ECOL 182R 1st Edition Lecture 23 Outline of Last Lecture I Predator adaptations A Generalized specialized B Choosing prey II Prey adaptations A Life Dinner Principle III Coevolution IV Community A Food webs Outline of Current Lecture I Chili peppers II Biomass A Consequences B Exceptions III Species diversity vs richness IV Diversity gradients A Hypotheses Current Lecture Why are chili peppers hot The compound Capsaicin Clearly hasn t evolved because we like spicy food But shouldn t the heat deter mutualistic seed dispersers too Bird taste receptors don t detect capsaicin They are the mutualists they disperse the seeds Mammalian taste receptors do They chew up seeds destroy them They are the seed predators Thus the heat is a deterrent to mammal antagonists that is completely undetected by bird mutualists The plant keeps away enemies while not affecting the mutualists Great example of why a community level approach is critical for understanding the world Biomass the weight of living stuff The efficiency with which food is converted to biomass is very low as you move up in trophic levels Producers 1 5 of solar energy biomass Primary consumers 10 of food biomass Secondary consumers 10 of food biomass Tertiary consumers 10 of food biomass Consequence 1 The total number of organisms biomass at any place on earth is limited by the amount of solar energy there So more living things in tropics than at poles And more living things in shallow water than deep Consequence 2 As you move up trophic levels there are fewer organisms and less biomass A pyramid of numbers there s less individual as you move up That s why there are so few species that are high up in food webs there is just less for them to eat Pretty much why dragons don t exist You also get a pyramid of biomass obviously because fewer individuals will weigh less Exception 1 Most forest vegetation is locked up unusable to herbivores bark wood can be eaten by very few species Exception 2 Marine producers represent little biomass whales weigh so much more than everything else Consequence 3 Substances entering at the bottom of the food chain get more concentrated as you move up called biomagnification Species diversity another way to describe compare communities What species are present how many Species richness how many species there are know the difference Problem with species richness places with identical species richness don t seem equally diverse Community 2 seems more diverse because individuals are more evenly spread across species look at lecture slides to understand this sentence Species diversity measure 2 things species richness and the spread of individuals across species A lot more complex than species richness Shannon Wiener Diversity Index it takes into account how many individuals the total number of individuals it s a weighted measure of the number of individuals by the number of species More diverse communities will have a higher H value Patterns in space Distribution of an individual genus Comparisons of species richness over space Diversity Gradients Diversity richness caries in predictable ways as you move globally scientific literature sometimes uses these words interchangeably Altitudinal gradient fewer species at higher altitudes Ocean depth gradient MORE species in DEEPER water but LESS biomass Latitudinal gradient More species as you move towards the tropics Anomaly 1 Higher number than expected where topography is complex Anomaly 2 Lower number than expected on peninsulas Exceptions Species evolved in adapted to cold climates are least common in tropics Also desert mountain ranges have low species numbers at top and at bottom too Why do diversity gradients exist 1 Productivity hypothesis more species can be supported where plants are more productive where there s more biomass there are more species Doesn t explain why some gradients are reversed Also doesn t tell us why shallow waters have high biomass but low species number Also doesn t tell us why polluted water has really high biomass and low species number 2 Time stability hypothesis species rich areas have been more stable over time But doesn t explain why tropics DO have high disturbance And doesn t explain the lack of disturbance now thought to lower diversity
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