Establishing what factors control diversity in nature has long been an important theme of research in ecology (Huston 1994, Rosenzweig 1995, Hubbell 2001). Although many factors can affect diversity (Palmer 1994, Vinson and Hawkins 1998), there is still
considerable debate over how they might interact (Hubbell 2001). In particular,
several studies in a variety of ecosystems have demonstrated that disturbance and productivity can interact to affect diversity, but the form of this interaction varies (Sousa 1984, Pickett and White 1985, Currie 1991, Wootton 1998, Waide et al. 1999, Mittelbach et al. 2001, Death and Zimmermann 2005, Cardinale et al. 2006).
Disturbance is one of the major structuring forces in lotic systems (Resh et al.
1988, Lake 2000, Death 2010). A dominant paradigm in disturbance ecology, the
Intermediate Disturbance Hypothesis (IDH), has been amongst the most widely applied
ecological theories (Grime 1973b, Connell 1978, Sousa 1979). However, there has
been little empirical support for this hypothesis in streams, possibly because many stream organisms are highly mobile (but see Townsend et al. 1997). In fact, the literature suggests that the disturbance‐diversity relationship is highly variable in nature generally (Mackey and Currie 2001). What is more surprising is that many studies show no or weak effects of disturbance on diversity or community structure (Mackey and Currie 2000). In lotic systems, the disturbance‐diversity relationship can
be confounded by the fact that disturbance not only acts directly on benthic
invertebrates but indirectly by the removal of food resources (Death 2002). Thus
several authors have proposed that the nature of the disturbance‐diversity
relationship is controlled by habitat productivity altering population growth rates
(Huston 1979, 1994, Kondoh 2001, Cardinale et al. 2006).
The relationship between productivity and diversity has also been an important research theme in ecology (Currie 1991, Abrams 1995, Waide et al. 1999, Mittelbach et al. 2001). However, just as with the disturbance‐diversity relationship, the form of the relationship can be quite variable (Abrams 1995, Waide et al. 1999, Mittelbach et al.
2001). The most commonly reported relationships are unimodal (e.g. Grime 1973a,
al. 2001) or linear (e.g. Currie and Paquin 1987, Currie 1991, Abrams 1995, Waide et al. 1999, Gaston 2000, Mittelbach et al. 2001) increases in diversity with increasing productivity. The variation in observed patterns may be a result of the scale of observation which has ranged from local, to regional and global comparisons (e.g.
Currie 1991, Chase and Leibold 2002). However, local scale studies often find
unimodal relationships (Waide et al. 1999, Mittelbach et al. 2001, Chase and Leibold
2002) which are potentially explained by many mechanisms (Rosenzweig and
Abramsky 1993, Abrams 1995, Waide et al. 1999) that often require some form of competitive trade‐off.
An extension of the IDH, the dynamic equilibrium model (DEM) (Huston 1979, 1994), predicts that the level of disturbance maximising diversity changes with habitat productivity. Using the patch occupancy models of Hastings (1980) and Tilman (1994), Kondoh (2001) expanded this model to account for metapopulation dynamics, multiple trophic levels and patchy disturbances. This modified model provides an alternative to prior models by allowing disturbances to create niche opportunities for the expression of various life‐history traits. However, like the DEM, it also predicts that diversity will peak at intermediate levels of productivity and disturbance with the position of the peak on one scale depending on the level of the other. Models such as the IDH, DEM and Kondoh’s model assume a trade‐off between competitive and colonisation ability
whereby organisms are either good colonisers or good competitors but not both
(Chesson and Huntly 1997, Roxburgh et al. 2004, Cadotte 2007). There appears to be
little evidence for the generality of competitive exclusion in stream communities
(Reice 1985, Death and Winterbourn 1995), and it appears more likely that carrying capacity may be determined by the productivity/resource supply rates of a site (Gross and Cardinale 2007, Death 2008). Accordingly, Death (2002) proposed that in the absence of disturbance, resource levels rather than competitive interactions set an upper limit to the richness of communities through colonisation, whilst disturbance resets the colonisation process by removing animals and thus taxa, and resetting resource levels.
The presence or absence of canopy cover in small streams can influence the
productivity (Robinson and Minshall 1986, Death 2002, Death and Zimmermann 2005, Fuller et al. 2008). As a large proportion of lotic invertebrates have good colonising abilities after disturbance (Mackay 1992, Allan 1995), it is likely that post‐flood recovery of the food base is the major determinant of invertebrate diversity in autotrophic streams (Death and Zimmermann 2005). This may not be the case in heterotrophic streams if the resource base is relatively unaffected by disturbance (i.e. as much organic matter is washed in as is washed out). Fuller et al. (2008) suggest that when periphyton recovery is not limited by nutrients, open canopy sites are more resilient to disturbance than sites with canopy cover. Conversely, sites with canopy
cover are more resistant because the invertebrate community recovery is not as
dependent on periphyton re‐growth post disturbance as in open canopy sites (Fuller et
al. 2008). The contrasting response of lotic invertebrates to productivity and
disturbance between autotrophic and heterotrophic streams is likely to hinder the
generalised application of models such as the DEM. That is, where retention of organic matter in forested streams is sufficient, macroinvertebrate communities are likely to
respond differently to disturbance than in autotrophic streams as they are less
dependent on primary productivity.
Several studies have recently assessed the DEM in a variety of ecosystems (e.g. Scholes et al. 2005, Cardinale et al. 2006, Svensson et al. 2007, Haddad et al. 2008, Sugden et al. 2008). However, the results have been equivocal with the response of
communities to productivity and disturbance varying inconsistently between
ecosystems and little evidence of interactive effects (but see Cardinale et al. 2006). In
this study we test the DEM on benthic invertebrate communities from mountain
streams in the central North Island of New Zealand. We investigate whether the observed levels of productivity and disturbance are sufficient to explain diversity patterns exclusively in these streams, or whether the relationship is modulated by the presence of canopy cover over the stream. We hypothesise that benthic invertebrate diversity is a product of the interaction between substrate disturbance and primary productivity, assessed as bed stability and periphyton biomass, respectively, and that this relationship will be stronger at open canopy sites than at sites with canopy due to tighter coupling with algal food resources. We discuss whether diversity patterns in
these streams can be better explained by a modified productivity‐disturbance‐diversity model that is not constrained by the competitive‐colonising trade‐off inherent with the
DEM.