Community Composition, Interactions, and Productivity

1. Community Composition, Interactions, and Productivity Biodiversity Population Interactions Productivity Controls • Understanding the patterns of and controls on distribution of organisms in aquatic habitats is essential to the study of ecology, particularly in the fields of conservation biology and fisheries management. • Species over-exploitation, habitat destruction, and introduction of exotic (alien) species by human activities has lead to dramatic community alterations and species extinction (locally and globally).

2. Biodiversity • Measures of biological diversity help define patterns and infer controls on community structure over various scales: • spatially (globally to between and within habitats). • temporally (evolutionary to seasonal) • These measures permit monitoring of ecosystem stability and/or impacts from outside disturbance (e.g. human activities). • Species Richness (S) • Total number of species in an area. • Evenness (or equitability; E): • Degree of equal representation for each species. • Shannon-Weaver Index (H’) • Incorporates information on both S and E. • H’ increases when either S or E increases.

3. Where p is the proportion of species j to the total of all individuals (= Nj / N) Where lnS is the maximum diversity; or maximum evenness for S species.

4. Biodiversity over Spatial Scales Within-Habitat (α diversity) versus Between-Habitat (β-diversity) • Consider the two sets of four ponds A-D and E-H. • Overall diversity of each set is similar. • Set A-D has lower α diversity; one species per habitat dominated community. • Set E-H has lower β diversity; little difference in community between habitats.

5. Global Scale Ecoregions: classification of large geographic areas based on their distinct assemblages of natural communities. Information on organisms and abiotic characteristics are considered. Presently, only particular animal taxa (fish, amphibians, crayfish, mussels) are used for distinguishing ecoregions. North America has been divided into 76 ecoregions. (1999)

6. Evolution as the Source of Biodiversity • Uninterrupted time and reproductive isolation are key to evolution of new species. • Few freshwater ecosystems have fulfilled this criteria (contrast marine ecosystems) due to climate variation (e.g., glaciations). • Most freshwater ecosystems have “cosmopolitan” species (wide spread geographically), and few have many “endemic” species (unique to a particular habitat).

7. Tectonic lakes (deep and old) have a much greater proportion of endemic species as compared to glacier lake. • Compare Lake Baikal (high endemic crustacean diversity) and the African Rift Lakes (high endemic teleost diversity). • Both show examples of adaptive radiation (many species from a single founder). Baikal Gammarids (amphipods) Tanganyika Cichlidea family

8. Short-term Variation in Diversity 1) Habitat diversity (many types in a single ecosystem). 2) Size of habitat (positive relationship with diversity). 3) Connectivity of habitats (ecotones; colonization conduits).

9. 4) Sources of recruitment (dormancy and dispersal). 5) Species interactions (specialize to avoid competition; niche). 6) Productivity (timing and location coincident with recruitment). Species space spawning activity to limit competition.

10. Phytoplankton Diversity • Phytoplankton require light, CO2 (inorganic carbon) and nutrients (P, N, etc.) to grow through photosynthesis; most aquatic environments are nutrient limited. • Many species competing for the same nutrient resources in the same areas should lead to competition and ultimately competitive exclusion. • Instead, MANY different species of plankton co-exist at once. This has been termed “The Paradox of the Plankton.”

11. Disturbance • One mechanism proposed to explain this paradox is the fact that lake conditions are not in a state of equilibrium for more than 1 month before the system is disturbed; it would take longer than this for 1 species to become dominant. • Disturbances can be difficult to characterize (vary in magnitude from slight shifts from equilibrium to punctuated events. • Lakes, groundwaters less prone to major disturbance events; but experience seasonal changes. • Streams, rivers, & wetlands experience regular disturbance (flooding, drying, etc.) • Systems prone to disturbance are less likely to achieve a classic “equilibrium” state (climax community); rather “dynamic equilibrium” is more normal.

12. Succession • Succession is the sequence of species colonizing newly available habitat and niches. • The sere (sequence of specific organisms) is based on an organism’s characteristics for colonization (recruitment), growth rate, resource competition, predator avoidance, physicochemical tolerances, disease resistance, and relative community scale. • Over time, the habitat may become modified so to favor the next organisms in the sere (e.g. nutrient depletion shifts competition). • Stages of Succession: • Early invaders: rapid reproducers and colonizers (r selection) • Mid- to late-succession: Better long-term competitors (K selection) • Maximum diversity occurs during mid-succession stages, as both early-stage and late-stage species are present and competing for resources. • Disturbance and succession within a larger ecosystem will favor an increase in diversity up to some limit.

13. Intermediate-Disturbance Hypothesis competition (K) recruitment/ colonization (r)

14. Long-Term Lake Succession“Lake Aging” • Over thousands of years, a newly formed lake will eventually fill with sediments and return to a more terrestrial state, regardless of trophic state. (30m lake at 1 mm/y will take 30,000 y to fill) • Although many exceptions exist; hypothetically lake succession proceeds from oligotrophic→ mesotrophic → eutrophic → senescence (marsh) → terrestrial. • Over decadal scale a subclimax may be observed. • Mean depth, lake size and watershed size and fertility are major facts on controlling the timing of lake succession. • Catastrophic change in watershed, climate, or nutrient loads can rapidly shift subclimax state. • Some manmade impacts on trophic state have been demonstrated to be reversible when appropriately mitigated (i.e. rejuvenation).

15. Population Interactions • Competition for Resources: • Exploitative competition: Both organisms competing for the same resource(s). • Interference competition (amensalism): Organism exert direct, negative effects on another (allelochemical and allelopathy) • Competitive interactions can get interesting when two species compete for more than one resource with differing capabilities. • Predation (mortality): • Prey population declines when growth rates slows below predation rate (and other mortality terms) • Predator Avoidance: • Mechanical defenses: spines, filaments, gelatinous aggregates. • Chemical defenses: allelochemical and allelopathy (taste nasty) • Life history defenses: growth rate / reproduction tradeoff • Behavioral defenses: diel vertical migrations (e.g. zooplankton) • Predator-Prey (Functional Response) Models.

16. Two species competing for Si and P resources. Curves represent growth rate under given nutrient concentrations. Note that the two species differ in their abilities to compete for different resources. Species 1 needs higher [Si] to survive competition. Species 2 needs higher [P] to survive competition.

18. Predator-Prey Models • Type I: e.g. Lotka-Volterra. For a given predator density, prey consumption increases linearly with prey density. • Type II: e.g. Holling disc equation. Includes search and handling time of prey, following structure of Michaelis-Menton equation. (e.g. microbes, zooplankton) • Type III: Introduces concept of “learning” and increase in predator efficiency with increase in prey density. (e.g. fish)

19. Trophic Cascades • Interactions at higher levels of the food chain have a cascading influence down through lower levels. • Bottom-up control: Primary production is controlled by limitations of abiotic factors (light, nutrients, etc.) • Top-down control: Primary production is controlled by predation on herbivores. • Trophic cascades in aquatic systems; e.g. piscivores and phytoplankton biomass. • With piscivore, larger population of zooplankton crustaceans, graze down phytoplankton. • Removal shift dominance to planktivorous fish and loss of large zooplankton and shitch to rotifers; phytoplankton bloom that are resistant to rotifer grazing.

21. Controls of 1º Productivity • Tolerance to temperature, pH and other physical chemical conditions. 2) Light: • Decreases with depth. • Decreases faster with turbid water. • Compensation depth: • Depth when cell photosynthesis = respiration. • More turbidity causes a shallower (lower) compensation depth. • At a depth where only 1% photosynthetic light remains = Euphotic Zone

22. 2. Turbulence (mixing): • Low when stratified. • Population stays in the light and grows. • High when stormy. • Population mixed too deep will die/declines. • Critical depth. • Population alive above • Population death below

23. 3) Nutrients (P, N, Si): • Deep winter mixing replenishes surface nutrients • Stratification minimizes supply from deep waters 4. Grazing: • Refers to the process of primary production being eaten by herbivores (e.g. cow). • Crustaceans like copepods and krill. • Grazing zooplankton populations typically increase after phytoplankton increase.

24. Seasonal Phytoplankton Succession * An annual cycle of species dominance in response to abiotic and biotic factors associated with seasonal changes in a temperate / cold-temperate lake. * Eight stage model: • Mid-winter • Late winter • Spring circulation • Initial summer stratification • Summer “clearwater” phase • Late summer stratification • Fall circulation • Late autumn decline

25. 1. Midwinter • Low temperature • stable water column (inverse thermal stratification) • high light reflectance due to snow cover (low penetration) • moderate to high nutrient availability Phytoplankton community dominated by small, motile, low-light adapted phytoplankton Though not common, in some cases rates of primary production under ice cover can be constitute a significant portion of annual production when there is no snow.

26. 2. Late winter • Low temperature • Stable water column • Moderate to high nutrient availability • Increasing light availability due to longer days, ice melt Rapid increase in motile species, particularly dinoflagellates In lakes that do not ice over (e.g. temperate monomictic lakes), phytoplankton biomass remains low due to deep mixing and decreased light levels.

27. 3. Spring Circulation • Low but increasing temperature • Mixing water column with low stability • Low (but variable and increasing) light availability • high nutrient availability (why?) Rapid growth and increases in phytoplankton biomass, particularly diatoms. Often represents period of highest annual biomass. Increasing light is dominant contributing factor; zooplankton grazing remains low for now.

28. 4. Initial Summer Stratification • Rapidly increasing temperature • Water column stabilizes • Light availability increasing rapidly to maximum • Declining nutrient availability (why and what nutrients?) Phytoplankton biomass declines rapidly due to sedimentation of diatoms, compensated by rapid growth of small flagellates Grazing by zooplankton increases rapidly during this period, due to hatching and response to prey density.

29. 5. Summer “Clearwater” Phase • High temperatures • High water column stability • High light availability • Sharply reduced nutrient availability (why?) Precipitous decline in phytoplankton populations due to nutrient limitations and high zooplankton grazing (clearance rate exceeds reproductive rates). • Zooplankton biomass high due to timing of hatching, high production in response to spring bloom; silica limitation common due to sedimentation of diatoms

30. 6. Late Summer Stratification • High temperature • Stable but decreasing water column stability and deepening of metalimnion • High but decreasing light availability • Low but increasing nutrient availability Increasingly diverse phytoplankton community, especially cyanobacteria and green algae (diatoms still silica-limited)

31. 7. Fall Circulation • Rapidly declining temperatures • Rapid vertical mixing, no water column stability • Decreasing light availability • High nutrient availability (why and what nutrients in particular?) Phytoplankton dominated by large algae, particularly diatoms Zooplankton populations in decline, grazing pressure is reduced.

32. 8. Late Autumn Decline • Low temperature • Decreased mixing of water column • Light availability rapidly declining to annual minimum • Rapidly decreasing nutrient availability (why?) Rapid decline in phytoplankton biomass due to reduction in light and nutrient levels. Grazing rates decreasing to annual minimum

33. Seasonal community structure beyond phytoplankton… Fig. 20.5

34. Seasonal community structure in stream systems…