4 Food and Cover
Food, cover, water, and space are the basic environmental components necessary for a species’ survival. Together these components comprise the species habitat. The habitat includes the physical and biological resources necessary to support a self-sustaining population.
It is useful to think of habitat meeting not only an individual’s needs but also a population’s needs. It is, after all, the population that can be sustained. Individuals, though clearly essential to population maintenance, just come and go in the process. A number of things drive the success of a population, especially energy. Although nutrients, water, and other factors clearly have a role in maintaining the fitness of individuals and populations (e.g., Jones 1992), the lion’s share of the system is driven by energy. A population gains energy from food resources and conserves energy by exploiting cover resources (Figure 4.1).
Figure 4.1: The concept of energy flow through individuals to influence individual and population fitness. (Based on Mautz, W.W. 1978. Big Game of North America, Ecology and Management. Stackpole Books, Harrisburg, PA, pp. 321–348; adapted from McComb, W.C. 2001. Wildlife Habitat Relationships in Oregon and Washington. OSU Press, Corvallis, OR.)
Food
Energy is the currency for population sustainability. Populations with sufficient energy sources grow; while populations without will decline. When an individual goes energetically bankrupt, they die. When a population goes energetically bankrupt, it goes extinct.
Food provides the source of energy (and nutrients) for individuals and populations and is therefore necessary for survival. The size and health of an individual is dependent on food availability and quality. When compared to individuals in poor health due to nutritional deficiencies, individuals in good condition are likely to benefit from increased resistance to disease, predator avoidance, and higher rates of reproduction. This is why food quality matters. Tree species vary in their ability to provide protein and carbohydrates to herbivores. Some parts of plants are more digestible than others and some plant species are more digestible than others (Mautz et al. 1976). Overall food type affects digestibility as animal matter is more digestible than plant matter. Species prefer food items that they are better adapted to digest and we can better understand these preferences by comparing ratios of utilized food items to available food items. Digestible energy and the net energy remaining after digestion and metabolism that influences the fitness of individuals and populations. But fitness, the ability to survive and reproduce, is also influenced by cover quality.
Cover
It is advantageous for an animal to conserve any energy that it acquires. Mammals and birds are endotherms that maintain a constant body temperature and expend a large amount of energy to maintain that temperature. Cover provides a mechanism for conserving energy and contributes to an animal’s survival or reproduction. The thermal neutral zone is the range of ambient temperatures where an animal has to expend the least amount of energy maintaining a constant body temperature. Thermal cover places the animal closer to the thermal neutral zone. Energy expenditures are minimized in an animal’s thermal neutral zone to maintain body temperature (Figure 4.2, e.g., Mautz et al. 1992).
Any departure from the thermal neutral zone results in increased expenditure of energy; so animals often select habitat that reduces climatic extremes. There are upper and lower critical temperatures beyond which exposure for a prolonged period would be lethal. Cover from overheating is especially important to large animals with a low surface-area-to-body-mass ratio because they may find it particularly difficult to release excess heat unless water is available to aid in evaporative cooling. Cover from severe cold is especially important to a species with a high surface-area-to-bodymass ratio (e.g., small birds and mammals). Cover that allows an animal to stay within an acceptable range of temperatures (particularly those that approach the thermal neutral zone) is important to maintaining a positive balance of net energy and hence influences animal fitness. For instance, imagine yourself standing in a field wearing summer clothes in mid-January in Minnesota. Without any measurements, you know that you are expending a significant amount of energy to stay warm. Now imagine you are in a field in Arizona in August at noon. You must expend energy to stay cool and not let your body temperature rise too high (e.g., heat stroke). In either case, moving into a building where the temperature is 18°C (65°F) allows you to spend less energy keeping your body at the appropriate temperature. Refer to Figure 4.2 and plot the metabolic rate for a small mammal or bird at a low temperature and then the metabolic rate for a temperature near the thermal neutral zone. The difference in metabolic rates along the y-axis is an index to the amount of energy that the individual can conserve by staying closer to the thermal neutral zone. For a small animal with a high metabolic rate and high surface area to body mass that conserved energy can mean the difference between life and death on a cold winter night.
Figure 4.2: Relationship between metabolic rate and ambient temperature in a hypothetical mammal. (Based on Gordon, M.S. 1972. Animal Physiology: Principles and Adaptations, 2nd ed. The MacMillan Co., New York, 591 pp.; adapted from McComb, W.C. 2001. Wildlife Habitat Relationships in Oregon and Washington. OSU Press, Corvallis, OR.)
But the relationship portrayed in Figure 4.2 is different for species that do not maintain a constant body temperature. Most reptiles, amphibians, and some nestling birds (birds that have not yet fledged) do not use large amounts of energy to maintain a constant body temperature. They are ectotherms—they receive most of their body heat from the surrounding environment, unlike endotherms that generate their own body heat. For ectotherms, metabolic rates and food requirements vary as ambient temperature varies. The evolutionary advantage of such an approach is that these ectotherms require less food to survive, but they can be restricted from extreme environments that otherwise would be inhabitable by endotherms (some bird and mammal species). Hence, reptiles and amphibians often use cover to adjust the ambient temperature to allow them to survive, reproduce, and move in places and times when they otherwise would be unable to (Forsman 2000). Consequently, cover is an important component of habitat for these species, to both conserve energy and place them at a temperature where they can be active.
Cover can also refer to the portion of habitat that an animal uses for nesting and escaping from predators. Hiding cover protects an animal from predation. Cottontail rabbits often spend resting hours in dense shrubby cover adjacent to grassy fields and meadows (Bond et al. 2001). The dense shrub cover protects them from predation by red-tailed hawks whose body size and wing spread do not allow them to penetrate dense vegetation.
Nesting cover provides the conditions necessary for raising young—appropriate temperature and protection from predators and competitors. The effectiveness of nest box programs for wood ducks, eastern bluebirds, and other cavity-using species demonstrates that manipulation of the quantity, quality, and availability of nesting cover resources can be an effective management technique (McComb and Lindenmayer 1999). Wildlife managers can influence habitat for a species by altering food quality, quantity, and/or availability while also altering the quality, quantity, and/or availability of cover. This strategy can lead to drastic changes in habitat quality for the species.
Water
Water is needed for natural processes such as digestion, metabolism, temperature regulation, and waste elimination. Animals can obtain water from natural or manmade sources (i.e., free water), food items (i.e., preformed water), and natural processes (i.e., metabolic water). Natural sources of water include any naturally occurring sources such as springs, wetlands, snow, or dew. Manmade water sources include reservoirs, catchments, or livestock tanks. Food items vary in water content with fruit, nectar, insects, and animal tissue having the highest water content. Individuals lose water through urine, feces, and evaporation.
Water is differentially important to animal species and can vary under different environmental conditions. Some species require free water or high humidity (mountain beaver, e.g., have a primitive uretic system) (Schmidt-Neilsen and Pfeiffer 1970). Other species obtain most of their water from their food (e.g., pocket gophers). Some species use water as a form of cover to enhance evaporative cooling (e.g., elk) or to escape predators (e.g., white-tailed deer). Still others such as amphibians require free water or moist environments for reproduction.
Space
The size of habitat is also an important determinant of its suitability for a species. A patch of habitat must be sufficiently large to provide energy inputs and energy conservation features to sustain a population. Habitat may occur in one large unit, but more commonly it is distributed in patches embedded in other less suitable patches. If these habitat patches are too widely distributed, then the animal expends more energy moving among patches than it receives from those patches. The amount of habitat and its quality and distribution are therefore interrelated. Increasing any one or all of these attributes of habitat increases the net energy available to animals that use this energy to maintain body temperature, move to food and cover, and reproduce.
Adaptations
What happens when aspects of the habitat are limited? Why might these limitations occur? This next section addresses strategies that species use when resources (food, cover, water) are limited.
Food, cover, and water availability vary by season and location. In some areas or seasons these necessary habitat components may become scarce or change in quality due to lack of sun or rain. Additionally, cover may become more important during cold seasons. Species use different strategies to handle variations or limitations in resources.
Seasonal variation in resources may lead to simple adaptations such as switching food items to those more readily available. Diet switching is seen most readily in generalists, (species with a broad diet) such as coyotes, which focus on easily obtainable fruits and neonates (especially deer fawns) in summers and small mammals during winter. Figure 4.3 shows how coyote consumption of four different food types (deer, small mammal, rabbit, and vegetation) varies across three seasons. You will notice that vegetation and deer (most likely neonates, a.k.a fawns) are the primary components of the diet in summer and fall, while small mammals are the primary diet component in winter. Specialist species (species with narrow diet preferences) are less likely to be able to switch to alternative food sources and adapt to limited resources in other ways.
Figure 4.3: Comparisons of (a) deer, (b) small mammal, (c) rabbit, and (d) vegetation consumption (weighted occurrence) by male and female coyotes in South Carolina. All samples used in this analysis were from genetically individually identified scats, which is how we know the sex of the coyote. Statistical comparisons were between sexes within seasons, where different letters indicate significant differences. From Jensen et al. 2024.
Alternatives to diet switching for handling resource limitations include migration, hibernation, or torpor. These are behavioral and physiological adaptations that some species have to further conserve energy. Southern flying squirrels (see Appendix 1 for a list of scientific names of all plants and animals used in this book) and some species of cave-dwelling bats often will use communal roosts in winter to collectively maintain a lower surface-area-to-body ratio. Flying squirrels pack many small bodies together to make one bigger, more energetically efficient big body by huddling (Merritt et al. 2001). Other species such as eastern chipmunks hibernate (long-term, deep dormancy state) or, as in the case of striped skunks, enter a state of torpor (short-term, involuntary dormancy) where metabolic rates are reduced and energy is conserved. Black-capped chickadees, a small, 10-g bird that spends winters in very cold climates, will cache food, roost in cavities, and alter their metabolic rates seasonally to cope with temperature extremes (Cooper and Swanson 1994).
Contributors and Attributions
Modified from the following sources:
Wildlife Habitat Management by Brenda C. McComb licensed CC BY 4.0
Additional references and citations from the above sources can be found in References in the backmatter.
4. Food and Cover is shared under a CC BY-NC-SA license
