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Chapter 53 (Campbell)Chapter 47 (Starr/Taggart) Community Ecology
Overview: What Is a Community? • A biological community • Is an assemblage of populations of various species living close enough for potential interaction
Figure 53.1 • The various animals and plants surrounding this watering hole • Are all members of a savanna community in southern Africa
Concept 53.1: A community’s interactions include competition, predation, herbivory, symbiosis, and disease • Populations are linked by interspecific interactions • That affect the survival and reproduction of the species engaged in the interaction
Table 53.1 • Interspecific interactions • Can have differing effects on the populations involved
Competition • 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 • The competitive exclusion principle • States that two species competing for the same limiting resources cannot coexist in the same place
Ecological Niches • The ecological niche • Is the total of an organism’s use of the biotic and abiotic resources in its environment • If an organism’s “profession” and the habitat it its “address”. • An organism’s niche is its ecological role– how it “fits into” an ecosystem
The niche concept allows restatement of the competitive exclusion principle • Two species cannot coexist in a community if their niches are identical
EXPERIMENT RESULTS 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 Chthamalusrealized niche Balanus Chthamalusfundamental niche Balanusrealized niche Ocean Ocean Low tide Low tide In nature, Balanus fails to survive high on the rocks because it isunable 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. CONCLUSION The spread of Chthamalus when Balanus wasremoved indicates that competitive exclusion makes the realizedniche of Chthamalus much smaller than its fundamental niche. Figure 53.2 • However, ecologically similar species can coexist in a community • If there are one or more significant difference in their niches
As a result of competition • A species’ fundamental niche may be different from its realized niche • Fundamental Niche: is the niche potentially occupied by that species • Realized Niche: the niche it actually occipies in a particular environment
A. insolitususually percheson shady branches. A. ricordii A. insolitus A. distichus perches on fence posts and other sunny surfaces. A. alinigar A. christophei A. distichus A. cybotes A. etheridgei Figure 53.3 Resource Partitioning • Resource partitioning is the differentiation of niches • That enables similar species to coexist in a community Seven species of Anolis lizard live in close proximity, and feed on insects and other small arthropods. However, competition for food is reduced because each has a different perch, thus occupying a distinct niche
G. fortis G. fuliginosa Beak depth Santa María, San Cristóbal 40 Sympatric populations 20 0 Los Hermanos Percentages of individuals in each size class G. fuliginosa, allopatric 40 20 Daphne 0 40 G. fortis, allopatric 20 8 10 12 14 16 0 Beak depth (mm) Figure 53.4 Character Displacement • In character displacement • 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 An example is the variation in beak size between different population of the Galapagos finches.
Predation • Predation refers to an 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
Figure 53.5 • Cryptic coloration, or camouflage • Makes prey difficult to spot
Figure 53.6 • Aposematic coloration • Warns predators to stay away from prey
In some cases, one prey species • May gain significant protection by mimicking the appearance of another
(b) Green parrot snake (a) Hawkmoth larva Figure 53.7a, b • In Batesian mimicry • A palatable or harmless species mimics an unpalatable or harmful model Larva weaves head back and forth and hisses like a snake.
(a) Cuckoo bee (b) Yellow jacket Figure 53.8a, b • In Müllerian mimicry • Two or more unpalatable species resemble each other
Herbivory • Herbivory, 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
Parasitism • In parasitism, 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
Three types of parasitism • Endoparasites: parasites that live within the body of their host, such as tapeworms and malarial parasites • Ectoparasites: parasites that feed on the external surface of host, such as ticks and lice • Parasitoidism: Insects – usually small wasp-lay eggs on or in living host. The larvae then feed on the body of the host, eventually killing it.
Disease • The effects of disease on populations and communities • Is similar to that of parasites
Pathogens, disease-causing agents • Are typically bacteria, viruses, or protists
Figure 53.9 Mutualism • Mutualistic symbiosis, or mutualism • Is an interspecific interaction that benefits both species
Figure 53.10 Commensalism • In commensalism • One species benefits and the other is not affected
Commensal interactions have been difficult to document in nature • Because any close association between species likely affects both species
Interspecific Interactions and Adaptation • Evidence for coevolution • Which involves reciprocal genetic change by interacting populations, is scarce
However, generalized adaptation of organisms to other organisms in their environment • Is a fundamental feature of life
Concept 53.2: 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, particularly on the composition, relative abundance, and diversity of its species.
Species Diversity • 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
A B C D Community 1 A: 25% B: 25% C: 25% D: 25% Community 2 Figure 53.11 A: 80% B: 5% C: 5% D: 10% • Two different communities • Can have the same species richness, but a different relative abundance Ecologist would say that community 1 has greater species diversity, a measure that includes both species richness and relative abundance
A community with an even species abundance • Is more diverse than one in which one or two species are abundant and the remainder rare
Trophic Structure • Trophic structure • Is the feeding relationships between organisms in a community • The transfer of food energy up the trophic levels from its source in plants and other photosynthetic organism (primary producers) through herbivores (primary consumers) to carnivores (secondary and tertiary consumers) • Is a key factor in community dynamics
Quaternary consumers Carnivore Carnivore Tertiary consumers Carnivore Carnivore Secondary consumers Carnivore Carnivore Primary consumers Zooplankton Herbivore Primary producers Plant Phytoplankton Figure 53.12 A terrestrial food chain A marine food chain • Food chains • Link the trophic levels from producers to top carnivores
Humans Smaller toothed whales Baleen whales Sperm whales Elephant seals Leopard seals Crab-eater seals Squids Fishes Birds Carnivorous plankton Copepods Euphausids (krill) Phyto-plankton Figure 53.13 Food Webs • A food web • Is a branching food chain with complex trophic interactions
Juvenile striped bass Sea nettle Fish larvae Fish eggs Figure 53.14 Zooplankton • Food webs can be simplified • By isolating a portion of a community that interacts very little with the rest of the community
Limits on Food Chain Length • Each food chain in a food web • Is usually only a few links long (about 5 trophic levels) • 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 • Hypothesis predicts that food chains should be relatively longer in habitats of higher photosynthetic productivity • Generally known as the 10% Law: That states that only 10% of energy will be transferred from one trophic level to the next.
The dynamic stability hypothesis • Proposes that long food chains are less stable than short ones • Population fluctuations at lower trophic levels are magnified at higher levels potentially causing the local extinction of top predators. • In variable environment, top predators must be able to recover from environmental shocks (such as extreme winters) that can reduce the food supply all the way up the food chain.
6 6 No. of species 5 5 No. of trophic links 4 4 Number of species Number of trophic links 3 3 2 2 1 1 0 0 Low Medium High (control) Productivity Figure 53.15 • Most of the available data • Support the energetic hypothesis
Species with a Large Impact • 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
Dominant Species • Dominant species • Are those species in a community that are most abundant or have the highest biomass (the total mass of all individuals in a population) • Exert powerful control over the occurrence and distribution of other species • Example: Sugar maples in North American forest communities has major impact on abiotic factors such as shading and soil, which in turn affect which other species live there
There is no single explanation for why a species becomes dominant in a community. • 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
Keystone Species • Keystone species • Are not necessarily abundant in a community • Exert strong control on a community by their ecological roles, or niches
With Pisaster (control) 20 15 Number of species present 10 Without Pisaster (experimental) 5 0 1963 ´70 ´71 ´73 ´64 ´65 ´69 ´66 ´72 ´67 ´68 (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. (a) The sea star Pisasterochraceousfeeds preferentially on mussels but will consume other invertebrates. • Field studies of sea stars • Exhibit their role as a keystone species in intertidal communities A good way to identify the keystone species is to removal experiment (like the experiment here) Figure 53.16a,b
100 80 60 Otter number (% max. count) 40 20 0 (a) Sea otter abundance 400 300 Grams per 0.25 m2 200 100 0 (b) Sea urchin biomass 10 8 6 Number per 0.25 m2 4 2 0 1972 1985 1989 1993 1997 Year Food chain beforekiller whale involvement in chain (c) Total kelp density Food chain after killerwhales started preyingon otters Figure 53.17 • Observation of sea otter populations and their predation • Shows the effect the otters haveon ocean communities