Community interactions

A. Community characteristics

1. Factors shaping community structure
- community = association of interacting populations of different species living together in a particular habitat
- habitat = place where an organism lives; characterized by distinctive physical features and vegetation.
- factors to consider:

- climate, topography dictate rainfall, temperature, soil composition, etc.
- availability of food and other resources (e.g., space)
- adaptive traits of individuals that allow exploitation of specific resources (see essay on p. 822) => niche concept
- interactions between inhabitants such as competition, predation, mutualism [Table 47.2, p. 842]
- physical disturbances, immigration, episodes of extinction affect habitat
- community properties: feeding levels, diversity (number of different spp.), relative abundance (number of individuals of each sp.)

2. Niche
- niche = full range of physical and biological conditions under which individuals of a specie live, reproduce
- each specie has its own niche defined, in part, by its relationships with other organisms; in general, the niche allows different species to minimize their interactions -- particularly competition

B. Types of species interactions

- interactions can occur between different spp. in a community, or between different communities
- Effects of interaction between spp [Table 47.2]

- Neutral = neither species directly affect the other
- Commensalism = one species benefits, the other is not affected (birds nest in trees)
- Mutualism = symbiotic relationship where both spp. benefit (yucca moth)
- Interspecific competition = both spp. harmed by interaction
- Predation, parasitism = one species benefits, the other is harmed

C. Examples of interactions

1. Mutualism
- yucca moth and yucca plant; each sp. of yucca plant is pollinated exclusively by one sp. of yucca moth
- fungal hyphae and young roots
- flaggelates in termite hindgut; paramecium and internal algae
2. Competitive interactions
- competitive exclusion: all individuals have equal access to a resource, but differ in ability to exploit resource --> example of paramecium expt. of Gause [Fig. 47.3]
- exclusion is not always necessary; coexistance possible if niches do not overlap --> physical ranges (elevation) differ, but overlap partly and food is same; physical range the same, but food resources overlap partly
- competition demonstrated if (and only if) increase in density of one species results in a decrease in density of presumed competitor
- when competitive species coexist, they do so at reduced densities
- resource partitioning: two competing species may coexit by sharing a resource in different ways or at different times [Fig. 47.5]
3. Predation
- predator = animal that feeds on other living organisms (prey), but doesn't reside in or on them; parasite resides in or on its prey
- prey may not always die
- some predator-prey populations coexist at more-or-less steady population levels; others undergo recurring cycles (abundance followed by crashes), erratic fluctuation, prey extinction
- dynamic determinants:

- carrying capacity of prey (max. # individuals that can be indefinitely maintained by the resources in a given environment)
- relative reproductive rates of predator and prey (predator usually has lower reproductive rate (= annual number of progeny per individual, on average))
- behavioral response of individual predators to changes in prey density (eat up everything or move to new territory?)

[When reproductive rate of predator is much slower than that of prey, consumption of prey by predator is limited, and carrying capacity is high, populations of both tend to fluctuate more-or-less regularly. Why?] [Fig. 47.7]
- prey-predator interactions exert selective pressure on both species --> evolution of new defense --> evolution of new predator behavior or appearance to counter prey defense
- adaptive responses include warning coloration, mimicry, camouflage, chemical defenses, etc.

D. Succession

1. Pioneer species --> Climax community
- succession = the predictable sequence of species that occupies a particular area
- pioneer species = opportunistic colonizer that first occupies vacant or vacated habitats; typically has high dispersal rate, rapid growth. First to colonize an area, followed by more competitive species
- over time, the new species are replaced by other species in an orderly progression until the array of species in the community stabilizes under prevailing conditions
- climax community = most persistant array of species that results after a considerable passage of time (100 yrs or more)
2. Primary and secondary succession
- primary succession occurs in an area originally devoid of life [Fig. 47.14]
- pioneer species help improve soil fertility; usually small, low-growing plants with short life cycle and abundance of seeds; many are mutualists w/ nitrogen-fixing bacteria; adapted to exposed areas, intense sunlight, large temperature swings, nutrient-difficient soil
- in secondary succession, a community reestablishes itself to a climax state after a disturbance that allows sunlight to penetrate (examples: abandoned farmland that was cleared forest; regrowth after forest fire in a national park)

E. Community Stability

- climate or other environmental variables may undergo long-term changes that destabilize a community; when this happens, a community may change [What does this mean?] in ways that persist even after the destabilizing condition has returned to "normal" --> member species that are rare or poor competitors may become exctinct
1. Cyclic non-directional change
- stability of a community may require episodes of instability that permit cyclic replacement of equilibrium species to maintain the climax community; example: periodic fires to clear underbrush in sequoia forests in Calif.
2. Predictability of stabile communities
- not always possible to predict configuation of a community
- predation may promote diversity by reducing prey population densities (example: +sea star --> 15 species of invertebrate prey -- mussels, limpets, chitons, barnacles; -sea star --> 8 species with mussels dominant)
- same predator may increase diversity in one situation, decrease diversity in another (example: periwinkle grazing on algae in tidepools maintain a diverse population of algae; on emergent rocks grazing reduces diversity due to avoidance of dominant algal species) [Fig. 47.15]

F. Patterns of Biodiversity

1. Mainland and marine patterns
- number of spp. increases from Artic regions to the temperate zone, to the tropics
- diversity in tropics favored:

- more rainfall and sunlight --> more food reserves
- species diversity tends to be self-reinforcing
- rate of speciation tends to exceed rate of extinction

2. Island patterns [Fig. 47.18]
- distance effect: islands distant from source areas receive fewer colonizing species
- area effect: larger islands tend to support more species
- species numbers increase on a new island and reach a stable number that is a balance between the immigration rate for new species to the island, and the extinction rate for established species