Download File - BIO271: Field Ecology at Fontbonne, Spring 2014

Chapter 9 Molles Notes
Introduction
Monarch butterflies migrate over several generations. Eggs are laid and hatch in one place, new
butterflies travel further than their parents, mate, lay eggs, and die, and the new offspring travel yet
further.
Populations are dynamic, and time magnifies those dynamics.
Population – group of individuals of a single species inhabiting a specific area within a defined time
frame. May consist of highly localized group of individuals representing a fraction of the total
population of a species, or it may consist of all the individuals of a species across its entire range.
Distribution – the size, shape, and location of the area a population occupies. This will also include
the spacing of the individuals within the area.
Density – number of individuals per unit area
Abundance – the total number of individuals, or biomass, of a species in a specified area.
9.1 Distribution Limits
The physical environment places limits on the distributions of populations. The environmental limits
of a species are based on its niche.
Niche – all of the factors necessary for a species’ existence – when, where, and how a species lives.
Niche influences growth, survival, and reproduction of a species
Fundamental niche – hypothetical, theoretical niche. Includes the values of all environmental
factors permitting a species to survive and reproduce. Defines the physical conditions under which a
species might live, in the absence of interactions with other species.
Realized niche – actual niche, restricted to consider all environmental and interspecies connections
and influences. Includes competition, predation, disease, and parasitism.
The fundamental niche is more hypothetical. No species or population lives in a vacuum, without the
influences and connections of other species. While it describes what *might* happen under a perfect
set of circumstances, it’s a poor model for real life.
Q: Why, then, is fundamental niche a useful/important concept??
Kangaroos
Conclusion – populations are limited by climate, but indirectly. Rather, agents of climate act directly
on the population – food production, water supply, habitat, parasite, pathogens, and competitors.
Probably also predators, but indirectly through another degree of separation.
Q: Is this one reason why climate change is a big deal? What about human population problems?
Caughley (1987). Three species of kangaroos live in minimally overlapping areas of the Australian
continent. No species live in the northernmost region, where it is too hot for the eastern gray, too
wet for the red, and too hot and dry for the western gray. These semi-overlapping distributions have
been stable for at least 100 years.
Tiger Beetles
Cicindela longilabris lives at higher latitudes and elevations that any other tiger beetles. Distribution
coincides with boreal and northern temperate forests throughout Canada and upper US. But it also
extends lives in New Mexico and California. The southern populations are confined to the
mountains, where the temperature in montane coniferous forests is similar to the northern forests.
Conclusion: Glacial melt pushed a once-wide and continuous population into higher latitudes, but
also into higher elevations, leaving behind scattered populations in the
Q: What will happen to these isolated populations over time? (Think about Chapter 4!)
Compared physiological characteristics of beetles from Maine, Wisconsin, Colorado, and Arizona.
Metabolic rates are higher and preferred temps lower than those of most other beetle species. This is
probably because beetles are adapted to the cool climates of boreal and montane forests. These
similarities existed even 3,000 km and 10,000 years apart.
The physical environment has restricted and controlled the variation allowed in the species by
controlling the niche in which it can exist. These limits may be stable for a long time.
Encelia/California daisy
Geographic distribution limits correspond to variations in temperature and precipitation. Much like
the kangaroo, species partially overlap where favorable conditions meet. Sparsely pubescent (hairy)
leaves absorb more solar radiation that non-pubescent leaves. It makes sense that these plant could
only exist in areas where solar radiation is not damaging in high doses. But E. frutescens, which has
sparsely pubescent leaves does live in high radiation areas. It compensates with high transpiration
rates.
Q: wouldn’t this cause water loss, especially in hot climates? What to do about that?
It solves this problem by living in stream channels, with lots of easy access to water. So, upon further
inspection, they occupy different micro climates, even though the two species appear to overlap.
Barnacles
An organism’s environment consists of more than abiotic (physical and chemical) factors. Biological
factors, such as predation and competition can affect the niche and fitness of a population.
Organisms in intertidal zones have adapted mechanisms to avoid overdrying. It appeared that Balanus
is more susceptible to dessication, and that Chthamalus is excluded from lower zones. When
transplanted, Chthamalus survived the lower zones very well. More likely, they outcompete each other
in different zones.
CR 9.1.1. How may a species respond to climate change?
If the climate shifts faster than natural selection can allow for adaptations to emerge, the population
will remain in its best suited niche. That will often mean increased or restricted travel, depending on
the circumstances.
CR 9.1.2. How might biological and physical aspects of the environment interact to influence a
species’ geographic distribution?
All factors are involved in defining the niche. Competition with other species will define a
distribution to maximize fitness while minimizing cost.
Concept 9.1 Review Answers
1. A species may shift its geographical distribution with climate change; it may adapt to the new
environmental conditions; or do both. Large and rapid climate change may lead to extinction.
2. A combination of physical and biological factors may restrict a distribution more than would
physical factors alone, since less than optimal physical conditions may make a species more
vulnerable to attack by pathogens, parasites, or predators, or less effective in competition with
other ecologically similar species.
9.2 Patterns on small scales
On small scales, individuals within populations are distributed in patterns that may be random,
regular, or clumped.
Large-scale – areas over which there is variation in the environment, but still within the range of the
target species or population
Small-scale – areas over which there is no variation in the environments, but the size will still
depend on the size of the target species or population
Random distribution – no apparent pattern or cause to distribution. All individuals have an equal
chance of being anywhere
Regular distribution – pattern follows a uniformly spaced arrangement. Likely repelling forces
between individuals
Clumped distribution – individuals have a high probability of being near each other. Likely due to
patchy clusters of resources or inter-individual attraction, or high risk in the negative spaces
Distributions form based on the environment, but the environment can also reinforce or dampen
other existing forces. And patterns created by social interactions may be reinforced or dampened by
the structure of the environment.
Concept Review
9.2.1. Are the concepts of “small” vs “large” scale the same for all organisms?
9.2.2. How could you test the hypothesis that low overlap in root systems in creosote bush
populations is the result of ongoing competition?
9.2.3. In the study of the distribution of stingless bee colonies, why were measurements of the
number and distribution of potential nest trees necessary?
Concept 9.2 Review Answers
1. No. Relative sizes and dispersal abilities of species must be taken into account when
distinguishing between large and small scale. Mammals and birds of the same size use the
environment on significantly different scales, as do mice and elephants.
2. To test the hypothesis, selectively remove, or kill, individual shrubs and determine if the root
systems of surrounding creosote bushes grow into the vacated area.
3. The measurements demonstrated that the number of colonies in the study area was not limited
by availability of suitable nest trees and that clumped or regular distributions would not be the
result of an underlying clumped or regular distribution of potential nest sites.
9.3 Patterns on large scales
Large-scale observations almost always reveal patchy (clumped) distributions across the widest
possible range, which may or may not include smaller areas of random or regular distributions.
Bird studies of Root and Brown showed patch distributions across a larger area. Specifically, 25% of
the locations sampled included half the populations. The best hypothesis for this observation is that
the environment varies and individuals aggregate in areas where the environment is favorable.
Whitaker’s study of plant distributions along moisture gradients on mountain sides showed that
density is clumped at areas of optimal resource concentration. Optimal concentration, though, is
different for different species, depending on their different requirements. Density decreases
substantially at the edges of the hot spots.
Concept Review
9.3.1. What factors might be responsible for the aggregation of American crows in winter?
9.3.2. Why might the winter aggregations of crows occur mainly along river valleys?
9.3.3. What does the position of pines along moisture gradients in both the Santa Catalina Mountains
of Arizona and the Great Smoky Mountains of Tennessee suggest about pine water relations?
Concept 9.3 Review Answers
1. Some factors favoring winter aggregation by crows, and other birds, include protection from
predators, more effective foraging in flocks, and choosing pairing of males and females for the
spring nesting season.
2. Food is generally abundant along river valleys.
3. The observation that pines (Pinus cembroides in Arizona and Pinus pungens in Tennessee) inhabit the
driest sites in both mountain ranges suggests that pines are relatively drought tolerant.
9.4 Organism size and population density
Big rule: Population density decreases as organismal size increases.
SR’s aside: Large predators live mainly solitarily, or in small groups. They need a wide range of
herbivores as food items, which in turn require a large(r) range of plants as food. (Remember the
10% rule of food pyramids?) Good local examples are wolves and mountain lions, which live
sparsely. As always, food requirements trump all else. This is true for non-carnivores, and nonanimals as well.
But, beyond this, it’s important to determine how much population density increases as average
body size decreases and vice versa. Also, how is this correlation stronger or weaker in different types
of organisms? Further, does the rule hold as a single organism’s body size increases indefinitely over
a lifetime (as in old, large trees)?
Self-thinning was introduced as an explanation for the changes in the distribution patterns in the
creosote bushes in section 9.2. Self-thinning also, reasonably, has a lasting effect on population
density.
Thus, the size-density relationship changes within populations and differs significantly between
populations of individuals that grow to different sizes at and past maturity.
Population density is a useful tool in conducting field research because it gives us a baseline control
level to which we can compare changes in a population over time.
CR 9.4.1. What are some advantages of Damuth’s strict focus on herbivorous mammals in his
analysis of the relationship between body size and population density? Consider how feeding ecology
might be a factor.
9.4.2 How might energy and nutrient relations explain the lower population densities of birds to
comparable sized mammals?
Concept 9.4 Review Answers
1. By restricting his analysis to herbivorous mammals, Damuth controlled for the potential
influence of feeding ecology and evolutionary history on factors that affect density.
2. Birds may live at lower densities because their food is less abundant, because they require more
food to support their activity and metabolic rate, or both. SR: another explanation is that
mammals have a biological need to remain as a family unit longer than organisms in other
classes.
Chapter 9
1. Encelia farinosa may be confined to upland slopes in the Mojave Desert by competition with E.
frutescens and physical disturbance by flash flooding in the washes. The deeper penetrating roots
of E. frutescens may provide it access to deeper groundwater and prevent it from being washed
out of the system by floods.
2. Because temperatures are lower and precipitation at high elevations and on north facing slopes on
these southern mountains, spruce populations have been able to survive on high desert
mountains. These southern populations are highly fragmented because they are limited to very
high elevations.
3. Figure 9.10 summarizes the answer to this question.
4. If plants are distributed mainly in response to the distribution of soil characteristics, there should
be a strong correspondence between the distribution of soil characteristics and the distribution
of individuals within a plant population. Competition between plants should act to space
individuals regularly through the environment. If the sites for seedling establishment are limited,
individuals will tend to be clumped.
5. The plant reproducing from seeds should have a less clumped distribution pattern compared to
the plant species reproducing mainly by budding.
6. Assuming that the habitat could be restored, the refuges should be established in areas that once
supported high densities of fish crows (historic hotspots). Several refuges should be stocked
simultaneously in case one or more fails.
7. Mammals tend to have higher densities than birds of similar size. Terrestrial invertebrates tend to
live at lower densities than aquatic invertebrates. More than half (the increments on figure 9.19
indicate 10 fold differences in mass and density).
8. Consult figure 9.22. Species that have an extensive geographic range, broad habitat tolerances, and
large local populations are the least vulnerable to extinction, while those with restricted
geographic range, narrow habitat tolerance, and small local populations are the most vulnerable.
9. Yes, at least somewhat since large organisms tend to live at the lowest population densities, they
automatically show at least one aspect of rarity. However, size of geographic range and habitat
tolerances are not included in the analyses by Damuth and Peters and Wassenberg.
Terms to Know:
Abundance
Clumped distribution
Density
Distribution
Fundamental niche
Large-scale phenomena
Niche
Pheromone
Population
Random distribution
Realized niche
Regular distribution
Small-scale phenomena