems. • Beavers, for example, create habitats for other species by cutting down trees and building dams.

55 Process and Pattern in Communities and Ecosystems • Large grazing mammals, such as bison, change the structure and composition of vegetation. • Bison prefer grasses to forbs (small broad-leaved plants). When bison are excluded from an area of prairie, grasses dominate the ecosystem. When bison are present, they eat the grasses and make space forbs. • Bison urine is broken down quickly, providing nitrogen for plant uptake. Plants in areas grazed by bison have higher leaf nitrogen levels and grow faster.

Figure 55. 10 Grazing Increases Plant Species Richness and Productivity (Part 1)

Figure 55. 10 Grazing Increases Plant Species Richness and Productivity (Part 2)

55 Process and Pattern in Communities and Ecosystems • Another keystone species is the sea star Pisaster ochraceous of the North American Pacific coast. • In the absence of the sea star, mussels take over the intertidal zone and crowd out other animals. • By consuming mussels, the sea star creates bare spaces for a variety of other species. • When sea stars were removed experimentally from parts of the intertidal zone, 28 species of animals and algae disappeared.

Figure 55. 11 Sea Stars are Keystone Predators

55 Disturbance and Community Structure • A disturbance is an event that changes the survival rate of one or more species in an ecological community. • A disturbance can be limited to a small area or it can be large in its effects (e. g. , a hurricane or volcanic eruption). • Disturbances have different effects according to how often they occur and the pattern of the damage that they cause. • Forest fires in Yellowstone National Park created a mosaic of patches that burned with varying intensity.

Figure 55. 12 Fires Create Mosaics of Burned and Unburned Patches

55 Disturbance and Community Structure • In general, communities with very high levels of disturbance and those with very low levels have fewer species than communities with intermediate levels. • This observation generated the intermediate disturbance hypothesis: § There is low species richness in areas with high disturbance because only species with great dispersal abilities and rapid reproductive rates can persist. § Species richness declines with low levels of disturbance because competitively dominant species displace other species.

55 Disturbance and Community Structure • In an experiment with different-sized boulders on intertidal beaches, it was determined that mediumsized boulders had more species on them than small or large boulders. • The idea that the small boulders were disturbed by wave action too frequently to have many species was tested by gluing down small boulders. The secured small boulders had more species than unsecured small boulders. • This experiment also showed that the number of species in a community changes over time following a disturbance.

Figure 55. 13 Species Richness Is Greatest at Intermediate Levels of Disturbance

55 Disturbance and Community Structure • Ecological succession is the sequence of changes in the species composition of a community over time. • Primary succession begins with the establishment of organisms on newly available sites that previously had no organisms. • Secondary succession begins when organisms reestablish themselves on disturbed sites where some organisms survived the disturbance.

55 Disturbance and Community Structure • A good example of primary succession can be found in glacial deposits (moraines) in Alaska that were left by the retreat of a glacier over the last 200 years. • By comparing moraines of different ages, ecologists have been able to infer the order of primary plant succession on them.

Figure 55. 14 Primary Succession on a Glacial Moraine

55 Disturbance and Community Structure • The changes that take place when all or part of the dead body of an animal or plant is decomposed are examples of secondary succession. • The needle litter under pine trees is decomposed by a succession of fungal species. • Each group of fungi gets energy by decomposing certain compounds and converting them to other compounds that are used by the next group of fungal species.

Figure 55. 15 Secondary Succession on Pine Needles

55 Dispersal, Extinction, and Community Structure • Immigration and emigration influence the structure of communities, and species introduced by humans often come to dominate the communities they invade. • Throughout the history of Earth, species have colonized new areas and others have gone extinct. • The rate of introduction of new species and the extinction of existing ones has been increased greatly by human activities over the past few centuries.

55 Dispersal, Extinction, and Community Structure • Before the Central American land bridge formed about 4 million years ago, South America had evolved a distinctive mammalian fauna. . • Thereafter, many mammals dispersed across the newly established land bridge, mostly North American mammals going south. • The North American invasion caused the extinction of several kinds of marsupial carnivores and the large herbivores they preyed on. • Subsequently, the invaders formed new species that today exist only in South America.

Figure 55. 16 North and South America Exchanged Mammals (Part 1)

Figure 55. 16 North and South America Exchanged Mammals (Part 2)