How Was Diversity Of Rf Generated How Does It Coexist

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museum hypothesis

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o (Wallace 1878)
o Larger area= accumulation of more spp
o Low extinction rates in climatically stable env= slow accumulation of spp
o Tropical lineages more time to diversify
o However: in Africa, precipitation and temp changed during last ice age (livingstone 1967). Andes, pollen history of warm and cold oscillations (Hammen and Gonzalez 1959)

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speciation speed

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o Warmer tropics- higher speciation rates. Temperature increases mutation rates, shortens generation times, leads to faster rates of pop divergence and higher speciation rates. Minimal variation in endothermic taxa body temperature stands at odds with pronounced latitudinal diversity gradients they exhibit

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Environmental instability: species pumps and refugia

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o Pleistocene glacial cycles of Amazonian forest contraction>> wet forest refugia within matrix dry savannah>> allopatric speciation. Cycle of expansion and contraction drives species pump>> accumulation in tropics.
o No in Amazonia- doubts refugia existed: taxonomic groups evolved pre-pleistocene climate changes. Moritz 2000- molecular methods indicate spp radiation pre-pleistocene.
o Brazil’s Atlantic forests, Madagascar, Australian wet tropics and Africa- feasible. Anthony 2007- climatic models and palynological studies: refugia consistent w disjunct distributions and patterns of genetic differentiation.
o Refugia as museum hypothesis- high richness of older mntns in East Africa and rich flora and fauna of geologically ancient Eastern Arc mntns contrast w poorer, younger mtns.

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Riverine barrier hypothesis

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o Wallace 1852- major rivers Amazon= speciation by pop isolation. Support from distributional patterns of marmosets, tamarins and various birds and lizards. Colwell 2000- strongest support from genetic studies is from large rivers, otherwise differentiation of pops across river barriers is ltd= partial explanation

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Geological instability

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o Mtn building led to high levels of endemism and spp diversity. Geologically active/ topographically complex regions promote speciation via creation of new habitats and steep environmental gradients. Shifts in spp distribution driven by climate and tectonic uplift- habitat expansion and contraction= population isolation, divergence and mixing= speciation.
o Hall 05- more recently evolved spp of Neotropical butterfly genus Ithomiola occur in montane regions, older basal spp occupy geologically stable lowland.

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Environmental heterogeneity

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o Taxa often differentiate along topographical, edaphic and geological gradients.
o Whinnett 2005- freq occurrence of hybrid zones of sister spp within ecotones that straddle habitat boundaries= support for theory.
o Little agreement about spatial and temporal structure and location of different forest formations in relation to historical and contemporary habitat heterogeneity/ degree to which boundaries represent sufficient barriers to dispersal and gene flow

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Ecological mechanisms maintaining diversity - Niche assembly models

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 Environmental heterogeneity, spp specialise to conditions leads to species coexistence reflecting nature of env variation
1) Trade-offs btw growth rate in high light and survival in low light conditions can lead to pioneer-climax dichotomy in tropical trees
2) Trade-offs among competitive and colonisation abilities allow inferior competitors to coexist w superior competitors as they are better colonisers (Coomes and Grubb 03 seed size and number trade off)
3) Spp segregate along environmental gradients such as soil moisture, nutrients etc
a. Specialisation to particular soil environment- variation contributes to beta-diversity. Distributions are well predicted by soils. Studies- difficult to detect sig soil related associations- large scale study- spatial distribution of 51% trees in Neotropics associated w local diff in soil nutrient distributions (John et al 2007)
b. Environmental history factors and dispersal process operate at different scales- structure community composition

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Ecological mechanisms maintaining diversity- • Spatial and temporal resource availability

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Growth survival trade-off well established among co-existing tropical trees: allow for spp coexistence based on temporal changes in resource availability.

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Ecological mechanisms maintaining diversity- Role of disturbance and productivity

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o Intermediate disturbance hypothesis
o Undisturbed plots- succession- competitive exclusion- low diversity. Disturbance= reversion of community to younger successional states, lets outcompeted spp grow. Disturbance facilitates coexistence in various ways:
 Within patch
• Homogenous closed system
• Disturbance regime provides intermittent opps for recruitment of spp at all points along life history spectrum.
• Fugitive spp can endure periods of severe competition in a dormant or durable life stage- temporal niche sharing/ storage effect
 Between patch
• Incl space and dispersal
• Local disturbance allows recolonisations from patches that are at earlier stages
• Poor dispersal by dominant spp can also slow excl if fugitive spp are v mobile in comparison- can take adv.

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IDH- Molino and Sabatier 01

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- forests where absence of extrinsic disutbrance would= loss of spp

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IDH Bongers 09

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IDH plays lesser role in wet as opposed to dry forest regions in Ghana. Disturbance regimes hard to measure. Local disturbance patterns cant always be readily detected

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Ecological mechanisms maintaining diversity- Lack of competition

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o Dispersal limitation may affect most tropical trees. Potential to prevent competition among spp
o However abundant widespread spp may escape limitation due to heavy seed set, effective dispersal and shade tolerant seedlings
o Absence of direct plant comp in understorey only reduces one avenue to exclusion.

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Ecological mechanisms maintaining diversity- • Negative density dependence

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o Increase in plant performance w increasing isolation from conspecifics
o Janzen Connell hypothesis
 Incidence/ impacts of pests and diseases should be greater a higher host density and among juveniles closer to mother tree.
 Due to non-random density dependent mortality, spatial distribution of host spp will become less aggregated through time
 There should be greater recruitment of susceptible spp within the host plant’s neighbourhood
 Over time, plant diversity greater than expected compared to random establishment and survival
o Support: damping off and canker disease= density/ distance dependent for seedlings of many diverse spp- mediate shifts in spatial patterns of surviving seedlings= more dispersed.
o Hubbell and foster 1990- growth performance of saplings 9/11 tree spp, lower beneath conspecific adults
o Density dependent effects underestimated, negative effects may unfold over longer time intervals and spatial scales than those of studies

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Ecological mechanisms maintaining diversity- Neutral theory and ecological drift

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o Ecological equivalence of spp and random drift processes to slow competitive excl and maintain comm structure.
o Models generate rank abundance curves similar to real ones (Hubbell 2001)
o Not sufficient to explain tropical tree diversity as a period of time pre dating origin of flowering plants needed to explain diversity of trop forests.
o Insufficient knowledge of potential role that multiple biotic interactions and trade-off might contribute to spp differentiation
o Real patterns consistent w niche and neutral models
o Ecological equivalence of spp must be determined directly
o Some niche structuring of communities cannot be doubted but patterns of spp abundances within communities may match that predicted by ecologically neutral drift.