Habitat
This week's exercise deals with two very important concepts in ecology that are relevant to observational studies in natural history. Habitat is an old concept in ecology. Habitat is variation in the landscape (generally in space) that is relevant to the biological processes of an organism. Habitat is operationally defined as being constituted by conditions, or factors, which can be either biological or environmental. Some conditions that can be used to describe a habitat are temperature regime, light intensity, humidity regime, vegetation structure and composition, altitude, exposure (aspect), soil type, etc. Habitat may be described by specifying the plant association of the area of interest, plus additional environmental conditions. For example, the habitat provided at 500 m elevation in pinyon-juniper woodland or open, gravelly prairie at 100 m elevation, on a SW facing slope, in the Puget trough.
Microhabitat
Additionally, within a habitat in which a species is found, some sectors of that area may be more suitable to the organism than are others. These especially suitable portions of the landscape vary in one or more measurable conditions and are known as microhabitats. For example, narrow canyons in pinyon-juniper woodland may be particularly amenable to the growth of Cowania mexicana, a fragrant shrub in the southern great basin. Or while a stream of moderate gradient in mature conifer forest may provide habitat for Cope's giant salamander, pools with large cobbles are a microhabitat type that supports higher salamander densities because of the lower energy required of the salamander when hunting, as compared to hunting in riffles. Either the environment or the organism may determine which activities are performed in which microhabitat. For example, a terrestrial isopod ('pill bug') may choose to live under logs in an intermediate state of decomposition. Hemlock seeds may germinate at a much higher frequency on cedar logs than on maple logs or leaf litter (the seeds don't 'choose'!). The habitat used by spotted owls is mature and old growth forest, however the establishment of nests is done exclusively in the microhabitat provided by the interior of broken, hollow snags. One might additionally specify that microhabitat variation is perceptible to an individual during the course of one complete lifecycle, whereas habitat variation generally extends beyond the home range of an individual (although this distinction is not always specified).
Ecological scale
While we can define habitats or microhabitats as we may, however whether or not they are relevant to a species depends on the ecological scale at which that organisms perceives its environment. That which constitutes suitable habitat for one species may be only an unsuitable microhabitat for another. The pertinent ecological scale of a species is not directly knowable and it depends on many aspects of the species: its size, vagility (ability to travel), energy requirements, etc. Even though it requires work to construct a quantitative description of the scale of environmental conditions to which an organism responds, one can observe where one finds a species, its vagility and where the species chooses to perform certain behaviors, in order to get a rough idea of both the habitat requirements of the species and the relevant microhabitat variation. Herein lies this week's exercise.
Goals, objectives, etc.
The goal of this week's exercise is for you to learn about the kind of variation in conditions (habitat) that is pertinent to organisms (in your taxonomic group of choice or any other). The objective you are to complete is to observe and describe (qualitatively) some aspects of habitat variation that are relevant to two species that you believe perceive their environment at different ecological scales. For each of these two species, describe their general habitat requirements. Substantiate your description based on at least some direct observation, plus any additional information you can access. Then describe the use of one or more microhabitats by this organism. Be sure to specify what use the organism has for that microhabitat. Base this on at least some observation.
For example, you find that the red backed salamander is only found in forested areas. Even though they may be found on the surface at night, during the day they are found under objects larger than 30 cm x 30 cm, and under these objects there is considerably more moisture than on top of the object, or on the soil surface in general. You might observe that maiden hair fern is found in forests over 30 yrs in age, and that they are especially frequent in mineral soil that is saturated with water for 6 months out of the year (I'm not sure if that is actually true, but it could be J ). In contrast, you might observe that suitable habitat for Douglas fir is on well-drained soil, below 800 m throughout the Olympic Peninsula. However, young Douglas firs (<5 yr old) are only found in the microhabitat provided by bare mineral soil in forest openings greater than 10 m x 10 m.
Please Note: These observations, like those of last week, should go in the species account section of your journal, not somewhere else. Be sure to record in your Journal section the excursions you did in order to make these observations. It may take you more than one outing to make enough observations in order to complete this assignment, but plan on having it done by the end of the quarter.
You may want to read on environmental conditions, habitat variation, habitat selection, and related topics in one of the ecology texts that are on reserve in the library for our program. Quantification of the selection of microhabitats by a species could be a valid project for the later part of the program.
Bottom line: Also, these exercises will be the primary basis upon which credit will be awarded in natural history this quarter. Thus it would be helpful, in cases where it is not obvious, if you would note in your journal any entries that pertain particularly to these exercises.