Chapter 53: Community Ecology

Community - the assemblage of species living close enough together for potential interaction

most significant factors in community structure:

species composition (both absolute and relative abundance)

two theories of how this is determined (Fig. 53.1):

1. individualistic theory (Gleason)--community as a chance assemblage of species as they have similar abiotic requirements – temperature, ranfall, soil type

b. interactive theory (Clements)--species are closely linked, mandatory biotic interactions, more of a “superorganism”

Fig. 53.1 expected distribution patterns for each hypothesis

composition of most plant communities closer to individualistic (most environmental gradients change gradually, and plant species gradients do, too) areas of abrupt change may lead to abrupt species changes, but still independently of each other

Animals often more linked than plants, depending on how narrowly- or widely-adapted they are eg. food species for the grey squirrel (wide) vs. limpkin (narrow)

The rivet model (Ehrlich and Ehrlich) is an extension of the Clements’ interactive model, applied to animals – it suggests that most species in a community are closely associated.

The redundancy model (Walker) suggests just the opposite; supports Gleason’s individualistic model

I. Interspecific Interactions - Table 53.1

between populations of different species within the same community

four principle categories of interactions:

1. interspecific competition

2. predator-prey

3. symbiotic interactions

4. coevolution

1. Interspecific Competition

density-dependent effect, negative effect on population growth of competing species

· interference competition--actual fighting over resources

· exploitative interference--utilizing similar resources

The Competitive Exclusion Principle states that:

two species with similar needs for the same limiting resource cannot coexist in the same place ie. if their niches are identical

Ecological niche:

· the fit (role) of an individual population in its ecosystem

· the sum total of that species’ use of the biotic and abiotic resources (not limited simply to habitat)

· may be a) fundamental niche--resources a population is capable of using under ideal circumstances, or

· b) realized niche--the resources actually used under the constraints of competition, predation, limited availability

eg. Fig. 53.2 two spp. of barnacle – experiments by Joseph Connell

· Balanus with fundamental, realized niches virtually the same; limited only from upper regions of intertidal zone by dessication at low tide

· Chthamalus with a wider fundamental niche, is outcompeted (competitively excluded) by Balanus at lower strata

Resource partitioning allows coexistence of competitors differentiation of niches that enables similar species to coexist in a community

· eg. lizards (7 spp. of Anolis) adapted to different microhabitats within the same community; suggests the work of natural selection to segregate them (inhabit dif. levels of the forest canopy with related morphological changes) - Fig. 53.3

· eg. Darwin finches exhibit character displacement in beak size ( and seeds eaten) in sympatric locations (where coexist in same geographical area)

· allopatric populations (geographically isolated) are more similar

· see: The Beak of the Finch by Jonathan Weiner, Vintage Books/Random House, 1994

2. Predator-Prey Interactions

comprises interspecific :

· animal on animal; animal on plants; even plant on animal! (sundew, Venus fly trap)

· generally are density-dependent, help to regulate populations (animal on plant predation often density-independent, with resources virtually unlimited)

· may enhance diversity of community, weeding out weaker indivs, preventing overshooting of food sources

· leads to defensive responses in prey

a. plant defenses against predation

· mechanical -- thorns, crystals in tissues, hooks and spines on leaves

· chemical--mostly secondary compounds (metabolic byproducts of major pathways) eg. strychnine from Strychnos; morphine from poppy; nicotine from tobacco (Nicotiana); mescaline from peyote; plant extracts such as cinnamon, cloves, peppermint; tannins from oaks; even simulated insect hormones that alter growth of predators

· selective pressure to overcome defenses strong, yields counteradaptations, ongoing changes

b. animal defenses against predation

· active--escape, defense

· passive--hiding, camouflage (“cryptic coloration” eg. English peppered moth, canyon tree frog, anole lizards)--Fig. 53.5

· mechanical or chemical defenses (“passive-aggressive”)

eg. porcupine, skunk, bombardier beetle

eg. accumulating toxins from plants eaten eg. Monarch butterfly larvae accumulate cardiac toxins from milkweed, retained into adulthood--birds “learn” to avoid taste

Aposematic (warning) coloration--a warning to predators eg. poison dart frog, coral snake (Fig. 53.6)

teach predators to avoid certain bright color patterns leads to mimicry of the model by other species:

1. Batesian mimicry--defenseless prey mimic unpalatable model (Fig. 53.7)

2. Mullerian mimicry--2 aposematically colored spp. resemble each other, reinforce each other’s message for double protection – Fig. 53.8

3. Symbiotic Interactions “living together”

a) parasitism--ecto- and endo-

b) commensalism--one partner benefits, the other neutral

c) mutualism--may evolve from predatr-prey and host-parasite relationships

a. Parasitism

· endoparasites often passive, rely on large number of eggs to “find” host

· ectoparasites evolve elaborate sensing and pursuit mechanisms eg. leeches use body temperature, movement, chemical cues on host’s skin

eluding parasites--defense mechanisms

immune system in vertebrates

secondary metabolites in plants to resist bacterial, fungal, insect attack

evolution between parasite and host to avoid killing host

eg. rabbits and myxoma virus in Australia has progressively selected for more resistant rabbits, less virulent virus to yield stable levels

b. Mutualism--both adapted toward mutually beneficial relationship (Fig. 53.9)

· mycorrhizal fungi and plant roots

· nitrogen-fixing bacteria and plant roots

· algae and fungi in lichens

· algae living within corals, jellyfish (Phylum Cnidaria)

· flowering plants and their animal pollinators

· ants and acacia trees--ants live in hollow thorns, feed on nectaries; protect the tree from other insects, fungal spores, overgrowing vegetation

c. Commensalism

few true examples of one-sided benefit

· cattle egrets and cattle

· ramora and shark

4. Coevolution

“interaction over ecological time leads to adaptations (heritable) over evolutionary time”

must be able to prove adaptation-counteradaptation in evolution of both species

must be reciprocal to be true coevolution:

change in 1 causes a selective force; adaptation in other species; counteradaptation in 1^st

eg. peppered moth and lichen--escape from predation

eg. passion flower (Passiflora) and Heliconius butterfly

· Passiflora leaves make toxins to retard insect feeding--selective pressure

· Heliconius larvae can feed due to special digestive enzymes--adaptation

· Passiflora evolved yellow spots to imitate eggs--counteradaptation

· yellow spots actually nectaries that attract ants and wasps that prey on eggs and larvae

II. Community Structure

Community characteristics influenced by species composition, interactions between species

Emergent properties unique at this level of organization (not seen at lower levels of organization):

1. Vegetation structure (vertical profile)

· Determines animals found there

· more complex the vegetation, more microhabitats for animals

2. Trophic structure—feeding relationships

· Elements of competition (interference, exploitative)

· Flow of energy, cycling of nutrients from one trophic level to the others

3. Species diversity = species richness (# different spp.)

relative abundance

Determinants of Community Characteristics

a. Role of Competition is a major factor in limiting diversity of spp.

· Herbivores eg. insects less likely to be limited by competition ie.. Food supply—are opportunistic, more density-independent

· Competition more likely a factor for carnivores, plants (only important if a) resources are limited and b) population size is near the carrying capacity)

· Dominant species are those with the highest biomass

b. Role of Predation

· Predator-prey populations usually self-regulate in the wild

· Prey develop defensive mechanisms

· Predators often switch to other food as one prey dwindles

Predators may actually enhance prey population

· eliminate the weak

· prevent overshooting of food resources

· moderate competition among prey species (prevent competitive exclusion)

A keystone predator is one which plays an important regulatory effect in the community.

Experiments by Robert Paine show that the sea star Pisaster is a keystone predator to its prey, the mussel Mytilus, and several other prey spp. (removing Pisaster allows Mytilus to become dominant, corresponding loss is species diversity from 15 to 8 prey species (Fig. 53.14)

DISTURBANCE AND COMMUNITY STRUCTURE

Ecological Succession - transition of species composition over ecological time

Transitional stages are sometimes called seres

Pattern of succession based on abiotic conditions (Table 53.2):

· Latitude

· Altitude

· Moisture

· Soil composition

a. primary succession—in new (lifeless) areas without soil eg. new volcanic rock, glacial till, new mountain ranges

in Alaska, would expect to see series from lichens to mosses to larger vascular plants (willow, alder) to climax forest (spruce, hemlock, aspen)

OR, see Enchanted Rock near Fredericksburg for primary succession closer to home

b. secondary succession occurs where an existing community has been cleared by some disturbance that leaves the soil intact eg old field succession

in this area of Texas, would see annual weeds and pine seedling to pine and hardwood saplings to mixed pine/hardwood forest (sweetgum, magnolia, tupelo, oak, holly)

Early stages of succession marked by predominance of opportunistic species: good colonizers, usually same as r-selected ie.. High fecundity and dispersal; short life history, little or no parental care

these fugitives often not good competitors, depend on disturbed areas to be well-adapted

Biogeography : (Introduced in Chapter 50) The past and present distribution of individual species (within and between continents)--Deals with distant evolutionary history, continental drift

Limits of species range - understanding the determinants of a species’ range

· Over evolutionary time, species may have retracted from a broader to a more restricted area eg. fossil evidence of camel, elephant relatives in North America

Reasons for high species diversity in tropics

· Communities are very old, allowing evolution of complex interactions

· Intermediate disturbances, glaciations, there yield more complexity

· Narrower ranges due to stability and predictability of environment yields small niches, reduced competition, more resource partitioning, all yielding more species

· Increased solar radiation yields higher PSN rates, a greater resource base

· Greater structural complexity of forest canopy, more microhabitats

· Diversity is self-propagating due to complex interactions, prevention of dominance

· Species in terrestrial environments tend to increase in species diversity from Arctic to tropics

· At given latitude, diversity tends to decrease with increasing altitude or with increasing depth in marine environments

Island Biogeography—how the species composition of an island is determined ( Fig. 53.26) any isolated habitat cut off from others of its kind eg mountain tops, lakes, oceanic islands)

Species diversity influenced by:

· Rate of immigration

· Rate of extinction

· Both affected by:

· Size of the island (more immigrants will “hit” a bigger target)—directly proportional (Fig. 53.27)

· Distance from the mainland—inversely proportional

· Number of species already present.

Interesting reading:

Interview with Dr. David Schindler – pages 1090-1091

The Song of the Dodo by David Quammen, Touchstone Books, 1997

Silent Spring by Rachel Carson

======================== OLD NOTES =========================

Effects on succession:

1. autogenic changes—those induced by the organisms themselves; either

· inhibition of own or other species, or

· facilitation—change abiotic factors such as soil, shade, pH, enhancing conditions for other species

· eg. pine inhibits own seedlings, facilitates germination and growth of hardwood seedlings

c. allogenic changes—outside changes which may disturb succession—logging, farming

eg. prairie fires interrupt succession toward climax forest, maintain grassland sere

traditional concept of succession : equilibrium model

· expect that equilibrium species (K-selected, better competitors, longer-lived) eventually replace colonizers (opportunists)

· community attains equilibrium with some addition or new species, loss or extinction of others at a slower rate

· species diversity increases

· community interactions become more extensive and complex

vs. flux model (non-equilibrial )

· species composition constantly changing

· localized disturbances keep some patches from ever achieving “climax”

· instead, end up with a patchwork, or polyclimax

· local environmental patchiness also contributes to the polyclimax effect

pattern of disturbance important

· frequent disturbances yield mostly good colonizers predominating, characteristics of early sere of succession (Fig. 53.17)

· mild, rare disturbance yields more late-stage species (Fig. 56.18)

· intermediate disturbances may yield the most species diversity (Fig. 53.19)

Eg. Krakatoa explosion in 1883—all life eliminated

Supports theories of size, distance

Equilibrium of bird species at 30 spp. after 35 years