41 Species Interactions

Overview: Communities in Motion A biological community is an assemblage of populations of various species living close enough for potential interaction For example, the “carrier crab” carries a sea urchin on its back for protection against predators 2

Figure 41.1 Figure 41.1 Which species benefits from this interaction? 3

Concept 41.1: Interactions within a community may help, harm, or have no effect on the species involved Ecologists call relationships between species in a community interspecific interactions Examples are competition, predation, herbivory, symbiosis (parasitism, mutualism, and commensalism), and facilitation Interspecific interactions can affect the survival and reproduction of each species, and the effects can be summarized as positive (), negative (−), or no effect (0) 4

Competition Interspecific competition (−/− interaction) occurs when species compete for a resource that limits their growth or survival 5

Competitive Exclusion Strong competition can lead to competitive exclusion, local elimination of a competing species The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place 6

Ecological Niches and Natural Selection Evolution is evident in the concept of the ecological niche, the specific set of biotic and abiotic resources used by an organism An ecological niche can also be thought of as an organism’s ecological role Ecologically similar species can coexist in a community if there are one or more significant differences in their niches 7

Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community 8

A. distichus perches on fence posts and other sunny surfaces. Figure 41.2 A. distichus perches on fence posts and other sunny surfaces. A. insolitus usually perches on shady branches. A. ricordii Figure 41.2 Resource partitioning among Dominican Republic lizards A. insolitus A. aliniger A. christophei A. distichus A. cybotes A. etheridgei 9

Figure 41.2a A. distichus Figure 41.2a Resource partitioning among Dominican Republic lizards (part 1: A. distichus) 10

Figure 41.2b A. insolitus Figure 41.2b Resource partitioning among Dominican Republic lizards (part 2: A. insolitus) 11

A species’ fundamental niche is the niche potentially occupied by that species A species’ realized niche is the niche actually occupied by that species As a result of competition, a species’ fundamental niche may differ from its realized niche For example, the presence of one barnacle species limits the realized niche of another species 12

Experiment High tide Chthamalus Balanus Chthamalus realized niche Figure 41.3 Experiment High tide Chthamalus Balanus Chthamalus realized niche Balanus realized niche Ocean Low tide Results High tide Figure 41.3 Inquiry: Can a species’ niche be influenced by interspecific competition? Chthamalus fundamental niche Ocean Low tide 13

Character Displacement Character displacement is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species An example is variation in beak size between populations of two species of Galápagos finches 14

Santa María, San Cristóbal Sympatric populations 40 Figure 41.4 G. fuliginosa G. fortis Beak depth 60 Los Hermanos 40 G. fuliginosa, allopatric 20 60 Daphne Percentages of individuals in each size class 40 G. fortis, allopatric 20 Figure 41.4 Character displacement: indirect evidence of past competition 60 Santa María, San Cristóbal Sympatric populations 40 20 8 10 12 14 16 Beak depth (mm) 15

Predation Predation (/− interaction) refers to an interaction in which one species, the predator, kills and eats the other, the prey Some feeding adaptations of predators are claws, teeth, stingers, and poison 16

Prey display various defensive adaptations Behavioral defenses include hiding, fleeing, forming herds or schools, and active self-defense Animals also have morphological and physiological defense adaptations Cryptic coloration, or camouflage, makes prey difficult to spot Video: Sea Horses 17

(c) Batesian mimicry: A harmless species mimics a harmful one. Figure 41.5 (a) Cryptic coloration (b) Aposematic coloration Canyon tree frog Poison dart frog (c) Batesian mimicry: A harmless species mimics a harmful one. Nonvenomous hawkmoth larva (d) Müllerian mimicry: Two unpalatable species mimic each other. Venomous green parrot snake Cuckoo bee Yellow jacket Figure 41.5 Examples of defensive coloration in animals 18

(a) Cryptic coloration Canyon tree frog Figure 41.5a Figure 41.5a Examples of defensive coloration in animals (part 1: cryptic coloration) 19

Animals with effective chemical defenses often exhibit bright warning coloration, called aposematic coloration Predators are particularly cautious in dealing with prey that display such coloration 20

(b) Aposematic coloration Poison dart frog Figure 41.5b Figure 41.5b Examples of defensive coloration in animals (part 2: aposematic coloration) 21

In some cases, a prey species may gain significant protection by mimicking the appearance of another species In Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful model 22

(c) Batesian mimicry: A harmless species mimics a harmful one. Figure 41.5c (c) Batesian mimicry: A harmless species mimics a harmful one. Nonvenomous hawkmoth larva Venomous green parrot snake Figure 41.5c Examples of defensive coloration in animals (part 3: Batesian mimicry) 23

Nonvenomous hawkmoth larva Figure 41.5ca Figure 41.5ca Examples of defensive coloration in animals (part 3a: Batesian mimicry, hawkmoth) Nonvenomous hawkmoth larva 24

Venomous green parrot snake Figure 41.5cb Figure 41.5cb Examples of defensive coloration in animals (part 3b: Batesian mimicry, snake) Venomous green parrot snake 25

In Müllerian mimicry, two or more unpalatable species resemble each other 26

(d) Müllerian mimicry: Two unpalatable species mimic each other. Figure 41.5d (d) Müllerian mimicry: Two unpalatable species mimic each other. Cuckoo bee Yellow jacket Figure 41.5d Examples of defensive coloration in animals (part 4: Müllerian mimicry) 27

Figure 41.5da Figure 41.5da Examples of defensive coloration in animals (part 4a: Müllerian mimicry, cuckoo bee) Cuckoo bee 28

Yellow jacket Figure 41.5db Figure 41.5db Examples of defensive coloration in animals (part 4b: Müllerian mimicry, yellow jacket) Yellow jacket 29

Herbivory Herbivory (/− interaction) refers to an interaction in which an herbivore eats parts of a plant or alga In addition to behavioral adaptations, some herbivores may have chemical sensors or specialized teeth or digestive systems Plant defenses include chemical toxins and protective structures 30

Figure 41.6 Figure 41.6 A marine herbivore 31

Symbiosis Symbiosis is a relationship where two or more species live in direct and intimate contact with one another 32

Parasitism In parasitism (/− interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process Parasites that live within the body of their host are called endoparasites Parasites that live on the external surface of a host are ectoparasites 33

Many parasites have a complex life cycle involving multiple hosts Some parasites change the behavior of the host in a way that increases the parasites’ fitness Parasites can significantly affect survival, reproduction, and density of host populations 34

Mutualism Mutualistic symbiosis, or mutualism (/ interaction), is an interspecific interaction that benefits both species In some mutualisms, one species cannot survive without the other In other mutualisms, both species can survive alone Mutualisms sometimes involve coevolution of related adaptations in both species Video: Clownfish and Anemone 35

(a) Ants (genus Pseudomyrmex) in acacia tree Figure 41.7 Figure 41.7 Mutualism between acacia trees and ants (a) Ants (genus Pseudomyrmex) in acacia tree (b) Area cleared by ants around an acacia tree 36

(a) Ants (genus Pseudomyrmex) in acacia tree Figure 41.7a Figure 41.7a Mutualism between acacia trees and ants (part 1: ants) (a) Ants (genus Pseudomyrmex) in acacia tree 37

(b) Area cleared by ants around an acacia tree Figure 41.7b Figure 41.7b Mutualism between acacia trees and ants (part 2: acacia tree) (b) Area cleared by ants around an acacia tree 38

Commensalism In commensalism (/0 interaction), one species benefits and the other is neither harmed nor helped Commensal interactions are hard to document in nature because any close association likely affects both species 39

Figure 41.8 Figure 41.8 A possible example of commensalism between cattle egrets and African buffalo 40

Facilitation Facilitation (/ or 0/) is an interaction in which one species has positive effects on another species without direct and intimate contact For example, the black rush makes the soil more hospitable for other plant species 41

With Juncus Without Juncus Figure 41.9 8 6 Number of plant species 4 2 Figure 41.9 Facilitation by black rush (Juncus gerardi) in New England salt marshes (a) Salt marsh with Juncus (foreground) With Juncus Without Juncus (b) 42

(a) Salt marsh with Juncus (foreground) Figure 41.9a Figure 41.9a Facilitation by black rush (Juncus gerardi) in New England salt marshes (photo) (a) Salt marsh with Juncus (foreground) 43

Concept 41.2: Diversity and trophic structure characterize biological communities Two fundamental features of community structure are species diversity and feeding relationships Sometimes a few species in a community exert strong control on that community’s structure 44

Species Diversity Species diversity of a community is the variety of organisms that make up the community It has two components: species richness and relative abundance Species richness is the number of different species in the community Relative abundance is the proportion each species represents of all individuals in the community 45

A B C D Community 1 Community 2 A: 25% B: 25% C: 25% D: 25% A: 80% Figure 41.10 A B C D Community 1 Community 2 Figure 41.10 Which forest is more diverse? A: 25% B: 25% C: 25% D: 25% A: 80% B: 5% C: 5% D: 10% 46

H  −(pA ln pA  pB ln pB  pC ln pC  …) Two communities can have the same species richness but a different relative abundance Diversity can be compared using a diversity index Widely used is the Shannon diversity index (H) H  −(pA ln pA  pB ln pB  pC ln pC  …) where A, B, C . . . are the species, p is the relative abundance of each species, and ln is the natural logarithm 47

Molecular tools can be used to help determine microbial diversity Determining the number and relative abundance of species in a community is challenging, especially for small organisms Molecular tools can be used to help determine microbial diversity 48

Figure 41.11 Results 3.6 3.4 3.2 Shannon diversity (H) 3.0 2.8 Figure 41.11 Research method: determining microbial diversity using molecular tools 2.6 2.4 2.2 3 4 5 6 7 8 9 Soil pH 49

Diversity and Community Stability Ecologists manipulate diversity in experimental communities to study the potential benefits of diversity For example, plant diversity has been manipulated at Cedar Creek Natural History Area in Minnesota for two decades 50

Figure 41.12 Figure 41.12 Study plots at the Cedar Creek Natural History Area, site of long-term experiments in which researchers have manipulated plant diversity 51

Communities with higher diversity are More productive and more stable in their productivity Able to produce biomass (the total mass of all individuals in a population) more consistently than single species plots Better able to withstand and recover from environmental stresses More resistant to invasive species, organisms that become established outside their native range 52

Trophic Structure Trophic structure is the feeding relationships between organisms in a community It is a key factor in community dynamics Food chains link trophic levels from producers to top carnivores 53

Quaternary consumers: carnivores Figure 41.13 Quaternary consumers: carnivores Tertiary consumers: carnivores Secondary consumers: carnivores Figure 41.13 Examples of terrestrial and marine food chains Primary consumers: herbivores and zooplankton Primary producers: plants and phytoplankton 54

A food web is a branching food chain with complex trophic interactions Species may play a role at more than one trophic level Video: Shark Eating a Seal 55

Humans Smaller toothed whales Baleen whales Sperm whales Elephant Figure 41.14 Humans Smaller toothed whales Baleen whales Sperm whales Elephant seals Crab- eater seals Leopard seals Birds Fishes Squids Figure 41.14 An Antarctic marine food web Carnivorous plankton Copepods Krill Phyto- plankton 56

Species with a Large Impact Certain species have a very large impact on community structure Such species are highly abundant or play a pivotal role in community dynamics 57

Dominant species are those that are most abundant or have the highest biomass One hypothesis suggests that dominant species are most competitive in exploiting resources Another hypothesis is that they are most successful at avoiding predators and disease 58

Keystone species exert strong control on a community by their ecological roles, or niches In contrast to dominant species, they are not necessarily abundant in a community Field studies of sea stars illustrate their role as a keystone species in intertidal communities 59

Number of species present Figure 41.15 Experiment Results 20 15 With Pisaster (control) Number of species present Figure 41.15 Inquiry: Is Pisaster ochraceus a keystone predator? 10 Without Pisaster (experimental) 5 1963’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 Year 60

Figure 41.15a Experiment Figure 41.15a Inquiry: Is Pisaster ochraceus a keystone predator? (part 1: experiment) 61

Number of species present Figure 41.15b Results 20 15 With Pisaster (control) Number of species present 10 Without Pisaster (experimental) 5 Figure 41.15b Inquiry: Is Pisaster ochraceus a keystone predator? (part 2: results) 1963’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 Year 62

Ecosystem engineers (or “foundation species”) cause physical changes in the environment that affect community structure For example, beaver dams can transform landscapes on a very large scale 63

Figure 41.16 Figure 41.16 Beavers as ecosystem engineers 64

Bottom-Up and Top-Down Controls The bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levels In this case, the presence or absence of mineral nutrients determines community structure, including the abundance of primary producers 65

The bottom-up model can be represented by the equation where N  mineral nutrients V  plants H  herbivores P  predators N V H P 66

The top-down model, also called the trophic cascade model, proposes that control comes from the trophic level above In this case, predators control herbivores, which in turn control primary producers N V H P 67

Biomanipulation is used to improve water quality in polluted lakes In a Finnish lake, blooms of cyanobacteria (primary producers) occurred when zooplankton (primary consumers) were eaten by large populations of roach fish (secondary consumers) Removal of roach fish and addition of pike perch (tertiary consumers) controlled roach populations, allowing zooplankton populations to increase and ending cyanobacterial blooms 68

Polluted State Restored State Fish Abundant Rare Zooplankton Rare Figure 41.UN02 Polluted State Restored State Fish Abundant Rare Zooplankton Rare Abundant Figure 41.UN02 In-text figure, biomanipulation, p. 855 Algae Abundant Rare 69

Concept 41.3: Disturbance influences species diversity and composition Decades ago, most ecologists favored the view that communities are in a state of equilibrium This view was supported by F. E. Clements, who suggested that species in a climax community function as an integrated unit 70

Other ecologists, including A. G. Tansley and H. A Other ecologists, including A. G. Tansley and H. A. Gleason, challenged whether communities were at equilibrium Recent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after being buffeted by disturbances A disturbance is an event that changes a community, removes organisms from it, and alters resource availability 71

Characterizing Disturbance Fire is a significant disturbance in most terrestrial ecosystems A high level of disturbance is the result of a high intensity and high frequency of disturbance Low disturbance levels result from either low intensity or low frequency of disturbance 72

High levels of disturbance exclude many slow-growing species The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster greater diversity than either high or low levels of disturbance High levels of disturbance exclude many slow-growing species Low levels of disturbance allow dominant species to exclude less competitive species 73

In a New Zealand study, the richness of invertebrate taxa was highest in streams with an intermediate intensity of flooding 74

Index of disturbance intensity (log scale) Figure 41.17 35 30 25 Number of taxa 20 15 Figure 41.17 Testing the intermediate disturbance hypothesis 10 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 Index of disturbance intensity (log scale) 75

The Yellowstone forest is an example of a nonequilibrium community The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance The Yellowstone forest is an example of a nonequilibrium community 76

(a) Soon after fire (b) One year after fire Figure 41.18 Figure 41.18 Recovery following a large-scale disturbance (a) Soon after fire (b) One year after fire 77

(a) Soon after fire Figure 41.18a Figure 41.18a Recovery following a large-scale disturbance (part 1: soon after) (a) Soon after fire 78

(b) One year after fire Figure 41.18b Figure 41.18b Recovery following a large-scale disturbance (part 2: one year after) (b) One year after fire 79

Ecological Succession Ecological succession is the sequence of community and ecosystem changes after a disturbance Primary succession occurs where no soil exists when succession begins Secondary succession begins in an area where soil remains after a disturbance 80

Early-arriving species and later-arriving species may be linked in one of three processes Early arrivals may facilitate the appearance of later species by making the environment favorable Early species may inhibit the establishment of later species Later species may tolerate conditions created by early species, but are neither helped nor hindered by them 81

Retreating glaciers provide a valuable field research opportunity for observing succession Succession on the moraines in Glacier Bay, Alaska, follows a predictable pattern of change in vegetation and soil characteristics The exposed moraine is colonized by pioneering plants, including liverworts, mosses, fireweed, Dryas, and willows 82

Glacier Bay Alaska 0 5 10 15 Kilometers Figure 41.19-1 Figure 41.19-1 Glacial retreat and primary succession at Glacier Bay, Alaska (step 1) 0 5 10 15 Kilometers 83

1941 1 Pioneer stage Glacier Bay Alaska 0 5 10 15 Kilometers Figure 41.19-2 1941 1 Pioneer stage Glacier Bay Alaska Figure 41.19-2 Glacial retreat and primary succession at Glacier Bay, Alaska (step 2) 0 5 10 15 Kilometers 84

1941 1907 1 Pioneer stage 2 Dryas stage Glacier Bay Alaska 0 5 10 15 Figure 41.19-3 1941 1907 1 Pioneer stage 2 Dryas stage Glacier Bay Alaska Figure 41.19-3 Glacial retreat and primary succession at Glacier Bay, Alaska (step 3) 0 5 10 15 Kilometers 85

1941 1907 1 Pioneer stage 1860 2 Dryas stage Glacier Bay Alaska 3 Figure 41.19-4 1941 1907 1 Pioneer stage 1860 2 Dryas stage Glacier Bay Alaska Figure 41.19-4 Glacial retreat and primary succession at Glacier Bay, Alaska (step 4) 0 5 10 15 Kilometers 3 Alder stage 86

1941 1907 1 Pioneer stage 1860 2 Dryas stage Glacier Bay Alaska 1760 4 Figure 41.19-5 1941 1907 1 Pioneer stage 1860 2 Dryas stage Glacier Bay Alaska Figure 41.19-5 Glacial retreat and primary succession at Glacier Bay, Alaska (step 5) 1760 0 5 10 15 Kilometers 4 Spruce stage 3 Alder stage 87

Figure 41.19a Figure 41.19a Glacial retreat and primary succession at Glacier Bay, Alaska (part 1: pioneer) 1 Pioneer stage 88

After about three decades, Dryas dominates the plant community 89

Figure 41.19b Figure 41.19b Glacial retreat and primary succession at Glacier Bay, Alaska (part 2: Dryas) 2 Dryas stage 90

A few decades later, alder invades and forms dense thickets 91

Figure 41.19c Figure 41.19c Glacial retreat and primary succession at Glacier Bay, Alaska (part 3: alder) 3 Alder stage 92

In the next two centuries, alder are overgrown by Sitka spruce, western hemlock, and mountain hemlock 93

Figure 41.19d Figure 41.19d Glacial retreat and primary succession at Glacier Bay, Alaska (part 4: spruce) 4 Spruce stage 94

Succession is the result of changes induced by the vegetation itself On the glacial moraines, vegetation increases soil nitrogen content, facilitating colonization by later plant species 95

Human Disturbance Humans have the greatest impact on biological communities worldwide Human disturbance to communities usually reduces species diversity Trawling is a major human disturbance in marine ecosystems 96

Figure 41.20 Figure 41.20 Disturbance of the ocean floor by trawling 97

Figure 41.20a Figure 41.20a Disturbance of the ocean floor by trawling (part 1: before) 98

Figure 41.20b Figure 41.20b Disturbance of the ocean floor by trawling (part 2: after) 99

Concept 41.4: Biogeographic factors affect community diversity Latitude and area are two key factors that affect a community’s species diversity 100

Latitudinal Gradients Species richness is especially great in the tropics and generally declines along an equatorial-polar gradient Two key factors in equatorial-polar gradients of species richness are probably evolutionary history and climate 101

Temperate and polar communities have started over repeatedly following glaciations The greater age of tropical environments may account for their greater species richness In the tropics, the growing season is longer, so biological time runs faster 102

Climate is likely the primary cause of the latitudinal gradient in biodiversity Two main climatic factors correlated with biodiversity are sunlight and precipitation They can be considered together by measuring a community’s rate of evapotranspiration Evapotranspiration is evaporation of water from soil plus transpiration of water from plants 103

Vertebrate species richness Figure 41.21 200 100 Vertebrate species richness (log scale) 50 Figure 41.21 Energy, water, and species richness 10 500 1,000 1,500 2,000 Potential evapotranspiration (mm/yr) 104

Area Effects The species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more species 105

Species richness on islands depends on island size, distance from the mainland, immigration, and extinction The equilibrium model of island biogeography maintains that species richness on an ecological island levels off at a dynamic equilibrium point 106

Studies of species richness on the Galápagos Islands support the prediction that species richness increases with island size 107

Area of island (hectares) Figure 41.22 Results 400 200 100 Number of plant species (log scale) 50 25 10 Figure 41.22 Inquiry: How does species richness relate to area? 5 10 100 103 104 105 106 Area of island (hectares) (log scale) 108

Concept 41.5: Pathogens alter community structure locally and globally Ecological communities are universally affected by pathogens, disease-causing organisms and viruses 109

Effects on Community Structure Pathogens can have dramatic effects on community structure when they are introduced into new habitats For example, coral reef communities are being decimated by white-band disease Sudden oak death has killed millions of oaks that support many bird species 110

Community Ecology and Zoonotic Diseases Zoonotic pathogens have been transferred from other animals to humans The transfer of pathogens can be direct or through an intermediate species called a vector Many of today’s emerging human diseases are zoonotic 111

Identifying the community of hosts and vectors for a pathogen can help prevent disease For example, recent studies identified two species of shrew as the primary hosts of the pathogen for Lyme disease 112

Figure 41.23 Figure 41.23 Identifying Lyme disease host species 113

Avian flu is a highly contagious virus of birds Ecologists are studying the potential spread of the virus from Asia to North America through migrating birds 114

Figure 41.24 Figure 41.24 Tracking avian flu 115

Community ecology is needed to help study and combat pathogens Human activities are transporting pathogens around the world at unprecedented rates Community ecology is needed to help study and combat pathogens 116

Figure 41.UN01a Figure 41.UN01a Skills exercise: using bar graphs and scatter plots to present and interpret data (part 1) 117

Figure 41.UN01b Figure 41.UN01b Skills exercise: using bar graphs and scatter plots to present and interpret data (part 2) 118

Figure 41.UN03 Figure 41.UN03 Summary of key concepts: species interactions 119