BS3120 - Ecosystems

BS3120 - Ecosystems

1. Diversity

   • Stability
     • Resistance - degree to which a community changes post-disturbance Resilience - Rate of recovery
     • Directly Proportional (Frank & McNaughton 1991)
       • Increased diversity leads to increased stability. Supported the hypothesis of MacArthur (1950s). Heterogeneity of landscape = wide range of trophic pathways.
     • Redundancy (Walker 1992)
       • Species can be lost with the community remaining constant. 
       • Functional Groups
         • Groups of species which perform similar biogeochemical processes. Core biotic or abiotic, and then peripheral. More complex ecosystems are less reliable. Further addition to functional groups are redundant.
       • Criticisms
         • What defines constant? Biomass should remain constant (Walker 1992).
         • Temporal
           • Pollinators are technically redundant when not in spring (Gitay et al. 1996).
   • Richness
     • Estimations of 8.7m sp. (Mora et al. 2011).
     • Species-Area Relationship (MacArthur & Wilson 1967)
       • Larger areas have more species. Greater diversity of habitats, less prone to migratory effects, less prone to invasives, less likely to be isolated.
     • Latitude-Diversity Gradient (Hillebrand 2004)
       • Peak of normal distribution at the equator. 
       • Mid-Domain (Colwell & Lees 2000)
         • Random placement of species within a domain with hard boundaries predicts a peak in the middle, without ecological processes.
       • Evolutionary
         • Three main factors: Organisation, extinction and immigration. 
         • Effective Evolutionary Time (Rohde 1992)
           • Higher energy increases mutations (increased temperature). Larger areas have greater speciation. Higher temporal stability geologically enhances clade persistence.
         • Out Of The Tropics (Jablonski et al. 2006)
           • Tropics are a cradle, with taxa preferentially originating in the tropics and spreading their latitudinal range into the extra-tropics without losing initial distributions. Marine bivalve age increases with 10 degree bin shifts.

2. Disturbance

   • Intermediate Disturbance Hypothesis (Connell 1978)
   • Extinction
     • Climate change (6%), pollution (11%), overharvesting (22%), habitat loss (30%), introduced species (34%).
     • Anthropogenic (Scott 2008)
   • Invasive Species
     • Brown Tree Snake (Fritts & Rodda 1998)
       • B. irregularis > Guam (1950s). Directly and indirectly to native vertebrate extinctions. Almost all birds went extinct.
     • Temporal Scales (Strayer et al. 2006)
       • Most studies carried out within 12 months, vast majority didn't even specify time since invasion occurred. 
       • Red Important Fire Ant (Strayer et al. 2006)
         • Initially causes pop. declines of native insects. However, in some years, native populations have rebounded to pre-invasion levels (12 years later). 
     • Not To Blame?
       • Handful of Detrimental Invasives (Brown & Gurevitch 2004)
         • Pigs, rats, cattle, goats, snakes, some grasses etc.
       • IUCN (Gurevitch & Padilla 2004)
         • Just 6% of IUCN listed species are indirectly or directly threatened by invasives. 30% threatened by habitat loss.
       • Symptoms, Not Causes (Vershuren et al. 2002)
         • Nile Perch + Water Hyacinth invasive to Lake Victoria due to industrialisation causing eutrophication which allowed these species to invade and reduced cichlid pop.

3. Abundance

   • Body Size
   • Rarity
     • Vagrancy
       • Non-resident, breeding species which contribute to 'rare' species.
     • Density
       • Crude - total area of a study/survey. Ecological - area in which organisms of interest were found.
     • Cryptic Species
       • Species which look similar recorded as one species (increase abundance). A. fulrigator butterfly been recorded as 10 species previously! (Hebert et al. 2004).
     • Recorder Effort
       • Reduces supposed abundances in areas which are hard to monitor/haven't been monitored.
   • Niche
     • Fundamental - Potential spectrum Realised - Actual spectrum No two species can occupy the same niche if resources are limiting (Gause).
     • Partitioning
       • Geometric Series
         • Fraction of resource fixed and > 0.5. Competition very important. Each new invader takes a constant fraction of the remaining resource.
       • Broken Stick
         • Proportion of resource random and < 0.5. Competition non-existent.
       • Neutral Model (Hubbell 2001)
         • Biological differences don't exist between animals on similar trophic levels - factors affecting species abundance distributions are purely stochastic. Fundamental biodiversity constant.
       • Geospiza sp. (Grant & Grant 1973)
       • Peppered/Oak Moths (Jaenike & Holt 1991)
   • Conservation
     • Abundant Bioturbators (Solan et al. 2004)
       • Sediment bioturbation levels are disproportionately affected by abundant benthic invertebrates.
     • Commonness Is Rare (Gaston & Fuller 2007)

4. Interactions

   • Food Webs
     • Stability
       • Increased length reduces stability. Equilibrium around 3/4 links. 
     • Energy Channels (Rooney & McCann 2012)
       • Fast energy channel - smaller, faster growing populations with high biomass turnover rates. Slow - greater number of weak links due to more competition Variation allows asynchronicity = stability and predator persistence.
     • Keystone Species
       • Effect of ecosystem is disproportionately great compared to abundance. 
       • Introductions
         • Additive Effects (Van Son & Thiel 2006)
           • Alterations to food web due to direct introduction. MPEs (Soluk 1993).
         • Non-Additive Effects
           • Propagated effects through the food chain
           • Trophic Cascades
             • More likely in less-complex food webs. Introducing carnivores can lead to reduced plant damage, increased plant biomass (Shurin et al. 2006)
             • Timber Wolves (Ripple & Beschta 2011)
               • Reintroduced in 1995. Caused 75% reduction in Aspen grazing due to predation on Elk. Bison, beaver and bear populations all increased.
           • Parity Effects
             • 1 step = decline, 2 steps = growth
   • Herbivores
     • Insect-Herbivore Load
       • Number of insect sp. which attack a given plant species
     • Biodiversity Hotspots (Myers et al. 2000)
       • 44% of plants and 35% of vertebrates live in 25 hotspots. 1.4% of surface. 17/25 hotspots have human pop. > animal pop. (Cincotta et al. 2000).