Maintenance of Diversity Maintenance of species diversity 1. Ecological succession 1. Succession A) Definition: the sequential, predictable change in species 2. Loss of Diversity composition over time following a disturbance • Primary succession – succession starts from a completely empty 3. General Mechanisms that Maintain Diversity community (i.e. bare substratum) such as that following glaciations or a volcanic eruption 4. Specific Mechanisms that Maintain Diversity • Secondary succession – when the majority of individuals are removed by a disturbance of lesser intensity, often leaving propagules (seeds, spores, larvae) only (e.g., flooding, forest fire) Succession results in a climax community (if given sufficient time), in which the competitive dominants prevail
B) Why is there succession? i) Species differ in life history characteristics ii) Species cannot optimize all characters, so there appears to be trade-offs among characters that influence how a species responds to a disturbance
Ecological succession Ecological succession C) Comparison of early and late successional species: D) Models of succession: (Connell and Slatyer 1977 American Naturalist) Life History Early Successional Late Successional i) Facilitation: early species modify the environment… Character (“r-selected”) (“K-selected”) - make it more suitable for later species Reproduction semelparous (once) iteroparous (multiple) - later species can’t colonize until environment modified Fecundity high low - modified environment sometimes not so good for early species Dispersal ability good / long distance poor / short distance ii) Inhibition: early species inhibit later species from colonizing… Growth rate fast slow - later species colonize when early species die Life span short long - as they colonize, later species out-compete earlier species Generation time short long Competitive ability POOR GOOD iii) Tolerance: no interactions (positive or negative) between earlier and later species… • not all species fit these categories but it is a useful general scheme - earlier species are quick to colonize (arrive earlier) • Early species are good at dispersing to and colonizing newly disturbed sites, - later species are slow to colonize (arrive later) grow rapidly, reproduce, but are out-competed. - later species “tolerate” earlier species and lower resource availability • Late species are poor at dispersing to and colonizing newly disturbed sites, grow slowly, and out-compete earlier species.
Succession in California rocky intertidal D) Models of succession: (Connell and Slatyer 1977 American Naturalist)
Succession in California rocky intertidal
Spring 1992 Spring 1993
Facilitation, Inhibition, or Tolerance?
Spring 1995 Fall 1999
1 Succession includes upward shifts in the rocky intertidal Comparison of early and late successional species in intertidal
Barnacles 2000 (+0.5 meters) Species Lifespan Dispersal Recruitment
barnacles short long (30-40 d planktonic) regular Endocladia 2000 (+0.6 meters) Endocladia intermediate medium (<10 d planktonic) intermediate Silvetia long (years) short (< 50 m usually) irregular
Barnacles 1992
Endocladia 1992
Occulto
Succession includes upward shifts in the rocky intertidal Facilitation during succession: Silvetia and Endocladia Barnacles 2000 Endocladia 2000 (+0.3 meters)
Endocladia 1992
Silvetia 2000 (+0.3 meters)
Silvetia 1992 Saguaro cactus Boathouse and “nurse plants”
2. Losses of diversity 2. Losses of diversity
Interspecific Competition (Competitive Exclusion) General Causes (main focus in ecology) • Competitive Exclusion • Predation (including overharvesting by humans) -- Examples of competitive exclusion • Loss of mutualist / commensal i) Connell showed that Semibalanus out-competed and • Loss of habitat (e.g., disturbance) excluded Chthamalus from the mid-intertidal • Modification of habitat (e.g., disturbance, pollution) ii) Menge showed that mussels out-competed and excluded • Climate Change barnacles and algae in the mid-intertidal at exposed sites
Thus, to understand why some communities have high diversity & others low diversity, we need to consider what factors can keep interspecific competition from occurring...
2 3. Maintenance of species diversity 3. Maintenance of species diversity
Some general mechanisms: (Connell 1978 Science) Competitive Exclusion will not occur when: A) Intraspecific competition > Interspecific competition i) Resources are unlimited (“recruitment limitation”) C) Circular networks
ii) Populations reduced by biotic interactions (e.g., C) Compensatory mortality predation, intraspecific competition) to levels where resources are not limited D) Gradual change
iii) Disturbance interrupts competitive exclusion
A) Intraspecific competition > Interspecific competition C) Compensatory mortality
i) If KA (carrying capacity for species A) is lower than population size i) mortality is disproportionately greater in competitive dominant of species A that would exclude species B and vice versa, both because it is more abundant (“frequency dependent”) species will coexist ii) sources of mortality could include predation, disturbance ii) example: - behavioral interactions: territorial individuals less tolerant of conspecifics than other species D) Gradual change i) Competitive rank varies with changing environmental conditions: Abundance B) Circular networks (largely hypothetical) A … B … C … D i) Competitive hierarchy: A > B > C > A Abundance ii) Hutchinson’s “paradox of the plankton” TIME
TIME iii) Species must not decline to extinction before environment ii) Only example is intraspecific (Sinervo & Lively 1996, lizard) changes to favor it again
Maintenance of species diversity Predation i) keeps population level of dominant competitor below 4. Specific Mechanisms that which would cause competitive exclusion
ii) or can create patchiness in the environment A) Predation A) Proportional predation
B) Disturbance B) “Switching” predator
C) Keystone predator
3 A) Proportional predation C) Keystone predation (e.g., Paine 1974, American Naturalist) i) generalist predator eats prey as they are encountered (like a i) predator prefers to eat competitive dominant species disturbance!) ii) predator is a specialist on competitive dominant ii) most abundant species (competitive dominant) is encountered most often… so eaten most often iii) no species can exclude another (if there is enough predation) Contrasting predicted patterns of predation mechanisms Proportional Switching Keystone B) “Switching” predator (e.g., Murdoch 1969, Ecology) 1:1 i) predator preference for prey (predator behavior) switches with relative abundance of prey species % ii) common species eaten disproportionately more than species A uncommon ones in diet iii) rare species can therefore become more abundant iv) once rarer species becomes more abundant, predator switches % species A in field to feeding on it A Coexistence Coexistence Coexistence Abundance Result: B and Stability TIME
Predation E) Examples (switching and keystone) Keystone predation (Paine 1966 American Naturalist)
a) Pattern: i) Presence of competitively dominant mussels leads to exclusion of other primary space holders lower diversity ii) Pisaster is a mussel specialist
mussels (1 andspecies) 25 species mid lower Pisaster b) Working hypothesis: Pisaster keeps mussels from excluding all other species in mid- to lower intertidal
Keystone predation (Paine 1966 American Naturalist) Predation c) Specific hypothesis: Example: Ha: In areas where Pisaster is removed, species diversity will decrease compared to unmanipulated controls Switching (Raimondi et al., a work in progress) i) System: rocky intertidal in northern Gulf of California Chthamalus – barnacle d) Test: removed Pisaster (you know the story…) Brachiodontes – mussel Acanthina – predatory snail (w/ spine) Morula – predatory snail e) Results: ii) Pattern: barnacle and mussel coexist i) diversity in control areas unchanged ii) diversity in removal areas declined from 25 to 1 barnacle Percent species cover mussel
TIME
4 v) Results: Switching (Raimondi et al.)
iii) General hypothesis: Acanthina or Morula are Morula Acanthina switching predators
HA: proportion of prey in diet will be disproportionate to prey Percent abundance barnacles in diet HO: proportion of prey in diet will NOT be disproportionate to prey abundance % barns in cage % barns in cage iv) Design: Percent a) manipulate frequency of barnacles and mussels in cages mussels with each species of predator in diet
b) count number of prey killed of each species % mussels in cage % mussels in cage
a) Morula switches but prefers mussels b) Acanthina switches but prefers barnacles
Maintenance of species diversity
vi) Conclusions: B) Disturbance Abiotic environmental perturbation to a population or i) Switching predation may be (in part) responsible for community maintaining coexistence A) Marine examples: 1) Waves ii) Other factors may also be important: 2) Rolling boulders • mussels require barnacles to settle on 3) Logs • mussels out-compete barnacles, but 4) Freshwater (low salinity) • barnacles settle 10x the rate of mussels 5) Freezes (thermal) 6) Exfoliation (removal of rock surface) 7) Sedimentation 8) Sand burial / scour 9) Eutrophication-anoxia 10) Desiccation
Disturbance Disturbance
B) Consequences to populations and communities B) Consequences to populations and communities ii) Alternatively, can be a source of density-dependent i) generally, considered a source of density-independent mortality if constant amount of refuge from mortality mortality exists (leads to changing proportion of mortality) e.g., a log lands on half a rock surface and kills half the barnacles regardless of how many are there e.g.,
Relationships: linear = constant rate Constant proportion are Scour Mortality Percent killed rate lost regardless of density density Percent As the number of individuals outside refuge increases, the cover proportion killed increases!
5 Disturbance Disturbance C) Example of Disturbance (Dayton 1971 Ecological Monographs) B) Consequences to populations and communities i) System: rocky intertidal of Washington state; looked at competition, predation, & disturbance in determining the size and structure of barnacle Say, refuge holds 50 individuals… 50/50 (Semibalanus cariosus) populations 100 %
Survival ii) examined effect of disturbance by waves & logs rate 50/100 banging against rocky intertidal 50 % 50/200 25 % iii) example of how disturbance leads to a renewal of resources (space) Density-dependent survival refuge 100 200 (mortality rate) can regulate upper Density (in and out of refuge) bounds of population size
C) Example of Disturbance (Dayton 1971 Ecological Monographs) v) Results cont’d: 100 low iv) Approach: high • used nails as proxy for barnacles percent • compared nail “survival “ nails “surviving” • set out nails in areas of: high w/ 1. low wave exposure time logs 2. high wave exposure 3. high wave with logs For population structure… v) Results: compare % cover of large mature barnacles 100 100 100 percent percent mature percent nails free juveniles “surviving” space cover
low high high w/ low high high w/ low high high w/ logs logs logs
D) Intermediate Disturbance Hypothesis
vi) Conclusions: i) Connell (1978) – proposed to explain maintenance of species diversity ii) species diversity is greatest at intermediate levels 1) disturbance can be important determinant of of diversity: population size and structure high
2) logs more important than wave exposure as Species diversity source of disturbance H’
low high low Disturbance (intensity / frequency)
6 D) Intermediate Disturbance Hypothesis D) Intermediate Disturbance Hypothesis
iii) Logic iii) Logic cont’d: a) disturbance interrupts successional sequence by creating e) at intermediate levels of disturbance, get a mix of patches of different ages different states of succession patches with climax (old), early-fugitive (new), and middle succession w/ mix of both: b) in absence of disturbance, succession leads to climax community characterized by monospecific stand of high competitive dominant (or a few) low diversity Species diversity both H’ c) but, at extremely high levels of disturbance, nothing or only climax a few species of rapid colonizers (or disturbance tolerant fugitives species) would persist low diversity low Would get similar (recall that competitively superior species typically have poor high low pattern for generalist colonizing capabilities) Disturbance (intensity / frequency) predator. Predation would be a disturbance
D) Intermediate Disturbance Hypothesis D) Intermediate Disturbance Hypothesis iii) Example: Wayne Sousa 1979 iii) Example: Sousa 1979
a) System: macroalgae in intertidal boulder field at b) Pattern cont’d: Ellwood, CA physical disturbance - boulders of different sizes roll b) Pattern: (succession) at different frequencies: successional stage: early mid late high
species: Ulva Gelidium Chondracanthus frequency Mastocarpus of Gigartina rolling
low small med large boulder size
D) Intermediate Disturbance Hypothesis e) Results: high
Species diversity iii) Example: Sousa 1979 H’ c) General hypothesis: low species diversity of macroalgae is maintained by small med large intermediate frequencies of disturbance boulder size: disturbance frequency: high mod low
d) Specific hypothesis: (observational based) 42% rolled 9% rolled 0.1% rolled boulders of intermediate size roll at intermediate per month frequencies and will have highest species diversity
f) Conclusions: pattern consistent with intermediate disturbance hypothesis
7 Alternative approach… an experiment
g) Specific hypothesis: • boulders of similar sizes manipulated to alter their frequency of disturbance will exhibit patterns of species diversity predicted by IDH
h) Test: i) collected and sterilized 32 small boulders ii) glued 16 to the bottom to prevent rolling iii) monitored succession of algae
i) Results: • after 1 year, small boulders glued down developed algae communities similar to larger boulders that rolled infrequently
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