Abstract
Disturbances often control community structure by removing large dominant species, allowing new species to colonize. Disturbances vary in intensity and extent, and their effects on resident communities can depend on local environmental conditions. We tested the effects of disturbance intensity and extent on different functional groups of understory species in kelp forests at 4 locations along an ocean climate gradient in Western Australia. We hypothesized that, compared to intact canopies, increasing disturbance intensities (50 and 100% of kelp removal) and extents (2, 4 and 8 m diameter) would promote light-dependent competitors (turf, foliose, articulated coralline and fucoid seaweeds) at the expense of less light-dependent functional groups (invertebrates and encrusting seaweeds). We also hypothesized that these effects would be most pronounced at warmer relative to cooler locations, where metabolic and ecological rates are faster. The first hypothesis was supported; light-dependent understory groups (turfs, in particular) increased, while less light-dependent groups (crusts in particular) decreased with increasing disturbance regimes. However, the second hypothesis was not supported; even though understory communities differed between locations and turf covers were highest at the warmest location, we found no significant interactions between locations and disturbance regimes. Importantly, our results revealed that even small-scale partial canopy loss can have significant effects on kelp-associated communities. The implied community-wide, density-dependent effects have implications for the management and conservation of kelp forests, because restoration of ecological functions must also consider the density of kelp forests, not simply their presence or absence.
Highlights
Natural and human-mediated disturbances modify the structure and functioning of communities and ecosystems (Grime 1977, Diaz-Pulido et al 2009, Wernberg et al 2013a)
The Principal coordinates ordination (PCO) showed that the cover of turf and foliose algae typically were associated with disturbances, whereas crusts were associated with canopy cover (Fig. 2)
Fucoids were negatively associated with control plots in Jurien Bay (Jur) and Mar (Fig. 2B,C); this pattern was less clear for Hamelin Bay (Ham) (Fig. 2B), while a weak positive association was observed at Kalbarri in the north (Kal) (Fig. 2A; this effect was likely due to very low abundances of fucoids on these reefs, see Fig. 3 and Section 4.1)
Summary
Natural and human-mediated disturbances modify the structure and functioning of communities and ecosystems (Grime 1977, Diaz-Pulido et al 2009, Wernberg et al 2013a). Disturbance extent and intensity could produce non-linear interactions through their effects on effective dispersal distance and survival, respectively, of colonizing species or their predators (e.g. Farrell 1989, Petraitis & Dudgeon 1999, Gagnon et al 2003, Layton et al 2019). Environmental gradients, such as wave exposure or temperature, could add a third dimension of potential interaction by affecting metabolic and ecological rates (e.g. dispersal, recruitment, growth, reproduction, mortality) differentially, often in non-linear ways (e.g. Gaylord et al 2002, Wernberg et al 2010, Kordas et al 2011)
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