Ecology: Community Interactions

Why Are Community Interactions Important? The interactions among populations within a community serve to maintain a balance between available resources and the number of … individuals using them As the interactions among the populations serve to limit population size, they also lead to … changes in characteristics and behaviors, increasing the fitness of the total population. This is … evolution

When changes in one species results in adaptive changes in an interacting species ... coevolution has occurred 

What Are the Effects of Competition Among Species? Competition among species, or interspecific competition, has such a strong effect on the species involved that each evolves ways to reduce any overlap in needs In other words, each species specializes within the community, developing its own well-defined, ecological niche 

Adaptations Reduce the Overlap of Ecological Niches Among Coexisting Species The Competitive Exclusion Principle - no two species can inhabit exactly the same ecological niche simultaneously and continuously - just as no two organisms can occupy exactly the same physical space When two or more species with similar requirements coexist, they typically occupy a smaller niche than either would if by themselves. This is … Resource Partitioning Figure 40-2 

Figure: 40-2 Title: Resource partitioning Caption: Each of these five insect-eating species of North American warblers searches for food in different regions of spruce trees. They reduce competition by occupying very similar, but not identical, niches.

If one of the competing species is removed from the community, the other species may expand its niche since the competition pressure has been reduced 

What Are the Results of Interactions Between Predators and Their Prey? Predation interactions have intense effects on the species involved Predators have evolved ways to best capture their prey, while the prey have evolved mechanisms to elude their predators This coevolution has resulted in some very complex physical characteristics and behaviors Know the examples from your text!

Bats and their moth prey have developed complex "cat and mouse" behaviors, while other species camouflage themselves to avoid predators or detection by prey (Figure 40-4) In contrast to camouflaged species, others stand out with bright or warning coloration (Figure 40-7) 

Figure 40-4 Figure: 40-4 Title: Camouflage by blending in Caption: (a) The sand dab is a flat, bottom-dwelling ocean fish with a mottled color that closely resembles the sand on which it rests. (b) This nightjar bird on its nest in Central America is barely visible among the surrounding leaf litter.

Figure 40-7 Figure: 40-7 Title: Warning coloration Caption: The South American poison arrow frog, with its poisonous skin, advertises its unpleasant taste with bright and contrasting color patterns. Figure 40-7

These species advertise their presence ... Their warning coloration are bright colors that warn potential predators that they are poisonous or otherwise distasteful and are to be avoided Species with common characteristics may share warning patterns as well; for example, stinging insects tend to be bright yellow with black stripes, and poisonous frogs from the tropics display very colorful skin pigments 

Some harmless species have evolved to mimic their poisonous relatives Mountain King Snakes (non-venomous), for example, have colorful stripes similar to Coral Snakes (venomous) (Figure 40-8)

the poisonous coral snake Figure: 40-8 part a Title: Warning mimicry part a Caption: The warning coloration of (a) the poisonous coral snake is mimicked by (b) the harmless mountain king snake. the poisonous coral snake

is mimicked by the harmless mountain king snake Figure: 40-8 part b Title: Warning mimicry part b Caption: The warning coloration of (a) the poisonous coral snake is mimicked by (b) the harmless mountain king snake. is mimicked by the harmless mountain king snake

“Devious” predators exists as well: Aggressive mimicry has evolved among species that resemble harmless species Predators, however, may be caught off-guard Some prey make use of color patterns that mimic a larger organism 

These species use their startle coloration to scare away predators (Figure 40-10) Some prey species have the ultimate defense: "chemical warfare" Coevolution, however, has also lead to a few predator species that are not harmed by the chemical produced and may even use it as its own defense mechanism 

Figure: 40-10 part c Title: Startle coloration part c Caption: (c) Predators of this caterpillar larva of the swallowtail butterfly are deterred by its resemblance to a snake. The caterpillar's head is the "snake's" nose.

What Is Symbiosis? Within a community, interacting with other species is unavoidable; however, some species have such close interactions that they have developed symbiotic relationships When one species of the relationship benefits and the other is unaffected, the relationship is ... commensalistic If one species benefits and the other is harmed, the relationship is ... parasitic If both species benefit, the relationship is mutualistic Let’s Summarize the possible interactions among organisms (Table 40-1)

Table: 40-T1 Title: Interactions Among Organisms

How Do Keystone Species Influence Community Structure? The influence of species on community structure is not necessarily equal When one species has a role that is out of proportion to its population size, that species is a keystone in the community 

Example 1: Pisaster as a keystone species The sea star Pisaster helps maintain diversity in the middle intertidal zone Without Pisaster, mussels overgrow the area

Often, a keystone species cannot be identified until it has actually been … removed from the community At this point it may be too late to reduce the impact its absence will have on the community 

Example: Sea otters as a keystone species Sea otters help maintain kelp forest communities Their decline allows overgrowth of sea urchins and loss of the kelp forest Sea otters have declined due to orca predation Why are orcas now preying on sea otters?

Succession: How Do Community Interactions Cause Change over Time? The interactions among members of a community lead to structural changes within that community; changes that are identified as stages in succession of the community Primary succession begins with pioneer species such as lichen and mosses establishing a hold on bare rock Figure 40-15

As soil slowly forms, additional species move into the young community in a recognizable pattern Secondary succession occurs after an established community has been disturbed perhaps by fire, wind storm, or farming If left undisturbed, succession will continue to a stable endpoint, the climax, determined in a large part by ... the geography and climate of the area Figure 40-16 

If a community is regularly disturbed, it will be maintained at a succession point below the climax, a subclimax Climax communities covering broad geographical regions are biomes Biomes are distinguished by specific climatic conditions and characterized by specific plant communities 

Summary: Major Points This chapter has covered: 1. How organisms interact with members of their own species, as well as with members of different species 2. That these interactions include mechanisms that have evolved to deal with competition for limited resources, predator/prey relationships, and symbiotic relationships 3. That the species that are present in a community depends on the ecosystem that is present 4. How an ecosystem changes over time (succession) 