Ecology Employs the Scientific Method
induction observation hypothesis or model experiment prediction deduction 60 1 What is an hypothesis? An hypothesis is an idea about how theworld works: e.g., Frogs sing on warm nightsafter periods of rain. We often wish to understand twocomponents of such a phenomenon: how? (encompasses physiologicalprocesses) [how does a frogrespond to temperature andrainfall?] why? (encompasses costs andbenefits of the behavior to theindividual) (how to answer..) 61 2 Experiments test predictions.
Hypotheses generate predictions: if observations confirm the prediction, the hypothesis isstrengthened (not proven) if observations fail to confirm the prediction, the hypothesis isweakened (or rejected) Best tests of hypotheses are experiments: independently manipulate one/few variables (or trick frogsinto singing on a wrong night) establish appropriate controls 62 3 Some Potential Pitfalls
A correlation between variables does not establish causation. Many hypotheses cannot be tested by experimental methodsbecause: the scale is too large: patterns may have evolved over long periods the spatial extent is too large for manipulation causal factors cannot be independently tested 63 4 Some Approaches to Difficult Problems
Mathematical models are powerful tools: researcher portrays system as set of equations model is an hypothesis and yields predictions that can be tested;examples include: models of disease spread models of global carbon Microcosms are sometimes useful: microcosms replicate essential features of the system in a laboratoryor field setting 64 5 Microcosms communities of freshwater invertebrates
6 Hypothesis: bird predation on insect herbivores reduces the amount of leaf area consumed. Field Study: construct bird-proof cages to allow insects freedom of movement. 7 Yes: insects increased 70% -> leaf area % lost increased 22% to 35%
So? Leads to important question: Will the decreases in bird populations caused by fragmentationof forests in the eastern US and elsewhere result in increasedinsect damage to forests? Other questions? 67 8 Statistics and scientific rigor
True of false: you can prove anything with statistics You cannot prove anything with statistics Statistical analysis attaches a level of confidence toconclusions that can be drawn In ecology as in all sciences we search for confidentconclusions, not provable truths Rigorous (accurate, exhaustive) Science
Based on conclusions that are the results of investigationscarried out with the express purpose of deriving thoseconclusions Based on conclusions to which a level of confidence can beattached, measured on an agreed scale -- note Boxes 1.2 and 1.3 and 1.4 Ecology in practice Lets examine some more real research programs
3 main points Ecological phenomena occur at a variety of scales Ecological evidence comes from a variety of different sources Ecology relies on truly scientific evidence and the application ofstatistics Brown troutin NZ Effects on individuals,populations, communitiesand ecosystems Understanding enhanced(naturally) when linksbetween all these levels aremade clear(er) Brown trout (Salmo trutta) introduced to NZ in from Europe Much can be learned bycomparing ecology ofstreams containing troutwith those occupied bynon-migratory native fish inthe genus Galaxias Question: is the nativeGalaxias (on the right)hiding from the introducedpredator? Start here Brown Trout: examining the individual level
What are the consequencesfor invertebrate feedingbehavior? Mayfly nymphs of variousspecies: are theredifferences in their activityrhythms depending onwhether they are inGalaxias or trout steams? In a Galaxias stream: activeboth day and night In a trout stream: daytimeactivity most reduced Brown Trout: examining the individual level
Trout rely primarily onvision to capture prey Galaxias rely on mechanicalcues Thus: invertebrates in atrout stream more at risk ofpredation during daylighthours Conclusions derived fromboth readily controlledconditions of a labexperiment and from themore realistic and morevariable circumstances of afield experiment Brown trout: population level
How does brown trout impact the distribution of native fish? At each site, a variety ofphysical variables weremeasured Using statistics, ?: whichphysical variables (if any)distinguished one type of sitefrom another ? A: Galaxias restricted to sitesupstream of waterfalls [cannotbe climbed by trout]. Why?Direct predation by trout onnative fish below waterfall 198 sites selected. Streams of similardimensions chosen atrandom in each of 3tributaries from each of 8subcatchments of the river Sites: (1) no fish; (2)Galaxias only; (3) trout only;(4) both Galaxias and trout Brown trout: community level
? Do these changes have community consequences thatimpact other species ? Do trout affect the streamfood web differently fromthe displaced Galaxias? 3 treatments established (no fish;Galaxias; trout) at naturallyoccurring densities in severalrandomized blocks in a stretch ofstream. Algae and invertebrates and then fish introduced andthen algae and invertebratessampled Trout -> lowerinvertebrates -> higheralgae Brown trout: ecosystem and energy flow
? Will the rate at whichradiation energy is capturedthrough photosynthesis bythe algae be greater in thetrout stream? Annual net primaryproduction (rate of productionof plant i.e. algal biomass)six times greater in the troutstream than in the Galaxiasstream ? In the trout stream, will thehigher primary productionbe associated with a fasterrate of uptake by algae ofplant nutrients (nitrate,ammonium, phosphate)from the flowing streamwater? Also yes. Conclusion: a trophic cascadeis responsible for the patternsobserved at the ecosystemlevel Succession What is the natural successional sequence of plants?
From this understanding:an artificial manipulationcan be planned; historicalrecords can be examined Ecological succession what is it? Excellent place to study:old agricultural fields inthe eastern US,abandoned by farmers What have the studies atCedar Creek illustrated? 22 fields at differentstages in an old-fieldsuccession(abandonedbetween 1927 and1982) were surveyedin 1983; 22snapshots Correlations! ?: are thecorrelations in (a) (d) the result of aneffect of field age or is the causalagent nitrogen withwhich age iscorrelated? Artificial experiments: search for causation
6 replications. Two fields (one abandoned for 46 yrs; one for 14 yrs);nitrogen manipulations Questions: (1) do patches receiving different supply rates of nitrogen becomeless similar in species composition over time? Yes. But 10 years later, plots receiving different amounts of N haddiverged in species composition. The greater the difference in N input,the greater the divergence (2) do patches receiving similar supply rates of nitrogen becomemore similar in species composition over time? At the start, different. After 10 years, plots within the two fieldssubjected to similar rates of N had become remarkably similar Artificial experiments: search for causation
Time itself is not the only cause of successional changes inspecies composition Differences in available nitrogen cause successions to diverge;similarities cause them to converge much more quickly thanthey would otherwise do Time (= opportunity to colonize) and nitrogen are clearlyintimately intertwined. More unanswered ecological questions. Know the Hubbard Brook study A modeling study: why Asian vultures were heading for extinction?
Vulture populations were declining by 22 to 50 %each year Loss of vultures ? And ? Common element: each had suffered fromvisceral gout (uric acid accumulation) followed bykidney failure. Why? Carcasses of domesticanimals treated with diclofenac were lethal tocaptive vultures (diclofenac is a non-steroidal anti- inflammatory drug (NSAID) and when given toworking animals it can reduce joint pain and sokeep them working for longer. ) Other elements? A modeling study: what proportion of carcasses (C) would have to contain lethal doses of diclofena to cause the observed population declines? Physical and Biological Principles 1
Ecological systems are physical entities: life builds on physical properties and chemical reactions ofmatter all processes obey the physical laws of thermodynamics (?) but life still pursues many varied options 54 25 Physical and Biological Principles 2
Ecological systems exist in dynamic steady states: despite substantial fluxes of energy and matter, ecologicalsystems remain more or less unchanged gains and losses are more or less balanced steady states apply to fluxes of materials and energy at alllevels of ecological organization 55 26 Physical and Biological Principles 3
The maintenance of living systems requires theexpenditure of energy: life forms exist out of equilibrium with their physicalenvironment losses must be replaced by energy or materials procured bythe organism the price of maintaining a dynamic steady state is energy 56 27 Physical and Biological Principles 4
Ecological systems undergo evolutionary changethrough time: physical and chemical properties, and physical laws, are immutable,but life exhibits remarkable diversity structures and functions of organisms (adaptations) areevolutionary products of natural selection, recognized by CharlesDarwin complexity builds upon complexity 57 28 Costa Rican mantids cryptic coloration
58 29 Summary Ecology is the scientific study of the natural environment and the relationships oforganisms to one another and to their surroundings. Ecologists study a variety of organisms and processes, spanning a wide range of spatialand temporal scales. Individual organisms live in habitats and have unique niches reflecting conditionstolerated and functional role. All ecological systems obey natural laws and are subject to evolutionary change. Ecologists employ the scientific method. Humans are part of the global ecosystem and have created numerous environmentalproblems.Solving these problems will require application of ecological principles. 71 30