Maintenance of Diversity

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