Chapter 45 Community Ecology

45.1 Fighting Foreign Fire Ants Native to Brazil, imported fire ants (Solenopsis invicta) nest in the ground and have painful stings Global trade and shipping brought fire ants to the US and to other countries around the world Fire ants have a negative impact on native species of plants, insects, birds, and other animals

Red Imported Fire Ants Figure 45.1 Red imported fire ants. The photo at the right shows mounds made by coloniesof fire ants in a Texas pasture.

Communities Species interactions such as competition or predation are one focus of community ecology A community is all the species that live in a region Species interactions and disturbances can shift community structure (types of species and their relative abundances) in small and large ways

45.2 Which Factors Shape Community Structure? Community structure refers to the number and relative abundances of species in a habitat Habitat The type of place where a species normally lives Community All species living in a habitat

Species Diversity Communities vary in their species diversity Two components of species diversity: Species richness: the number of species Species evenness: the relative abundance of each species

Community Structure Many factors influence community structure Abiotic factors such as climate Gradients of topography Species interactions (direct and indirect) Symbiosis refers to direct, long-term interactions: Commensalism: One species benefits and the other is neither benefited nor harmed Mutualism: Both benefit Parasitism: Parasite benefits, host is harmed

Table 45-1 p810

Commensalism Figure 45.2 Commensalism. Epiphytic orchids grow on a tree trunk. The tree provides the orchids with an elevated perch from which they can capture sunlight, and it is neither helped nor harmed by their presence.

Take-Home Message: What factors affect species in a community? The types and abundances of species in a community are affected by physical factors such as climate and by species interactions. A species can be benefited, harmed, or unaffected by its interaction with another species.

45.3 Mutualism Mutualism is a species interaction in which each species benefits by associating with the other Flowering plants and animal pollinators Birds that disperse seeds Lichens, mycorrhizae, and nitrogen-fixing bacteria that help plants obtain nutrients Animals share nutrients with mutualistic microorganisms in their gut Two species may protect one another

Obligate Mutualism: Yucca and Moth Figure 45.3 An obligate mutualism. Each species of yucca plant (left) has a relationship with one species of yucca moth (right). After a female moth mates, she collects pollen from a yucca flower and places it on the stigma of another flower, then lays her eggs in that flower’s ovary. Moth larvae develop in the fruit that develops from the floral ovary. When mature, they gnaw their way out and disperse. Seeds that larvae did not eat give rise to new yucca plants.

Mutual Protection Figure 45.4 Mutual protection. The stinging tentacles of this sea anemone (Heteractis magnifica) protect its partner, a pink anemonefish (Amphiprion perideraion) from fish-eating predators. In return, the anemonefish chases away fish that eat sea anemone tentacles. The anemonefish secretes a special mucus that pre-vents the anemone from stinging it.

Take-Home Message: What are the effects of participating in a mutualism? A mutualism benefits both participants. In some cases, two species form an exclusive partnership. In others, a species provides benefits to, and receives benefits from, multiple species. Participating in a mutualism has both benefits and costs. Selection favors individuals who maximize their benefits while minimizing their costs.

45.4 Competitive Interactions Resources are limited; individuals of different species often compete for access to them Interspecific competition hurts both species Competition among individuals of the same species is more intense than interspecific competition

The Niche Each species requires specific resources and environmental conditions that we refer to as its ecological niche Both physical (abiotic) and biological (biotic) factors define the niche The more similar the niches of two species are, the more intensely the species will compete

Interspecific Competition Interference competition One species actively prevents another from accessing a resource Exploitative competition Species reduce the amount of a resource available to the other by using that resource

Interference Competition Figure 45.5 Interspecific competition among scavengers. After facing off over a carcass (top), an eagle attacked a fox with its talons (bottom). The fox then retreated, leaving the eagle to exploit the carcass

Effects of Competition Competitive exclusion When two species require the same limited resource to survive or reproduce, the better competitor will drive the less competitive species to extinction in that habitat Competitors can coexist when their resource needs are not exactly the same Competition suppresses growth of both species

Competitive Exclusion in Paramecium Time (days) population density Relative 0 4 8 12 16 20 24 P. caudatum alone Time (days) population density Relative 0 4 8 12 16 20 24 Both species together Time (days) population density Relative 0 4 8 12 16 20 24 P. aurelia alone Figure 45.6 Animated Competitive exclusion. Growth curves for two Paramecium species when grown separately and together. Stepped Art

ANIMATED FIGURE: Competitive exclusion To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Competing for Pollinators Figure 45.7 Competing for pollinators. Mimulus and Lobelia grow together in damp meadows. To test for competition, researchers grew Mimulus plants either alone or with Lobelia. In mixed plots, pollinator visits to Lobelia plants frequently intervened between visits to Mimulus. As a result, Mimulus in mixed plots produced 37 percent fewer seeds than those grown alone Mimulus Lobelia

Resource Partitioning Resource partitioning is an evolutionary process by which species become adapted to use a shared limiting resource in a way that minimizes competition (directional selection) Example: Eight species of woodpecker in Oregon feed on insects and nest in hollow trees, but the details of their foraging behavior and nesting preferences vary

Character Displacement Over generations, directional selection leads to character displacement – the range of variation for one or more traits is shifted in a direction that lessens the intensity of competition for a limiting resource Example: Where two species of salamanders coexist, differences in body length becomes more pronounced

Character Displacement in Salamanders Figure 45.8 Animated Possible evidence of character displacement in salamanders (Plethodon). Where P. cinereus (shown at right) and P. hoffmani coexist, their average body lengths (purple bars) differ more than they do in habitats where each species lives alone (orange bars)

ANIMATED FIGURE: Hairston's experiment To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Take-Home Message: What happens when species compete for resources? In some interactions, one species actively blocks another’s access to a resource. In other interactions, one species is simply better than another at exploiting a shared resource. When two species compete, selection favors individuals whose needs are least like those of the competing species.

45.5 Predator–Prey Interactions Predation is an interspecific interaction in which one species (predator) captures, kills, and eats another species (prey) Relative abundances of predators and prey shift over time in response to species interactions and changing environmental conditions

Predator Responses to Changes in Prey Density Type I response (passive predators) Number of prey killed depends on prey density Type II response Number of prey killed depends on the predator’s capacity to capture, eat and digest prey Type III response Number of kills increases only when prey density reaches a certain level

Responses of Predators to Prey Density Figure 45.9 Animated Functional responses of predators to changes in prey density.

A Type 2 Response

Figure 45.9 Animated Functional responses of predators to changes in prey density. B Example of a type II response from one winter month in Alaska, during which B. W. Dale and his coworkers observed wolf packs (Canis lupus) feeding on caribou (Rangifer tarandus). Figure 45-9b2 p814

ANIMATED FIGURE: Predator-prey interactions To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Cyclic Changes in Abundance Time lag in predator response to prey density can lead to cyclic changes in abundance When prey density is low, predators decline, prey are safer, prey numbers increase When prey density is high, predator numbers increase, prey numbers decline

Canadian Lynx and Snowshoe Hare Figure 45.10 Population cycles in an arctic predator and its main prey. A. Abundance of Canadian lynx (dashed line) and snowshoe hares (solid line), based on the numbers of pelts sold by trappers to Hudson’s Bay Company during a ninety-year period.

Predator and Prey Figure 45.10 Population cycles in an arctic predator and its main prey. B. Canadian lynx pursuing a snowshoe hare.

Take-Home Message: How do predator and prey populations change over time? Predator populations show three general patterns of response to changes in prey density. Population levels of prey may show recurring oscillations. The numbers in predator and prey populations vary in complex ways that reflect the multiple levels of interaction in a community.

45.6 An Evolutionary Arms Race Predators select for better prey defenses, and prey select for more efficient predators Prey defenses include exoskeletons, unpleasant taste, toxic chemicals or stings, and physical adaptations such as camouflage

Coevolution of Predators and Prey Predator and prey populations exert selective pressures on one another Genetic traits that help prey escape will increase in frequency Defensive improvements select for a countering improvement in predators Example: Spraying beetles and grasshopper mice

Defense and Counter Defense Figure 45.12 Defense and counter defense. (a) Eleodes beetles defend themselves by spraying irritating chemicals at predators. (b) This defense is ineffective against grasshopper mice, who plunge the chemical-spraying end of the beetle into the ground and devour the insect from the head down.

Some Physical Adaptations of Prey Warning coloration Many toxic or unpalatable species have bright colors and patterns that predators learn to avoid Mimicry A harmless animal looks like a dangerous one Camouflage Body shape, color pattern and behavior that make an individual blend in with its surroundings

Warning Coloration and Mimicry Figure 45.11 Examples of mimicry. Edible insect species often resemble toxic or unpalatable species that are not at all closely related. (a) A yellow jacket can deliver a painful sting. Non stinging wasps (b), beetles (c), and flies (d) benefit by having a similar appearance.

Camouflage in Prey and Predators Figure 45.13 Camouflage in prey and predators.

Coevolution of Herbivores and Plants With herbivory, an animal feeds on plants Two defenses have evolved in response to herbivory: Some plants withstand and recover quickly from the loss of their parts Some plants have physical deterrents (spines, thorns, tough leaves); or chemical deterrents (secondary metabolites that taste bad or sicken herbivores)

Take-Home Message: How do predation and herbivory influence community structure? In any community, predators and prey coevolve, as do plants and the herbivores that feed on them. Defensive adaptations in plants and prey can limit the ability of predators or herbivores to exploit some species in their community.

45.7 Parasites and Parasitoids With parasitism, one species (parasite) benefits by feeding on another (host), without immediately killing it Endoparasites such as parasitic roundworms live and feed inside their host An ectoparasite such as a tick feeds while attached to a host’s external surface

Endoparasites Figure 45.14 Parasites inside and out. A. Endoparasitic roundworms in the intestine of a host pig.

Ectoparasites Figure 45.14 Parasites inside and out. B. Ectoparasitic ticks attached to and sucking blood from a finch.

Parasite Diversity Parasitism has evolved in members of a diverse variety of groups Bacterial, fungal, protistan, and invertebrate parasites feed on vertebrates Lampreys attach to and feed on other fish Parasitic plants that withdraw nutrients from other plants

Dodder: A Parasitic Plant Figure 45.15 Dodder (Cuscuta), also known as strangleweed or devil’s hair. This parasitic flowering plant has almost no chloro-phyll. Leafless stems twine around a host plant during growth, as shown in the close-up at right. Modified roots penetrate the host’s vascular tissues and absorb water and nutrients from them.

Strangers in the Nest With brood parasitism, one egg-laying species benefits by having another raise its offspring Examples: European cuckoo, cowbird One cowbird can parasitize 30 nests per season, decreasing the reproductive rate of the host species

Cowbird with Foster Parent Figure 45.16 A cowbird with its foster parent. A female cowbird minimizes her cost of parental care by laying her eggs in the nests of other bird species.

Parasitoids Parasitoids are insects that lay eggs in other insects Their larvae develop in the host’s body, feed on its tissues, and eventually kill it As many as 15 percent of all insects may be parasitoids Example: parasitoid wasps

Biological Pest Controls Some parasites and parasitoids are raised commercially for use as biological pest control agents Example: Parasitoid wasps lay eggs in aphids Introducing a species into a community as a biological control has both advantages and risks

Biological Pest Control Agent Figure 45.17 Biological control agent: a commercially raised parasitoid wasp about to deposit a fertilized egg in an aphid. The wasp larva will devour the aphid from the inside.

Take-Home Message: Effects of parasites, brood parasites, and parasitoids Parasites reduce the reproductive rate of host individuals by withdrawing nutrients from them. Brood parasites reduce the reproductive rate of hosts by tricking them into caring for young that are not their own. Parasitoids reduce the number of host organisms by preventing reproduction and eventually killing the host.

45.8 Ecological Succession Ecological succession is a process in which one array of species replaces another over time It can occur in a barren habitat such as new volcanic land (primary succession) or a disturbed region in which a community previously existed (secondary succession)

Pioneer Species Primary succession begins when pioneer species such as lichens and mosses colonize a barren habitat with no soil Pioneer species are opportunistic colonizers of new or newly vacated habitats Pioneers help build and improve soil for later successional species

Primary Succession: Alaska’s Glacier Bay Figure 45.18 Animated One pathway of primary succession in Alaska’s Glacier Bay region.

ANIMATED FIGURE: Succession To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE