assemblages
Abstract
There is much debate as to what the form of the productivity‐diversity relationship is, particularly in lotic systems which have received few assessments of this relationship. One factor that assessments of primary productivity do not account for is the growth form of algae that drives production. Thus, we set out to (i) examine the productivity‐ diversity relationship in 24 streams of Cantabria, Spain, in July 2007 and (ii) determine
whether this relationship was underpinned, and better explained, by specific
responses to the form of the periphyton community. Due to recent evidence in streams suggesting productivity simply sets the upper limit to richness, rather than to increase the effect of competitive interactions, we hypothesized that diversity would be a log‐linear function of productivity. We predicted that diversity would respond opposingly to two coarse measures of the periphytic community; i.e., positively to % diatom cover and negatively to % filamentous algae cover. There was no relationship between productivity and diversity in these streams but, as predicted, this relationship was underpinned by responses to the growth form of periphyton community. Diatom cover was the best predictor of invertebrate diversity patterns. Generally, diversity responded positively to % diatom cover and negatively to % filamentous algae cover. However, results were variable and % EPT exhibited a greater sensitivity to higher levels of diatom cover. These findings highlight two important implications: firstly, productivity‐diversity relationships in streams can be underpinned by interactions with specific forms of periphyton. Secondly, rapid assessment of periphytic forms is useful for managers as these coarse measures are highly relevant to higher trophic levels. We suggest a threshold of 40 % filamentous algae cover for managers wishing to minimise deleterious effects of eutrophication on stream communities.
Introduction
Although the relationship between productivity and diversity is a central theme in ecological research (Currie 1991, Abrams 1995, Waide et al. 1999, Mittelbach et al. 2001), we are far from reaching a consensus on the form of the relationship either empirically or in theory (Waide et al. 1999, Mittelbach et al. 2001). Unimodal (e.g. Grime 1973, Huston 1979, Rosenzweig 1992, 1995, Leibold 1999, Waide et al. 1999, Mittelbach et al. 2001) and 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 prevail as the most common forms of this relationship. Differences in the observed patterns may be a result of several factors including the spatial scale of observation (Currie 1991, Mittelbach et al. 2001, Chase and Leibold
2002), disturbance (Huston 1979, 1994), and history of community assembly (Fukami
and Morin 2003). However, the relationship has also been shown to differ between ecosystems and organisms studied (Waide et al. 1999, Mittelbach et al. 2001). These
differences suggest individual patterns are driven by underlying ecosystem specific
interactions. Furthermore, the methods used to collate the information on the
productivity‐diversity relationship have recently come under heavy criticism due to inconsistent classification and errors in meta‐analyses (Hillebrand and Cardinale 2010,
Whittaker 2010).
While abiotic forces (floods, hurricanes etc.) are considered to dominate biotic forces (competition, predation) in many systems, plant‐herbivore relationships remain the basis of most ecosystems. As such the control of diversity is likely to be an interaction between disturbance and the productivity of an ecosystem (Huston 1979, 1994, Kondoh 2001). No system is more heavily influenced by regular disturbance events than streams (e.g. flooding) (Resh et al. 1988, Lake 2000, Death 2008). Death (2002) suggests that floods remove taxa and their resources and opens the habitat for recolonisation whilst primary productivity sets the upper limit to how many animals can return to these habitats after the disturbance. Recent support has been found for Death’s hypothesis but these studies found no evidence of interactive effects of disturbance and productivity (Tonkin and Death In prep., Tonkin et al. In prep.), but
rather that diversity is a result of the additive effects of disturbance and productivity.
Without the interactive effects of disturbance, making sense of the productivity‐
diversity relationship in streams should be clear‐cut. However, compared to lentic systems and in fact most other environments, few studies have investigated whether higher productivity leads to greater diversity in lotic systems. The few to look specifically at this have found both unimodal (Death and Zimmermann 2005) and log‐ linear (Death 2002, Barquin 2004, Tonkin and Death In prep., Tonkin et al. In prep.) increases in diversity with productivity.
Assessing the relationship between primary productivity and diversity is
essentially testing the response of higher trophic levels to the rate of energy production at lower trophic levels. Primary productivity is typically provided in streams by periphytic algae. Algal forms vary greatly in streams from prostrate and stalked diatoms through to filamentous green algae (Allan 1995), all of which respond
differently to environmental conditions. It is now common practice to use the
periphyton community as an index for biomonitoring environmental conditions (Kelly
and Whitton 1995, Pan et al. 1996, Kelly 1998b, a, Kelly and Whitton 1998, Hill et al. 2000). Typically the focus of these assessments of biotic integrity have been diatoms (Kelly and Whitton 1995, Pan et al. 1996, Kelly 1998b), but Whitton and Kelly (1995) advocated the use of the full community of plants including bryophytes.
Not only do various forms of periphytic algae respond differently to
environmental conditions but they can provide diverse habitat and resources for
higher trophic levels. Different periphyton growth forms can also fulfil different functional roles in benthic communities (Steinman et al. 1992). When levels of periphyton reach greater densities and epilithic diatoms are replaced by macroalgae such as filamentous green algae, interactions between grazers and periphyton can shift from simple plant‐herbivore interactions to more complex relationships. As well as providing food for a few specialist taxa, macroalgae can both provide and remove habitat and compete for space with invertebrates. Dudley et al. (1986) classed
invertebrates into those negatively affected by macroalgae due to competition for
space, positively affected due to habitat provision, and positively affected by food provision. This can be reflected in the typical shift from pollution (nutrient) sensitive
taxa associated with thin diatom films, to pollution tolerant taxa and filamentous algae growth forms often associated with nutrient eutrophication (Suren et al. 2003). Of course, grazing can have a large impact on periphyton in aquatic systems and this top‐
down control has been the central focus of periphyton‐invertebrate community
relationships to date (Hillebrand 2009). This grazing control can differ between growth forms of periphyton (Feminella and Hawkins 1995), such as that between diatom and filamentous forms (Suren and Riis 2010). Lamberti (1989) suggests this differential response of algae may initially occur where productive capacity of algae is high, but may be later regulated through the arrival of specialist grazers, or physical disturbance (Fisher et al. 1982). This indicates that vigilance is needed when inferring top‐down or bottom‐up control as it is likely to change through time and may be dependent on animals present in the system at hand.
Compared to other ecosystems, the productivity‐diversity relationship has
received relatively little attention from lotic ecologists. Thus, we set out to (i) test the response of stream invertebrate diversity to primary productivity; and (ii) because productivity measurements do not account for variation in the form of producers, we
examine if this productivity‐diversity relationship can be better explained by
underlying responses to different forms of periphyton (i.e. diatoms/periphytic mats
and filamentous green algae). We also use a common stream specific metric, % EPT, to assess whether this metric is more sensitive to environmental gradients in streams than simple diversity measures. As a result of previous work in streams (Death 2002, Barquin 2004, Tonkin and Death In prep., Tonkin et al. In prep.), and the fact that productivity does not appear to increase rates of competitive exclusion in streams as it often does in lakes, we hypothesise that diversity will increase logarithmically with
increasing productivity. However, we predict that this relationship will be
underpinned by particular responses to different forms of periphyton. Specifically, due to the view that diatoms are considered favourable to grazers and filamentous algal forms poor habitat for many invertebrates (Suren and Riis 2010), diversity will respond positively to % diatom cover and negatively to % filamentous algae cover.
Methods
Study sites
Twenty four streams were sampled in the Cantabrian Mountains of Northern Spain
(Fig. 1, Appendix 1). The Cantabrian Mountains span ~483 km east to west along the northern coast of Spain reaching 2,648 m asl at Torre de Cerredo. Average rainfall ranges from ~1,200 to 1,600 mm p.a. depending on location within the region. Land‐
use surrounding sampling sites varied from Atlantic deciduous forest consisting
predominantly of oak (Quercus spp.) and European beech (Fagus spp.) to pasture and small urban settlements. Sites were selected from six river catchments: Rio Besaya, Rio Saja, Rio Pas, Rio Pisueña, Rio Nansa, and Rio Ebro. Within each catchment sites were selected a priori so that one low and one high productivity site in close
geographic proximity were sampled. As these were selected prior to sampling,
productivity estimates for the selection of a priori high and low productivity streams were based on visual estimates of periphyton which are detailed below. All sites were cobble bottom streams. Altitude of the sites ranged from 163 to 1061 m asl and average channel width ranged from 1.9 to 30.7 m (Appendix 1).