Ecology, Ecosystems, and Biodiversity (LC 2027) (Leaving Cert Biology): Revision Notes

Measuring Features of an Ecosystem

Species diversity index

A species diversity index is a mathematical tool that helps ecologists measure the variety of life in a community. It's much more sophisticated than simply counting the number of different species present. The index considers two crucial factors that together give us a complete picture of biodiversity:

• Species richness - the total number of different species in the community
• Relative abundance - how evenly the individual organisms are spread among the different species

Understanding species diversity is essential for assessing ecosystem health. An ecosystem where species have similar population sizes is generally more stable and diverse than one dominated by just a few species, even if both contain the same total number of species.

Simpson's diversity index equation

Simpson's diversity index is one of the most widely used methods for calculating species diversity. The mathematical formula may look complex at first, but it's quite logical:

$D_s = 1 - \frac{\sum n(n-1)}{N(N-1)}$

Where:

• $D_s$ = Simpson's diversity index value
• $n$ = the number of individuals of each particular species
• $N$ = the total number of individuals of all species combined
• $\sum$ = sum of (you calculate $n(n-1)$ for each species, then add them all together)

Understanding diversity through examples

Let's examine how this works using woodland data comparing two different forest areas:

Practical applications of diversity indices

Species diversity indices serve many important purposes in ecological research and conservation:

• Habitat monitoring - tracking how biodiversity changes over time in response to environmental pressures
• Impact assessment - measuring how pollution, invasive species, or climate change affect local ecosystems
• Conservation decisions - providing scientific data to inform planning for new developments, road construction, or habitat protection projects

Population curves

Population curves are graphs that show how the number of individuals in a species changes over time. These patterns help ecologists understand population dynamics and predict future changes. There are two fundamental types of population growth that create distinctly different curve shapes.

J-shaped population curves

J-shaped curves represent exponential growth, where the population size increases at an ever-accelerating rate. This dramatic growth pattern occurs under specific conditions:

• Abundant resources with no apparent limits
• Ideal environmental conditions (temperature, pH, etc.)
• Absence of significant predators or diseases
• Unlimited space for expansion

S-shaped population curves

S-shaped curves demonstrate logistic growth, which more accurately reflects how most populations behave in nature. This growth pattern shows three distinct phases:

1. Initial slow growth - the population is small and reproduction is limited
2. Rapid exponential phase - conditions improve and the population expands quickly
3. Levelling off phase - growth slows as environmental limits are reached

The horizontal dashed line in the diagram represents the carrying capacity - the maximum population size that the environment can sustain over the long term. As populations approach this limit, their growth rate naturally slows and eventually stabilises.

Carrying capacity and limiting factors

Carrying capacity is determined by various limiting factors that restrict how large a population can grow in a particular environment:

When populations approach their carrying capacity, increased competition for these limited resources causes growth rates to decline and populations to stabilise around the maximum sustainable level.

Pyramids of numbers

Pyramids of numbers are visual diagrams that display the quantity of organisms present at each trophic level within a food chain. These diagrams help us understand how energy and nutrients flow through ecosystems by showing the numerical relationships between producers and consumers.

Shapes of number pyramids

Depending on the specific ecosystem characteristics, pyramids of numbers can take several different forms:

• Upright pyramids represent the most common pattern, where organism numbers decrease at each higher trophic level. This makes biological sense because many producers are needed to support fewer primary consumers, which in turn support even fewer secondary and tertiary consumers.
• Inverted pyramids occur when there are fewer organisms at the producer level than at consumer levels. This seemingly unusual pattern can happen when individual producers are very large (such as oak trees) and can support many smaller consumers (like insects and birds).
• Spindle-shaped pyramids have a narrow base and top but a broader middle section. This pattern appears when intermediate trophic levels contain more organisms than either the producers or top-level consumers.

Limitations of number pyramids

While pyramids of numbers provide useful information, they have several important limitations:

Pyramids of biomass

Biomass refers to the total mass of all living tissue in a given area. Pyramids of biomass show the total biomass present at each trophic level, providing a more accurate representation of energy relationships within ecosystems than simple organism counts.

Typical biomass pyramids

In most terrestrial ecosystems, biomass decreases as you move up through the trophic levels, creating the classic pyramid shape. This pattern occurs because:

• Approximately 90% of energy is lost between each trophic level through metabolic processes
• Large amounts of producer biomass are required to support smaller amounts of primary consumer biomass
• Each successive trophic level can sustain progressively less total biomass

Inverted biomass pyramids

In some ecosystems, particularly aquatic ones, pyramids of biomass can appear inverted or upside-down:

This occurs when:

• Rapid reproduction rates - producers like phytoplankton reproduce extremely quickly
• Short producer lifespans - producers are consumed rapidly but constantly replaced
• Long-lived consumers - organisms like fish accumulate biomass over extended periods while feeding continuously on fast-reproducing producers

Comparing measurement approaches

Each measurement method offers distinct advantages and faces specific challenges:

Remember!