A new article about facilitation cascades has been published by Christine Angelini and Brian Silliman from the University of Florida. It is such an interesting contribution to our understanding of epiphyte ecology that I'm going to dedicate four blogs to it. This week: application in New Zealand.

The application of this model to the Tillandsia of Georgia worked really well, now I want to see if it will work for temperate New Zealand forests.

The primary foundation species is the host tree because epiphyte species mostly rely on host trees for access to light. Let's say we have a tawa (Beilschmiedia tawa) host tree. The bare branches will support minimal flora and fauna communities but as it grows away from the shady forest floor it will become the first step in a facilitation cascade.

Our secondary foundation species is often moss. Moss provides substrate and moisture that facilitates the arrival of invertebrates and other epiphytes such as orchids and ferns. It therefore increases both the diversity and abundance of plants and animals that can establish (type C cascade). 

But that isn't the end of the story! If a nest epiphyte species such as kahakaha (Collospermum hastatum) arrives on the moss, it will again significantly increase the diversity and abundance of flora and fauna communities (still type C). These large plants can host many plants and invertebrates as well as the occasional bird and reptile. If nest epiphytes do not establish the community will generally be smaller and less diverse. There are even species that depend on nest epiphytes, for example: tawhiri karo (Pittosporum cornifolium). Therefore we might consider nest epiphytes to be tertiary foundation species, adding another step to Angelini and Silliman's process:

I suppose it all depends on your interpretation! If you consider the moss to be the foundation species it works more like the Georgia example but then the moss requires the host tree... what are your thoughts? 

To finish, here is a photo illustrating the significant habitat that nest epiphytes can create for many other species.

A new article about facilitation cascades has been published by Christine Angelini and Brian Silliman from the University of Florida. It is such an interesting contribution to our understanding of epiphyte ecology that I'm going to dedicate four blogs to it. This week: a model to explain the application.

To understand and predict species assemblages and interactions in different facilitation cascades we can use the Foundation Species-Biodiversity model: 

According to this model, facilitation cascades can be described using three categories based on the difference that secondary foundation species make to the community:

Type A: 
Secondary foundation species support more individuals of species that are already in the community (increased abundance) by providing functional traits that are similar to those of the foundation species.

Type B: 
Secondary foundation species provide habitat for different individuals (increased diversity) by providing different functional traits than the foundation species.

Type C: 
Secondary foundation species support more individuals of the existing species AND individuals of new species (increased abundance AND diversity) through the provision of a range of traits. 

Notes: some secondary species may not significantly increase either abundance or diversity, and facilitation cascades can switch if environmental conditions change.

Examples of each facilitation cascade. Simplified from Angelini & Silliman:

Type A: 

Foundation species: shoal grass
Secondary foundation species: manatee grass

Manatee grass facilitates an increase in the abundance of resident invertebrates and fish by providing more habitat of a similar nature. In other words, more of the same services.

Type B:

Foundation species: turtle grass
Secondary foundation species: pen shells

Pen shells facilitate increased diversity by providing places for fish to lay eggs. In other words, new and different services.

Type C:

Foundation species: mangroves
Secondary foundation species: oysters

Oysters facilitate both an increase in diversity and abundance. They provide more habitat for species that already inhabit mangrove roots but also provide habitat for species that are oyster-dependant.

Next week: application of all of this theory to New Zealand epiphytes!

An exciting new article about facilitation cascades has been published by Christine Angelini and Brian Silliman from the University of Florida. It is such an interesting contribution to our understanding of epiphyte ecology that I'm going to dedicate four blogs to it. This week: an epiphyte study.

Angelini & Silliman (In Press) conducted a range of experiments in Southern USA to investigate the relationships between Southern live oak trees (Quercus virginiana) and Spanish moss (Tillandsia usneiodes) a common epiphyte that is infact in the bromeliad, not moss, family. I'll briefly run through some of the interesting things that they discovered.

Oak-Tillandisa-insect facilitation cascade:

• Oak trees are a foundation species that provide habitat for Tillandsia 
• Tillandsia is a secondary foundation species that provides habitat for a diverse and abundant community of invertebrates
  

Key evidence:
Tillandsia relies on oak trees to provide conditions that it can establish and survive in. In this part of the world the conditions on the ground are hot and dry and very unfavourable for Tillandsia.

Within an oak tree, clumps of Tillandsia (called festoons) increase the structure, or physical space, available for insect habitat as well as stabilising temperature and humidity. This increases not only the abundance of invertebrates already present in the tree, but also in the number of species, the different types of feeding guilds (e.g. decomposers, herbivores), and the life stages (e.g. juveniles, adults). These invertebrates cannot survive on the tree without the epiphytic Tillandsia.

One example of the Tillandsia providing for invertebrates also happens to be my favourite part of this study: 
The researchers found that Tillandsia acts as a nursery by protecting juvenile crickets from spider predation. This experiment involved releasing baby crickets onto A) a branch with no epiphytes and B) a branch with Tillandsia, then letting the spiders loose! After 12 hours they counted live crickets and inspected dead crickets to find that the branch with Tillandsia had a 95 % cricket survival rate while the bare branch had only 60 % and all dead crickets had spider wounds... can't argue with that!

Next week: a model to understand the roles that different species play in facilitation cascades.

Facilitation cascade theory applies when harsh environmental conditions are improved by the arrival of a foundation species. An example is the establishment of seagrass on rocky coastlines which stabilises rocks, softens the impact of breaking waves, and provides shade.

The foundation species provides habitat for a secondary foundation species. Seagrass provides stable substrate for the establishment of mussels that cannot successfully establish on bare rocks.

The combination of foundation and secondary foundation species provides the necessary conditions for many other species to establish and interact. In a coastal system the seagrass and mussels provide substrate and refuge for a diverse and abundant community that includes barnacles, snails and amphipods. 

The key concepts here are that the seagrass does not require the presence of other species to establish but the mussels need the seagrass, and the other species need BOTH the mussels and seagrass. 

The positive interactions between these species occur in a hierarchical order and, unlike successional theory, the foundation species is not replaced by subsequent species.

Next week: an example of a facilitation cascade in the canopy!