Ecology is the study of relationships of organisms with one another and their environment. Organisms are organized in populations, communities, ecosystems, biomes and the biosphere.
A population of organisms is a sum of all individuals of a single species living in one area at one time.
Individuals in a population can occupy space in three basic patterns: clumped spacing, random spacing and uniform spacing.
Metapopulations are collections of populations of the same species spread over a greater geographic area. There is some migration (ths gene-flow) between populations. Larger populations are sources and smaller populations are sinks of individuals within a metapopulation.
Population size is determined by four general factors: natality, mortality, immigration and emigration.
Natality depends on a number of factors: the proportion of the population that are at a reproductive age (as opposed to pre-reproductive and post-reproductive), proportion of the reproductively mature individuals that get to reproduce, sex-ratio of the reproductives, the mating system, the fertility of individuals (sometimes affected by parasites), the fecundity (number of offspring per female), the maturation rate (the amount of time needed for an individual to attaint sexual maturity), and longevity (amount of time an individual can live after reproducing).
Mortality is affected by bad weather, predation, parasitism and infectious diseases. It depends on the mortality of pre-reproductive stages (from eggs and embryos, through larva and juveniles), mortality of reproductive stages, and mortality of post-reproductive stages (often from disease or aging).
A population can, theoretically, grow exponentially indefinitely. However, in the real world, the growth is limited by the amount of space, food (energy) and predators. Thus, the population size often plateaus at an optimal number – the carrying capacity of that population.
Some organisms produce a large number of progeny, most of which do not make it to maturity. This is r-strategy. The population size of such species often fluctuates in boom-and-bust patterns.
Other organisms produce a small number of progeny and make a heavy investment into parenting and protecting each offspring, This is K-strategy. The population size of such species grows more slowly and tends to stabilize around the carrying capacity.
All populations show small year-to-year fluctuations of population sizes around the optimum number. Some species, however, exhibit regular oscillations in population sizes. Such oscillations often involve populations of two different species, usually a predator and its prey, the most famous example being that of the snowshoe hare and the lynx.
Correct prediction of future changes in a population size is essential for the assessment of the populations viability and for its protection.
A biological community is a collection of all individuals of all species in a particular area. Those species interact with each other in various ways, and have evolved adaptations to life in each others’ presence.
Niche is a term that describes a life-role, or job-description, or one species’ position in the community. An example may be a large herbivore, a nocturnal burrowing seed-eater, a seasonal fruit-eater, etc.
Within one community only one species can occupy any particular niche. If two species share some of their niche, they are in competition with each other. If two species occupy an identical niche, they cannot coexist – one of the species will be forced to move out or go extinct.
If two species compete for the same resource (food, territory, etc.), one will utilize the resource better than the other. Competitive exclusion is a process in which one species drives another species out of the community.
Complete exclusion is not inevitable. The competition between two species can be reduced by natural selection, i.e., one of the species will be forced to assume a slightly different niche. For instant, two species can geographically partition the territory, e.g., one living at higher altitude than the other on the same mountain-side. Two species can also temporally partition the niches, for instance one remaining active at night and the other becoming active during the day.
Predation is one of the most important interaction between species in a community. Predation often causes evolutionary arms-races between predators and prey. For instance, by killing the slowest zebras, lions select for greater speed in zebras. Greater speed in zebras selects for greater speed in lions.
The most interesting examples of evolutionary arms-races between pairs of enemies are those in which the prey is dangerous to the predator, often by being toxic or venomous. For example, garter snakes and tiger salamanders on the West coast are involved in one such arms-race. Prey – the salamander – secrete tetrodotoxin from its skin. This toxin paralyzes the snake. Locally, some snakes have evolved an ability to tolerate the toxin, but the side-effect of such evolution is that these snakes are slow and sluggish – themselves more vulnerable to predation by birds.
Ground squirrels (prey) in the Western deserts have evolved immunity to rattlesnake venom, so the rattlesnakes (predators) are becoming more venomous. Similarly, and in the same area, desert mice have evolved immunity to the toxin of their prey – the scorpions, resulting in increasing toxicity of the scorpion venom in that region (but not in areas where these two species do not overlap). A Death’s-head sphynx moth steals honey from beehives and has evolved partial immunity to honey-bee venom.
Many plants have evolved thorns or toxic chemicals to ward off their enemies – the herbivores. Monarch butterflies are capable of feeding on milkweed despite this plant’s toxic content. Moreover, the Monarchs store the noxious chemical they extracted from milkweed and that chemical makes the butterflies distasteful to their own predators.
The shape and color of the prey often evolves to protect from predation. Warning coloration, usually in very bright colors, informs the predators that the prey is dangerous. Aposomatic coloration is one commonly found kind of warning coloration – the black and yellow stripes on the bodies of many bees and wasps are almost a universal code for dangerous venomous stings.
Cryptic coloration, or camouflage, on the other hand, allows an animal to blend in with its surroundings. Many insect look like twigs, leaves or flowers, effectively hiding them from the eyes of predators. Some animals have evolved behavioral color-change, e.g., chameleons, some species of cuttlefish and the flounder.
Batesian mimicry is a phenomenon in which non-toxic species evolve to resemble a toxic species. Thus, some butterflies look very similar to Monarch butterflies and some defenseless flies and ants have aposomatic coloration.
Mullerian mimicry is a phenomenon in which two or more dangerous species evolve to look alike. This is “safety in numbers” strategy as a predator who tastes and spits out one of them, will learn to avoid all of them in the future.
Co-evolution does not occur only between enemies. It can also occur between species that positively affect each other. The best example is co-evolution of flowers and insect pollinators.
Symbiosis is a relationship between organisms that are not direct enemies (e.g,. predator and prey) to each other. Commensalism, mutualism and parasitism are forms of symbiosis.
In commensalism, one partner benefits, while the other one is not affected at all. For instance, birds building nests in a tree do not in any way affect the fitness of the tree.
Mutualism benefits both partners. The best known examples are lichens, mycorrhizae, and legumes. Birds that clean the skin or teeth of crocodiles, hippos or rhinos are protected by their hosts.
Parasitism is detrimental to one of the partners. Parasites that are too dangerous, i.e., those that kill their host, are not successful since they also die without leaving offspring. Thus, parasites evolve to be minimally harmful to their hosts. The same logic goes for infectious agents – the disease should help propagate the microorganism (e.g, by causing sneezing, diarrhea, etc.) without killing the host.
The organisms that make up ecosystems change over time as the physical and biological structure of the ecosystem changes. Right now, one of the effects of global warming is that some species migrate and others do not. Thus, old ecosystems break down and new ones are formed. The ecosystems are in a process of remodelling. During that process, many species are expected to go extinct.
When an ecosystem is disturbed to some extent, but not completely eradicated, the remodelling process that follows is called primary succession.
When an ecosystem is completely wiped out (e.g,. a volcanic eruption on an island), secondary succession occurs, with a predictable order in which species can recolonize the space. One species prepares the ground (quite literally) for the next one. The process may start with bacteria, lichens and molds, continuing with mosses, fungi, ferns and some insects, etc, finally ending with trees, birds and large mammals. The final structure of the ecosystem is quite stable over time – this is a mature ecosystem.
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapters 26, 27, 28 and 29