Models for the maintenance of diversity

How do you account for the maintenance of many similar species in local communities (e.g. corals on reefs, plankton in lakes, trees in forests, endophytic fungi in leaves)??? The problem with classical equilibrium theory…

Why cannot classical niche theory account for species rich communities?

Hutchinson’s ‘Paradox of plankton’ and ‘Homage to Santa Rosalia’ (1959)

Perhaps because community ecology has not paid sufficient attention to historical explanations of ecological phenomena (fear loss of “predictability”?)

Hubbell and Foster (1986) “Theoreticians abhor unique events”! Take the case of plants…

What will determine the success of a plant in occupying a unit of space?

Might argue that:

Similarity in traits seen among many tree species might reflect convergent generalization in response to the long-term average diffuse selective forces imposed by the sum of all competitors Hubbell random drift model

If species are competitively equivalent generalists

Then: Diversity in a local community is simply a consequence of regional tree species richness and availability of potential immigrants (= climate, historical biogeography, local dispersal, and speciation on regional scales)

A similar idea to community drift is the lottery model (Chesson and Warner 1981).

Recruitment sites (territories) also become available at random Potential colonists vary in recruits available to fill territories Adults long-lived relative to availability of territories Related to the lottery model is the ‘storage effect’ of Warner and Chesson (1985).

What if recruitment depends on environmental conditions, but environmental conditions fluctuate? All species have some conditions under which their populations can grow = ‘good years’. What if Different species benefit in different years. Why would this facilitate coexistence? Cáceres (1997, 1998) Long-term coexistence of Daphnia species

Previous work had suggested equilibrial mechanisms for coexistence - partitioning of water depth in Michigan lakes

Also coexist in Lake Oneida (NY) without habitat heterogeneity

Daphnia pulicaria Daphnia galeata 30 yr record of Daphnia density in the water column and 20 yr record of recruitment data to persistent egg bank (in lake sediment)

Predation (fish, inverts), competition and resource/abiotic conditions probably all affect Daphnia success.

Long-lived egg bank allows D. galeata to Evidence for storage mediated coexistence? recruit in favorable years Pake and Venable (1996) looked at dormancy and seed germination of Sonoran Desert annual plants.

Conditions for storage:

1. Many species produce a super-annual seed bank 2. Germination responses to temp. are species-specific

Species at low densities occasionally experience a favorable year in the absence of competition (because germination responses are not completely correlated among species) Rare species sometimes attain high population growth rates and ‘re-invade’ a community... Facelli et al. (2005) Explored seed germination as key to storage effect in arid South Australia Intermediate Disturbance Hypothesis (Connell 1978)

Frequency and intensity of various kinds of abiotic disturbances affect patterns of diversity.

Why would disturbance be important for coexistence?

Weak/infrequent disturbances ______

Intense/frequent disturbances ______

Intermediate disturbances ______Intermediate Disturbance Hypothesis Wayne Sousa (1979) Grad student of Joe Connell

Needed a system where disturbance easily manipulated and response is fast.

How does disturbance to boulders affect diversity of algae?

Waves in winter storms turn over boulders exposing new substrate for colonization and smother algae on the underside of the rock.

What influences local disturbance rate?? Frequency of disturbance determined by rock size

Classify rocks according to force categories.

Observations consistent with IDH

Recently turned over boulders are colonized by a green alga (Ulva) and barnacles

At an intermediate time post-disturbance boulder space is shared between Ulva and several mid-successional red algae.

Undisturbed boulders become dominated by the late successional alga Gigartina Mid-successional algae most abundant at intermediate disturbances

Note: Numbers above columns are sample sizes

x axis ‘pull’ is force required to move a boulder

Figure shows density of mid-successional red algae Sousa also performed Connell-style experiment:

Are there alternate explanations for differences in algal occurrence patterns?

Sterilized boulders, cemented half in place and looked at colonization

Unstabilized rocks dominated by Ulva after 18 mo (early successional species).

Stabilized rocks had diverse cover of red algae (mid-successional species). Application to IDH to forest communities

Disturbance in forests is mostly from treefalls (blow-downs and sometimes landslides)

If IDH operates what tree communities would be expected to have highest diversity?

What problems might arise in explaining tree diversity using IDH? Recruitment limitation

“Failure to arrive, or to establish at a site suitable for onward growth”.

This encompasses both failure of dispersal (dispersal limitation) and absence of suitable microsites for establishment (establishment limitation). Establishment limitation may be imposed by either absence of suitable abiotic conditions (e.g exposed soil, sufficient light) or biotic conditions (e.g. seed or seedling pathogens)

Good evidence for both dispersal limitation and establishment limitation in many community types (trees, fish, others?) Example of recruitment limitation in a marine system (Birkeland 1982)

‘Crown of Thorns’ starfish Acanthaster planci What determines when crown of thorns is abundant?

Adult Acanthaster outbreaks occur, suddenly and irregularly. Coral cores suggest that is a long-term dynamic

Outbreaks have become more frequent and long-lasting in recent years

Adult populations reach very large numbers and munch prodigious quantities of coral…e.g. ~500,000 removed in 1 year from Tutuila, American Samoa

Individual COTS can cover 20 km2 of coral a year. Hypothesis: Outbreaks of COTS represents periodic escape from recruitment limitation

Birkeland figured out that outbreaks occur 3 years after typhoons bringing heavy rain (but do not follow “dry” typhoons)

Hypothesis: terrestrial run-off from storms provide nutrients to for phytoplankton which supports survival of Acanthaster larvae.

Increased survival of larvae results in high settlement rates on reefs across a large area and high adult densities 3 yr later.

Adult COTS produce 65 million eggs! Small increases in larval survival probability could result in rapid increases in recruitment Recruitment limitation in context of other traits

Remember example of Drosophila competition - colonization trade-off??

Tilman (1994) Space allows coexistence of unlimited diversity if a strict trade-off exists between dispersal ability and competitive ability (and longevity).

Trade-off between competitive ability and dispersal is likely (resources used for roots and shoots cannot be used for seeds) Species differ in R* value for N - the limiting resource in young successional grasslands

Support for competition- colonization trade-off for plants at Cedar Creek, MI (Tilman 1994) What about colonization alone (‘steady state’ dispersal limitation)? Hurtt and Pacala (1995) simulation models to look at how dispersal would slow competitive exclusion. Rank species according to competitive ability but give all species equivalent and restricted dispersal ability.

Limited dispersal can dramatically slow population and community dynamics in species rich communities

Could high diversity originate through dispersal limitation? So, how dispersal limited are communities?

Long-term seed trapping studies can be used to see how frequently seeds arrive at potential recruitment sites BCI 50 ha plot Two hundred 1 m2 seed traps arrayed along trails over 50 ha Hubbell et al. (1999)

Analysis of seed trap captures over 10 years

300 tree species in the plot that can contribute seeds to plots

1.3 million seeds collected over 10 years!

On average, 31 species were recorded in a single trap over 10 YEARS A small number of wind-dispersed species reach nearly all traps.

HALF of the species had seeds dispersed to 6 traps or fewer

40 species no captures at all

How long are the reproductive lifespans of individual trees? Summary: Many theories for the maintenance of diversity explore mechanisms that can prevent species exclusion through limiting similarity.

Classical niche differentiation may not be necessary.

Species coexistence can be mediated by repeated disturbance (IDH), temporal fluctuations in recruitment success (Lottery model and storage effect), competition-colonization trade-offs or simply by steady-state dispersal limitation.

Models need not stop competitive exclusion, just slow it down sufficiently such that species loss is balanced by species replacement Coming soon… the niche strikes back!