Biological barriers to restoration testing the bi

Authors: Jessica L Frame J Iwan Jones Steve J Ormerod Jonathan P Sadler Mark E Ledger

Publish Date: 2016/04/26

Volume: 777, Issue: 1, Pages: 161-170

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Abstract

The biotic resistance hypothesis provides one of several explanations for the limited biological recovery of streams recovering chemically from acidification The hypothesis proposes that acidification has changed the presence abundance and interactions among species in acidified streams to the extent that acidsensitive colonists cannot reinvade even where acidity has ameliorated As a first step in testing for biotic resistance in streams we conducted a field experiment to determine whether the success growth rate of acidsensitive recolonists mayfly nymphs Baetis rhodani is reduced by competition with abundant acidtolerant residents stonefly nymphs Leuctra inermis in a chemically recovering Welsh stream UK Gut contents analysis revealed a marked overlap in resource use between the two species However when Baetis was exposed to several 0 025 05 and 1 times ambient densities of its putative competitor Leuctra growth rates of the colonist were not affected by the residents at any of the densities tested These results do not support the hypothesis that resident species constrain colonist populations by affecting growth rates through competition for limited resources or interference Further work is required to assess whether independent and/or interactive ecological effects of other common residents might affect colonists in ecosystems recovering from past stressorsDeclining acid deposition across the northern hemisphere over recent decades has led to widespread improvements in the water quality of formerly acidified surface waters Battarbee et al 2014 Monteith et al 2014 In some areas mitigation programmes have attempted to restore water quality by applying calcium carbonate to catchments or river channels through ‘liming’ McKie et al 2006 yet despite continuing deacidification in many river systems there has been only partial biological recovery eg Ormerod Durance 2009 Battarbee et al 2014 Several hypotheses have been proposed to explain the failure of acidsensitive species to recolonise watercourses that are apparently recovering chemically Yan et al 2003 These hypotheses which are not necessarily mutually exclusive include 1 dispersal limitation 2 abiotic resistance 3 resource limitation and 4 biotic resistance eg Ledger Hildrew 2005 Murphy et al 2014Dispersal limitation could be an important consideration for less mobile aquatic organisms eg Gastropoda but recent research indicates it is unlikely to be a major factor constraining recolonisation of insects with highly dispersible winged adults Masters et al 2007 Murphy et al 2014 Abiotic resistance may also limit the persistence of recolonists where continued acid episodicity arises after heavy rainfall or snowmelt Lepori et al 2003 Kowalik Ormerod 2006 but episodes may not be a straightforward explanation for limited biological recovery where acidity is ameliorating Monteith et al 2014 Resource limitation may occur where the low quantity and or quality of available food constrains recovery Ledger Hildrew 2001a b Thomsen Friberg 2002 For example the absence of herbivorous invertebrates in acid streams has been attributed to the impoverished nature of the biofilms at these sites eg Townsend et al 1983 Dangles Guérold 2000 Whilst much evidence points to marked changes at acidified sites in the taxonomic composition of algal assemblages Hirst et al 2004 Bray et al 2008 consequent effects on food quality/quantity remain uncertain Ledger Hildrew 2001a b Murphy et al 2014 Evidence for biotic resistance to recovery has been found in lakes eg Vinebrooke et al 2003 but the hypothesis has received little consideration in rivers and streams but see Ledger Hildrew 2005 Layer et al 2013 For instance we do not know whether changes in the presence and abundance of species and biotic interactions which occur under acid conditions in turn influence the invasibility of riverine communities as acidity ameliorates Ledger Hildrew 2005 Murphy et al 2014Acidification leads to major shifts in the presence and abundance of species across the food web Changes in assemblage structure and resulting shifts in the strength and number of species interactions may impart a degree of biotic resistance to the reestablishment of acidsensitive populations once water quality is restored ‘Community closure’ sensu Lundberg et al 2000 may arise where species loss occurs unevenly across functional groups altering trophic structure Hildrew et al 1984 Planas 1996 Evidence of ‘competitive resistance’ has been found in acidified lake communities eg Keller Yan 1998 Holt Yan 2003 Vinebrooke et al 2003 Frost et al 2006 where delayed recovery of algal and zooplankton communities following chemical restoration has been attributed to competition with persistent acidtolerant species but support in streams remains circumstantialIn this paper we examine the potential for biotic resistance to recovery within the benthic macroinvertebrate community of a chemically recovering stream in midWales UK Acidified streams in the catchments of the River Wye are improving chemically both through declining acid deposition and restoration projects liming but to date evidence for biological recovery is scarce with limited persistence of many acidsensitive invertebrates especially mayflies eg Bradley Ormerod 2002 Ormerod Durance 2009 One of the untested mechanisms of community resistance to reinvasion in streams is competition with residents and it could be that changes in species abundances and interactions occurring as streams acidified place tight constraints upon the subsequent return of sensitive species when acidity ameliorates Ledger Hildrew 2005 Layer et al 2013Stoneflies occur commonly in upland streams and abundances can increase dramatically under high acidity reflecting compensatory dynamics arising as resources are released by species deletions Ledger Hildrew 2005 Frost et al 2006 Tixier et al 2012 We have shown elsewhere that species within the order feed Ledger Hildrew 2000a and grow Ledger Hildrew 2001a b on the sparse algal layers of impoverished acid streams and it could be that as acidity declines recolonists such as baetid mayflies face competition with these highly abundant herbivorous residents Stoneflies can depress the abundance of algal resources available on stones Ledger Hildrew 2000b and where they are abundant could limit the availability of algal food for new colonists As a first step in testing for biotic resistance in streams we examined the effect of a numerically dominant acidtolerant resident Plecoptera Leuctra inermis Kempny 1899 on the success growth of a widespread acidsensitive and early recolonist Ephemeroptera Baetis rhodani Pictet 1845 that has so far shown limited persistence in deacidifying streams eg Bradley Ormerod 2002 Several field experiments have demonstrated the potential for competition within the stoneflies eg Ledger Hildrew 2000a b and mayflies eg Kohler 1992 characteristic of upland streams but the extent to which competition occurs between these groups has not been tested within the context of acidification Specifically we assessed the degree of resource overlap between Leuctra inermis and Baetis rhodani and using a field experiment tested the hypothesis that the growth rate of a recolonist Baetis could be affected by Leuctra its putative competitor Consistent with the biotic resistance hypothesis we predicted that the growth of Baetis would be constrained by Leuctra and that mayfly growth rates would decrease with the increasing abundance of the stonefly residentResearch was conducted in a reach of the upper River Wye site WY34 52414772N −3659245W in August 2007 Catchment land use was a combination of pasture with acid grassland and forests of exotic species such as Picea sitchensis Bong Carrière Forestry Commission 2002 The site has been monitored routinely as part of longterm research in the Wye and Usk catchments see Lewis et al 2007 Frame 2010 In July 2003 hydrological source areas of the Wye were treated with limestone as part of a largescale mitigation programme designed to improve water quality and ecological status of sites impacted by acidification Lewis et al 2007 Although the main channel of the river was not limed directly it was subject to the cumulative influence of limetreated tributaries and was characterised by stable pH during the summer months River water at the study site was moderately acidic minimum pH 57 mean pH 60 maximum pH 63 for 2003–2006 study pH 61 August 2007 and nutrient poor mean dissolved calcium 2 mg l−1 nitrate 04 mg l−1 phosphate 0012 mg l−1 for 2003–2006 with relatively low mean dissolved aluminium concentrations maximum 0080 mg l−1 minimum 0009 mg l−1 mean 0032 mg l−1 2003–2006 Frame 2010 Research was conducted in a physically unstable Pfankuch index = 73 Pfankuch 1975 stony riffle dominated by cobbles and gravel where the river was 10 m wide and 05 m deep Frame 2010

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