Ecology

Albuquerque‐Lima, Sinzinando; Zappi, Daniela C.; Taylor, Nigel P.; Lowry, Martin; Winberg, Mats; Machado, Isabel Cristina

doi: 10.1002/ecy.4058pmid: 37078705

doi: 10.1002/ecy.3752pmid: N/A

Maitland, Bryan M.; Rahel, Frank J.

doi: 10.1002/ecy.4103pmid: 37203414

Changing ecological conditions along environmental gradients influence patterns of biodiversity and ecosystem functioning. However, how networks of interacting species respond to these changes remains unclear. We quantified aquatic food webs along longitudinal stream gradients spanning the Rocky Mountain–Great Plains ecotone using community composition, functional traits, and stable isotopes. We predicted that increasing ecosystem size, productivity, and species richness along the gradient would positively influence aquatic trophic diversity (e.g., expanded vertical and horizontal trophic niche breadths). We also predicted that trophic redundancy among fish species would decrease moving downstream as species partition food resources (e.g., reduced trophic niche overlap). Consumer stable isotope data (δ13C and δ15N) revealed nonlinear changes in trophic diversity along the gradient. Invertebrate trophic diversity had a dome‐shaped relationship with the gradient, strongly linked to an expanding then contracting δ13C range. Fish trophic diversity initially increased and then plateaued downstream, despite linearly expanding δ13C and δ15N ranges. Trophic redundancy within the fish community decreased downstream along the gradient. However, trophic redundancy also showed a nonlinear relationship with fish species richness; it initially declined, then began to increase when more than nine species were present, indicating a shift from niche partitioning to niche packing at intermediate species richness levels. This result suggests that while δ13C and δ15N ranges for fish communities increased across the gradient, niche packing within communities in the Great Plains caused overall trophic diversity to saturate. Our results demonstrate that food web structure along stream gradients reflects an interaction between factors that decrease trophic redundancy, such as increased living space and niche partitioning, versus factors that increase trophic redundancy, such as increased species richness and niche packing. Our study helps to explain how multiple mechanisms shape food web properties along longitudinal stream gradients, and where niche partitioning or niche packing may be dominant. Understanding the functional roles of organisms across similar environmental gradients in other ecosystems will be increasingly important because they determine how food webs, and thus ecosystem function, will respond to environmental change, biodiversity loss, or species invasions.

Lopez, Laura K.; Cortez, Michael H.; DeBlieux, Turner S.; Menel, Ilona A.; O'Brien, Bruce; Cáceres, Carla E.; Hall, Spencer R.; Duffy, Meghan A.

doi: 10.1002/ecy.4063pmid: 37186234

The healthy herds hypothesis proposes that predators can reduce parasite prevalence and thereby increase the density of their prey. However, evidence for such predator‐driven reductions in the prevalence of prey remains mixed. Furthermore, even less evidence supports increases in prey density during epidemics. Here, we used a planktonic predator–prey–parasite system to experimentally test the healthy herds hypothesis. We manipulated density of a predator (the phantom midge, Chaoborus punctipennis) and parasitism (the virulent fungus Metschnikowia bicuspidata) in experimental assemblages. Because we know natural populations of the prey (Daphnia dentifera) vary in susceptibility to both predator and parasite, we stocked experimental populations with nine genotypes spanning a broad range of susceptibility to both enemies. Predation significantly reduced infection prevalence, eliminating infection at the highest predation level. However, lower parasitism did not increase densities of prey; instead, prey density decreased substantially at the highest predation levels (a major density cost of healthy herds predation). This density result was predicted by a model parameterized for this system. The model specifies three conditions for predation to increase prey density during epidemics: (i) predators selectively feed on infected prey, (ii) consumed infected prey release fewer infectious propagules than unconsumed prey, and (iii) sufficiently low infection prevalence. While the system satisfied the first two conditions, prevalence remained too high to see an increase in prey density with predation. Low prey densities caused by high predation drove increases in algal resources of the prey, fueling greater reproduction, indicating that consumer–resource interactions can complicate predator–prey–parasite dynamics. Overall, in our experiment, predation reduced the prevalence of a virulent parasite but, at the highest levels, also reduced prey density. Hence, while healthy herds predation is possible under some conditions, our empirical results make it clear that the manipulation of predators to reduce parasite prevalence may harm prey density.

Berthelot, Sylvie; Bauhus, Jürgen; Dormann, Carsten F.; Gravel, Dominique; Messier, Christian; Nock, Charles A.; Paquette, Alain; Reich, Peter B.; Fründ, Jochen

doi: 10.1002/ecy.4070pmid: 37127925

It is commonly expected that exotic plants experience reduced herbivory, but experimental evidence for such enemy release is still controversial. One reason for conflicting results might be that community context has rarely been accounted for, although the surrounding plant diversity may moderate enemy release. Here, we tested the effects of focal tree origin and surrounding tree diversity on herbivore abundance and leaf damage in a cross‐Atlantic tree‐diversity experiment in Canada and Germany. We evaluated six European tree species paired with six North American congeners in both their native and exotic range, expecting lower herbivory for the exotic tree species in each pair at each site. Such reciprocal experiments have long been called for, but have not been realized thus far. In addition to a thorough evaluation of overall enemy release effects, we tested whether enemy release effects changed with the surrounding tree diversity. Herbivore abundance was indeed consistently lower on exotics across all six tree genera (12 comparisons). This effect of exotic status was independent of the continent, phylogenetic relatedness, and surrounding tree diversity. In contrast, leaf damage associated with generalist leaf chewers was consistently higher on North American tree species. Interestingly, several species of European weevils were the most abundant leaf chewers on both continents and the dominant herbivores at the Canadian site. Thus, most observed leaf damage is likely to reflect the effect of generalist herbivores that feed heavily on plant species with which they have not evolved. At the German site, sap suckers were the dominant herbivores and showed a pattern consistent with enemy release. Taken together, the consistently lower herbivory on exotics on both continents is not purely a pattern of enemy release in the strictest sense, but to some degree additionally reflects the susceptibility of native plants to invasive herbivores. In conclusion, our cross‐Atlantic study is consistent with the idea that nonnative trees have generally reduced herbivory, regardless of tree community diversity and species identity, but for different reasons depending on the dominant herbivore guild.

Glass, Thomas W.; Bishop, Louise; Fraley, Kevin M.

doi: 10.1002/ecy.4097pmid: 37165905

Wang, Ai‐Ying; Peng, Yan‐Qiong; Cook, James M.; Yang, Da‐Rong; Zhang, Da‐Yong; Liao, Wan‐Jin

doi: 10.1002/ecy.4062pmid: 37186391

Ecological interactions among plants, insect herbivores, and parasitoids are pervasive in nature and play important roles in community assembling, but the codiversification of tri‐trophic interactions has received less attention. Here we compare pairwise codiversification patterns between a set of 22 fig species, their herbivorous pollinating and galling wasps, and their parasitoids. The parasitoid phylogeny showed significant congruence and more cospeciation events with host insects phylogeny than with host plants. These results suggest that parasitoid phylogeny and speciation is more closely related to their host insects than to their host plants. The pollinating wasps hosted more parasitoid species than gallers and indicated a more intense interspecific competition among parasitoids associated with pollinators. Closer matching and fewer evolutionary host shifts were found between parasitoids and galler hosts than between parasitoids and pollinator hosts. These results suggest that interspecific competition among parasitoids, rather than resource availability of host wasps, is the main driver of the codiversification pattern in this community. Therefore, our study highlights the important role of interspecific competition among high trophic level insects in plant–insect tri‐trophic community assembling.

Stratmann, Theresa S. M.; Forrest, Matthew; Traylor, Wolfgang; Dejid, Nandintsetseg; Olson, Kirk A.; Mueller, Thomas; Hickler, Thomas

doi: 10.1002/ecy.4071pmid: 37128704

Long‐distance movements are hypothesized to positively influence population size and stability of mobile species. We tested this hypothesis with a novel modeling approach in which moving herbivores interact with the environment created by a dynamic global vegetation model using highly mobile Mongolian gazelles in the eastern Mongolian grasslands as a case study. Gazelle population dynamics were modeled from 1901 to 2018 under two scenarios, one allowing free movement and one restricting movement. Gazelles were 2.2 times more abundant when they could move freely and were extirpated in 71% of the study area when mobility was restricted. Mobility resulted in greater population increases during times of abundant forage and smaller population decreases during drought. Reduced thermoregulatory costs associated with climate change, combined with an increase in vegetation biomass, increased gazelle abundance. Since high abundances often resulted in overgrazing and, thus, extirpation when movement was restricted, mobility had an important role in maintaining higher densities. The novel modeling approach shows how accounting for not just herbivore but also plant ecophysiology can improve our understanding of the population dynamics of highly mobile herbivores, in particular when examining the effects of habitat and climate change. Since the model simulates herbivores based on general physiological mechanisms that apply across large herbivores and the vegetation model can be applied globally, it is possible to adapt the model to other large‐herbivore systems.

Vleminckx, Jason; Barrantes, Oscar Valverde; Fortunel, Claire; Paine, C. E. Timothy; Bauman, David; Engel, Julien; Petronelli, Pascal; Dávila, Nállarett; Rios, Marcos; Valderrama Sandoval, Elvis Harry; Mesones, Italo; Allié, Elodie; Goret, Jean‐Yves; Draper, Freddie C.;

Van Moorter, Bram; Kivimäki, Ilkka; Panzacchi, Manuela; Saura, Santiago; Brandão Niebuhr, Bernardo; Strand, Olav; Saerens, Marco

doi: 10.1002/ecy.4105pmid: 37212446

Niche modeling is typically used to assess the effects of anthropogenic land use and climate change on species distributions and to inform spatial conservation planning. These models focus on the suitability of local biotic and abiotic conditions for a species in environmental space (E‐space). Although movements also affect species occurrence, efforts to formally integrate geographic space (G‐space) into niche modeling have been hindered by the lack of comprehensive theoretical frameworks. We propose the “functional habitat” framework to define areas that are simultaneously of high quality in E‐space, and functionally connected to other suitable habitats in G‐space. Originating in metapopulation ecology, approaches have been developed to assess the amount of suitable connected habitats, based on the proximity between pairs of locations. Using network theory, which operates in topological space (T‐space, defined by a network), we extended these metapopulation approaches to integrate movement constraints in G‐space with niche modeling in E‐space. We demonstrate the functional habitat framework using empirical data (GPS tracking and population monitoring) throughout the European wild mountain reindeer (Rangifer t. tarandus) distribution range. We show that functional habitat outperforms traditional suitability in explaining the species' distribution. This approach integrates effects from habitat loss and fragmentation for spatial conservation planning, and avoids overemphasizing small, inaccessible areas with locally suitable habitats. The functional habitat framework formally integrates biotic, abiotic, and movement constraints in niche modeling using network theory, thus opening a wide range of applications in spatial conservation planning.