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.

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.

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?

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.

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