In the introduction to this series we looked at all the tiers of complexity that an ecologist
may study, starting with an individual organism and zooming out to consider the entire biosphere.
Let’s now take a closer look at all of these tiers one at a time so that we can get a better
understanding of what ecologists study, starting at the most fundamental level, an organism. What
can an ecologist study when it comes to a single organism? First, they can study an organism’s
life history, or cycle. This pertains to how the organism grows, develops with time, reproduces
and, ultimately, survives. Life histories vary tremendously from one species to the next. For
example, some organisms die immediately after reproducing, such as salmon, many insects, and
all grain crops, while others live on to reproduce repeatedly, like most plants and vertebrates. Why
is there so much variation? Researchers who study ecology at the organism level ask this question
and many others, like: How have different life cycles developed? How do they serve
the organism? They also attempt to understand adaptations they observe in an organism. These
are beneficial features or behaviors arising by natural selection which allow organisms to thrive
in specific habitats. They range from varying body coverings, like fur, feathers, and scales;
to patterns of color and camouflage; to defenses like a skunk’s spray and a snake’s venom;
and finally, to body parts like beaks, claws, and antlers. Adaptations can take multiple forms,
including alterations to an organism’s morphology, which refers to a structural change which gives
an organism a greater chance of survival in its particular habitat. A great example is the Fennec
fox, which lives in the desert. It has large ears. You might think it’s so the fox can hear better,
but it’s actually to allow heat to radiate away from the body, helping to cool the
fox down in the intensely hot environment. Adaptations can be physiological in nature as
well: these are internal processes that regulate and maintain homeostasis, so that an organism is
able to survive in the environment in which it exists. Examples of this include temperature
regulation, release of toxins or poisons for protection, and even releasing natural antifreeze
in the form of proteins, to avoid freezing in cold environments. When it comes to humans, we can find
some familiar examples. Calluses on our hands can be considered physiological adaptations to
repeated contact or pressure, as can tanning or darkening of the skin after repeated
exposure to the sun. There are also behavioral adaptations, which are changes in behavior that
certain organisms or species use to survive in their environment. There are two types: learned
behaviors and instinctive behaviors. Learned behaviors include habituation, or getting used
to a particular stimulus after being exposed to it multiple times; and sensitization, or responding
to certain stimuli in an enhanced or more reactive manner. Think about becoming allergic
to something – like a certain type of food or pollen. Your body, by becoming more sensitive,
protects you from exposing yourself to these irritants in the future by producing a noticeable
and worsening response. Then there’s imprinting – an adaptive function that allows a young animal to
distinguish its own mother from other females of the same or different species – and to remain near
her for safety and protection. Learned behaviors can also come about from play and observation.
Crows, for example, are notorious for their knack for observation-based learned behaviors, including
solving puzzles and using tools. Instinctive behaviors are those behaviors that animals are
born knowing how to do. Changes usually come about over many generations until they finally become
a part of instinctive behavior. Some examples include the migration of birds like geese during
the cold seasons of the year to areas further south, in search of food; the hibernation of some
mammals during the winter, like bears; courtship, or mating patterns; and foraging behaviors,
which include knowing ways of finding food, depending on the habitat or environment.
Moving on from organisms, population ecologists are scientists who study the size, or total number
of individuals, as well as density and structure of populations, and how they change over time.
Of interest are a population’s geographic range, which has limits that a species can tolerate, like
fluctuations in temperature or moisture. The study of population ecology also includes understanding,
explaining, and predicting species distributions. Why do species inhabit particular areas, and how
are they prevented from establishing beyond their range limits? Such range questions have become
popular in the last decade or so in response to concerns about climate change. Additionally,
what are the birth and death rates of a particular species? What urges populations to immigrate,
or explore new areas beyond their previous range? What causes them to emigrate,
when they leave familiar territory altogether? What are the long-term probabilities of species
persisting and surviving in certain habitats? What factors play a role in deciding this?
Now let’s discuss population growth. Populations can grow in two different modes: at geometric
rates or at exponential rates. While exploring these, keep in mind that we will be assuming
the presence of unlimited resources which urge population growth. Exponential growth happens
continuously, with reproduction happening at any time, like in human populations. Geometric
populations grow through pulsed reproduction instead. An example of this is deer which have
a constrained mating and reproduction season, keeping them from constantly reproducing. When
exponential growth occurs, it is in favorable environments and at low population densities.
Because of this, exponential growth may apply to populations establishing new environments;
during transient, favorable conditions; and by populations with low initial population density.
Of course, neither geometric nor exponential growth can continue indefinitely. In nature,
population growth must eventually slow, and population size eventually ceases to increase.
As resources are depleted, the population growth rate slows and eventually stops. This is known as
logistic growth. The population size at which growth stops is generally called the carrying
capacity, which is the number of individuals of a particular population that the environment
can “carry”, or support. Carrying capacity is characterized by equal birth and death rates,
creating a population growth of zero. In some populations, organisms in lower trophic levels,
meaning the positions that organisms occupy in a food web, are controlled by organisms at
the top. This is known as top-down control. For example, carnivores at the top keep the herbivore
population in check. If herbivores weren’t controlled, all the plants would be eaten,
leading to a potential collapse of the entire ecosystem. In bottom-up control,
it’s the producers at the lower trophic levels who drive the changes instead. If
plant populations change, then the population of all species will be impacted. With organisms
and populations better understood, let’s move forward and look at communities of organisms.