Habitat structure alters topdown control in litte

Authors: Gregor Kalinkat Ulrich Brose Björn Christian Rall

Publish Date: 2012/11/28

Volume: 172, Issue: 3, Pages: 877-887

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Abstract

The question whether topdown or bottomup forces dominate trophic relationships energy flow and abundances within food webs has fuelled much ecological research with particular focus on soil litter ecosystems Because litter simultaneously provides habitat structure and a basal resource disentangling direct trophic and indirect nontrophic effects on different trophic levels remains challenging Here we focussed on shortterm per capita interaction strengths of generalist predators centipedes on their microbidetritivore prey springtails and addressed how the habitat structuring effects of the leaf litter modifies this interaction We performed a series of laboratory functional response experiments where four levels of habitat structure were constructed by adding different amounts of leaf litter to the experimental arenas We found that increased leaf litter reduced the consumption rate of the predator We interpreted this as a dilution effect of the augmented habitat size provided by the increasing leaf litter surface available to the species Dilution of the prey population decreased encounter rates whereas the capture success was not affected Interestingly our results imply that topdown control by centipedes decreased with increasing resource supply for the microbidetritivore prey ie the leaf litter that simultaneously provides habitat structure Therefore effective topdown control of predators on microbidetritvore populations seems unlikely in litterrich ecosystems due to the nontrophic habitatstructuring effect of the basal litter resourceProgress in foodweb ecology is critically based upon information about bioenergetic flows of energy between consumer and resource pairs These interaction strengths and their distributions across the myriads of links in natural food webs are vital for community structure population dynamics and ecosystem functioning eg McCann et al 1998 Neutel et al 2002 2007 Otto et al 2007 Rall et al 2008 Berlow et al 2009 Binzer et al 2011 The biotic mechanisms shaping and structuring interaction strengths are complex and might be driven by basal resources eg detritus or consumers eg predators One major question in the ecology of soil food webs therefore deals with the regulation of detritivore populations and whether they are controlled by bottomup mechanisms ie energy and nutrient supply or topdown regulated by their multiple predators Both hypotheses are supported by studies Bengtsson et al 1997 found topdown control whereas the results of Scheu and Schaefer 1998 and Ponsard et al 2000 provided evidence for bottomup control Major progress in this field requires insights in consumer–resource interactions with a particular focus on the strength of such interactions Scheu 2002 Due to the natural composition of soil and litter habitats with their porous fractal structure and opaqueness the direct observation of species interactions in the natural context is almost intractable Indirect observation via gut or stomach content analysis a standard procedure in freshwater eg Elliott and Persson 1978 Woodward and Hildrew 2002 and marine eg Daan 1973 Aljetlawi et al 2004 Smout and Lindstrøm 2007 systems is hampered by the fact that a large fraction of soil predators rely on extraintestinal digestion Cohen 1995 and therefore deep understanding of predator–prey interaction strengths in these systems remains challenging While different methods of tracking feeding links qualitatively were developed and improved over the past decades—particularly stable isotope analyses molecular gut content analyses and fatty acid trophic markers Post 2002 King et al 2008 Ruess and Chamberlain 2010—they have scant ability for tracking feeding interactions quantitatively Therefore we have to rely on laboratory experiments to determine per capita impacts of litter and soildwelling predators on their preyGenerally two different approaches to determine capture rates and handling times can be distinguished 1 direct observation and 2 indirect derivation through model fitting In carefully designed experiments both approaches result in congruent parameter estimates Tully et al 2005 While direct observation is feasible particularly for larger predators eg fishes in laboratory experiments Persson and Brönmark 2002 studies working with diminutive organisms in opaque environments have to either rely on adequate model fitting techniques to reveal functional response parameters or reduce the structural complexity of the experiment to improve the visibility of the interactions As Jeschke et al 2004 highlighted the majority of functional response studies are carried out in simplified laboratory systems The resulting problem that feeding rates might differ in more complex experiments has been addressed by several functional response studies in recent years there experimental complexity was introduced by variation of numbers of predator individuals predator interference eg Kratina et al 2009 Lang et al 2012 the number of prey species alternative prey eg Colton 1987 Elliott 2004 Kalinkat et al 2011 or even the additional presence of nonprey species Kratina et al 2007 Another lack of reality in laboratory studies is due to oversimplified environmental conditions that are typically provided within artificial arenas There are only a limited number of studies focussing on the effects of habitat complexity on the functional response of terrestrial predators Kaiser 1983 Munyaneza and Obrycki 1997 Pitt and Ritchie 2002 Hoddle 2003 Hohberg and Traunspurger 2005 Hauzy et al 2010 VucicPestic et al 2010a While some of these studies focussed on the fractal complexity of an artificially structured habitat Kaiser 1983 Pitt and Ritchie 2002 Hoddle 2003 and others made qualitative comparisons of withstructure versus nonstructuretreatments Hohberg and Traunspurger 2005 VucicPestic et al 2010a there is only one study to our knowledge with a qualitative comparison between a simplified unstructured laboratory setting and field conditions Munyaneza and Obrycki 1997 This study indicated reduced capture rates of terrestrial arthropod predators by a factor of roughly two under greenhouse and field conditions compared to the experimental setting with controlled conditions in the laboratory experimentBeyond these specific functional response studies a broader look at the literature reveals that habitat structure effects on predator–prey interactions have been the focus of many studies especially in aquatic ecosystems eg Crowder and Cooper 1982 Gotceitas and Colgan 1989 and references therein There predation rates are also reduced in highcomplexity treatments and tend to be highest in habitats with intermediate structural complexity Crowder and Cooper 1982 However a continuous framework that is suitable to link trophic and nontrophic effects between basal resources eg litter firstorder consumers eg detritivores and predators is still missing This applies particularly to leaf litter systems where pulses of incoming material and longlasting decay of the litter yield a continuously changing amount and complexity of habitat structure Therefore our understanding of dynamics and functioning of such ecosystems is challenged by a general lack of studies addressing how habitat structure modifies interaction strengths and topdown control of microbidetritivores by predatorsIn this study we aimed to fill this gap by studying the effects of systematic variation in leaf litter density on the functional response of the centipede Lithobius mutabilis Chilopoda Lithobiidae as a ubiquitous and frequent generalist predator of the leaflitter system on its microbidetritivore prey the springtail Heteromurus nitidus Collembola Entomobryidae Springtails have been shown to be flexible foragers that can feed on fungal hyphae bacteria or detritus depending on the available resources Scheu 2002 According to Lawrence and Wise 2000 they might be assigned to the functional guild of detritivores as higher abundances of springtails cooccurred with increased rates of litter disappearance in this experiment Within the model framework of the functional response we expected prey refuges of the additional habitat structure to cause a shift from type II to type III functional responses Real 1977 Scheffer and De Boer 1995 as has already been shown for other predator–prey pairs from litter systems de Ruiter et al 1988 VucicPestic et al 2010b Furthermore we anticipated that the capture rate should be negatively affected by increasing habitat structure as encounter rates are directly dependent on movement patterns and velocities of predators and prey Muirhead and Sprules 2003 Gergs and Ratte 2009 Therefore habitat complexity should only affect the encounter rate and not the mechanisms involved once the two species are in close contact which includes handling timeIn consequence we hypothesised that the increased complexity of leaf litter should 1 provide additional prey refuges therefore resulting in more sigmoid type III functional responses 2 decrease the capture rates and 3 not affect the handling times

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