Ecological Concepts: Biomagnification, Biodiversity, and Population Dynamics

Ecological Concepts

Biomagnification and Bioaccumulation

Biomagnification is the process in which chemical substances become more concentrated at each trophic level. This occurs because organisms at higher trophic levels must consume more biomass to meet their requirements.

Bioaccumulation refers to the build-up of a chemical substance in the tissues of a single organism.

Indicator Species and Biotic Index

Indicator species are sensitive to specific environmental conditions and consequently have a limited range of tolerance.

A high biotic index indicates the presence of many pollution-sensitive organisms, denoting an unpolluted environment.

A low biotic index indicates a polluted environment, due to a relative abundance of pollution-tolerant organisms.

Biodiversity

Biodiversity describes the variety and variability of all living organisms within a given ecological area.

Species richness describes the number of different species present in an area (more species = greater richness).

Species evenness describes the relative abundance of the different species in an area (similar abundance = more evenness).

Simpson's Reciprocal Index

The Simpson’s reciprocal index can be used to measure the relative biodiversity of a given community.

A high index value suggests a stable site with many different niches and low competition (high richness and evenness).

A low index value suggests a site with few potential niches where only a few species dominate (low richness and evenness).

The index value may change in response to an ecological disturbance (such as human intervention or natural disasters). This is known as the edge effect, whereby species distribution is influenced by the divergent environmental conditions.

Edges and Habitat Corridors

Edges tend to have greater biodiversity, as different habitats with different abiotic factors exist in close physical proximity. However, edges tend to have more competition than central regions, which may restrict survival prospects of certain species.

Habitat corridors between parts of a fragmented habitat can connect disparate regions to improve genetic diversity.

Population Dynamics

Capture-Mark-Release Recapture

The capture-mark-release recapture method is a means of estimating the population size of a motile species.

Exponential Population Growth

Exponential population growth will occur in an ideal environment where resources are unlimited. In such an environment, there will be no competition to place limits on a geometric rate of growth.

Initially, population growth will be slow as there is a shortage of reproducing individuals that may be widely dispersed. As population numbers increase, the rate of growth similarly increases, resulting in an exponential (J-shaped) curve.

This maximal growth rate for a given population is known as its biotic potential. Exponential growth can be seen in populations that are very small or in regions that are newly colonized by a species.

Logistic Population Growth

Logistic population growth will occur when population numbers begin to approach a finite carrying capacity. The carrying capacity is the maximum number of a species that can be sustainably supported by the environment.

As a population approaches the carrying capacity, environmental resistance occurs, slowing the rate of growth. This results in a sigmoidal (S-shaped) growth curve that plateaus at the carrying capacity (denoted by κ). Logistic growth will eventually be seen in any stable population occupying a fixed geographic space.

Exponential Growth Phase: N + I >>> M + E (much greater than)

Transitional Phase: N + I > M + E (greater than)

Plateau Phase: N + I = M + E (equal)

Top Down and Bottom Up Factors

Top down factors are pressures applied by a higher trophic level to control the population dynamics of the ecosystem. The top predator either suppresses the abundance of its prey or alters its behavior to limit its rate of population growth.

Top down control results in an oscillating trophic cascade (suppression at one level increases numbers at the next level). Keystone species commonly exert top down control by preventing lower trophic levels from monopolizing essential resources.

Bottom up factors are pressures that limit the availability of resources to lower trophic levels (e.g., producers). A lack of resources at lower trophic levels suppresses the abundance of organisms at higher trophic levels.

Population growth will be reduced for all higher levels as the suppression of the 'bottom' restricts energy supply to the 'top'. Human activity can often limit resource availability and hence inadvertently exert bottom up pressure on an ecosystem.

Algal Blooms and Eutrophication

Algal blooms are typically caused by the sudden enrichment of nutrients in the water due to run-off (eutrophication).

Algal blooms generally have a detrimental effect on the wider aquatic ecosystem:

• The spread of algae will block out sunlight below the surface and reduce photosynthesis by phytoplankton and seaweeds.
• The reduction in light will cause algae to respire instead of photosynthesize, reducing levels of dissolved oxygen in the water.
• As algae begin to die, an increase in the numbers of bacterial decomposers will further reduce levels of dissolved oxygen.
• Without adequate levels of light or dissolved oxygen, most aquatic organisms within the environment will struggle to survive.

Algal blooms can be limited by measures that exert either bottom up control or top down control. The most success will be had if bottom up and top down control measures are used in combination.

Bottom Up Control

• Algal blooms can be reduced by limiting the supply of nutrients such as nitrogen and phosphorus in the water.
• This may involve reducing the use of fertilizers for agricultural practices to limit the nutrient input from surface runoff.
• Nutrient reduction can be expensive to implement and difficult to police, as it requires a concerted community effort.

Top Down Control

• Algal blooms can be reduced by introducing piscivorous (fish-eating) fish into the aquatic ecosystem.
• The piscivores will feed on zooplanktivores – and by reducing their numbers, will increase the number of zooplankton.
• Zooplankton (such as Daphnia) feed on algae, and hence will reduce the population of algae via herbivory.
• Introducing piscivores can have unintended consequences on food webs and should be done with caution.

Eutrophication is the enrichment of an ecosystem (typically aquatic) with chemical nutrients (nitrates, phosphates, etc.).

An increase in nutrient supply within waterways will result in several ecological consequences:

• A rapid growth in algal populations will occur (algal blooms) as a result of the increased availability of nutrients.
• As the algae die, there will be a subsequent spike in the numbers of saprotrophic microbes (decomposers).
• The high rate of decomposition will result in an increased biochemical oxygen demand (BOD) by saprotrophic bacteria.
• The saprotrophs will consume available quantities of dissolved oxygen, leading to deoxygenation of the water supply.
• Eutrophication will also increase the turbidity of the water, which will reduce oxygen production by photosynthetic seaweeds.
• This will stress the survival of marine organisms, potentially leading to a reduction in biodiversity within the ecosystem.