Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Molecular ecology wikipedia , lookup
Habitat conservation wikipedia , lookup
Ecological fitting wikipedia , lookup
Unified neutral theory of biodiversity wikipedia , lookup
Biodiversity action plan wikipedia , lookup
Introduced species wikipedia , lookup
Occupancy–abundance relationship wikipedia , lookup
Theoretical ecology wikipedia , lookup
Island restoration wikipedia , lookup
Latitudinal gradients in species diversity wikipedia , lookup
Community Ecology What Is a Community? • biological community • an assemblage of populations of various species living close enough for potential interaction • The various animals and plants surrounding this watering hole are all members of a savanna community in southern Africa Figure 53.1 • A community’s interactions include: • • • • • competition predation herbivory symbiosis disease • Populations are linked by interspecific interactions that affect the survival and reproduction of the species engaged in the interaction • Interspecific interactions can have differing effects on the populations involved Table 53.1 • Interspecific competition • Occurs when species compete for a particular resource that is in short supply • Strong competition can lead to competitive exclusion • The local elimination of one of the two competing species The Competitive Exclusion Principle • States that two species competing for the same limiting resources cannot coexist in the same place • Is the total of an organism’s use of the biotic and abiotic resources in its environment • The niche concept allows restatement of the competitive exclusion principle • Two species cannot coexist in a community if their niches are identical • Ecologically similar species can coexist in a community If there are one or more significant differenc.e in their niches e 53.2 RESULTS EXPERIMENT Ecologist Joseph Connell studied two barnacle speciesBalanus balanoides and Chthamalus stellatus that have a stratified distribution on rocks along the coast of Scotland. When Connell removed Balanus from the lower strata, the Chthamalus population spread into that area. High tide High tide Chthamalus Balanus Chthamalus realized niche Chthamalus fundamental niche Balanus realized niche Ocean Ocean Low tide Low tide CONCLUSION In nature, Balanus fails to survive high on the rocks because it is unable to resist desiccation (drying out) during low tides. Its realized niche is therefore similar to its fundamental niche. In contrast, Chthamalus is usually concentrated on the upper strata of rocks. To determine the fundamental of niche of Chthamalus, Connell removed Balanus from the lower strata. The spread of Chthamalus when Balanus was removed indicates that competitive exclusion makes the realized niche of Chthamalus much smaller than its fundamental niche. Resource Partitioning • the differentiation of niches that enables similar species to coexist in a community A. insolitus usually perches on shady branches. A. ricordii A. distichus perches on fence posts and other sunny surfaces. A. insolitus A. alinigar A. christophei A. distichus A. cybotes A. etheridgei Figure 53.3 • There is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species G. fortis G. fuliginosa Beak depth Percentages of individuals in each size class Santa María, San Cristóbal 40 Sympatric populations 20 0 Los Hermanos 40 G. fuliginosa, allopatric 20 0 Daphne 40 20 G. fortis, allopatric 0 8 Figure 53.4 10 12 Beak depth (mm) 14 16 • Interaction where one species, the predator, kills and eats the other, the prey • Feeding adaptations of predators include • Claws, teeth, fangs, stingers, and poison • Animals also display • A great variety of defensive adaptations • Feeding adaptations of predators include Claws, teeth, fangs, stingers, and poison • Animals also display A great variety of defensive adaptations • Cryptic coloration, or camouflage • Makes prey difficult to spot Figure 53.5 • Aposematic coloration • Warns predators to stay away from prey Figure 53.6 • In some cases, one prey species may gain significant protection by mimicking the appearance of another • Batesian mimicry: A palatable or harmless species mimics an unpalatable or harmful model (b) Green parrot snake Figure 53.7a, b (a) Hawkmoth larva • In Müllerian mimicry • Two or more unpalatable species resemble each other (a) Cuckoo bee Figure 53.8a, b (b) Yellow jacket • the process in which an herbivore eats parts of a plant. • Has led to the evolution of plant mechanical and chemical defenses and consequent adaptations by herbivores • one organism, the parasite derives its nourishment from another organism, its host, which is harmed in the process • Parasitism exerts substantial influence on populations and the structure of communities • The effects of disease on populations and communities is similar to that of parasites • Pathogens, disease-causing agents are typically bacteria, viruses, or protists • Mutualistic symbiosis, or mutualism is an interspecific interaction that benefits both species Figure 53.9 • One species benefits and the other is not affected Figure 53.10 • Commensal interactions have been difficult to document in nature because any close association between species likely affects both species Species Diversity • Dominant and keystone species exert strong controls on community structure. • In general, a small number of species in a community exert strong control on that community’s structure • The species diversity of a community Is the variety of different kinds of organisms that make up the community Has two components • Species richness •Is the total number of different species in the community • Relative abundance •Is the proportion each species represents of the total individuals in the community • Two different communities can have the same species richness, but a different relative abundance A B C D Figure 53.11 A: 25% Community 1 B: 25% C: 25% D: 25% A: 80% Community 2 B: 5% C: 5% D: 10% • A community with an even species abundance is more diverse than one in which one or two species are abundant and the remainder rare • the feeding relationships between organisms in a community • a key factor in community dynamics • Food chains – Link the trophic levels from producers to top carnivores Quaternary consumers Carnivore Carnivore Tertiary consumers Carnivore Carnivore Secondary consumers Carnivore Carnivore Primary consumers Zooplankton Herbivore Primary producers Plant Figure 53.12 A terrestrial food chain Phytoplankton A marine food chain Humans – a branching food chain with complex trophic interactions Smaller toothed whales Baleen whales Crab-eater seals Birds Sperm whales Elephant seals Leopard seals Fishes Squids Carnivorous plankton Copepods Euphausids (krill) Phytoplankton Figure 53.13 • Food webs can be simplified by isolating a portion of a community that interacts very little with the rest of the community Juvenile striped bass Sea nettle Fish larvae Figure 53.14 Fish eggs Zooplankton • Each food chain in a food web is usually only a few links long • There are two hypotheses that attempt to explain food chain length • The energetic hypothesis suggests that the length of a food chain • Is limited by the inefficiency of energy transfer along the chain The dynamic stability hypothesis Proposes that long food chains are less stable than short ones • Most of the available data supports the energetic hypothesis 6 No. of species 5 No. of trophic links 4 4 3 3 2 2 1 1 0 0 High (control) Medium Productivity Figure 53.15 5 Low Number of trophic links Number of species 6 • Certain species have an especially large impact on the structure of entire communities • Either because they are highly abundant or because they play a pivotal role in community dynamics • Are those species in a community that are most abundant or have the highest biomass • Exert powerful control over the occurrence and distribution of other species • One hypothesis suggests that dominant species are most competitive in exploiting limited resources • Another hypothesis for dominant species success Is that they are most successful at avoiding predators • Are not necessarily abundant in a community • Exert strong control on a community by their ecological roles, or niches • Field studies of sea stars Number of species present • Exhibit their role as a keystone species in intertidal communities 20 With Pisaster (control) 15 10 Without Pisaster (experimental) 5 0 1963 ´64 ´65 ´66 ´67 ´68 ´69 ´70 ´71 ´72 ´73 (a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates. Figure 53.16a,b (b) When Pisaster was removed from an intertidal zone, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which Pisaster was not removed, there was little change in species diversity. • Observation of sea otter populations and their predation Otter number (% max. count) 80 60 40 20 0 (a) Sea otter abundance 400 Grams per 0.25 m2 – Shows the effect the otters have on ocean communities 100 300 200 100 0 Number per 0.25 m2 (b) Sea urchin biomass 10 8 6 4 2 0 1972 1985 1989 1993 1997 Year Figure 53.17 Food chain before killer whale involvement in chain (c) Total kelp density Food chain after killer whales started preying on otters • Some organisms exert their influence by causing physical changes in the environment that affect community structure • Beaver dams can transform landscapes on a very large scale Figure 53.18 • Some foundation species act as facilitators • That have positive effects on the survival and reproduction of some of the other species in the community Number of plant species 8 6 4 2 0 Figure 53.19 Salt marsh with Juncus (foreground) With Juncus Without Juncus Conditions • The bottom-up model of community organization: • Proposes a unidirectional influence from lower to higher trophic levels • In this case, the presence or absence of abiotic nutrients • Determines community structure, including the abundance of primary producers Bottom-Up and Top-Down Controls • The top-down model of community organization • Proposes that control comes from the trophic level above • In this case, predators control herbivores • Which in turn control primary producers • Long-term experiment studies have shown • That communities can shift periodically from bottom-up to top-down Percentage of herbaceous plant cover 100 75 50 25 0 0 Figure 53.20 100 200 Rainfall (mm) 300 400 • Pollution • Can affect community dynamics • But through biomanipulation • Polluted communities can be restored Polluted State Fish Restored State Abundant Rare Zooplankton Rare Abundant Algae Abundant Rare • Disturbance influences species diversity and composition. • Decades ago, most ecologists favored the traditional view • That communities are in a state of equilibrium • However, a recent emphasis on change has led to a nonequilibrium model • Which describes communities as constantly changing after being buffeted by disturbances •Is an event that changes a community •Removes organisms from a community •Alters resource availability • Fire • Is a significant disturbance in most terrestrial ecosystems • Is often a necessity in some communities Figure 53.21a–c (a) Before a controlled burn. A prairie that has not burned for several years has a high proportion of detritus (dead grass). (b) During the burn. The detritus serves as fuel for fires. (c) After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living. • The intermediate disturbance hypothesis • Suggests that moderate levels of disturbance can foster higher species diversity than low levels of disturbance • The large-scale fire in Yellowstone National Park in 1988 • Demonstrated that communities can often respond very rapidly to a massive disturbance (a) Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance. Figure 53.22a, b ( b) One year after fire. This photo of the same general area taken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground. • most widespread agents of disturbance • Human disturbance to communities usually reduces species diversity • Humans also prevent some naturally occurring disturbances which can be important to community structure • Is the sequence of community and ecosystem changes after a disturbance Primary succession Occurs where no soil exists when succession begins Secondary succession Begins in an area where soil remains after a disturbance • Early-arriving species •May facilitate the appearance of later species by making the environment more favorable •May inhibit establishment of later species •May tolerate later species but have no impact on their establishment • Retreating glaciers • Provide a valuable field-research opportunity on succession Canada Grand Pacific Gl. 1940 Alaska 0 1912 1948 1879 1949 1935 Miles 1941 1 899 1907 5 1879 1948 1931 1911 1900 1892 1879 1913 1860 Reid Gl. Johns Hopkins Gl. 1879 Glacier Bay 1830 1780 1760 Pleasant Is. Figure 53.23 McBride glacier retreating 10 • Succession on the moraines in Glacier Bay, Alaska • Follows a predictable pattern of change in vegetation and soil characteristics (a) Pioneer stage, with fireweed dominant (b) Dryas stage 60 Soil nitrogen (g/m2) 50 40 30 20 10 0 Figure 53.24a–d Pioneer Dryas Alder Spruce Successional stage (d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content. (c) Spruce stage • Biogeographic factors affect community diversity. • Two key factors correlated with a community’s species diversity •Are its geographic location and its size • The two key factors in equatorial-polar gradients of species richness • Are probably evolutionary history and climate • Species richness generally declines along an equatorialpolar gradient • especially great in the tropics • The greater age of tropical environments • May account for the greater species richness • Climate is likely the primary cause of the latitudinal gradient in biodiversity • The two main climatic factors correlated with biodiversity • Are solar energy input and water availability 180 140 120 100 80 60 200 Vertebrate species richness (log scale) Tree species richness 160 100 50 40 20 0 900 500 700 1,100 300 100 Actual evapotranspiration (mm/yr) (a) Trees Figure 53.25a, b 10 1 (b) Vertebrates 1,500 2,000 1,000 500 Potential evapotranspiration (mm/yr) • The species-area curve quantifies the idea that all other factors being equal, the larger the geographic area of a community, the greater the number of species • A species-area curve of North American breeding birds • Supports this idea Number of species (log scale) 1,000 100 10 1 1 Figure 53.26 10 100 103 104 105 106 Area (acres) 107 108 109 1010 • Species richness on islands • Depends on island size, distance from the mainland, immigration, and extinction • The equilibrium model of island biogeography maintains that Equilibrium number Number of species on island (a) Immigration and extinction rates. The equilibrium number of species on an island represents a balance between the immigration of new species and the extinction of species already there. Figure 53.27a–c Rate of immigration or extinction Rate of immigration or extinction Rate of immigration or extinction • Species richness on an ecological island levels off at some dynamic equilibrium point Small island Large island Number of species on island (b) Effect of island size. Large islands may ultimately have a larger equilibrium number of species than small islands because immigration rates tend to be higher and extinction rates lower on large islands. Far island Near island Number of species on island (c) Effect of distance from mainland. Near islands tend to have larger equilibrium numbers of species than far islands because immigration rates to near islands are higher and extinction rates lower. • Studies of species richness on the Galápagos Islands • Support the prediction that species richness increases with island size FIELD STUDY Ecologists Robert MacArthur and E. O. Wilson studied the number of plant species on the Galápagos Islands, which vary greatly in size, in relation to the area of each island. RESULTS 400 Number of plant species (log scale) 200 100 50 25 10 5 0 0.1 1 10 100 1,000 Area of island (mi2) (log scale) CONCLUSION Figure 53.28 The results of the study showed that plant species richness increased with island size, supporting the species-area theory. Integrated and Individualistic Hypotheses • Two different views on community structure • Emerged among ecologists in the 1920s and 1930s • The integrated hypothesis of community structure • Describes a community as an assemblage of closely linked species, locked into association by mandatory biotic interactions • The individualistic hypothesis of community structure • Proposes that communities are loosely organized associations of independently distributed species with the same abiotic requirements • The integrated hypothesis Population densities of individual species • Predicts that the presence or absence of particular species depends on the presence or absence of other species Environmental gradient (such as temperature or moisture) Figure 53.29a (a) Integrated hypothesis. Communities are discrete groupings of particular species that are closely interdependent and nearly always occur together. • The individualistic hypothesis Population densities of individual species • Predicts that each species is distributed according to its tolerance ranges for abiotic factors Environmental gradient (such as temperature or moisture) Figure 53.29b (b) Individualistic hypothesis. Species are independently distributed along gradients and a community is simply the assemblage of species that occupy the same area because of similar abiotic needs. • In most actual cases the composition of communities Number of plants per hectare • Seems to change continuously, with each species more or less independently distributed 600 400 200 0 Wet Figure 53.29c Moisture gradient Dry (c) Trees in the Santa Catalina Mountains. The distribution of tree species at one elevation in the Santa Catalina Mountains of Arizona supports the individualistic hypothesis. Each tree species has an independent distribution along the gradient, apparently conforming to its tolerance for moisture, and the species that live together at any point along the gradient have similar physical requirements. Because the vegetation changes continuously along the gradient, it is impossible to delimit sharp boundaries for the communities. • The rivet model of communities • Suggests that all species in a community are linked together in a tight web of interactions • Also states that the loss of even a single species has strong repercussions for the community Rivet and Redundancy Models • The redundancy model of communities • Proposes that if a species is lost from a community, other species will fill the gap