In the previous section on Daisyworld, the interactions among daisies appear very simple. Yet Daisyworld has some very interesting behaviors due to feedbacks among black and white daisies and temperature . In real ecosystems, the interactions are more varied and complex. These interactions commonly produce feedbacks in the respective populations of various species. Competition for resources and predation can limit populations of organisms. Mutually beneficial relationships can increase the populations of both species interacting. And the organisms influence their environment, shaping Earth's surface and the resources available to ecosystems. Thus, interactions among species are a major component of how like on Earth persists and evolves through time.
Some Interaction Types
Most interactions between species have produce harms and benefits for the organisms involved, but sometimes the results are closer to neutral. There are typically 6 types of interactions when considering the harms and benefits to each species (Figure), but there are also other ways to frame interactions (see this Khan Academy video for example).
Figure: The benefits and harms to organisms with different types of interactions. Predation has the equivalent effects as parasitism in this diagram. (Diagram by Ian Alexander, Wikipedia, CC BY-SA 4.0)
The following sections include these interactions types, with examples. For the first three interactions types, mutualism, neutralism, and competition, the effects of the interaction are similar on both of the species. For the next three, the effects are asymmetric, with one species benefiting more than the other from the interaction. Note that these categories are neither the only interaction types nor are they able to capture the intricacies of actual interactions among organisms. For example, how does an opossum pushing a skunk into the water fit within this classification? (Confession: I just wanted to include a link to the video.)
Many interactions result in benefits to both organisms, and for a relationship to be mutually beneficial, both species need to do better when interacting with the other. These relationships include organisms providing resources and protection for each other. Ecosystems are commonly built around these relationships, and mutualistic relationships allow large amounts of biomass to accumulate, say in a rainforest. Organisms helping each other increases the amount and diversity of life that can be supported.
Examples: Insects pollinate flowers, which provide the insects with nectar and pollen as food. Fungi obtain their energy from breaking down dead wood to release nutrients for the living trees. Bacteria in our guts help us digest our food, and we provide them with a stable environment with plenty of food for themselves. The examples are limitless.
Some species exist in the same environment without directly affecting each other. If they use different resources in the environment and do not prey on each other, their direct interactions are neutral - neither good nor bad for either species. Even if their interactions are neutral, they are part of the same ecosystem, so changes in the ecosystem can affect both species similarly or differently.
Example: California ground squirrels and mule deer both live in many California ecosystems. The squirrels generally eat seeds, barley, oats, and acorns with some insects and bird eggs. In contrast, mule deer largely browse on bushes and trees, with acorns being an important part of their diet in summer and fall (Sommer et al., 2007). These two species might compete for acorns when other resources are sparse, but they mostly exist in neutral proximity to each other. However, both are hunted by coyotes, bobcats, and pumas, so their populations are linked through other species. For example, if the population of squirrels declines, say from predation by red tailed hawks, there might be more predator pressure on deer from their common predators, e.g. coyotes, bobcats, and pumas. Thus, a large population of squirrels might reduce predation on deer, indirectly benefiting them, while also increasing competition for acorns if other food resources are low.
Competition for resources and space is very common among species that occupy similar roles in an ecosystem. When species compete with each other, both pay a price for that competition because they are sharing resources. When interactions are predominantly competitive, either species would do better if the other was not present. Competition tends to drive selection of the organisms that are best suited for a particular ecosystem role, with the population of the better adapted species increasing relative to that of the less well adapted species. As the population of the more successful species increases, it is less affected by the competition because each organism experiences fewer interactions with its competitors. In contrast, each individual of the less successful species experiences relatively more competitive interactions, making the harm experiences greater.
Example: California grasslands have been invaded by new species of grass introduced by humans. These new species out competed the native species due to their greater ability to reproduce in many of the environments (see for example, Corbin and D'Antonio, 2010). The new species are taking up the space and nutrients previously used by the native grasses. In this case, the native species experienced a significantly greater harm during the competition than the invasive species.
Example: Virginia opossums and striped skunks actually do compete with each other to some degree. Both are opportunistic feeders, eating almost anything that is available, with opossums eating more small vertebrates and plants and skunks focusing more on insects. They are also about the same size. Thus, they play a similar role in the ecosystem and can compete for both food and dens, although opossums can also den in trees. In contrast to the example of the grasses, opossums and skunks commonly co-exist, with the competition less intense and their abilities to reproduce more equal in many ecosystems.
Sometimes interactions benefit one organism while being of neither benefit nor harm to the other organism. These types of relationships are also very common and help promote diverse ecosystems.
Examples: A tree frog in a rainforest benefits from the habitat created by the tree it lives in without harming or benefiting the tree. A cattle egret can catch half again as much food while expending two thirds of the energy if it searches for insects near grazing animals who disturb the insects. This activity does not directly benefit the grazing animals, nor does it harm them.
Interactions can have no effect on one species while harming another. These interactions are usually incidental to a behavior of the species that does not experience harm or benefit. They can include interactions such as a change in the environment, incidental killing of organisms, and other influences.
Examples: Hippos consume a significant amount of undigestible organic matter, and they create a very large volume of poop. They often poop in ponds, and these ponds can become anoxic, killing the aquatic organisms that require oxygen to live (Pennisi, 2018). Most animals have stepped on insects, killing them.
Parasitism and Predation
Parasitism and predation both have a strong benefit for one species and a significant harm to the other. In both cases, one organism takes resources from another. Parasitism involves one organism living off the resources of another for an extended period of time without causing their death. Parasites benefit from the survival of their host organism. In contrast, predation usually involves killing the prey organism and consuming it. Often, parasites are small relative to their hosts, whereas predators are frequently about the same size as or larger than their prey. This difference in size is related to the energy needs of an organism relative to its size: in general, larger organisms need more energy to accumulate their biomass and to maintain their activity. A small parasite can take energy from a larger host for an extended period of time without killing it. In contrast, a larger organism generally needs more energy, so it is difficult to maintain itself without killing the organism it is preying on. Similarly, it is easier to kill an organism that is of similar size or smaller, so predators tend to prey on smaller organisms. In contrast, parasites typically kill by causing systemic problems for their hosts, such as diseases, extracting too many resources, or taking over cellular processes in the case of viruses.
Parasitism Examples: Parasites include organisms like ticks that live by sucking the blood of mammals. Other parasites include mistletoe living in trees; mistletoe has evolved to plant its "roots" into the circulation system of the trees, extracting nutrients and water from the host. This can cause significant damage to the tree.
Predation Examples: Obvious examples include animals that prey on other animals. (see https://necsi.edu/predator-prey-relationships for some examples.)
There are a lot of questions as to how herbivory (eating of plants) fits into this relationship scheme. If an herbivore kills the plant, the interaction is similar to predation. However, many plants survive and some have even evolved to do better when fed on by other species. For example, fruits often attract other species to help distribute the seeds for the plant. Thus, the relationships between plants and the things that eat them can be evaluated in terms of harms and benefits much like those involving other species interactions.
Relationships among organisms can be more complicated than who benefits or loses from the direct interactions. Both direct and indirect interactions have been driving forces for evolution, leading to deeply interconnected communities within ecosystems. The relationships often become more intricate and interdependent through time as species help and harm each other, responding to the organisms around them. These interactions often lead to stabilizing feedbacks and persistent ecosystems. Sometimes, however, a geological event, evolution of a revolutionary process, or the introduction of a new species can disrupt the established relationships and lead to ecological change that triggers an amplifying feedback.
Return to Daisyworld
In the introduction to this chapter, I posed some questions about the interactions among daisies in Daisyworld:
How many different types of interactions are there? Can they be classified into the interaction types described in the second part of the chapter? If this was a real ecosystem, what other organisms would be needed to allow the daisies to grow? How would any of these needs affect the simple Daisyworld model?
How would you answer these questions after reading about different interaction types? What would you like to add to a Daisyworld model?
Sommer, M. L., R. L. Barboza, R. A. Botta, E. B. Kleinfelter,
M. E. Schauss and J. R. Thompson. 2007. Habitat Guidelines for Mule Deer: California Woodland Chaparral Ecoregion. Mule Deer Working Group, Western Association of Fish and Wildlife Agencies. PDF
Pennisi, 2018 https://www.sciencemag.org/news/2018...-across-africa