This is a purposefully vague category (most of the other sections involve interactions) where either part of the ‘trick’ to the answer is to work out what interaction is actually occurring (so putting it in the section on Predation, for example, takes away part of the exercise) or the data show more than one interaction occurring (for example predation affecting competitive effects).
6.1 Wild dogs and dik dik in Laikipia
Laikipia in Kenya was recolonised by wild dogs following a 20 year absence. 60% of the diet of wild dogs is dik-dik, the most abundant ungulate in the region. Dik-dik limit the biomass of trees. Other grazers in this system include impala, elephants and giraffes. Around the time that wild dogs recolonised, there was a 30% increase in rainfall in this area.
Researchers followed changes in the system for 7 years after wild dogs recolonised. What would you have expected to happen in this situation?
Do the data presented here support this expectation?
Figure 2 shows the results across the park in the 7 years after wild dogs returned
Figure 3 shows experimental work to determine the response of trees to herbivores in test plots
Source: Ford et al. 2015. Ecology; 96: 2705-2714
Figures 2 and 3 reproduced with permission of John Wiley & Sons.
©2015 by the Ecological Society of America.
Where top predators have been reintroduced (naturally or by people) to ecosystems from which they had been lost, there is sometimes a phenomenon known as trophic cascade. The top predators reduce numbers of herbivores which enables additional growth of vegetation that was previously suppressed by heavy grazing. Thus the top predators have an indirect effect on vegetation. The effect of wolf reintroduction on willow and aspen through their predation of elk, in Yellowstone National Park has become a classic example of a trophic cascade.
When wild dogs recolonised Laikipia in Kenya, researchers expected to see a similar trophic cascade. They predicted that wild dogs would suppress populations of dik-dik, a browser which feeds on trees. Smaller populations of dik-dik would then have a reduced impact on tree growth, resulting in an increased numbers of trees. However, although the grazing was clearly having an impact on tree growth (graph 1), the researchers could not detect any difference in tree growth between the number of trees before and after the wild dogs recolonised the area (graph 2). This suggests that there was not a trophic cascade.
Possible reasons for the lack of a trophic cascade could be that these effects take more than 7 years to be detectable. Additional rainfall might have increased the growth of trees in areas experiencing higher grazing pressure thus counter-acting the effect of heavy browsing. Finally, other grazers may have filled the gap left by dik-dik.
6.2 Mink and otters in the UK
Otter disappeared from much of the UK around 100 years ago due to hunting and poisoning. Mink escaped from various fur farms and established themselves across much of the UK during the same time.
Otters are now recovering. Researchers speculate that mink and otter will compete and that otter are likely to be more successful as they are larger than mink.
What do the graphs opposite suggest has happened?
Source: Harrington et al 2009. Ecology; 90: 1207-1216
Figures 1 and 3 reproduced with permission of John Wiley & Sons.
© 2009 by the Ecological Society of America.
6.4 Snails in gardens
As a gardener, I get upset when slugs and snails eat my plants. For five years, I removed snails from my garden. Last year I gave up. The number of snails that I observed in my garden remained the same as in previous years and there was no increase to the amount of damage to my plants. What’s going on?
6.6 Chalk grassland diversity
The figure shows data from a chalk grassland. Chalk grasslands generally have high species diversity. This dataset shows the change in species richness over 15 years following abandonment of management (grazing). Brachypodium pinnatum is a native grass species that is usually found at low frequencies in chalk grassland.
Cover of Brachypodium is shown by the green line and left hand y-axis; number of species in the grassland shown by the dashed orange line and the right hand y-axis
What ecological hypothesis could explain the pattern shown in the figure?
Source of data: Bobbink & Willems. 1987. Biological Conservation, 40: 301-314.
6.7 Seabird feeding interactions
Seabird populations are generally regulated by bottom-up processes. As their food sources tend occur patchily in the environment, these birds often live in large colonies. The downside of this is that intra- and inter-specific competition for food can be very intense. Different options to overcome these issues include foraging at different levels of the ocean and at different distances from the shore.
Pratte et al studied this by observing the food types in the bills of auks feeding their chicks (top graph). They also analysed stable isotopes of C and N in the blood of adult birds (this gives an indication of the prey on which the birds had been feeding). The bottom graph represents these data and interprets the values of δ15N‰ and δ13C‰. What do these data suggest is happening in this seabird colony?
Source: Pratte et al. 2017. Marine Ecology Progress Series, 572: 243-254.
Lower figure reproduced with the permission of Inter Research.
© Inter-Research 2017