Paper: “A sixth-level habitat cascade increases biodiversity in an intertidal estuary”


Mads S. Thomsen, Thomas Hildebrand, Paul M. South, Travis Foster, Alfonso Siciliano, Eliza Oldach, David R. Schiel



Many studies have documented habitat cascades where two co-occurring habitat-forming species control biodiversity. However, more than two habitat-formers could theoretically co-occur. We here documented a sixth-level habitat cascade from the Avon-Heathcote Estuary, New Zealand, by correlating counts of attached inhabitants to the size and accumulated biomass of their biogenic hosts. These data revealed predictable sequences of habitat-formation (= attachment space). First, the bivalve Austrovenus provided habitat for green seaweeds (Ulva) that provided habitat for trochid snails in a typical estuarine habitat cascade. However, the trochids also provided habitat for the non-native bryozoan Conopeum that provided habitat for the red seaweed Gigartina that provided habitat for more trochids, thereby resetting the sequence of the habitat cascade, theoretically in perpetuity. Austrovenus is here the basal habitat-former that controls this “long” cascade. The strength of facilitation increased with seaweed frond size, accumulated seaweed biomass, accumulated shell biomass but less with shell size. We also found that Ulva attached to all habitat-formers, trochids attached to Ulva and Gigartina, and Conopeum and Gigartina predominately attached to trochids. These “affinities” for different habitat-forming species probably reflect species-specific traits of juveniles and adults. Finally, manipulative experiments confirmed that the amount of seaweed and trochids was important and consistent regulators of the habitat cascade in different estuarine environments. We also interpreted this cascade as a habitat-formation network that describes the likelihood of an inhabitant being found attached to a specific habitat-former. We conclude that the strength of the cascade increased with the amount of higher-order habitat-formers, with differences in form and function between higher and lower-order habitat-formers, and with the affinity of inhabitants for higher-order habitat-formers. We suggest that long habitat cascades are common where species traits allow for physical attachment to other species, such as in marine benthic systems and old forest.


FCs are pretty widespread phenomena but they have attracted interest only in the last 15-20 years. Then, my research begin where the few information about them stop.

In order to have a broad understanding of the role of HC in ecology, I am currently working in different marine environments (soft-bottom estuarines and rocky shores), with different 1st habitat formers (seaweeds, seageasses, invertebrates) and different 2nd habitat formers and clients (epiphytes, invertebrates).

a and b reppresent two typical situations in soft-bottom estuarines. Due to lack of hard substrates, seaweeds can only rely in molluscs to settle (a) or seagrasses to be entagled (b) becoming 2nd habitat former. In rocky shores, a similar situation can be explained through epiphytes (c) but lack of substrate is not a problem anymore unless considering the competition.

What among seaweeds?

Even if Facilitation Cascade seems to be simple concept to understand it is in real much more complicate than the expected… and even if it looks very complex, it is in real much easier than the expected…

After a quick explanation about my research, especially when I talk about hundreds or thousands of invertebrates among seaweeds, people usually look at me with that typical face like wondering “What is he talking about? Where does he see all these organisms?“.

Unfortunately, our mind (scientific and not) has always been pretty restricted to allow us to give importance to what is invisible, small… (I remind the “conquest of the invisible” by Pasteur) neglecting a huge amount of species, interactions, functions, biological processes, etc… in a single word… “information“.

It seems to be hard accepting the idea that a little piece of seaweed can host a huge amount of species. That is why in this short clip I try to visually demonstrate what happen when we wash a little piece of seaweed collected from a rocky shore.
The seaweed is a Cystophora scalaris, one of the species I am working most with, and it is a very common canopy-forming seaweed in the rocky shores of New Zealand (Agardh used to relate it to Cystopseira sp. and Sargassum sp. from Mediterranean Sea because of their common role as facilitators in coastal habitats).

The majority of little dots at the bottom of the box are gastropods… but there are still hundreds of invertebrates pretty invisible to the naked eye in a box like this, like amphipods and copepods. The pictures show a clearer view of the epifauna more accurately sieved.


In the following clip I show what happen when we compare the amount of invertebrates in two pieces of the same seaweed species. The first one is a piece of C. scalaris while the second one is a piece of C. scalaris with attached, during a recolonization experiment, a little piece of a natural epiphyte of Cystophora, Jania micrarthrodia.

The difference in the amount of invertebrates is pretty clear. Even if I manually attached the epiphyte, Jania is a natural epiphyte of Cystophora, so what is experimentally happening here is just the emulation of a natural condition. But there is something more…
After deattaching Jania from Cystophora, I can rinse the epiphyte more accurately…

Obviously this quick clip does not want to have any scientific relevance and it is mean to give a very rough idea of what happen when an epiphyte increases the complexity of a seaweed structure and architecture with full advantage for the associated epifaunal assemblage.
Several are the factors here not included: the different size of the seaweed, the rest of smaller and “invisible” invertebrates for naked eye, the accuracy of the washing, etc.
Anyway, a more true view of the invertebrates can be offered by the next pictures, after an accurate sieving.

The epifauna I am considering has usually a size range between 250 μm and 2 mm and the majority of these are amphipods, copepods and gastropods.
In my PhD I am focus on the whole community of invertebrates. Nonetheless my primary focus are gastropods since they can be considered very reppresentative of the epifauna community for my purposes:
– they are slow-moving invertebrates compared with amphipods, copepods or crabs, so they can be collected easly without any virtual lack of information;
– they are relatively easy to identify, at least with a categorization based on morpho-species;
– they reppresent a very high proportion of the community, statistically strong, and a very diverse category of invertebrates: in my studies, gastropods reppresent more and less the 60-70% of the whole biodiversity.