The important role of indirect facilitation, like trophic cascade and keystone predation, in structuring communities have been documented over many decades and across ecosystems. By contrast, indirect facilitation mediated by habitat cascades (where ‘inhabitants’ organisms are facilitated through sequential habitat formation or modification) is less studied, and these processes are not covered in ecological text books or conservation practices. This could be because habitat cascades are ecologically unimportant, or, alternatively, highlights a major research gap.
In this thesis, I investigated the core hypothesis that habitat cascades can be key drivers of biodiversity in marine benthic ecosystems. To test this hypothesis, I combined descriptive and experimental field and laboratory studies aimed at improving our understanding of the mechanisms underpinning habitat cascades via three broad research objectives: (i) quantifing the variability in habitat cascades under different environmental conditions, (ii) testing mechanisms that increase or decrease habitat cascades, (i) testing how habitat cascades can be affected by human stressors.
In Chapter 2, I described two new habitat cascades from relatively ‘simple’ sedimentary estuarine shell beds, where small infaunal bivalves (Austrovenus stutchburyi, primary habitat former) provide substrate for large and form-functionally different seaweeds (Ulva sp. and Gracilaria chilensis, secondary habitat formers). To date, most research on habitat cascades has focused on interactions between a single primary and secondary habitat former studied on small spatio-temporal scales, thereby questioning if habitat cascades have broad ecological relevance. I tested if habitat cascades, when standardized by seaweed biomass, are stronger at high than low abundances of the secondary habitat former and when the secondary habitat former has high (Gracilaria) compared to low (Ulva) morphological complexity. I also tested if habitat cascades are stronger at higher latitudes, where intertidal desiccation stress is stronger, and when secondary habitat formers are alive compared to mimics. In contrast to my hypotheses, I found weaker habitat cascades at high abundances and for the coarsely branched habitat formers, and I found no patterns across latitudes; however, as expected I did find stronger habitat cascades for living than mimic of secondary habitat formers.
Chapter 3 described, from the same estuarine sedimentary system, a rare example of a ‘higher-level habitat cascade’. Virtually all habitat cascade studies have tested if and how two co-occurring habitat-forming species affect biodiversity compared to systems dominated by a single habitat-forming species. My aim here was to document a new ‘long habitat formation cascade’ where the primary bivalve Austrovenus provides attachment space for the secondary seaweed Gracilaria, that again provides substratum for the tertiary epiphytic seaweed Ulva. I tested if this long bivalve-seaweed-seaweed cascade affected mobile invertebrates and if it is a general process operating across Gracilaria biomasses, seasons, elevation levels, sites and estuaries. My study confirmed that Ulva increased invertebrate abundances and altered community structures, whereas increases in taxonomic richness only was observed under a smaller subset of environmental conditions. These positive effects were, however, not supported for non-living Ulva mimics, suggesting that common invertebrates graze on Ulva.
In Chapter 4 I described a new habitat cascade from a seagrass-dominated system where unattached seaweeds (Ulva, secondary habitat former) can become entrapped and entangled around seagrass leaves (Zostera muelleri, primary habitat former). I tested the hypotheses that (i) the presence of seaweeds entangled in estuarine seagrass beds modify biodiversity via cascading habitat formation, (ii) similar processes occur across a wide range of spatial and temporal conditions, and (iii) the biomass and the structural attributes of seaweeds (comparing living vs artificial mimics) modify the strength of habitat cascades. I found that entangled seaweeds, across latitudes, elevation levels and seasons, consistently increased the abundance and richness of invertebrates and I also found stronger facilitation of invertebrates in high than low seaweed biomass and by live than mimic seaweeds. Furthermore, an experiment, using different seaweed mimics showed consistent facilitation of invertebrates with increasing mimic biomass between estuaries and across latitudes, thereby supporting all three hypotheses in a single experiment. I concluded that entangled seaweeds, by adding biomass and different physical structures, can support strong habitat cascades in sedimentary estuarine seagrass beds.
In Chapter 5 I tested, again in a seagrass-dominated system, if and how anthropogenic stressors, like fertilization and enhanced sedimentation, affect seagrass performances and seagrass-seaweed habitat cascades. I found that fertilization had little impact whereas even low sedimentation levels had strong negative effects on both seagrass and fauna. Furthermore, I found strong negative effects of sediments, across seasons and elevation levels, but also that negative effects of sediments on invertebrates were elevated in the presence of the secondary habitat former. These results thereby provide rare evidence of how a habitat cascade can break down under high anthropogenic stress.
In Chapter 6, I studied habitat cascades from more diverse rocky intertidal shores. Primary habitat formers with different morphologies affect secondary habitat-forming epiphytes and epifauna differently. However, no studies have tested the opposite hypothesis; do morphologically ‘similar’ congeneric primary habitat formers support similar epiphytes with similar direct and indirect cascading effects on invertebrate communities? This hypothesis was tested by sampling co-existing congeneric habitat-forming fucoid seaweeds, Cystophora torulosa, C. scalaris, and C. retroflexa, with and without epiphytes across reefs and latitudes. The survey was then followed by field experiments, where defaunated Cystophora species and the morphologically different fucoid Hormosira banksii, with and without living and mimics of epiphytes, were out-transplanted to quantify the impact on colonizing gastropods. I found that the three Cystophora species supported different gastropod communities and had different cascading effects, and that these results can be, in part, explained by their physical structures. I also found that epiphytic biomass had strong positive effect on gastropods abundances, and that artificial mimics and live epiphytes were colonized by similar gastropod communities, suggesting that structural effects are more important than whether the habitat is ‘edible’.
In Chapter 7, I tested, again from rocky intertidal systems, if habitat cascades affect secondary (animal) production. Secondary production of small mobile invertebrates inhabiting Cystophora seaweed, with and without epiphytes, was estimated from published productivity models. More specifically, I tested if (i) the three Cystophora species support similar secondary production, (ii) finely branched epiphytes increase secondary production, (iii) production is greatest in warmer locations and seasons, and (iv) secondary production is higher on living epiphytes than non-living epiphyte mimics. The first two hypotheses were rejected as the three Cystophora species supported different secondary production and because epiphytes, when its biomass was taken into consideration, did not increase secondary production. Nevertheless, the two latter hypotheses were both supported, as production was highest in the northern location and in summer months and on living than mimic epiphytes. Thus, similar looking congeneric primary habitat formers supported different secondary production and epiphytes did not increase secondary production per seaweed-biomass, but will increase areal-based production when epiphytes enhance total standing plant biomass.
I conclude that poorly studied habitat cascades were ubiquitous in marine benthic systems on the South Island of New Zealand, modifying animal biodiversity across habitats, seasons, years, latitudes, sites and elevations levels. I also conclude that data on the abundances, morphologies and types (live or not) of co-existing habitat formers were strong mechanistic descriptors of habitat cascades. I finally suggest that habitat cascades, like other important indirect facilitation processes, should be covered in ecological text books and conservation practices.