INTRODUCTION TO ENVIRONMENTAL SCIENCES

LECTURE NOTES

Ecosystems III: Species Interactions, Population Dynamics,

and Ecological Succession

I. Species Interactions

Five basic types of interactions: interspecific competition, predation, parasitism, mutualism, and commensalism.

A. Interspecific Competition

Two or more species compete for the same resources (food, water, territory) either through interference competition or exploitation competition.

Interference competition: One species limits another's access to a specific resource. Usually involves the establishment of defined and defended territories.

Exploitation competition: One species exploits a resource more effectively than another. May result in competitive exclusion where one species is forced out.

Can lessen competition by resource partitioning. Different species can exploit resources at different times (day versus night), in different ways (small versus large), or in different places (different heights in a tree).

B. Predation

One species (predator) feeds directly on another (prey). However predator does not live in or on the prey.

Form a predator-prey relationship. Predator can only survive as long as there is prey. Cannot over hunt. Relationship often follows a boom-bust population cycle.

Predation improves the genetic stock of the prey (weak and sick killed) and improves the chances of those prey not killed to survive since there is less demand for food by the reduced prey population.

C. Parasitism

One species (parasite) feeds on the other (host) by living in (endoparasite) or on (ectoparasite) the host. The parasite benefits and the host is harmed. Examples include tapeworms, lice, and fungus.

D. Mutualism

Both species benefit. An example is the pollination of flowers by insects and birds.

E. Commensalism

One species benefits, the other is neither harmed nor helped in any obvious way (the harm or benefit may not be obvious, but still very real). Examples include clownfish (benefits) and sea anemones (neither) or trees and epiphytes (orchids).

The last three types of interactions are examples of symbiosis, where species live together in an intimate association (one in or on the other).

II. Population Dynamics and Carrying Capacity

A. Population Controls

Species populations are constantly changing in size (or number of individuals), density, dispersion, and age distribution. Known as population dynamics.

Population change = (births + immigration) - (deaths + emigration)

These variables are dependent on the balance between biotic (reproductive) potential and environmental resistance. Determines the carrying capacity of an ecosystem.

Usually, populations show early exponential growth (slow then fast; J-shaped curve), which is controlled by reproductive potential, until the carrying capacity for that species is neared or reached. Then growth levels off due to environmental resistance. This is known as logistic growth (S-shaped curve). Often the population fluctuates slightly above and below the carrying capacity.

Sometimes a species overshoots its carrying capacity and the population declines dramatically because of a lack of resources. Often a time lag involved between the depletion of the resources and the increase in mortality rate. Often occurs in resource-limited environments such as islands or restricted habitats where migration out after resource depletion has occurred is not possible.

Three general types of population change over time curves: stable, cyclic, and irruptive.

Some environmental stresses on populations are density dependent (i.e., resources, diseases, predation). As population becomes too dense, birth rates decline and deaths increase. This acts to lower density. Other environmental stresses are density independent (drought, flooding, fire).

B. Reproductive Strategies

There are two extreme modes of reproductive strategies: R-strategists or K-strategists. Many species fall somewhere in between the two extremes.

R-strategists: Reproduce early and in great quantities. They have short generation times and reach reproductive age early. Their large birth rate overcomes large mortality rate (i.e. insects). Their population may fluctuate wildly above and below the carrying capacity. They often are opportunistic generalists. Typically have early-loss survivorship curves. That is, most mortality occurs early and very few individuals survive to a relatively old age.

K-strategists: Reproduce late and in small numbers. Their low birth rate usually requires parental care. Have long generation times and reach reproductive age late. Populations remain fairly stable around the carrying capacity. Specialists prone to extinction. Tend to have late loss survivorship curves (fig. 5-32). That is most individuals survive to adulthood and most mortality occurs late.

Intermediate species also have intermediate loss survivorship curves at which the mortality rate remains fairly constant over time (Fig. 5-32)

III. Ecological Succession

Communities and ecosystems constantly respond to changing conditions. The plant and associated animal species found in one place will change over time. Known as ecological succession or community development. Plant growth rate, productivity, species diversity, stratification, etc. change as succession progresses.

A. Primary Succession

This is where an ecosystem develops from scratch. There was no prior ecosystems, soil, or bottom sediments in place (examples are a lava flow or a newly created reservoir). First life forms often are pioneer plant species (usually reproduce quickly), such as lichen and moss.

These pioneer plants facilitate the establishment of early successional plants (shrubs), then mid-successional plants (trees), and finally late successional plants (shade tolerant trees as part of a climax community). Animals succeed one another in step with plants.

B. Secondary Succession

This is where an ecosystem develops from a disturbed or destroyed, previously existing ecosystem, such as an abandoned farm or burned area). Usually colonized first by weeds, then grasses, shrubs, and finally trees.

Early successional plants often hinder the establishment of later species. A disturbance of some kind (i.e. fire) is required for succession to proceed.

Succession often occurs in fits and starts and in a patchwork pattern rather than in a continuous, monolithic way. May never reach a true climax community. Conditions may stabilize at an intermediate condition.

IV. Ecological Stability

All natural systems have homeostatic processes (positive and negative feedback loops) that allow them to withstand externally imposed changes or stress. That is they all have a certain level of stability within limits.

Three components to stress response: Inertia (persistence), constancy, and resilience. Often there is a time delay between the stress and the response.

Inertia (persistence): Resistance to being altered.

Constancy: Ability to maintain a certain size.

Resilience: Ability to recover.

Stress often acts synergistically. The combination of two or more stresses is greater than the sum of each individually. Positive stimuli can also be synergistic.

High species diversity has often been considered inherently more stable than other situations. Sometimes this is not the case. Some ecosystems have high diversity and inertia (rain forest), but low resilience. Others have low diversity and inertia (grasslands), but high resilience. Yet their overall stabilities are roughly the same.

Species diversity is a function of immigration rate into an ecosystem minus the extinction rate. Known as the species equilibrium model or theory of island biogeography (ecosystem does not have to be literally an island).

Small islands (or ecosystems) have lower diversity because lower immigration rate and higher extinction rates than large islands (ecosystems). Islands close to mainland (source of species) also tend to have greater diversity than those far from the source because it is easier for individuals to migrate there. Argues for keeping an ecosystem at a minimum size and connected to the source of the species (via greenways) in order to remain viable.