Interactions n Populations do not exist in isolation. n All populations are tightly linked to other populations that share the same habitat. n Categories of interaction: parasitism +/- predation +/- herbivory +/- competition -/- mutualism +/+

Parasitism n Parasites are extremely diverse. (Fig. 49.1) n All parasites acquire resources from their host. This is always detrimental to the host. (Fig. 49.2) n A “coevolutionary arms race” exists between parasites and their hosts. Parasites develop better ways to attack and use the host, while hosts develop better defenses. Example: Plasmodium and the human immune system. (Fig. 49.3a,b) Parasites can manipulate the behavior of their host. (Fig. 49.4)

159 nm Figure 49.1 left

1437 nm Figure 49.1 middle

833 µm Figure 49.1 right

Human host 1. Sporozoites are injected from salivary gland of mosquito into human. 2. Sporozoites reproduce asexually to form merozoites in human liver. 3. Merozoites are released into blood- stream where they infect red blood cells. 5. Merozoites surviving human immune system become male or female gametocytes. 6. When a mosquito bites the human host, gametocytes enter the mosquito as part of a blood meal. 8. After meiosis, resulting cells develop into sporozoites and migrate to salivary glands. Mosquito host 7. Male and female gametocytes fuse in mosquito’s gut. Gametocytes Sporozoites in salivary glands Gut 4. Merozoites reproduce asexually until they cause red blood cells to rupture (causing anemia in human host). Figure 49.2

Cytotoxic T cell Infected liver cell HUMAN IMMUNE DEFENSE AGAINST PLASMODIUM Plasmodium sporozoites HLA-B53 Healthy liver cell HLA-B53 Infected liver cell cp26 protein from Plasmodium 1. Healthy liver cell has an HLA-B53 protein on its membrane. 2. HLA-B53 protein displays cp26 protein from Plasmodium, indicating that the liver cell is infected. 3. Cytotoxic T cell recognizes HLA-B53 and cp26 complex. 4. Cytotoxic T cell kills liver cell before merozoites are produced. Dead liver cell Cytotoxic T cell Figure 49.3a

In The Gambia, West Africa, different strains of Plasmodium have different versions of the cp protein. How successful are these different strains at infecting people? Infection rate Plasmodium strain Interpretation cp26Low HLA-B53 binds to these proteins. Immune response is effective. cp29Low cp26 and cp29 strains together High Immune response fails when these strains infect the same person. HLA-B53 does not bind to these proteins. Immune response is not as effective. cp27 cp28 High Average Figure 49.3b

Birds that prey on snails are the next host for the parasite Infected snails move to open sunny areas; tentacles wiggle. Uninfected snails stay in shaded areas; tentacles do not wiggle. Figure 49.4

Predation n When predation occurs, a predator kills and consumes a prey individual. n Predators can regulate prey populations and/or reduce them to below carrying capacity. (Fig. 49.5a,b) n Prey have a wide array of mechanisms that they use to defend themselves from predation. (Fig. 49.7) n Keystone predators are those that have an exceptionally great impact on all the other surrounding species. (Fig. 49.9, 49.10)

Regulated prey population Time Population fluctuates within a narrow range Figure 49.5a Carrying capacity (the max # that can be supported by available resources) Prey population size

Time Predator behavior Prey population size Figure 49.5b

Predation rate (number of moose killed/moose density) 5 0 HighMediumLow Moose density Figure 49.6

Camouflage Figure 49.7 upper

Mimicry Figure 49.7 center

Weapons Figure 49.7 lower

Figure 49.8a Prey and predator

Figure 49.8b Correlation between predation rate and prey defense Attachment strength (N) Shell mass (g) Low predationHigh predation Low predation Site type

Figure 49.8c Is prey defense induced by presence of predator? Are mussel defenses induced by the presence of crabs? Are mussel defenses induced by the presence of broken mussel shells? Seawater Crab (fed fish, not mussels) Mussels No crab Broken mussel shells Intact mussel shells Shell thickness HIGHShell thickness LOWShell thickness HIGHShell thickness LOW Yes

Keystone predator present Figure 49.9 left

Keystone predator absent Figure 49.9 right

Keystone predator present Figure left

Keystone predator absent Figure right

Herbivory n Unlike predators, herbivores are plant-eaters that remove tissue from their prey, but rarely kill them. n Why don’t herbivores eat more of the available plants than they do? Top-down hypothesis (Fig ) Poor nutrition hypothesis Plant defense hypothesis (Fig )

Predator Herbivore Primary producer (plants) Figure 49.11

Competition n Competition is detrimental to both of the individuals or species involved because it reduces available resources. n Every species has a unique niche, or set of habitat requirements. Competition occurs when niches overlap. Competitive exclusion results when niches completely overlap. (Fig c) Coexistence is possible if niches do not overlap completely and the species involved partition the available resources. (Fig b) Coexistence is also possible if other factors serve to limit the better competitor in some way.

One species eats seeds of one size range Number consumed Seed size Figure 49.13a

Partial niche overlap: competition for seeds of intermediate size Species 1 Species 2 Number consumed Seed size Figure 49.13b

Complete niche overlap Species 1: Strong competitor Species 2: Weak competitor, driven to extinction Number consumed Seed size Figure 49.13c

Consumptive competition occurs when organisms compete for the same resources. These trees are competing for nitrogen and other nutrients. Figure upper left

Preemptive competition occurs when individuals occupy space and prevent access to resources by other individuals. The space preempted by these barnacles is unavailable to competitors. Figure upper right

Overgrowth competition occurs when an organism grows over another, blocking access to resources. This large fern has overgrown other individuals and is shading them. Figure middle left

Chemical competition occurs when one species produces toxins that negatively affect another. Note how few plants are growing under these Salvia shrubs. Figure middle right

Territorial competition occurs when mobile organisms protect a feeding or breeding territory. These red-winged blackbirds are displaying to each other at a territorial boundary. Figure lower left

Encounter competition occurs when organisms interfere directly with each other’s access to specific resources. Here, spotted hyenas and vultures fight over a kill. Figure lower left

Intertidal competitors Chthamalus in upper intertidal zone Mean tidal level Balanus in lower intertidal zone Figure 49.15a

COMPETITION EXPERIMENT Upper intertidal Lower intertidal 1. Transplant rocks containing young Chthamalus to lower intertidal. 2. Let Balanus colonize the rocks. 3. Remove Balanus from one-half of each rock. Monitor survival of Chthamalus on both sides. On which side of the rocks do Chthamalus survive better? Chthamalus Balanus Figure 49.15b

Young ChthamalusOlder Chthamalus Competitor absent Competitor present Competitor absent Competitor present Figure 49.15c Chthamalus survives better without competition.

Mutualism n Mutualism is a type of interaction that is beneficial to both species involved. n It does not involve altruism. The benefits are a by-product of each species’ own self-interest. n The costs and benefits of mutualism vary widely between partners, over time, and from one area to the next. (Fig a–c)

Mutualism between fish Figure 49.16b

Treehopper excreting honeydew, which is harvested by ants Figure 49.17a

Are ants beneficial to treehoppers? 1000 m 2 study plot Plants with ants Plants with ants removed Figure 49.17b

Plants with ants Plants without ants AugustJuly Average number of young treehoppers per plant Figure 49.17c Which treatment contained more treehoppers?