Implications For Species Coexistence Research Articles

Assembly history modulates vertical root distribution in a grassland experiment

The order of arrival of plant species during assembly can affect the structure and functioning of grassland communities. These so‐called priority effects have been extensively studied aboveground, but we still do not know how they affect the vertical distribution of roots in the soil and the rooting depth of plant communities.To test this hypothesis, we manipulated the order of arrival of three plant functional groups (forbs, grasses and legumes) in a rhizobox experiment. Priority effects were created by sowing one functional group 10 days before the other two. Rhizoboxes in which all functional groups were sown simultaneously were used as controls. During the experiment, the total visible root length and the mean and maximum rooting depth of plant communities were monitored using image analysis and a new methodological approach using deep learning (RootPainter) for root segmentation. At harvest, we measured aboveground (community and species level) and belowground (community level) biomass, and assessed the vertical distribution of the root biomass in different soil layers.At the community level, all scenarios where one functional group was sown before the other two had similar shoot and root productivity. At the species level, two forbs (Achillea millefolium and Centaurea jacea) benefited from arriving early, and one legume (Trifolium pratense) had a disadvantage when it was sown after the grasses. Priority effect treatments also affected the vertical distribution of roots. When grasses were sown first, plant communities rooted more shallowly (lower mean and maximum rooting depth) than when forbs or legumes were sown first. In addition, roots moved down the soil profile more slowly in grasses‐first communities.Our results highlight that plant functional group order of arrival in grassland communities can affect the vertical distribution of roots in the soil and this may have implications for species coexistence.

Contrasting rhizosphere soil nutrient economy of plants associated with arbuscular mycorrhizal and ectomycorrhizal fungi in karst forests

PurposePlants growing in the soils of karst forests associate with arbuscular mycorrhizae (AM) or ectomycorrhizae (ECM) to acquire nutrients. We researched how these different mycorrhizal associations affect rhizosphere soil nutrient economy in these calcareous soils.MethodsBulk and rhizosphere soils were sampled beneath 25 AM and 9 ECM plants growing in primary forests at the Puding Karst Critical Zone Observatory. Nutrient contents and potential enzyme activities were analyzed to test the effect of different types of mycorrhizal association on rhizosphere soil nitrogen (N) and phosphorus (P) economies.ResultsThe contents of nitrate-N and available-P were markedly lower in the rhizospheres of ECM plants compared to AM plants. Ectomycorrhizal plants promoted relatively greater investment in N-acquisition enzymes, in contrast, AM plants caused relatively greater investment in P-acquisition enzymes. The decreased pH in the rhizospheres of AM plants likely promoted the greater P availability.ConclusionOur results revealed how plants that form contrasting mycorrhizal associations have fundamentally different effects on rhizospheric nutrient economies in the low fertility karst soils of southwest China. Differentiation in N- and P-acquisition capacity of these plants have implications for species coexistence and the high levels of plant biodiversity observed in these forests.

Plant-soil feedbacks in declining forests: implications for species coexistence.

Plant-soil feedbacks (PSFs) play a relevant role as drivers of species abundance, coexistence, and succession in plant communities. However, the potential contribution of PSFs to community dynamics in changing forest ecosystems affected by global change drivers is still largely unexplored. We measured the direction, strength and nature (biological vs. chemical) of PSFs experienced by coexisting tree species in two types of declining Quercus suber forests of southwestern Spain (open woodland vs. closed forest) invaded by the exotic soil pathogen Phytophthora cinnamomi. To test PSFs in a realistic community context, we focused not only on individual PSFs (i.e., comparing the growth of a tree species on conspecific vs. heterospecific soil) but also calculated net-pairwise PSFs by comparing performance of coexisting tree species on their own and each other's soils. We hypothesized that the decline and death of Q.suber would alter the direction and strength of individual and net-pairwise PSFs due to the associated changes in soil nutrients and microbial communities, with implications for recruitment dynamics and species coexistence. In support of our hypothesis, we found that the decline of Q.suber translated into substantial alterations of individual and net-pairwise PSFs, which shifted from mostly neutral to significantly positive or negative, depending on the forest type. In both cases however the identified PSFs benefited other species more than Q.suber (i.e., heterospecific positive PSF in the open woodland, conspecific negative PSF in the closed forest). Our results supported PSFs driven by changes in chemical soil properties (mainly phosphorus) and arbuscular mycorrhizal fungi, but not in pathogen abundance. Overall, our study suggests that PSFs might reinforce the loss of dominance of Q.suber in declining forests invaded by P.cinnamomi by promoting the relative performance of non-declining coexisting species. More generally, our results indicate an increase in the strength of net PSFs as natural forests become disturbed by global change drivers (e.g., invasive species), suggesting an increasingly important role of PSFs in forest community dynamics in the near future.

Species Size and Reproductive Economy in Herbaceous Vegetation

According to traditional theory, larger plants generaly have an advantage with respect to competition, especially for light. It seems a paradox then that most species that coexist within natural vegetation are relatively small; specis size distributions are right-skewed at virtually every scale. The critical question then becomes: if bigger is better in competition for resources, why then are there so many small plants? A potential explanation for this paradox is that smaller species may have greater reproductive economy-i.e. the ability to reproduce despite suppression from intense competition. Selection for greater reproductive economy may be associated with smaller seed sizes, increased rates of self-fertilization and/or clonality, and in the case of this study, smaller size at reproductive maturity. Random plots in an old field at Queen's University Biological Station were sampled and the largest and smallest reproductive individuals of each species were collected, dried and weighed - to test the hypothesis that smaller species can reproduce at a smaller proportion of their maximum potential plant sizes. The results did not support this, but the hypothesis that smaller plants have greater reproductive economy could not be rejected as it was not possible to record data for the largest possible plant size for each species (since even the largest plants were subjected to competition from neighbours). This provides a focus for future research. Understanding the role of plant size in affecting the process of species assembly has important implications for species coexistence and mechanisms of biodiversity preservation, and thus efforts involving conservation and ecosystem management.

Nested species-rich networks of scavenging vertebrates support high levels of interspecific competition.

Disentangling the processes that shape the organization of ecological assemblages and its implications for species coexistence is one of the foremost challenges of ecology. Although insightful advances have recently related community composition and structure with species coexistence in mutualistic and antagonistic networks, little is known regarding other species assemblages, such as those of scavengers exploiting carrion. Here we studied seven assemblages of scavengers feeding on ungulate carcasses in mainland Spain. We used dynamical models to investigate if community composition, species richness and structure (nestedness) affect species coexistence at carcasses. Scavenging networks showed a nested pattern in sites where highly efficient, obligate scavengers (i.e., vultures) were present and a non-nested pattern everywhere else. Griffon Vulture (Gyps fulvus) and certain meso-facultative mammalian scavengers (i.e., red fox, Vulpes vulpes, and stone marten, Martes foina) were the main species contributing to nestedness. Assemblages with vultures were also the richest ones in species. Nested species-rich assemblages with vulture presence were associated with high carcass consumption rates, indicating higher interspecific competition at the local scale. However, the proportion of species stopping the consumption of carrion (as derived from the competitive dynamic model) stabilized at high richness and nestedness levels. This suggests that high species richness and nestedness may characterize scavenging networks that are robust to high levels of interspecific competition for carrion. Some facilitative interactions driven by vultures and major facultative scavengers could be behind these observations. Our findings are relevant for understanding species' coexistence in highly competitive systems.

The geographic and seasonal dimensions of habitat use in Galerida larks: implications for species coexistence and range limits

In this paper, we investigated the patterns of habitat use in three sibling species of larks: the Thekla lark Galerida theklae which is sympatric with either of two parapatric species of crested larks (G. macrorhyncha replacing G. cristata in arid areas of Morocco). Specifically, we addressed the following questions: 1) do species show niche conservatism in space (France vs Morocco) and time (breeding vs winter period)? 2) Does habitat use change at different interspecific densities? 3) Which factors drive the narrow range limit between G. cristata and G. macrorhyncha? We found that G. theklae was strongly associated with sloping and rocky (rugged) habitats in comparison with crested larks, but habitat use was not entirely consistent in space and time. Evidences for ecological displacement were mitigated. In the only habitat shared by G. cristata and G. theklae in France (vineyards surrounded by low shrubland), we observed a one‐third increase of G. theklae from nearby garrigues in winter, which coincided with a significant decrease of G. cristata after we controlled for density changes in non‐shared habitats. However, in Morocco, the use of rugged habitats by G. theklae was not affected by the prevalence of G. cristata or G. macrorhyncha in surrounding areas, at least at the spatial scales investigated. Our results suggest that interspecific competition plays a role in the parapatric distribution of G. cristata and G. macrorhyncha, because: 1) the alternative of a recent contact receives little support; 2) transition between species coincides with aridity gradients but not with topographic barriers or a sharp ecotone; 3) G. cristata also occurs in arid areas wherein G. macrorhyncha is rare or absent. Overall, our results add support to the view that a better integration of biotic interactions and spatio‐temporal variations in ecological niches might increase the predictive power of ecological niche models.

Maintenance of diversity altered by a shift in dominant species: implications for species coexistence

Diversity can be maintained by the biological characteristics of species within commu- nities, particularly dominant species. Invasions often result in shifts of dominant species. Despite the number of studies on invasion ecology, few have determined their long-term effect on diversity. In the present study, we coupled short- and long-term studies to examine the relationship between invasive species (colonial ascidians or sea squirts) and diversity and to discuss their implications for species coexistence. Diversity patterns between 2 panel studies (1979 to 1982; 2003 to 2006), one conducted before the establishment of invasive colonial ascidians, revealed an increase in species diversity. Short-term recruitment studies designed to elucidate mechanisms behind the patterns observed in our long-term studies suggest contrasting methods of maintaining diversity. Between 1979 and 1982, Mytilus edulis was a consistent spatial occupant whose hard shell provided secondary space for col- onization by other species. In contrast, the 2003 to 2006 community was dominated by the seasonally abundant invasive colonial ascidians Botrylloides violaceus and to a lesser extent Didemnum vexil- lum, which do not provide secondary substrate. These species either senesce after reproduction or undergo seasonal regression and free space for colonization by other species. These studies suggest that the coexistence of species has shifted from a community in which diversity was maintained by secondary substrates to a community in which diversity is maintained by primary substrates. Addi- tionally, they suggest that the community is undersaturated and could support more species, particu- larly those that seasonally utilize a limited resource (free space).

Impacts of seedling herbivory on plant competition and implications for species coexistence.

Although the causes and consequences of seedling herbivory for plant community composition are well understood, the mechanisms by which herbivores influence plant species recruitment to the established phase remain less clear. The aim was to examine how variation in the intensity of seedling herbivory interacts with growth-defence trade-offs and herbivore feeding preferences to affect plant community development. Using 14-d-old seedlings of Trifolium pratense and T. repens, relative growth and susceptibility to herbivory by the snail Helix aspersa was quantified to elucidate putative growth-defence trade-offs for these species. Then mixed assemblages of 14-d-old Trifolium seedlings were exposed to herbivory by zero, two, five or ten snails and determined how variation in the intensity of herbivory affected competitive interactions into the mature phase (as measured by total plant biomass at 120 d old). In the absence of herbivory, communities were dominated by T. pratense; a result expected on the basis that it yielded larger and presumably more competitive seedlings. However, when seedlings were exposed to herbivory, the balance of competition shifted. At low levels of herbivory (two snails), both Trifolium species contributed equally to total plant biomass. More intense herbivory (five snails) resulted in almost total mortality of T. pratense and dominance of the mature community by T. repens. The most intense herbivory (ten snails) effectively removed all seedlings from the experimental community. The study illustrates a mechanism whereby spatio-temporal fluctuations in seedling herbivory, when coupled with species-specific variation in competitive ability and sensitivity to herbivore attack, can differentially influence plant recruitment into the mature phase. This mechanism may be a key element in our attempts to understand plant species coexistence, since fluctuations in plant recruitment are fundamental to the many theories that view coexistence as a consequence of a spatio-temporal lottery for dominance over regeneration micro-sites.

Weak Competition Among Tropical Tree Seedlings: Implications for Species Coexistence

ABSTRACTThe intensity of competition among forest tree seedlings is poorly understood, but has important ramifications for their recruitment and for the maintenance of species diversity. Intense competition among seedlings could allow competitively dominant species to exclude subordinate species. Alternatively, the low density and small stature of forest tree seedlings could preclude intense interseedling competition. In this case, other processes, such as size‐asymmetric competition with adults, interactions with consumers, or neutral dynamics would prevail as those structuring the forest understory. We tested the intensity of, and potential for, intraspecific competition among tree seedlings of three species (Brosimum alicastrum, Matisia cordata, and Pouteria reticulata) in two Neotropical rain forests. We reduced stem densities by up to 90 percent and monitored individual growth and survival rates for up to 24 mo. Individual growth and survival rates were generally unrelated to stem density. Contrary to the predicted behavior of intensely competing plant populations, the distribution of individual heights did not become more left‐skewed with time for any species, regardless of plot density; i.e., excesses of short, suppressed individuals did not accumulate in high‐density plots. We further measured the overlap of zones of influence (ZOIs) to assess the potential for resource competition. Seedling ZOIs overlapped only slightly in extremely dense monodominant plots, and even less in ambient‐density plots of mixed composition. Our results thus suggest that interseedling competition was weak. Given the low density of tree seedlings in Neotropical forests, we infer that resource competition among seedlings may be irrelevant to their recruitment.

Optimal photosynthetic characteristics of individual plants in vegetation stands and implications for species coexistence.

This paper reviews the way optimization theory has been used in canopy models to analyse the adaptive significance of photosynthesis-related plant characteristics and their consequences for the structure and species composition of vegetation stands. In most studies simple optimization has been used with trait values optimal when they lead to maximum whole-stand photosynthesis. This approach is subject to the condition that the optimum for one individual is independent of the characteristics of its neighbours. This seems unlikely in vegetation stands where neighbour plants strongly influence each other's light climate. Not surprisingly, there are consistent deviations between predicted plant traits and real values: plants tend to be taller, distribute nitrogen more evenly among their leaves and produce more leaf area which is projected more horizontally than predicted by models. By applying game theory to individual plant-based canopy models, other studies have shown that optimal vegetation stands with maximum whole-stand photosynthesis are not evolutionarily stable. They can be successfully invaded by mutants that are taller, project their leaves more horizontally or that produce greater than optimal leaf areas. While these individual-based models can successfully predict the canopy structure of vegetation stands, they are invariably determined at unique optimal trait values. They do not allow for the co-existence of more than one species with different characteristics. Canopy models can contribute to our understanding of species coexistence through (a) simultaneous analysis of the various traits that determine light capture and photosynthesis and the trade-offs between them, and (b) consideration of trade-offs associated with specialization to different positions in the niche space defined by temporal and spatial heterogeneity of resources.

Behavioral resource depression and decaying perceived risk of predation in two species of coexisting gerbils

Behavioral resource depression occurs when the behavior of prey individuals changes in response to the presence of a predator, resulting in a reduction of the encounter rate of the predator with its prey. Here I present experimental evidence on the response of two species of gerbils (Gerbillus allenbyi and G. pyramidum) to the presence of barn owls. I conducted the experiments in a large aviary. Both gerbils responded to the presence of barn owl predators by foraging in fewer resource patches (seed trays) and by quitting foraged resource patches at a higher resource harvest rate (giving-up density of resource; GUD). This reduced the amount of time gerbils were exposed to owl predation, and hence the encounter rate of owls with gerbils, i.e., behavioral resource depression. Thus, the presence of owls imposes a foraging cost on gerbils due to risk of predation, and also on the owls themselves due to resource depression. I then examined how resource depression relaxed over time following exposure to owls. In the days following an encounter with the predator, the reduction in foraging activity for both gerbil species eased. Increasing numbers of trays were foraged each day, and GUDs in seed trays declined. The two gerbils differed in their rate of recovery, with G. pyramidum returning to prepredator levels of foraging after 1 or 2 nights and G. allenbyi taking 5 nights or longer. Interspecific differences in recovery rates may be based on differences between the species in vulnerability to predation and/or ability to detect the presence of predators. The differences in recovery rates may be due to optimal memory windows or decay rates, where differences between species are based on risk of predation or on how perceived risk changes with time since a predator was last encountered. Finally, differences between or among competitors in recovery from resource depression may provide foraging opportunities in time for the species which recover most quickly and may have implications for species coexistence.

Complex Competitive Relationships Among Genotypes of Three Perennial Grasses: Implications for Species Coexistence

Competitive relationships were studied among genotypes of Agropyron repens, Poa pratensis, and Phleum pratense collected from a grassland community in southeastern Ontario. Field surveys revealed no significant correlations of abundance among the three species within randomly placed survey plots. The greater performance of target plants in removal plots versus control plots for all three species in a vegetation-removal experiment, however, suggests that each of these three species was suppressed by competitive interactions in the field. One clone (genotype) of each species was collected from each of 10 sites (7-cm2 neighborhoods) within the community. Each genotype was propagated vegetatively and grown in the greenhouse in monocultures and in pairwise mixtures with all other genotypes collected from the community. Differences in relative competitive ability among species or genotypes were measured as significant differences between the yield-suppression coefficients of the two components of a mixture. A significant interaction of target site of origin and associate site of origin (i.e., mixture effect) for yield-suppression coefficients of intraspecific mixtures for each species suggests genotypic variability among the collected clones. Competitive relationships at the species level were transitive. Genotypes collected from the same neighborhood exhibited competitive standoffs or transitive competitive relationships. Competitively subordinate genotypes from different neighborhoods, however, belonged to different species. Competitive relationships at the genotype level (across neighborhoods) were complex and often intransitive. These data provide preliminary support for the competitive-combining-ability hypothesis for species coexistence, which predicts that intransitive competitive abilities at the genotype level (spanning taxonomic boundaries) may prevent competitive exclusion at the whole-population level and hence contribute to the maintenance of both species and genotypic diversity within vegetation.