Life on earth

Learning Objectives

  1. Give some examples of the variety of life on earth and explain how organisms are similar and how they differ (traits), both within and between species.
  2. Notice and explain the various ways that organisms live in the world and meet their requirements for shelter, food, and safety throughout their life cycles.
  3. Recognize and identify ecosystem services in biological examples. 
  4. Know and be able to defend reasons for the importance of maintaining biodiversity.
  5. Explain and apply how ecologists define biodiversity as species richness and evenness and identify higher levels of ecological biodiversity such as biomes.

There is more to life than what we see

The European Honey bee, Apis mellifera, is popular worldwide as a honey producer and crop and plant pollinator. As its common name implies, honey bees are not native to North America, but you’ll find them in every state. Worker bees leave their hives and forage for nectar on flowers in the surrounding 1 to 3.5 mile radius to the hive, returning with their forage to supply the hive with the raw ingredients for bee bread (a mix of pollen and nectar) and honey. While foraging for nectar on plant flowers, bees bump their heads into the sticky flower pollen, which contains plant sperm. At their next plant stop, pollen rubs off of the bee and onto the flower, potentially fertilizing eggs for the plant. Worker bees live 60 days on average, during which time they assist plant sex and also make honey to rear more bees. The worker herself will never mate; she’s a member of a caste system in which only the queens and drones (male bees) have sex and produce offspring. In fact, a worker’s egg-laying organ develops into a stinger instead of an ovipositor (egg depositor).

If a worker bee stings a mammal, like us, the bee injects venom below the skin. Bee venom is not alive but does contain biochemical components like proteins and peptides, pheromones that attract other bees, and histamine, which causes the localized inflammatory response at the site of the sting. The proteins and peptides break down cell walls, causing pain and triggering your immune response. Venom also increases heart rate and adrenal production, among other system-wide effects.

A foraging worker visits flowers from a variety of plant species, from native to non-native, and from wild growing species to those cultivated as crops. As the worker forages, she encounters more than flowers, with their pollen and nectar. She may pick up parasites, become contaminated with pesticides or insecticides, or encounter a predator. The foraging worker acquires parasites like the Varroa mite. Mites are one of many natural sources of disease for bees. Many bee disease are bacterial, like Nosema, or viral, like deformed wing virus (DWV) or chronic bee paralysis virus (CBPV). These are both viruses with an RNA genome, instead of DNA like living organisms. Once inside a living host cell, they use the cell’s machinery to replicate themselves, making more virus and causing disease in the infected bee. Predation is a minor problem for a foraging bee because most predators attack the full hive instead of its sting-bearing workers. While skunks or bears may be more well known as bee predators, the insect predators are more destructive for the bees. For instance, the hive beetle lays its eggs in the honey comb to provision future larvae with honey.

While there are disease-causing bacteria in honey bees, the more common bacteria are beneficial to the bees, residing in the bee crop and gut or cultivated by the bees in the bee bread. Bacteria are employed by bees to preserve pollen for later consumption, fight disease, and preserve the bee bread by preventing growth of fungi, like mold. An example of a bacteria bees cultivate is Lactobacillus, which is one of the bacteria humans use to cultivate yoghurt (Donkersley et al 2018).

In the bee example above, list all of the living organisms that bees interact with, and any non-living elements that they encounter. Are bees neutral players in their interactions, or do they have strong affects on other species?

Life Cycles and Life Histories: be born, reproduce, and die

An organism’s life cycle indicates how new individuals are produced through  reproduction. The image below shows the human life cycle, which involves sexual reproduction. We’ll dig into the details of “diploid” and “haploid” later. For now, recognize that:

  • post-fertilization, a zygote develops into a fetus, with the eventual outcome of a juvenile offspring (a baby, in humans).
  • the baby grows and develops into an reproductively mature adult
  • once reproductive maturity is reached, egg or sperm can be produced
  • if egg and sperm meet, then sperm fertilizes egg to produce a zygote

Not all species have a sexual life cycle: the bacteria that live in the guts of bees and humans do not produce egg and sperm. Furthermore, not all sexual life cycles look like the one above. A bee’s life cycle involves some individuals who are not capable of reproduction, the workers. We’ll see diagrams of life cycles throughout the course to examine similarities and differences in how species reproduce.

The concept of life history addresses why organisms differ in their reproductive strategies because of their evolutionary history. Life history tracks a member of a species from birth to death, helping answer questions like:

  • how many babies should I have at once?
  • how many times should I have babies?
  • how long should I wait to have babies?
  • how long will I live?

These questions are not, of course, being actively pondered by fungi, bacteria, plants, or animals, except humans. Instead, you can think of them as trade-offs for the life time energy budget of an organism. Imagine that each reproductive task costs energy (it does!). If one strategy is to have lots of babies at a young age, the trade-off is a shorter lifespan, which we can flippantly summarize as “live fast, die young.” An alternate strategy involves a long life, but limits the number of kids to 1-2 with each reproductive round. Even though humans can ponder these existential questions, we are still held by the constraints of our life history to have a few offspring in singles (usually). Life histories can evolve, changing over several generations of selection pressure for, say, early reproduction.

Biological Diversity Defined Ecologically

The following Crash Course video gives a quick overview of where organisms live, and why. (Please be familiar with all the vocabulary in this video, except the term “physiognomy.”)

Temperature and moisture are the abiotic factors that dictate which species can survive and thrive in different areas, or biomes. Tropical regions maximize both of those abiotic factors, making the tropics potentially very diverse. For example, in its current state of restoration post civil war in Mozambique, Gorongosa National Park contains two different biomes–savanna grasslands and tropical rain forest–and about 6 ecosystems. The quality of these ecosystems is defined by their biodiversity, the variety of different species found interacting with each other and their environment within the ecosystem.

Diversity can be quantified

Ecologists define biodiversity as two things: species richness and species evenness. Species richness is the count of the number of different species of a type in an ecosystem. For instance, we might go out into metro Atlanta and count all the different bird species we find, from hummingbirds to pigeons. The total count of bird species in Atlanta tops 100 species, according to the 2018 Audubon Christmas Bird Count. Some of those are native (American Robin, Chickadee, Ruby-throated Hummingbird) and other are invasive (House Sparrow, Eurasian Collared Dove). Species richness is just the total count of all these different bird species.

Species evenness accounts for how proportionally represented different bird species are. If every bird species were equally common in Atlanta, then we’d see a lot more hummingbirds and a lot fewer pigeons and sparrows. Instead, pigeons and sparrows are common and have larger population sizes, while hummingbirds are rarer. If all bird species had the same population size in Atlanta, then evenness would be high. Because some birds are common while others are rare, evenness is lower. 

Why is biodiversity important?

No two species are exactly the same. Functionally, though, similar species may provide similar resources. For instance, all legume plants (like beans) fix nitrogen from the air to make it usable for living organisms. Other plants, like trees or grasses, do not fix nitrogen. So, a community of plants that includes a mix of grasses, trees, and legumes is more productive than a monoculture of just one of those types of plants (Tilman et al 1997). More diverse communities also seem to be more resistant to invasion by non-native species, more stable over time, and is able to recover more quickly from a natural disaster—think tornado or hurricane or drought—or a disturbance like a forest fire.

In fall 2019, fires swept through large areas of disturbed Amazon rainforest. Unlike coniferous forests, which burn as part of their natural cycle, undisturbed rainforests are usually too wet to burn. Where rainforest has been cleared for farming, the usual moisture and temperature regulation that the large Amazonian trees provide is absent. Disturbed or cleared areas were more likely to burn in 2019, and allowed fire to spread into adjacent undisturbed forest. Fire accelerated the loss of biodiversity and the damage to the ecosystem services that the forest provides (Montibeller et al 2020). For one, normally a forest is a “carbon sink,” a regulating service wherein carbon resides for many years in trees and out of circulation in the carbon cycle. Burned forest releases carbon into the atmosphere without a naturally occurring countermeasure to sequester the equivalent amount of carbon back into storage. An increase in carbon in the atmosphere is the primary contributor to global warming.

While carbon sinks benefit all life on earth, we use the term ecosystem services to describe ways that humans specifically benefit from biodiversity. As with much other life, humans use the “regulating services” of the earth’s oxygen in the air, water purified by soil and rock, and soil formed by erosion processes. The food grown in that soil grows via the “supporting service” of primary production, where plant biomass is made using photosynthesis of sunlight energy and CO2 from the air. Many plants reproduce by pollination, a supporting service wherein insects (usually) transfer pollen from one plant to another, enabling plants to reproduce.

Humans are also interested in the “provisioning services” that are a resource for new food crops and medicines. Other provisions include fibers that we turn into clothing and other materials, fuel resources like wood and plant-based ethanol, and domesticated animals for food and labor.

Scientists and policy makers have created categories like “provisioning” and “regulating,” but it is not always clear which service gets labeled as regulating versus supporting versus provisioning. For this course, we expect you to recognize and identify ecosystem services when you encounter them. In other words, we would like for you to make connections between science and society. Here’s a list of a few well documented ecosystem services:

Supporting ecosystem services promote general ecosystem function: nutrient cycling, soil formation, photosynthesis, biodiversity, pollination

Provisioning ecosystem services provide extractive resources: food, lumber

Regulating ecosystem services moderate natural phenomena: cool temperatures, control flooding, water purification, carbon sequestration, clean air

Cultural ecosystem services are non-material benefits gained from an ecosystem: sense of place, recreation, education, traditional/indigenous knowledge


Montibeller, B., Kmoch, A., Virro, H. et al. 2020. Increasing fragmentation of forest cover in Brazil’s Legal Amazon from 2001 to 2017. Sci Rep 10, 5803.

Tilman, D., Knops, J., Wedin, D., Reich, P., Ritchie, M., Siemann, E. 1997. The Influence of Functional Diversity and Composition on Ecosystem Processes. Science 29 AUG 1997 : 1300-1302. DOI: 10.1126/science.277.5330.1300