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Lecture 10

EESC 3310 Lecture 10: Week 10

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Earth & Environmental Sciences
EESC 3310

Community Diversity, Disturbance, & Stability ● Community Diversity ○ Species richness: # of different species ○ Evenness: relative distribution of species in the community - opposite of dominance ○ Spatial diversity: vertical (light management, canopy) and horizontal structure (overall more ecological niches open up and are created) ○ Temporal diversity: degree of cyclical changes; crop rotations ○ Structural diversity: number of locations (niches and trophic levels) and connections between trophic groups ○ Functional diversity: ecological processes (nutrient cycling, energy flow, biological control) ○ Alpha diversity is just the diversity of each site (local species pool). Beta diversity represents the differences in species composition among sites. ● Productivity of an Intercropped System ○ Land equivalent ratio (LER) ■ Σ (Ypi Ymi ■ Y = yield polyculture pi ■ Ymi yield monoculture ■ Looks at proportion of yield in poly vs. mono and adds them up ■ Take Partial LER for each crop (just LER formula for Crop A, and LER for Crop B) -- should be <1 ■ Then add up partial LERs to get LER (over 1) ■ Ex. Crop A Partial LER = .83, Crop B Partial LER = .80 so LER = .83 + .80 = 1.63 ○ Theory of Island Biogeography ■ Fumigated mangrove islands which killed invertebrates ■ Looked at how ecosystems/# of species reacted to this disturbance ■ Where your immigration = extinction → equilibrium in # of species ■ Colonization increases as distance to mainland decreases ■ Extinction increases as island size decreases ■ Larger island → more niches for species to get established → lowers your extinction rate ■ Your small far islands are gonna have much less species richness than big islands/islands close to the mainland ■ Applying the Theory to Land ● “Mainland” → source pool of species → matrix ● Patches of land are your “islands” ● Can help management of ag fields to make ag land less hospitable for pests and creating more natural habitat to support natural enemies ● Doutt and Nakata, 1965 ○ One of first accounts of successful conservation biological control ○ Vineyard pest overwinters in adult phase so it can’t be infested by pest until eggs are laid in spring whereas alternate host overwinters in egg stage so it can be ○ Due to this different life cycles of the hosts the parasite often attacks the alternate host which led to increased population of vineyard pest in spring time which destroyed grapes ○ Was due to blackberries in the field which attracted the alternate host ● In Field Annual Flower Strips ○ Takes pollen out of field ○ Syrphid flies need pollen to successfully reproduce ○ So when you add alyssum it decreases number of aphids which increases lettuce growth ● Combining Tactics in a Vineyard ○ Source pool of species is called matrix ○ Corridor resulted in correlation of lower pests and more predators ● Diversity Indices ○ Shannon Index (H’) ■ Minimum value = 0 (low diversity ■ Proportion(P) = n/N ■ Then take Ln(P) ■ Then multiply P*Ln(P) ■ Ex. Say Species A is made up of species with #s 2, 4, 6, 8 ● Your P of Species 1 = n/N = 2/20 ● Then take Ln(2/20) ● Then multiply 2/20* Ln(2/20) ○ Simpson’’s Reciprocal Index ■ Minimum value =1 (low diversity) ○ Where n = total abundance of a particular species, N = total abundance of all species ● Evenness of Natural Enemies ○ When you use organic evenness generally is greater than when using conventional ○ As evenness increased ■ Number of retrieved predators increased ● Which caused Number of herbivores to decline ○ Which caused Plant biomass increased ■ Caused a trophic cascade ■ So being more even had an ecological function → greater yield for that plant ● Disturbance ○ Natural: fire, landslide, tree fall, volcanic eruption ○ Fire ■ Swidden: Sustainable due to fallow period ○ Tillage ○ Harvesting ○ Flooding ○ After Disturbance: Natural Stages of Succession ■ Recovery stage ● R-selected species colonize and modify environmental conditions ■ K-selected species come back ○ Types of Succession: ■ Primary Succession: occurs where soil has not formed and seed bank is empty ■ Secondary succession: more common in agroecosystems ■ Climax Community: ending community after succession has restored ecosystem ○ Why do we care about succession in agroecology? ■ Keep systems in early stages of succession produces large amounts of harvestable material
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