Community Ecology Chapter 53 Overview: What Is a Community? A biological community is 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

Interspecific Interaction A community’s interactions 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 Interaction Table 53.1

Competition Interspecific competition occurs when species compete for a particular resource that is in short supply Strong competition can lead to competitive exclusion

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 The niche concept allows restatement of the competitive exclusion principle Two species cannot coexist in a community if their niches are identical

However, ecologically similar species can coexist in a community If there are one or more significant difference in their niches When Connell removed Balanus from the lower strata, the Chthamalus population spread into that area. The spread of Chthamalus when Balanus was removed indicates that competitive exclusion makes the realized niche of Chthamalus much smaller than its fundamental niche. RESULTS CONCLUSION Ocean Ecologist Joseph Connell studied two barnacle speciesBalanus balanoides and Chthamalus stellatus that have a stratified distribution on rocks along the coast of Scotland. EXPERIMENT 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. Low tide High tide Chthamalus fundamental niche Chthamalus realized niche Chthamalus Balanus realized niche Balanus Figure 53.2

Resource Partitioning Resource partitioning is the differentiation of niches That enables similar species to coexist in a community A. insolitus usually perches on shady branches. A. distichus perches on fence posts and other sunny surfaces. A. distichus A. ricordii A. insolitus A. christophei A. cybotes A. etheridgei A. alinigar Figure 53.3

Character Displacement In character displacement characteristics are more divergent G. fortis Beak depth (mm) G. fuliginosa Beak depth Los Hermanos Daphne Santa María, San Cristóbal Sympatric populations G. fuliginosa, allopatric G. fortis, allopatric Percentages of individuals in each size class 40 20 8 10 12 14 16 Figure 53.4

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

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 Mimicry In some cases, one prey species May gain significant protection by mimicking the appearance of another

In Batesian mimicry A palatable or harmless species mimics an unpalatable or harmful model (a) Hawkmoth larva (b) Green parrot snake Figure 53.7a, b

In Müllerian mimicry Two or more unpalatable species resemble each other (a) Cuckoo bee (b) Yellow jacket Figure 53.8a, b

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

In parasitism, one organism, the parasite Derives its nourishment from another organism, its host, which is harmed in the process

Pathogens, disease-causing agents 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 Bacteria Viruses Protists

Mutualistic symbiosis, or mutualism Is an interspecific interaction that benefits both species Figure 53.9

Commensalism In commensalism One species benefits and the other is not affected Figure 53.10

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

Species richness Relative abundance 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 Community 1 A: 25% B: 25% C: 25% D: 25% Community 2 A: 80% B: 5% C: 5% D: 10% D C B A Figure 53.11

Trophic Structure Trophic structure Is the feeding relationships between organisms in a community Is a key factor in community dynamics

A terrestrial food chain Trophic structure Is the feeding relationships between organisms in a community Food chains Link the trophic levels from producers to top carnivores Quaternary consumers Tertiary consumers Secondary consumers Primary consumers Primary producers Carnivore Herbivore Plant Zooplankton Phytoplankton A terrestrial food chain A marine food chain

Smaller toothed whales Food Webs A food web Humans Baleen whales Crab-eater seals Birds Fishes Squids Leopard seals Elephant Smaller toothed whales Sperm whales Carnivorous plankton Euphausids (krill) Copepods Phyto- plankton Figure 53.13 Is a branching food chain with complex trophic interactions

Limits on Food Chain Length Each food chain in a food web Is usually only a few links long 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 Support the energetic hypothesis High (control) Medium Low Productivity No. of species No. of trophic links Number of species Number of trophic links 1 2 3 4 5 6 Figure 53.15

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 Exert powerful control over the occurrence and distribution of other species

Keystone Species Keystone species Are not necessarily abundant in a community Exert strong control on a community by their ecological roles, or niches

Ecosystem “Engineers” (Foundation Species) 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

Bottom-Up and Top-Down Controls 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

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

Percentage of herbaceous plant cover Long-term experiment studies have shown That communities can shift periodically from bottom-up to top-down Figure 53.20 100 200 300 400 Rainfall (mm) 25 50 75 Percentage of herbaceous plant cover

What Is Disturbance? A disturbance 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 (a) Before a controlled burn. A prairie that has not burned for several years has a high propor- tion 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. Figure 53.21a–c

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. (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. Figure 53.22a, b

Ecological Succession Is the sequence of community and ecosystem changes after a disturbance

Two key factors correlated with a community’s species diversity Geographic location Size Climate Is likely the primary cause of the latitudinal gradient in biodiversity

Vertebrate species richness (log scale) The two main climatic factors correlated with biodiversity Are solar energy input and water availability (b) Vertebrates 500 1,000 1,500 2,000 Potential evapotranspiration (mm/yr) 10 50 100 200 Vertebrate species richness (log scale) 1 300 700 900 1,100 180 160 140 120 80 60 40 20 Tree species richness (a) Trees Actual evapotranspiration (mm/yr) Figure 53.25a, b