Removing Scarface Claw Doesn’t Make Rats Galore

ANDREW VEALE (Manaaki Whenua/Landcare Research, Auckland)

With the proposed removal of feral cats in the northern block of Aotea, it is a good time to discuss both the effects of cats on island ecosystems, and some of the myths around what will happen if they are controlled or eradicated.  The scientific literature is very clear that cats have significant effects on island ecosystems, and reviews of the effects of cat eradication have shown a multitude of benefits across taxa of these eradications(1), (2)  Few people debate that cats have a severe negative effect on some bird, bat and lizard species on Aotea, but one aspect of cat eradication that has received quite a lot of attention is the possibility of mesopredator release – whereby removing cats could cause undesired consequences, by releasing invasive prey species (in this case rats) which may have worse effects on native prey species.  

Introduction

One example of invasive species release that has often been cited is on Macquarie Island, where cats were removed before the rats and rabbits, and the rabbit population appeared to increase after cat removal.  While some researchers considered the removal of cats on Macquarie Island to be the primary factor in an increasing rabbit population(3), this ignored the rabbit population cycles that had occurred since early studies in the 1950s, and also discounted the fact that rabbit numbers were considered to be higher in the mid 1970s when cat populations were uncontrolled. More recent studies(4) have argued that actually cat eradication did not greatly affect the rabbit population, and instead researchers believe the natural fluctuations of seed and grass abundance was primarily responsible for any changes of abundance around this time for the rabbits.

What about in a New Zealand setting?

There are three kinds of study relevant to what will happen on Aotea:

1. Simulation modelling studies of mesopredator release

2. Studies of the effects of cat eradications, and

3. Studies of mesopredator release in NZ.  All of these point to the risk of mesopredator release being minimal.

A simulation study of mouse/rat/stoat dynamics in New Zealand found that simulated rat populations did peak higher when stoats were significantly decreased (>90 killed) however these effects were minor compared to the bottom up effects of removing mice and rats causing a crash in stoat abundance(5).  Also, in another simulated study found it was highly unlikely that mesopredator release would occur in most New Zealand systems because they are primarily bottom up regulated(6), whereby seed and fruit abundance leads to more rodents, leading to more cats, rather than being top down regulated where cats control rats, and rats control the amount of seed and fruit.  

The best empirical study to date looking at mesopredator of New Zealand pest mammal populations looked at mice/rat/stoat/possum interactions(7).  This was done with four study locations in the North Island, controlling stoats and possums and looking at their effects on rat and mouse densities.  They found that there was no measurable response of two mesopredators (rats and mice) following control of the top predator (stoats), but there was competitive release of rats following removal of a herbivore (possums), and competitive release of mice following removal of rats.  In another study, this time in grassland, high mouse abundance occurred only on the predator suppression site with regular production of pasture seed, indicating that this food resource was the main driver of mouse populations(8).

In another study cat/rat dynamics on islands off the coast of Madagascar(9), the scientists found that there was primarily a bottom up regulation of the relationship, meaning rats and cats having additive effects, but removing cats did not leading to better survival of seabirds.  Part of the reason for this is that for long lived species such as seabirds, population growth is much more sensitive to changes in adult survival than to changes in breeding success(6), (9).  Generally, if cats are eradicated on islands, it is unlikely that any decrease in seabird breeding success caused by rat predation on chicks would exceed the benefits accrued due to a significant increase in adult survival (10), (11), (12).

Conclusion

Altogether these studies indicate it is unlikely that the proposed cat removal could result in mesopredator release on Great Barrier Island.  It is unlikely that rats are primarily regulated by cat densities, and even if there is an effect, it is unlikely that the negative effects of any increase in rat population would have a greater effect than the previous rat and cat predation experienced.  One final important note is that on Aotea, while removal of cats is occurring in the northern block, rat control is also commencing, therefore even if there was a small effect of cats on rats, the rat populations are already going to be controlled.

The easiest way to imagine the question “do cats control rat populations” is “can you empty a river with a bucket?”.  If you had a huge number of buckets constantly being filled, and the river has a small flow, the level might go down a little, but it would never empty.  This is the same, for cats versus rats.  There will never be enough cats to kill the rats, because for every rat that is killed, a vacant position becomes available, which is rapidly filled with a new rat, because generally rats breed quicker than the cats can eat them.

References:

1. Medina, FM. et al. (2011). A global review of the impacts of invasive cats on island endangered vertebrates.  Global Change Biol 17: 3503–3510.

2. Nogales M, Martin A, Tershy BR, Donlan CJ, Veitch D, Puerta N, Wood B, Alonso J (2004). A review of feral cat eradication on islands. Conserv.Biol.18: 310–319.

3. Bergstrom DM, Lucieer A, Kiefer K, Wasley J, Belbin L, Pederson TK, Chown SL (2009). Indirect effects of invasive species removal devastate world heritage island. J Appl Ecol 46: 73–81. 

4. Springer, K (2016). Methodology and challenges of a complex multi-species eradication in the sub-Antarctic and immediate effects of invasive species removal. NZ J Ecol 40: 273-278.

5. Tompkins DM and Veltman CJ (2006). Unexpected consequences of a vertebrate pest control: predictions from a four species community model. Ecol Appl 16: 1050–1061.

6. Russell JC, Lecomte V, Dumont Y, Le Corre M (2009). Intraguild predation and mesopredator release effect on long-lived prey. Ecol Model 220: 1098–1104.

7. Ruscoe W. et al. (2011). Unexpected consequences of control: competitive vs. predator release in a four-species assemblage of invasive mammals. Ecol Lett 14: 1035–1042.

8. Norbury G, Byrom A, Pech R, Smith J, Clarke D, Anderson D, Forrester G. (2013). Invasive mammals and habitat modification interact to generate unforeseen outcomes for indigenous fauna. Ecolog Appl 23: 1707–1721.

9. Ringer D, Russell JC, Le Corre M (2016). Trophic roles of black rats and seabird impacts on tropical islands: Mesopredator release or hyperpredation? Biolog Cons 185: 75–84.

10. Dumont Y, Russell JC, Le comte V, Le Corre M (2010). Conservation of endangered endemic seabirds within a multi predator context: the Barau’s petrel in Réunion Island. Nat Resour Model 23: 381–436.

11. Bonnaud E, Zarzoso-Lacoste D, Bourgeois K, Ruffino L, Legrand J, Vidal E (2010). Top-predator control on islands boosts endemic prey but not mesopredator. Anim Conser 13: 556–567.

12. Hughes BJ, Martin GR, Reynolds SJ (2008). Cats and seabirds: effects of feral domestic cat Felis silvestris catus eradication on the population of sooty terns Onychoprion fuscata on Ascension Island, South Atlantic. Ibis 150: 122–131.