1.1 Definition and scales #
1.1.1 Three definitions, one job #
Haeckel coined ecology in 1866 from the Greek for household: the relations of an organism to its organic and inorganic surroundings. Andrewartha and Birch later defined it as the study of the distribution and abundance of organisms. Krebs added the interactions that determine that distribution and abundance. The third wording is the one these notes use. A map of where a species lives is a result. The subject is why the map has that edge, why the numbers are those numbers, and what happens to both when a neighbour, a nutrient, or the climate changes.
“Environment” includes abiotic conditions and other organisms. A predator is as much a part of a herbivore’s environment as temperature is. A definition that stops at “the green background” cannot ask about competition or disease.
1.1.2 Scales, and the question each one can hold #
An individual is born, grows, reproduces, and dies. Its physiology is the neighbouring subjects. Ecology asks how its survival and reproduction change along a gradient.
A population is a group of individuals of one species in a stated area, counted for a stated question. Gene flow and the fate of alleles inside it are Evolutionary Biology and Genetics. Ecological drift, below, is a sampling of individuals, not of gene copies.
A community is the populations in a place that can interact. The boundary is drawn by the investigator: the birds of a woodlot, the invertebrates of one tide pool, the microbes of a rhizosphere. A community has composition and relative abundances. It does not have a physiology of its own.
An ecosystem is those organisms plus the abiotic pools and the fluxes among them. Energy flow and element cycling are ecosystem questions even when the mechanism is a single metabolic guild.
A landscape is a mosaic of ecosystems, with patches, corridors, and a matrix between them. Area, isolation, and edge are landscape questions.
A biome is a climate-scale vegetation class, recognised by physiognomy. Tropical rainforests on different continents share a climate and a structure. They do not share a species list.
The biosphere is the global sum of ecosystems, including the atmosphere and the oceans insofar as organisms exchange gases with them.
A question that jumps scale usually becomes nonsense. “The carrying capacity of the Earth” is not the K of one logistic population. “The diversity of a forest” is unfinished until alpha, beta, and gamma have been distinguished (Chapter 6). Choose the scale before choosing the method.
1.1.3 What these notes leave in the neighbouring subjects #
Selection, drift, speciation, phylogeny, and deep time stay in Evolutionary Biology. The Calvin cycle, C4 and CAM, water potential, and hormone responses stay in Plant Physiology. Organ anatomy, alternation of generations, and crop domestication stay in Plant Biology. Nitrogenase, nitrifiers, methanogens, and the Winogradsky column stay in Microbiology. These notes call those results by name when a flux or a mutualism would otherwise have no agent.
1.2 A history that is still an argument #
1.2.1 Before the word #
Alexander von Humboldt, in the early nineteenth century, tied vegetation to climate with elevation and latitude. That comparative habit is Chapter 9. Darwin’s image of a bank of plants, birds, and insects, “dependent on each other in so complex a manner,” is a community sentence. The mechanism he proposed for how the inhabitants came to be those species is Evolutionary Biology, not a second copy of it here. Warming’s plant ecology, at the end of that century, asked how plant form tracks habitat. Haeckel supplied the name. He did not supply the modern experimental programme.
1.2.2 Two arguments that organise the later chapters #
Clements, in Plant Succession (1916), treated the community as a kind of organism with a development and a climax determined by climate. Gleason, in 1926, answered that species are individualistic: each has its own tolerances, and a community is the overlap of their distributions. The modern reading is closer to Gleason. Species do have individual ranges. There are also real assembly patterns, so “individualistic” is not “unpredictable.” Tansley, in 1935, coined ecosystem for the organisms together with the physical system, and he rejected the organism metaphor as a literal description. Chapter 6 keeps his warning. A community can be stable, in a defined sense, without being a superorganism.
Late Jurassic 150 Ma Koeppen Climate Types If Co2 1000 Ppmv 1 1 1 1
Elton, in Animal Ecology (1927), put food chains, the pyramid of numbers, and the niche-as-role into the subject. Lindeman, in 1942, moved the currency from numbers to energy. Hutchinson, in 1957, made the niche an n-dimensional volume. The Odums’ textbooks made ecosystem budgets a teaching framework. Lotka and Volterra supplied equations for competition and predation that are still the first models, because their assumptions are visible. MacArthur and Wilson, in 1967, did the same for islands. A model in these notes is a hypothesis with assumptions. It is not a law that a named forest is obliged to obey.
1.2.3 How a claim is tested #
Observation finds the pattern. An experiment, a comparison, or a model that can fail distinguishes processes that produce the same pattern. Connell’s cages on a rocky shore (Chapter 6) are the type of an ecological experiment: a distribution that looks like a physiological limit was partly a competitor. Models are tested by whether their assumptions hold and whether their qualitative predictions survive, not by curve-fitting alone.
Hurlbert, in 1984, named pseudoreplication: treating multiple samples from one treated lake and one control lake as if they were many independent tests of the treatment. The treatment was not replicated. The samples estimate what is in those two lakes. Scale can also change the answer. A process that dominates a square metre need not dominate a region. Wiens emphasised that the scale of the study is part of the result. These notes describe that logic. They do not give a field protocol.
1.3 Pattern, process, and the null #
1.3.1 What has to be explained #
Distribution and abundance are the two patterns. A mechanism is a process that could produce them and that would be rejected if a stated observation went the other way. “The species is adapted to its habitat” is not yet an explanation. Adaptation, as a tested historical hypothesis, is Evolutionary Biology. Here the ecological content is which factor sets the edge and which factor sets the density.
Correlation of abundance with a factor is a start. The factor may be a proxy for another (wet sites are also nutrient-rich, or also full of a predator’s alternative prey). A mechanism needs a link that can be broken: remove the predator, add the nutrient, transplant beyond the range.
1.3.2 Null models, including neutrality #
A null model asks what the pattern would be if the process of interest were absent. Diamond argued that island birds form “assembly rules,” including checkerboard distributions of competing species. Connor and Simberloff answered that some of those checkerboards appear in random placements that preserve the row and column totals. The lesson these notes keep is the method. A pattern that the null produces is not evidence for the favoured process. A pattern the null cannot produce is worth a mechanism.
Hubbell’s neutral theory is a null for communities of trophically similar species. Individuals of different species are treated as identical in per-capita birth, death, and dispersal. Diversity is maintained by speciation and by ecological drift, the chance loss of individuals. The theory does not claim that niches are imaginary. It claims that a niche story has to explain more than drift and dispersal already explain. Ecological drift is analogous in form to genetic drift and is not the same process. One samples individuals. The other samples gene copies. Genetic drift remains Evolutionary Biology.
Key terms. Distribution; abundance; population; community; ecosystem; landscape; biome; biosphere; individualistic hypothesis; pseudoreplication; null model; ecological drift; neutral theory.
Summary. Ecology explains distribution and abundance by interactions with conditions and with other organisms. The scale is part of the question: a biome is not a community, and an ecosystem budget is not a life table. Clements’s organism metaphor lost to Gleason’s individualistic species and to Tansley’s ecosystem, but assembly is still real. Models from Lotka, Volterra, and MacArthur are tools with assumptions. A pattern is not a process, and a null model, including neutral theory, is the comparison a mechanism has to beat.
Self-check.
1. Which of Krebs’s wording is missing from “ecology studies where organisms live,” and why does the gap matter for predation?
2. Why is the carrying capacity of one population not “the carrying capacity of the biosphere”?
3. What did Gleason deny in Clements, and what did Tansley add that neither of them had named?
4. A study takes twenty water samples from one fertilised lake and twenty from one unfertilised lake. Why is that not twenty replicates of fertilisation?
5. How does ecological drift differ from genetic drift, and what does neutral theory not claim?
