Paper: “Effects of local anthropogenic stressors on a habitat cascade in an estuarine seagrass system”

Siciliano et al., 2019 Effects of local anthropogenic stressors on a habitat cascade in an estuarine seagrass system_1

Alfonso Siciliano, David R. Schiel and Mads S. Thomsen



Recent research has shown that co-occurring primary and secondary habitat-forming species typically support higher biodiversity than do monocultures of the primary habitat-former alone. However, these ‘habitat cascades’ may not be universal and it is important to know whether, when and where positive effects on biodiversity from secondary habitat-forming species change to negative effects. Here, we tested how anthropogenic stressors (fertilisation and sedimentation) and unattached secondary habitat-forming Ulva seaweeds affected the primary habitat-forming seagrass, Zostera muelleri, and its associated invertebrates in the Avon–Heathcote Estuary, New Zealand. We experimentally stressed Zostera by adding different fertilisation and sediment levels. Fertilisation had little impact, whereas even low sedimentation levels had strong negative effects on Zostera and its associated fauna. In a second experiment, sediments and Ulva were added to seagrass beds and unvegetated mudflats to test whether sediment stress modifies habitat cascades. We found again strong negative effects of sediments on Zostera, irrespective of spatio-temporal conditions, and that negative effects of sediments on invertebrates were enhanced in the presence of the secondary habitat former. These results highlighted that anthropogenic stressors can destabilise habitat cascades; processes that may be of particular importance in estuaries that are characterised by low biodiversity and stressful environmental conditions.

Conference: “Habitat cascade destroyed in the Kaikoura earthquake”

New Zealand Marine Science Society conference (NZMSS) 2017, University of Canterbury, New Zealand


Mads S. Thomsen, Isis Metcalfe, Alfonso Siciliano, Tommaso Alestra, Stacie Lilley, Shawn Gerrity, David R. Schiel



It is well-described how anthropogenic activities and natural disaster can destroy primary habitat-forming species, like seagrasses, corals and kelps. However, less research and conservation effort has focused on how these types of disturbances affect associated secondary habitat-formers, like epiphytes, and animals depending on biogenic habitat. In this talk we will first introduce the concept of habitat cascades with examples from New Zealand rocky shores and compare them to habitat cascades from other ecosystems. We will then show that intertidal primary (fucoid hosts) and secondary (seaweed epiphytes) habitat-formers and their inhabitants (small mobile invertebrates) have been decimated on reef along a 100km swathe of coastline that were uplifted by 1-6m by the recent 7.8mW Kaikoura earthquake. Finally, we will discuss potential cascading ecological effects, future scenarios for natural recovery and whether restoration is a viable option to speed up the recovery of habitat cascades on these degraded reefs.

PNZ1 – Host variety enhances diversity: the role of multiple secondary habitat-forming seaweeds in facilitating estuarine invertebrate communities

This project describes the simplest habitat cascade system documented during my PhD, in soft-bottom shells-bed estuaries. Here, my aim was to test if habitat cascades are strongly affected by the biomass of the secondary habitat formers and if different secondary habitat former species (i.e., ecologically and morphologically different) facilitate clients differently. Feature of this project was the strong ecological and morphological difference between the primary and the secondary habitat formers (respectively a bivalve and two species of seaweeds). This difference makes the seaweeds relevant in creating a strong habitat cascade, providing a large number of features absent in the primary habitat formers and providing, therefore, additional opportunities of facilitation (such as sheltering within the fronds, moisture retention in the soft tissues for intertidal stress buffering, disproportionate increase of settlement space, etc.). Then, I expected that seaweeds increase enormously the benefits provided by the cockles for the local invertebrate community and, therefore, that habitat cascades are of paramount importance in these habitats.


The aim of this study was to quantify the variability in habitat cascades supported by two ecologically and morphologically different secondary habitat formers, the sheet-forming green seaweed Ulva sp. and the coarsely branched red seaweed Gracilaria chilensis, through the following hypotheses:

(1) the invertebrates’ abundance, diversity and community structure depend on the biomass and/or the identity of the secondary habitat former, and that Gracilaria typically have higher diversity and abundances because it has a more structural complex form;

(2) the invertebrates have different host-specificity for Ulva and Gracilaria in virtue of their ecological differences, and that herbivorous invertebrates generally prefer the simpler Ulva whereas small species that could be susceptible to high predation (e.g., juveniles and slow moving soft crabs) generally prefer the more complex Gracilaria;

(3) that these effects (see point 1 and 2) are stronger in northern regions and summer months because metabolic processes, feeding and predation rates are higher in warm and cold temperate conditions;

(4) the secondary habitat former morphology (foliose vs branched) and type (living vs mimic) are more ecologically relevant than the primary habitat former type (living vs dead vs mimic) in driving the invertebrates’ assemblage (where ecological relevance are calculated from sum of squares explained in Anova models);

(5) the seaweeds have an important role in reducing predation pressure providing shelter for gastropods and the predation rate is dependent on the biomass and morphology of the seaweeds.

PNZ2 – High level habitat cascades: a comparison in estuarine environments

As complementary to the previous project, here I described a higher-level habitat cascade, again in soft-bottom estuarine communities, testing how a higher order habitat cascade affects client diversity. Here, the primary habitat former was again a bivalve while both the secondary and tertiary habitat formers were seaweeds but different in species and morphologically features (see previous project). To date, there are only two cases of ‘long habitat cascades’ described but I believe that they are common and widespread even if not enough investigated.


In this study I hypothesized the existence of a 4-level habitat cascade based on the successive interaction between the bivalve Austrovenus stutchburyi, the green seaweed Ulva sp. and the red seaweed Gracilaria chilensis, testing the following hypothesis:

(1) the 4-level habitat cascade is more stable than the corresponding 3-level one previously described (AustrovenusGracilaria-invertebrates), i.e., it supports larger abundance and richness of invertebrates as a result of a more structurally complex interaction;

(2) this condition is consistent across season, with more noticeable effects in summer;

(3) the contribution of Ulva as 3rd habitat former is relevant across latitudes, with stronger effects in northern regions, irrespective of the condition of the second habitat former (here, artificial mimic);

(4) similar effects are reported when the habitat formers are non-living (here, artificial mimic), as a result of the contribution of Ulva’s mimics to the morphological features of the habitat cascade.

PNZ3 – Effects of drifting seaweeds on habitat cascades in soft-bottom seagrass systems

With a change of habitat, this project described habitat cascades supported by seagrass and drifting/entangled seaweeds, respectively as primary and secondary habitat formers, testing which factors can shape habitat cascades in soft-bottom seagrass systems. Compared to the project 1, here the differences between primary and secondary habitat formers are less pronounced as both of them are plants, able to provide shelter, resources, stress buffering in an analogous way. Then, I expected less noticeable effects of the secondary habitat former on the potential effect of the habitat cascade compared to project 1.


In this study, I described an habitat cascade based on the facilitation that the seagrass Zostera muelleri provides toward the drifting seaweeds entangled on its leaves, hypothesizing that:

(1) the presence of a secondary habitat former (here, Ulva) on the seagrass bed has a relevant role in controlling habitat cascades in soft-bottom estuaries, i.e., I expect to find more complex invertebrates’ communities when seagrass and Ulva are co-occurring;

(2) these habitat cascades occur over a wide range of spatio-temporal conditions, including across latitudes, estuaries, seasons;

(3) the secondary habitat former biomass and ‘type’ (here whatever the the seaweed is alive or an artificial mimic) control abundance and biodiversity of invertebrates, i.e., I expect to find more abundant and richer communities when living and abundant seaweeds occur;

(4) the invertebrates’ communities and HCs associated with morphologically different secondary habitat former mimics are different across latitudes;

(5) gastropods use the secondary habitat former as a predation shelter.

PNZ4 – Effects of local anthropogenic stressors on habitat cascades in soft -bottom seagrass systems

As complementary to the previous project, here I described how anthropogenic factors, such as nutrient enrichment and sediment pollution, can reduce the seagrass performances and how high rate of sedimentation can destabilize the habitat cascades existing in seagrass systems.


Here, I investigated the effects of eutrophication and sedimentation on the Zostera muelleri performances and tested how sedimentation affects the habitat cascade based on the interaction seagrass – drifting seaweeds, testing if:

(1) impacts of nutrients and sediments on seagrass performance and seagrass associated invertebrates are dose-dependent and interactive;

(2) sediment stress affect the strength and direction of how drift alga affect seagrass and seagrass asscoated invertebrates (i.e., if sediment modify the strength of habitat cascades).

PNZ5 – Are habitat cascades similar between morphologically comparable canopy-forming hosts and epiphytes?

This project described how habitat cascades supported on rocky shore by brown canopy-forming seaweeds were shaped in presence of epiphytes, respectively as primary and secondary habitat formers. In line with the other projects, here I tested how the presence of the secondary habitat former can create habitat cascades and affect the local epifaunal. Nevertheless, in this project the two key features consisted in (i) testing the effects of three similar looking primary habitat formers simultaneously (the brown seaweeds Cystophora spp.) and (ii) testing the effects of ecologically more similar primary and secondary habitat formers compared to the previous projects (here both seaweeds), i.e., able to provide analogous benefits.


This study described a rocky shore habitat cascade, based on the interaction between the seaweeds Cystophora spp. and the epiphytes naturally supported, addressing the following hypothesis:

(1) three congeneric Cystophora species support similar abundances, taxonomic richness and community structure of gastropods;

(2) presence of epiphytes on Cystophora changes community structures and increases abundances and richness of gastropods, i.e. that epiphytes provide HC on Cystophora species;

(3) these HCs occur over a wide range of spatio-temporal conditions, including across latitudes and between reefs and seasons;

(4) epiphyte biomass and epiphyte ‘type’ (here whatever the epiphyte is alive or an artificial mimic) modify the strength of HCs, with larger gastropod abundances and richness for living and abundant epiphytes (in part because many gastropods are herbivores that may consume the epiphyte);

(5) the gastropod communities and habitat cascades associated with Cystophora host are very different when compared with gastropods communities associated with the morphologically and taxonomically very different H. banksii host.

PNZ6 – Effects of habitat cascade on the secondary production in rocky intertidal seaweeds-dominated systems

This project, finally, described how habitat cascades (the same described in the previous project) can affect the secondary production in rocky shore systems. Compared to all the previous projects, where classical community descriptors (such as abundance, richness and community structure) were considered, here I estimated the secondary production as function of epifaunal body biomass and water temperature using an allometric equation.


Here, I am described a rocky shore habitat cascade, based on the interaction between the seaweeds Cystophora spp. and the epiphytes naturally supported, testing the effects on the secondary production:

(1) three congeneric Cystophora species provide similar secondary production as a result of similar epifaunal load resulting from their taxonomic relatedness;

(2) the presence of epiphytes on Cystophora increases the secondary production as a result of a higher abundance and richness of invertebrates;

(3) the production varies across latitudes and peaks in northern locations, due to the higher physiological rates;

(4) the production varies across seasons and summer productivity exceeds the winter productivity as a consequence of temperature on the metabolic rates;

(5) the secondary production is strongly dependent on the biological attributes of the epiphytes (here living vs artificial mimic) and I expect that productivity in treatments with living epiphytes exceeds the production of treatments with artificial epiphytes, because of the duplex function of epiphytes in providing habitat and trophic resources.

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.