Chapter 54 Community Ecology

Reminder: Earth Day April 22 nd, 2010

Earth Day 2010 (40th anniversary of Earth day)

Iowa DNR 2009 self- assessment The DNR also rated water quality at C-. Numerically the water quality rated a D, but they gave themselves extra credit for effort!

Iowa’s water quality (DNR self-assessment)

A biological community –Is an assemblage of populations of various species living close enough for potential interaction The animals and plants surrounding this watering hole are all members of a savanna community in southern Africa

Community Interactions 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

Interspecific Interactions

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 So if we take the niche concept into account –Then the competitive exclusion principle can be reworded as: 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 difference in their niches Fundamental niche (the niche potentially occupied) versus realized niche (the niche actually occupied) 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 Low tide High tide Chthamalus Balanus realized niche Balanus Ocean

As a result of competition –A species’ fundamental niche may be different from its realized niche

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 Resource Partitioning Resource partitioning is the differentiation of niches that enables similar species to coexist in a community

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 Character Displacement In character displacement there is a tendency for characteristics to be more divergent in sympatric (geographically overlapping) populations of two species than in allopatric (geographically separate) populations of the same two species The allopatric populations have similar beaks and use similar resources. The sympatric populations would potentially compete for resources and their characteristics are more divergent

Predation

Predation refers to an interaction where one species, the predator, kills and eats the other, the prey Adaptations –Feeding adaptations of predators include claws, teeth, fangs, stingers, and poison –animals also display a great variety of defensive adaptations to conteract predation

Predation (Hawk-snake)

Cryptic coloration, or camouflage makes prey difficult to spot

Aposematic coloration: warns predators to stay away from prey

Warning coloration in poisonous frogs (movie)

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

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

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

Fooling predators (movie)

Herbivory Parasitism Disease Mutualism Commensalism

Herbivory –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 can have a significant effect on survival, reproduction, density, and productivity of a population Effects can be direct or indirect Tomato Hornworm covered in cocoons of pupating parasitic wasp

Disease Pathogens (disease-causing agents) have effects on populations and communities that are similar to that of parasites Deer with Chronic Wasting Disease

Mutualism Mutualistic symbiosis, or mutualism is an interspecific interaction that benefits both species

Mutualism

Commensalism In commensalism: One species benefits and the other is not affected Difficult to positively document in nature, because in any association between organisms both are probably affected Cattle egrets and water buffalo

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 appears to be a fundamental feature of life

Species in the Structure of a Community In general, a small number of species in a community exert strong control on that community’s structure. Dominant and Keystone species (more on these soon)

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 and relative abundance 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 Community 1 A: 25%B: 25%C: 25%D: 25% Community 2 A: 80%B: 5%C: 5%D: 10% D C B A

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 –Is a key factor in community dynamics

Food chains link the trophic levels from producers to top carnivores Quaternary consumers Tertiary consumers Secondary consumers Primary consumers Primary producers Carnivore Herbivore Plant Carnivore Zooplankton Phytoplankton A terrestrial food chainA marine food chain

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

Limits on Food Chain Length 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

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

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 (human introduced invasive species as an example)

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

Field studies of sea stars e xhibit their role as a keystone species in intertidal communities (a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates. With Pisaster (control) Without Pisaster (experimental) Number of species present ´64´65 ´66 ´67 ´68 ´69 ´70´71 ´72 ´73 (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 shows the effect the otters have on ocean communities Food chain before killer whale involve- ment in chain (a) Sea otter abundance (b) Sea urchin biomass (c) Total kelp density Number per 0.25 m Grams per 0.25 m 2 Otter number (% max. count) Year Food chain after killer whales started preying on otters

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

Some foundation species act as facilitators –That have positive effects on the survival and reproduction of some of the other species in the community Salt marsh with Juncus (black rush) (foreground) With Juncus Without Juncus Number of plant species Conditions Juncus slows salt buildup in soils and thereby increases species richness

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

Disturbances

Disturbance Disturbance influences species diversity and composition A disturbance –Is an event that changes a community –Removes organisms from a community –Alters resource availability Decades ago, most ecologists favored the traditional view that communities are in a state of equilibrium The current view is a nonequilibrium model –Which describes communities as constantly changing after being buffeted by disturbances

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.

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.

Human Disturbance Humans are the most widespread agents of disturbance…no surprise there! Human disturbance to communities –Usually reduces species diversity Humans also prevent some naturally occurring disturbances which can be important to community structure (fire suppression)

Ecological Succession Ecological succession –Is the sequence of community and ecosystem changes after a disturbance Primary succession –Occurs where no soil exists when succession begins ( as in a glacier retreat) Secondary succession –Begins in an area where soil remains after a disturbance ( as in a post-fire forest)

Possible effects of early-arrivers 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

McBride glacier retreating 0510 Miles Glacier Bay Pleasant Is. Johns Hopkins Gl. Reid Gl. Grand Pacific Gl. Canada Alaska Casement Gl. McBride Gl. Plateau Gl. Muir Gl. Riggs Gl. Retreating glaciers –Provide an opportunity to study succession Glacier Bay, Alaska Retreating glaciers leave behind rocky moraines

Succession on the moraines in Glacier Bay, Alaska f ollows a predictable pattern of change in vegetation and soil characteristics

The soil conditions change during succession as well

Species Diversity Biogeographic factors affect community diversity Two key factors correlated with a community’s species diversity –Are its geographic location and its size

Equatorial-Polar Gradients The two key factors in equatorial-polar gradients of species richness –evolutionary history –climate Species richness generally declines along an equatorial- polar gradient (And is especially great in the tropics) The greater age of tropical environments may account for the greater species richness. Polar and temperate areas have had to repeatedly “start- over” after big disturbances like glaciers. Climate is likely the primary cause of the latitudinal gradient in biodiversity due to differences in solar energy availability and water availability.

Area Effects 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 Area (acres) Number of species (log scale) ,000

Island Equilibrium Model Species richness on islands –Depends on island size, distance from the mainland, immigration, and extinction

Studies of species richness on the Galápagos Islands –Support the prediction that species richness increases with island size The results of the study showed that plant species richness increased with island size, supporting the species-area theory. FIELD STUDY RESULTS 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. CONCLUSION Area of island (mi 2 ) (log scale) Number of plant species (log scale) ,

Community Structure Two different views on community structure 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

Rivet and Redundancy Models 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 The redundancy model of communities –Proposes that if a species is lost from a community, other species will fill the gap Community hypotheses and models represent extremes, and that most communities probably lie somewhere in the middle