Species Interactions and Community Structure • • • • Community Webs  Complexity and Structure Indirect Interactions Keystone Species  Effects on Diversity Mutualistic Keystones 1

Figure 17_02 2

Community Webs •

Winemiller

described feeding relations among tropical freshwater fish.

 Represented food webs in various ways:  Only included common species.

 Top-predator sink.

 Excluded weakest trophic links.

3

Winemiller food web Lowland stream in Venezuela; S > 88 4

Strong Interactions and Food Web Structure •

Paine

suggested feeding activities of a few species may have a dominant influence on community structure.

 Suggested criterion for strong interaction is degree of influence on community structure.

5

• Strong Interactions and Food Web Structure

Tscharntke

studied food webs associated with wetland reeds (

Phragmites australis

).

 Attacked by fly

Giraudiella inclusa

.

 & 14 species of parasitoid wasps.

 Blue tit 6

Figure 17_05 7

Indirect Interactions • Indirect interactions are the effects of one species on another through a third species  Examples:  Trophic cascades  Indirect commensalism  Apparent competition 8

Indirect Commensalism • • One species indirectly benefits another species (through a third species) while it is neither helped or harmed E.g., Martinsen et al. (1998)  Beavers cut cottonwood trees - trees produce stump sprouts  Beetles prefer sprout leaves  Beetles grow larger, faster and use defensive compounds in leaves 9

Figure 17_06 10

Apparent Competition • • Negative effects between two competitors who share a predator or herbivore  One species may facilitate presence or increased abundance of a predator which suppresses the second species Orrock et al. (2008)  Exotic plant

Brassica nigra

shelters mammals which increases herbivory on native bunchgrass

Nassella pulchra

11

Indirect Interactions 12

Keystone Species • If keystone species reduce likelihood of competitive exclusion, their activities increase the number of species that coexist in communities.

13

Food Web Structure and Species Diversity •

Paine

found: increased number of species in intertidal food webs = proportion of predators also increased.

 His hypothesis = higher proportion of predators produces higher predation pressure on prey populations, promoting higher diversity.

14

Keystone Species 15

Paine experiment: Remove top predator Monitor for 2 years 16

Consumer Effect on Local Diversity •

Lubchenko

proposed to resolve the effect herbivores have on plant diversity  Herbivore food preference.

 Competitive relationships between plant species in the local community.

 Variance in feeding preferences and competitive relationships across environments.

17

Consumers’ Effects on Local Diversity •

Lubchenko

: influence of intertidal snail (

Littorina littorea

) on algal community.  In Lab: snails prefer green (

Enteromorpha

spp.) over red (

Chondrus crispus

) algae.

 In field,

Enteromorpha

out-competes

Chondrus

in tide pools.

18

Consumers’ Effects on Local Diversity 19

Consumers’ Effects on Local Diversity • When snails are present in high densities,

Littorina

grazes down

Enteromorpha

, releasing

Chondrus

from competition.

 Green crabs,

Carcinus

, eat young snails, = no juveniles in tide pools.



Carcinus

are controlled by seagulls.

20

Consumers’ Effects on Local Diversity  Low snail density -

Enteromorpha

dominates tide pool.

 Medium snail density - Competitive exclusion eliminated, and algal diversity increased.

 High snail density - Feeding requirements are high enough that snails eat preferred algae and less-preferred algae.

 Algal diversity decreased.

21

Fish as River Keystone Species •

Mary Power:

do California roach

Hesperoleucas symmetricus

and steelhead trout

Oncorhhyncus mykiss

web structure?

influence food 22

Fish as River Keystone Species 23

•

Mary Power

experiment • Cages with or w/out predator fish, allow insects in • • Fish present = low algae No fish = high algae • Why?

24

25

•

Mary Power

predictions

:

 Predatory fish decrease algal densities.

 Low predator density increases midge production.

 Increased feeding pressure on algal populations.

– Thus, fish act as Keystone Species.

26

Mutualistic Keystones •

Power

: Keystone species exert strong effects on community structure, despite low biomass.

27

Seed Dispersal Mutualists as Keystone Species •

Christian

observed native ants disperse 30% of shrubland seeds in fynbos of South Africa.

 Seed-dispersing ants bury seeds in sites safe from predators and fire.

 Argentine ants displaced native ant species that disperse large seeds.

 = reductions in seedling recruitment by plants producing large seeds.

28

= Exotic species is mutualistic keystone species 29

Keystone Nutrient Recycler Idea that a species that cycles large amount of nutrients can affect community 30

31

Upstream Sura 43 fish species found here Frugivores, insectivores, piscivores, herbivores 32

Alfaro Astyanax Terrestrial insects Archocentrus Astatheros Aquatic insects Priapichthyes Insects 33

•

Astyanax

= tetra that excretes 10X P of entire community = Keystone Nutrient Recycler 5 common species

Astyanax Astatheros Archocentrus Priapichthys Alfaro

P excreted SRP 81 0.3

0.2

2.2

8.6

40.5

0.1

0.1

1.1

3.4

34

Review • • • • Community Webs  Complexity and Structure Indirect Interactions Keystone Species  Effects on Diversity Mutualistic Keystones 35