Herbivores Control Effects of Algal Species Richness on Community Biomass and Stability in a Laboratory Microcosm Experiment

Oikos 000: 001–009, 2016 doi: 10.1111/oik.03105 © 2016 Th e Authors. Oikos © 2016 Nordic Society Oikos Subject Editor: Christopher Swan. Editor-in-Chief: Dries Bonte. Accepted 11 February 2016 Herbivores control effects of algal species richness on community biomass and stability in a laboratory microcosm experiment Chase Rakowski and Bradley J. Cardinale C. Rakowski ([email protected]) and B. J. Cardinale, School of Natural Resources and Environment, Univ. of Michigan, 440 Church St, Ann Arbor, MI 48109, USA. Hundreds of studies that have explored how biodiversity aff ects the productivity and stability of ecosystems have produced a consensus that communities composed of more species tend to have higher biomass that is more stable through time. However, the majority of this work stems from studies performed using highly simplifi ed food webs, often composed of just primary producers competing for inorganic resources in the absence of trophic interactions. When studies have incorporated trophic interactions, diversity-function relationships have been more variable, leaving open the question of how biodiversity aff ects the functioning of ecosystems with more trophic levels. Here we report the results of a laboratory experiment that used freshwater microcosms to test for eff ects of algal diversity (one or four species) on community biomass and temporal variability in the presence and absence of two diff erent herbivore species (cladocerans Ceriodaphnia dubia and Daphnia pulex ). When no herbivores were present, we found the classic pattern observed in hundreds of other studies – as species richness of algae increased, algal biomass increased, and the temporal variation in biomass decreased. Th is pattern was retained when one of the herbivores (C. dubia ) was present. Ceriodaphnia dubia exhibited weak and non-selective grazing on the focal algae, leaving the eff ect of diversity on biomass and variability essentially intact. In contrast, D. pulex exhibited strong and selective grazing in algal polycultures that qualitatively altered both diversity – function relationships. As algal richness increased, total algal biomass decreased and variation through time increased. Th ese changes were coupled with larger and less variable populations of D. pulex . Our results show that herbivory leads to a richer array of diversity – function relationships than often observed in studies focused on just one trophic level, and suggests trophic interactions should be given more attention in work that seeks to determine how biodiversity impacts the functioning of ecosystems. In the face of accelerating loss of biodiversity (Murphy calls for researchers to begin considering ‘ vertical ’ diversity and Romanuk 2014, Newbold et al. 2015), an increasing (i.e. diversity of trophic levels) alongside the consideration amount of research has been dedicated to understanding of ‘ horizontal ’ diversity (i.e. diversity within a trophic level) how changes in biodiversity alter the functioning of eco- (Duff y 2002, Duff y et al. 2007). systems. Ecologists have paid particular attention to how While a growing number of researchers are now add- species richness infl uences community biomass and the ing trophic interactions to the design of their studies, there temporal variability of biomass, as productivity and sta- are still relatively few published results that we can draw bility are two metrics that are widely used to describe the inferences from, and the results of these few studies are functioning of ecosystems (Hooper et al. 2005). Although decidedly mixed (Duff y et al. 2005, Th ebault and Loreau a general consensus has emerged that species richness tends 2006, Jiang and Pu 2009). Consider, as one line of evidence, to increase the production and stability of biomass through the synthesis by Cardinale et al. (2011) who summarized time (Hooper et al. 2005, Cardinale et al. 2012), there is results of 13 studies that have manipulated the diversity of increasing recognition that most of our inferences about primary producers and then measured herbivory. Nine of biodiversity stem from studies performed using communi- these showed that herbivory increased, whereas four showed ties that have been greatly over-simplifi ed. Cardinale et al. that herbivory decreased, in more diverse assemblages of pro- (2009) summarized the characteristics of biodiversity experi- ducers. Similarly, other authors have found that increasing ments performed through 2005 and showed that Ͼ 90% of producer diversity is associated with reduced herbivory (see these had manipulated the diversity of seven or fewer species synthesis by Hillebrand and Cardinale 2004), with increased in just one trophic level, mostly primary producers. Th e nar- herbivory (Stein et al. 2010, Loranger et al. 2013), or with row focus on primary producers, coupled with the fact that no change in herbivory (DeMott 1998). trophic interactions are well known to regulate the biomass It is presently unknown why trophic interactions have and variability of producers (Borer et al. 2005, O ’ Gorman had such varying eff ects on diversity – function relationships. and Emmerson 2009, Estes et al. 2011), have prompted Some mathematical models and select empirical evidence EV-1 suggest that variation in herbivore traits such as body size inhabits lakes and ponds worldwide (USEPA 2002). Adult and feeding selectivity may help explain diff erences among C. dubia are Ͻ 1 mm and their small ( Ͻ 35 μ m) gape pre- studies (Duff y 2002, Th ebault and Loreau 2005, Narwani vents them from easily consuming larger algal cells (Burns and Mazumder 2010). For example, herbivores that exhibit 1968). A previous study performed with Ceriodaphnia selectivity in their feeding may encounter their preferred reticulata (Narwani and Mazumder 2010) suggests that food less often or spend more time handling food in a diverse C. dubia might feed at a similar or slower rate in polycul- community, leading to decreased herbivory rates (DeMott tures versus monocultures of algae. Th e Daphnia pulex com- and Kerfoot 1982, Vos et al. 2001, Kratina et al. 2007). plex is also common and globally distributed (Crease et al. On the other hand, generalist herbivores may benefi t from 2012). Daphnia pulex adults are larger and have a larger gape diverse diets if the species eaten are complementary in the (45 μ m) than C. dubia , allowing them to more easily consume nutrients they provide and/or if diversity tends to dilute larger particles (Burns 1968). In the same study by Narwani toxins (Pfi sterer et al. 2003). Unfortunately, few studies have and Mazumder (2010), D. pulex demonstrated accelerated quantifi ed the eff ects of herbivores on diff erent resource feeding in polycultures versus monocultures of algae. species in a way that allows us to estimate selectivity, or We acquired both cladocerans from Sachs Systems that can separate eff ects of diversity on the performance of Aquaculture, FL. Upon arrival, we inspected the zoo- diff erent trophic groups. Both of these measurements are plankton cultures and removed all contaminating species. required to understand how trophic interactions alter the We then housed the cladocerans in 1-l borosilicate glass diversity – function relationship for primary producers. bottles containing COMBO growth medium with animal Here we report the results of a laboratory experiment trace elements, a standard medium for culturing freshwa- performed with freshwater microcosms in which we tested ter plankton (Andersen 2005), which was refreshed every for eff ects of algal diversity (one or four species) on commu- fi ve days. We incubated the bottles under a 16:8 hour nity biomass and temporal variability in the presence versus light:dark cycle and fed the cladocerans a mixture of green absence of two diff erent herbivore species (the cladocerans algae ( Selenastrum capricornutum and Chlorella sorokiniana ). Ceriodaphnia dubia and Daphnia pulex ). Both C. dubia and In order to ensure that none of these algae would contami- D. pulex are abundant and widespread in lakes. We chose to nate the experiment, we rinsed and starved the cladocerans contrast these two herbivores because they are diff erent in prior to inoculation by placing individuals into fresh sterile their feeding habits, with D. pulex being larger and having COMBO medium three times over a period of eight hours. a bigger gape size than C. dubia that allows it to feed on a We examined cultures using microscopy to check for con- wider range of algae. With so little prior work on diversity – tamination and verify no unwanted species were present. function relationships in multi-trophic systems, it is diffi cult to make a priori hypotheses that are founded on solid logic Focal algae or prior ecological knowledge. Nevertheless, mathematical models of multi-trophic systems (Th ebault and Loreau We used fi ve species of green algae (division Chlorophyta) 2005) tend to make three qualitative predictions that we for the experiment: Chlorella sorokiniana , Scenedesmus acum- considered when designing our study: 1) in the absence of inatus , Pediastrum duplex , Monoraphidium minutum and herbivory, increasing species richness of primary producers Monoraphidium arcuatum . Th ese genera are found in 9 to tends to increase and stabilize producer biomass; 2) when 53% of North American freshwater lakes and range from producer richness has non-signifi cant or negative eff ects on the 67th to the 5th most commonly sampled taxa out of 262 herbivory, then relationships between producer richness, genera identifi

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