Assessing the role of large herbivores in the structuring and functioning of freshwater and marine angiosperm ecosystems

2 figuras, 3 tablasWhile large herbivores can have strong impacts on terrestrial ecosystems, much less is known of their role in aquatic systems. We reviewed the literature to determine: (1) which large herbivores (>10 kg) have a (semi-)aquatic lifestyle and are important consumers of submerged vascular plants, (2) their impact on submerged plant abundance and species composition and (3) their ecosystem functions. We grouped herbivores according to diet, habitat selection and movement ecology: (1) Fully aquatic species, either resident or migratory (manatees, dugongs, turtles), (2) Semi-aquatic species that live both in water and on land, either resident or migratory (swans), (3) Resident semi-aquatic species that live in water and forage mainly on land (hippopotamuses, beavers, capybara), (4) Resident terrestrial species with relatively large home ranges that frequent aquatic habitats (cervids, water buffalo, lowland tapir). Fully aquatic species and swans have the strongest impact on submerged plant abundance and species composition. They may maintain grazing lawns. Because they sometimes target belowground parts, their activity can result in local collapse of plant beds. Semi-aquatic species and turtles serve as important aquatic-terrestrial linkages, by transporting nutrients across ecosystem boundaries. Hippopotamuses and beavers are important geomorphological engineers, capable of altering the land and hydrology at landscape scales. Migratory species and terrestrial species with large home ranges are potentially important dispersal vectors of plant propagules and nutrients. Clearly, large aquatic herbivores have strong impacts on associated species and can be critical ecosystem engineers of aquatic systems, with the ability to modify direct and indirect functional pathways in ecosystems. While global populations of large aquatic herbivores are declining, some show remarkable local recoveries with dramatic consequences for the systems they inhabit. A better understanding of these functional roles will help set priorities for the effective management of large aquatic herbivores along with the plant habitats they rely on.This research was funded by the Spanish Ministry of Science and Innovation (CTM2013-48027-C3-3-R), an Intramural Project from the Spanish National Research Council (CSIC, 201330E062) and the Pew Marine Fellowship.Peer reviewe

Herbivore regulation of plant abundance in aquatic ecosystems.

Herbivory is a fundamental process that controls primary producer abundance and regulates energy and nutrient flows to higher trophic levels. Despite the recent proliferation of small-scale studies on herbivore effects on aquatic plants, there remains limited understanding of the factors that control consumer regulation of vascular plants in aquatic ecosystems. Our current knowledge of the regulation of primary producers has hindered efforts to understand the structure and functioning of aquatic ecosystems, and to manage such ecosystems effectively. We conducted a global meta-analysis of the outcomes of plant-herbivore interactions using a data set comprised of 326 values from 163 studies, in order to test two mechanistic hypotheses: first, that greater negative changes in plant abundance would be associated with higher herbivore biomass densities; second, that the magnitude of changes in plant abundance would vary with herbivore taxonomic identity. We found evidence that plant abundance declined with increased herbivore density, with plants eliminated at high densities. Significant between-taxa differences in impact were detected, with insects associated with smaller reductions in plant abundance than all other taxa. Similarly, birds caused smaller reductions in plant abundance than echinoderms, fish, or molluscs. Furthermore, larger reductions in plant abundance were detected for fish relative to crustaceans. We found a positive relationship between herbivore species richness and change in plant abundance, with the strongest reductions in plant abundance reported for low herbivore species richness, suggesting that greater herbivore diversity may protect against large reductions in plant abundance. Finally, we found that herbivore-plant nativeness was a key factor affecting the magnitude of herbivore impacts on plant abundance across a wide range of species assemblages. Assemblages comprised of invasive herbivores and native plant assemblages were associated with greater reductions in plant abundance compared with invasive herbivores and invasive plants, native herbivores and invasive plants, native herbivores and mixed-nativeness plants, and native herbivores and native plants. By contrast, assemblages comprised of native herbivores and invasive plants were associated with lower reductions in plant abundance compared with both mixed-nativeness herbivores and native plants, and native herbivores and native plants. However, the effects of herbivore-plant nativeness on changes in plant abundance were reduced at high herbivore densities. Our mean reductions in aquatic plant abundance are greater than those reported in the literature for terrestrial plants, but lower than aquatic algae. Our findings highlight the need for a substantial shift in how biologists incorporate plant-herbivore interactions into theories of aquatic ecosystem structure and functioning. Currently, the failure to incorporate top-down effects continues to hinder our capacity to understand and manage the ecological dynamics of habitats that contain aquatic plants

Population variability and extinction risk

Population models generally predict increased extinction risk (ER) with increased population variability (PV), yet some empirical tests have provided contradictory findings. We resolve this conflict by attributing negative measured relationships to a statistical artifact that arises because PV tends to be underestimated for populations with short persistence, Such populations do not go extinct quickly as a consequence of low intrinsic variability; instead, the measured variability is low because they go extinct so quickly. Consequently, any underlying positive relationship between PV and ER tends to be obscured. We conducted a series of analyses to evaluate this claim. Simulations showed that negative measured relationships are to be expected, despite an underlying positive relationship. Simulations also identified properties of data, minimizing this bias and thereby permitting meaningful analysis. Experimental data on laboratory populations of a bruchid beetle (Callosobruchus maculatus) supported the simulation results. Likewise, with an appropriate statistical approach (Cox regression on untransformed data), reanalysis of a controversial data set on British island bird populations revealed a significant positive association between PV and ER (p = O. 03). Finally, a similar analysis of time series for naturally regulated animal populations revealed a positive association between PV and quasiextinction risk (p \u3c 0.01). Without exception, our simulation results, experimental findings, reanalysis of published data, and analysis of quasiextinction risk all contradict previous reports of negative or equivocal relationships. Valid analysis of meaningful data provides strong evidence that increased population variability leads to increased extinction risk

Large herbivores and aquatic-terrestrial links in southern boreal forests

1. Concurrent measurement of population dynamics and associated spatio-temporal patterns of resource flow across aquatic-terrestrial boundaries are rare, yet necessary to understand the consequences of cross-habitat resource flux. Long-term study of the moose Alces alces (L.) population in Isle Royale National Park (Lake Superior, USA) provides an opportunity to examine the patterns of resource flux from aquatic to terrestrial habitats over ∼50 years. 2. We analysed the spatio-temporal dynamics of aquatic-derived nitrogen (N) that moose transfer to terrestrial systems by using excretion models, foraging parameters, moose densities, and moose carcass locations (n = 3616) collected from 1958-2005. 3. Results suggest that moose transfer significant amounts of aquatic-derived N to terrestrial systems, which likely increases terrestrial N availability in riparian zones. A seasonal increase in terrestrial N availability when moose are foraging on N-rich aquatic macrophytes would contrast with the depression of soil N mineralization previously attributed indirectly to moose. 4. Aquatic foraging by moose and moose carcass locations are significantly clustered at multiple scales, indicating that grey wolves Canis lupus (L.) and moose can create concentrated areas of resource transfer due to clustered predation and foraging patterns. 5. This study shows that patterns of faunal-mediated resource transfer can depend significantly on predator-prey dynamics, and that large predators in this system influence herbivore-controlled resource transfer between ecosystems. Given the circumpolar extent of moose, they constitute an important, unquantified aquatic-terrestrial resource vector in boreal systems. © 2008 The Authors

Experimental evidence that the ecosystem effects of aquatic herbivory by moose and beaver may be contingent on water body type

© 2015 John Wiley & Sons Ltd. Mammalian herbivores that consume both aquatic and terrestrial vegetation may have important but little understood effects on freshwater ecosystems. We assessed the effects of North American moose (Alces americanus) and/or beaver (Castor canadensis) on aquatic vegetation and abiotic conditions of three types of waterbodies. We established year-round aquatic exclosures and reference plots in glacial lakes, dammed lakes and riverine ponds (n = 3 in each case). Within plots, we monitored parameters including dissolved oxygen, light, plant diversity and species-specific plant biomass. The effects of herbivory by beaver and moose appear to be contingent upon abiotic and plant community characteristics. Exposure to herbivory decreased biomass in riverine ponds and dammed lakes but not in glacial lakes and decreased species richness in riverine ponds and glacial lakes but not in dammed lakes. Changes in macrophyte species diversity correlated with exposure to herbivory only in glacial lakes. Disaggregating the effects of herbivory according to abiotic conditions and plant community characteristics revealed outcomes that are not detectable when results are aggregated. In some catchments, the response of the aquatic plant community to beaver and moose herbivory may substantially differ from the response of the surrounding terrestrial plant community. Five of the six waterbodies created by beavers (dammed lakes and riverine ponds) maintained \u3e 80% macrophyte cover, despite seasonal reduction in biomass by both moose and beaver. Herbivores appear to cause a short-term reduction in plant biomass in dammed lakes, resulting in greater light availability, without depleting biomass over a number of years