Interactions Between Populations (OCR A-Level Biology A): Revision Notes

Interactions Between Populations

Interspecific competition

Interspecific competition occurs when individuals of different species compete for the same limited resources within an ecosystem. This type of competition can have significant impacts on population distribution, abundance, and evolutionary development of the species involved.

Competition between bluebell species

The English bluebell (Hyacinthoides non-scripta) faces a conservation threat from the Spanish bluebell (Hyacinthoides hispanica), which was introduced to UK gardens during the seventeenth century. Since 1909, Spanish bluebells have spread into woodland habitats across the UK, where they compete directly with the native species.

Both the Spanish bluebell and hybrids between the two species grow more vigorously than English bluebells. They produce highly fertile seeds that enable rapid colonisation of bluebell woodlands. Research from 2013 indicates that both invasive types flower earlier than English bluebells, potentially giving them a competitive advantage when establishing in new areas. The UK is home to approximately 50% of the world's population of H. non-scripta, making conservation of natural woodland populations particularly important.

Experimental evidence: Barnacle competition

Field experiments on rocky shores have provided clear evidence of interspecific competition. Chthamalus stellatus and Balanus balanoides are two barnacle species that inhabit rocky shore environments. While larvae of C. stellatus settle across the shore, adult barnacles only develop in certain zones, suggesting competition with B. balanoides.

Researchers designed an experiment using wire cages to exclude predators from rocks where adult C. stellatus were normally absent. In some caged areas, all B. balanoides individuals were removed, while in others they remained. Adult C. stellatus populations were monitored over two years.

Competitive exclusion

When two species have very similar requirements for their niche (the role of a species in a habitat, including its trophic level, interactions with the abiotic environment, and interactions with other species), the competition between them becomes intense. This does not necessarily involve direct fighting; instead, competition can be subtle, with the less successful species experiencing starvation or inability to reproduce.

Competitive exclusion is the principle stating that when individuals of different species compete for the same resources, one species may succeed while the other becomes excluded from the niche and completely disappears. This means each niche in an ecosystem is typically occupied by only one species, with competing species being excluded.

Resource partitioning

Species can coexist in the same ecosystem by avoiding direct competition through resource partitioning. This occurs when different species competing for similar resources occupy slightly different niches.

A clear example comes from anolis lizards in the Dominican Republic. Seven different species feed on the same prey animals (insects and small arthropods) but avoid direct competition by perching at different heights and locations to catch their food. Each species has evolved to prefer specific perching sites within the forest structure.

In tropical forests, grazers have evolved to feed at different heights, with some specialising on fruits and others on leaves.

Character displacement

Character displacement occurs when two species that compete with each other show differences in physical features where they coexist, but do not show these differences where they exist separately. This represents an evolutionary response to reduce competition.

The marine snails Hydrobia ulvae and Hydrobia ventrosa inhabit mud flat environments. When populations of these species live in separate habitats (allopatry), their shells are similar in size. However, where both species occupy the same habitat (sympatry), H. ulvae consistently develops larger shells. This size difference reduces direct competition for resources between the species.

Predator-prey relationships

Predator-prey interactions create dynamic population changes in ecosystems. When populations of primary consumers increase, such as aphids, they provide abundant food for their predators, in this case ladybird beetles.

Predator numbers typically increase following an increase in food supply. As predator populations grow, predation intensity increases, leading to a decrease in prey numbers. With fewer prey available, predator populations subsequently decline due to reduced food availability.

The lynx and snowshoe hare cycle

The classic example of predator-prey population cycles comes from Canada's Arctic regions, where lynx (predator) and snowshoe hares (prey) were extensively trapped for the fur trade during the nineteenth and early twentieth centuries. Data from various sources, including Hudson's Bay Company trapping records, revealed that both species' populations fluctuated with peaks occurring approximately every 10 years across different Canadian regions.

Understanding population fluctuations

The predator-prey cycle can be interpreted through the following sequence:

• The prey population increases when few limiting factors are present
• More food becomes available to predators, whose numbers increase after a lag period needed for reproduction
• Increased predator populations lead to more prey being consumed
• Intensified predation limits the prey population, whose numbers begin falling while predator numbers continue rising
• Fewer prey animals reduce food availability for predators, whose numbers fall through reduced reproduction and starvation
• The cycle repeats

Complexity in natural ecosystems

The lynx-snowshoe hare example represents a simplified ecosystem with one prey species and one main predator. Most natural ecosystems contain more complex feeding relationships where:

• Each predator species feeds on several prey species
• Each prey species faces predation from multiple predator species
• When one prey species declines, predators can switch to alternative food sources
• The dominant predator consuming any particular prey species may change over time

Selective predation

Predators often show size preferences when hunting prey. Research on shore crabs feeding on mussels demonstrates this selective behaviour. When offered mussels of different sizes, crabs showed preferences for particular size ranges:

Mussel groups offered	Mean smaller-size group consumed	Mean larger-size group consumed	Statistical significance
Groups 1 and 3 (15-20mm vs 26-30mm)	47	27	p < 0.05 (significant)
Groups 2 and 3 (21-25mm vs 26-30mm)	66	48	NS (not significant)
Groups 3 and 4 (26-30mm vs 31-35mm)	35	29	NS
Groups 3 and 5 (26-30mm vs 36-40mm)	47	7	p < 0.05 (significant)

These results indicate crabs prefer intermediate-sized mussels, avoiding the largest size class. This likely reflects the energy cost of breaking open larger shells versus the energy gained from consuming them.

Studies on gull predation of crabs reveal different patterns:

Crab species	Mean carapace width of living crabs (mm)	Mean carapace width predated by gulls (mm)
A	$30 \pm 3.5$	$33 \pm 2.7$
B	$65 \pm 6.4$	$48 \pm 6.2$
C	$40 \pm 3.9$	$42 \pm 4.1$
D	$27 \pm 2.8$	$25 \pm 2.4$

Species B shows gulls selecting smaller individuals from the population, possibly because larger crabs are more difficult to handle or transport. Gulls often carry crabs away from where they catch them, which may influence size selection.

Case study: Owl and vole populations

Other factors influencing vole populations might include:

• Food scarcity leading to starvation
• Competition from other herbivores (e.g., mice)
• Predation by other predators (e.g., weasels)
• Weather changes affecting plant growth and seed production
• Disease outbreaks
• Migration away from the study area

Similarly, owl populations are affected by availability of other prey species (mice, shrews) and factors causing mortality or migration beyond vole abundance.

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