Invasive Species: Passengers Or Drivers Of Native Declines?

Today I've decided to investigate a real doozy of an ecological question: whether or not invasive species actually cause the decline of natives or if other factors, such as habitat degradation, are responsible. 

Also, I've really enjoyed reading the blog Goodbye,birdsong?, so I thought inspecting invaders should also give some due attention to our feathered friends (wouldn't want any RSPB members to get in a flutter). 

And I'm doing it the style of a police report. I miss my past life sometimes...

CASE REPORT

The Question: Are invasive species drivers of native species decline or passengers of habitat modification? 

The Case: The impact of the invasive common myna (Acridotheres tristis) on native bird species. (Specifically, is the common myna impacting native species or is it down to forest density?)

Chirpy chappy: The common myna

Location: Canberra, Australia. 

No they aren't random cat heads, they're the distribution of the common myna in 1900-81 (left) and 1998-2000 (right), highlighting its territorial expansion.

Prior knowledge: Habitat modification and invasive species play significant roles with regard to the state of biodiversity. However, distinguishing between the two is often difficult. This is referred to as the 'driver-passenger' model ('passengers' take advantage of habitat modification whereas 'drivers' actually cause native species declines). 

The common myna is 1 of only 3 birds in the IUCNs 100 of the worst invasive species list (Lowe et al., 2000). Concern that it displaces natives through competitive territorial dominance, however, evidenced is mixed. 

Gurevitch and Padilla (2004): Native species impacts can result from invaders directly or compounded by a modified habitat. Research often investigates the anthropogenic habitat impacts or the effect of invaders on natives. This means drivers of change could be mistakenly identified.

Hypothesis: Common myna is both a passenger of habitat change and a driver of some native bird declines.  

The Evidence: Collected by Grarock et al., 2013 in a 2.5 year study investigating changes in native bird abundance in relation to different habitat types and common myna abundance. 

Illustration of the different habitats considered in the study.

Results: 

• Significant relationships uncovered between common myna abundance and tree density. Abundance declines as tree density increases.This suggests that the common myna is a passenger of habitat change.
• No significant relationship found between common myna abundance and total species richness, which is strange as invasive species normally dominate over native species. 
• However, there is a negative relationship between common myna abundance and the gang-gang cockatoo (Callocephalon fimbriatum), potentially driving its decline. 8/20 native small birds shown to be negatively impacted by the common myna. 
• Significant positive relationship between common myna abundance and large bird abundance, but not effect on species richness.

  

  Goodbye gang-gang? The common myna is implicated in its decline.

Key Findings:

1. The common myna is a passenger of habitat change.
2. In combination with habitat change, the common myna appeared to be a driver of some bird species declines.
3. It is essential to simultaneously investigate invasive species impact and habitat modification to assist with differentiating the impacts on natives.

Discussion points:

• Study not comprehensive, it does not include potential influence of other species. Acknowledged that it would be difficult to account for more complex ecological interactions.
• Fragmentation of native vegetation and reduced tree density may enhance habitat quality for the common myna, enabling the species to spread to new areas and compete with natives. This could be relevant to countries such as Madagascar, Indonesia and other countries experiencing deforestation in the presence of the common myna.  

Conclusions:

Results suggest impacts of habitat modification and invasive species are interrelated. Many species are strongly influenced by habitat, however, not all species appreciate the same habitat conditions. Both factors can substantially impact native taxa. It would be wrong to say either one is more important in causing declines as each species should be treated uniquely with regard to the threats it faces. 

Sorry for a cop-out, fence sitting conclusion, but I feel it's the most appropriate one. Are invaders drivers or passengers? The answer would have to be that they can be both. Case closed.

Over and out

The Invader Inspector