Community Ecology (Ch. 55) Community = group of species inhabiting a given area. Questions in community ecology: How do species affect each other? How do interactions between species affect species’ distributions (what lives where)? How is the overall community structured (diversity)? How do communities change over time?

Interactions between species Can be categorized by whether species are harmed or helped. This sounds like the behavioral categories of selfish, altruistic, etc. but it’s different because it applies to individuals of 2 different species. Competition: both species harmed (- / -) Mutualism: both species benefit (+ / +) Predation and Parasitism: one species benefits while the other is harmed (+ / -)

Competition Caution: Ecological definition is not the same as its general usage. (Metaphorical use, like “strategy.”) Def: An interaction in which organisms from two species attempt to use the same, limited resource, which harms both species. Formally this is “interspecific competition” meaning “between-species.” Intraspecific competition (within species) has already been discussed, in the form of carrying capacity.

Potentially limiting resources Food Water Light Space (inc. nest sites, hiding places, etc.) Oxygen (in aquatic systems)

Competition can structure communities Competitively inferior species may be restricted to growing in places where the better competitors cannot live. Classic example: barnacles on rocky coastlines. Small species is restricted to growing only in the driest habitats near the top of the rocks. They could live farther down, but are prevented from doing so by competition from the larger species.

Fig. 55.1

The Ecological Niche The “niche” of a species is the complete spectrum of resources that it can use. Fundamental niche = the resources a species could potentially use, in the absence of any competition. Realized niche = the resources actually used. This may be less than the fundamental niche, if a competing species is taking away some resources.

Niche is not the same as Habitat Habitat = the place an organism lives. Niche is a broader, more abstract concept. It includes not only the place the organism lives, but the food it eats, and all the other resources that it needs. When the niches of two species overlap, those two species compete.

Predation and Parasitism One organism feeds on another. Good for the diner; bad for the dinner. A predator kills its food organism (“prey”) before consuming it. A parasite feeds on a living organism (“host”) over an extended period of time.

Different consequences of predation and parasitism Since predators kill before they eat, there is an advantage to being an efficient killer. Parasites live for a long time in or on their host (maybe even for many parasite generations). Therefore, it is not to their advantage to harm the host. The most successful parasites are those that harm the host the least.

Predation affects the populations of both prey and predator. When prey are common, predators get lots of food and predator populations grow. But when predator populations get large, many prey are eaten, and the prey population goes down. As prey populations get low, there is little food and predator populations fall. This allows prey population to grow, and the cycle starts over again.

Coupled population cycles in Canadian Lynx and Snowshoe Hare Fig 55.5

Predation has evolutionary consequences Many potential prey are cryptic (camouflaged). We have already discussed defenses of plants against herbivores. Animal prey also have defenses Spines, claws, stings, poisonous chemicals, etc. Often, poisonous animals advertise the fact with warning coloration. (The opposite of cryptic.)

Mimicry of Warning Coloration Often, different species share the same warning coloration. Mullerian mimics – similar colors among different species, when both are defended. Batesian mimics – similar colors, when one is harmful (the model) and one is harmless (the mimic).

Mimicry among butterfly species Fig. 55.10

Mutualisms benefit both partners see examples on pp 983-985 Some already discussed: Nitrogen-fixing bacteria in plant roots Cellulose-digesting bacteria in cow stomach Fungus and alga forming a lichen Also – Plants and their animal pollinators Plants and animals that disperse their seeds Defensive mutualisms: one species feeds off the other, but also defends it against other enemies.

Symbiosis “Symbiotic” is often used as a synonym for “mutualistic” but that is really incorrect. Symbiosis means living together in a close, individual to individual relationship. Some mutualisms are symbiotic (bacteria in cow’s stomach) but many are not (bee and flower). Not all symbiosis is mutualistic: - Parasitism is a type of symbiosis!

Predators can affect community characteristics Bison increase the diversity of prairies, because they mostly eat grass, which favors the growth of many wildflowers (forbs). Starfish make tide-pools more diverse, because they mostly eat mussels, which otherwise would take over the habitat. In both examples, the predator increases diversity by preventing one species from out-competing everything else.

Succession – Community Change over Time Over time, communities change. New species arrive; some species disappear. Diversity usually rises over time. This is NOT the same as evolution. We are not talking about change in the species themselves, only about arrivals & departures.

Primary vs. Secondary Succession Primary succession starts with bare ground where nothing lived before – Beaches; Volcanic islands (Hawaii, Galapagos); land exposed by retreating glaciers, etc. Secondary succession follows a disturbance that disrupt a pre-existing community – Fire, Floods, Human activity (esp. farming) Secondary succession is much more common.

Old Field Secondary Succession Most of the forests of the southeastern coastal plain and piedmont are secondary; they were once farmed and have now been abandoned. Year 0 – Crabgrass Year 1 – Horseweed Year 2 – Aster and Goldenrod Years 3-5 – Broomsedge (tall brownish grass) Years 5-10 – Blackberries and young pine Years 10-50 – Pine forest Years 50-100 – Transition to oak-hickory forest