We just examined the two lowest tiers on the scale of ecology, organisms and populations.
When populations of different species can be found in the same location, this constitutes a
community. So how do ecologists study communities? Community ecologists focus on interactions between
populations of different species and how these interactions shape the communities where they
live. This includes not just the study of the interactions of species, such as mutualism,
predation, and competition, but also the dynamics and structure of the community.
How is it organized? How does it function? In a nutshell, community ecology examines how
coexisting organisms interact and compete in a particular niche or geographical location,
such as woodland, prairie, or lake. Let’s start by defining the different kinds of interactions
between populations of species. Mutualism is a symbiotic relationship that is beneficial
to both organisms involved. This occurs when two organisms of different species “work together”,
and each reaps benefits that increase survivability. The oxpecker, a kind of bird,
and the zebra are one such example: the bird gets food in the form of ticks and other parasites on
the zebra’s skin, while the zebra gets a kind of “pest control.” Going a little deeper,
mutualism can take two forms. When the species are entirely dependent on each other, this is called
obligate mutualism. An easy way to remember this is that both organisms are obligated,
or forced to, rely on one another. An example of this is mycorrhizal fungi with hyphae that
exchange crucial nutrients with plant roots. In other situations, where the species can survive
on their own, like the bird/zebra relationship we mentioned, this is called facultative mutualism.
Another example of obligate mutualism is the yucca plant and the moth: the flower of the yucca plant,
in the habitat of the arid climate of the southwestern United States,
relies on the moth for the pollination process. In turn, the moth benefits by laying its own eggs
on the flower and feeding the larvae with the seeds. Honeybees and plants are another example
of facultative mutualism, given that honey bees visit many different plant species for nectar,
and the plants receive a number of different insect visitors for pollination.
Now let’s look at the opposite of mutualism, predation and competition. The former is when
one population is the resource of the other, while the latter is when individuals seek to obtain the
same environmental resource. Think of competition as occurring horizontally on the same resource
level while predation takes place vertically between different resource levels. These are
still ecological relationships, but they’re not symbiotic ones. The best-known examples
of predation involve carnivorous interactions, in which one animal consumes another – like
when wolves hunt moose, or owls hunt mice. If there’s an increase in predation in a community,
or any other source of mortality, this will generally reduce the absolute change in density
of consumers, simply because the direct impact of predators is to reduce the numbers of their prey.
While the term “reducing” seems negative, predators are often a positive influence
on communities, and can increase diversity. They do this by hunting dominant species,
which reduces the pressure on or overconsumption of foundation species, those species that have
a strong role in structuring a community by providing habitat structure and food.
Some examples of foundation species are corals, kelp beds, seagrass meadows, or hardwood trees.
Predation can have a positive effect when the predators feed more on the dominant competitors,
that would otherwise use up all of the community’s resources, which organisms are competing for.
Animals fight over territory, food, water, and shelter. Plants compete for light exposure,
pollinators, and soil nutrients. Microbes compete for nutrients and chemical substrates.
Competition is important in an ecosystem as the best competitors are the ones who survive and
pass along their genes. Their offspring have an increased chance of survival because their
parents out-competed other species and passed down the genetic tools that helped them succeed.
What can affect population size? It comes down to the availability of food,
which can act as a “bottom-up” control. In years when preferred food is abundant, populations are
able to grow. As predator populations increase, in response to the growth of consumer populations,
they put greater strain on the prey populations and act as a top-down control,
pushing the prey towards a state of decline to keep the community in balance.
While the presence of a predator can decrease the growth rate of a population, that doesn’t
affect the overall carrying capacity, which is reliant on the availability of resources. So,
how does predation drive population dynamics? When predators are scarce,
prey increases in abundance. As their food source increases, predators increase in abundance. When
the predators reach sufficiently high numbers, the prey population is driven down to low numbers.
With a scarcity of food, the predator population declines and the cycle repeats. We should also
discuss how predation drives selection. Predators are effective agents of natural selection because
they possess key traits that allow them to capture and consume their prey. They adopt different
strategies to hunt which can drive prey evolution, to increase their likelihood of survival.
With communities covered, let’s now pull back even farther to take a look at ecosystems.