Ecology Lecture 10 - Communities & Ecosystems Food Webs (Fall 2023) PDF

Multispecies Interactions and Food Webs Indirect effects: effects of linked interactions among more than two species Occur when an interaction between two species is affected by another species One species affects a second species through a change in an intermediate, or m...

Multispecies Interactions and Food Webs Indirect effects: effects of linked interactions among more than two species Occur when an interaction between two species is affected by another species One species affects a second species through a change in an intermediate, or mediating, species Complex interactions! 1  Example: Entangling Exploitation with Competition Park found the presence/absence of a protozoan parasite (Adelina tribolii) influences competition in flour beetles (Tribolium).  Adelina lives as an intercellular parasite.  Reduces density of T. castaneum but has little effect on T. confusum.  T. castaneum is usually the strongest competitor, but with the presence of Adelina, T. confusum becomes strongest competitor. 2 From: Klingler and Bucher (2022): The red flour beetle T. castaneum: elaborate genetic toolkit and unbiased large scale RNAi screening to study insect biology and evolution 3 4  Food Chains Feeding interactions (trophic interactions) Food chain Primary Producer Primary Consumer Secondary Consumer Tertiary Consumer Top (apex) Predator Trophic Level Energy Flow Arrows represent the flow of energy from one trophic level to the next 5 Four-level Food Chain – Coachella Valley 6 Before non-native species were introduced, like the signal crayfish; before the food chain (community) was altered. 7  Trophic Energy Transfer Efficiency Start with primary producers, like plants: The availability and intensity of sunlight in different habitats, the availability of carbon dioxide or water, and the speed of chemical reactions all place limits on the rate of photosynthesis. 8  Trophic Energy Transfer Efficiency Heterotrophs are limited by the abundance of their food Thus, the biomass of primary producers should affect the biomass of herbivores, which in turn should affect the biomass of predators Of the energy that is taken in, only a fraction is used in production (converted to tissue) 9  Trophic Energy Transfer Efficiency Only 15% of the total energy consumed is turned into new boar tissue and is theoretically available to predators at the next higher trophic level (LOW energy transfer efficiency)10  Trophic Pyramids Inverted biomass pyramid in an aquatic ecosystem (standing crop of phytoplankton lower that primary consumer) The inefficiency of energy transferred up the food chain means that the top predator’s biomass is derived from an incredibly large amount of phytoplankton at the base 11  Bottom-up and Top-Down Control Bottom-up control: Organisms at the basal trophic levels (autotrophs) control the upward flow of energy, thereby constraining the abundance of species at the higher trophic positions More photosynthesis would mean more energy acquired; energy that can be moved up to higher trophic levels Phytoplankton growth could be increased through the addition of nutrients (N or P) 12 Top-Down Control: Influence of a Predator Trophic Cascade Effects of this top-down regulation appear to “cascade” (or propagate) down the food chain 13 Bottom-Up vs. Top-Down Addition of predators to an ecosystem is a form of biomanipulation (management) in systems that have been impacted by eutrophication (excessive nutrient input) 14 Oligotrophic vs. Eutrophic 15  Food Webs: Adding Compexity Omnivores vs. Trophic Specialists Ontogenetic diet shifts Detritivores Decomposers Scavengers 16  Food Webs Graphical representation of the energetic feeding relationships among species that coexist in a community  Complex conceptual models of multispecies interactions  Simplified food web: Scorpions actually consume more than 100 different arthropods! 17  Complex Food Webs Three-dimensional ball-and-stick model of the food web for El Verde rainforest in Puerto Rico Species are color- coded into functional groups, or trophic nodes Basal (red nodes), intermediate (orange nodes), and top predatory species (yellow nodes) 18  Trophic Subsidies Food webs sometimes receive much of its energy from an outside source Allochthonous inputs Island food webs in the Gulf of California are subsidized by marine sources (dead marine animals, algal wrack, and waste from seabird colonies) 19  Trophic Subsidies Allochthonous input (exogenous energy source) in shaded headwater streams Endogenous (within system) production (autochthonous production) in open mid-reach streams 20  Strongly Interacting Species Keystone Species: exert strong effects on their community structure, despite low biomass 1) Must have a large effect on community structure, function, and diversity 2) That effect must be disproportionately large relative to the biomass or abundance of the species Examples (often top predators): gray wolves in Yellowstone National Park, sea stars in the intertidal zone, sea otters in the kelp forests along the Pacific coast, tiger sharks in the marine food webs of Australia’s Shark Bay, and mountain lions in the Yosemite National Park Ecosystem 21 Keystone Species Keystone Predators Mutualistic Keystones (keystone mutualists) Green-backed firecrown hummingbird in the South American Patagonian grasslands 22