Hannah L. Buckley

A Critical Review of Twenty Years’ Use of the Resource‐Ratio Theory

A model of species interactions based on their use of shared resources was proposed in 1972 by Robert MacArthur and later expanded in an article (1980) and a book (1982) by David Tilman. This “resource‐ratio theory” has been used to make a number of testable predictions about competition and community patterns. We reviewed 1,333 papers that cite Tilman’s two publications to determine whether predictions of the resource‐ratio theory have been adequately tested and to summarize their general conclusions. Most of the citations do not directly test the theory: only 26 studies provide well‐designed tests of one or more predictions, resulting in 42 individual tests of predictions. Most of these tests were conducted in the laboratory or experimental microcosms and used primary producers in freshwater systems. Overall, the predictions of the resource‐ratio theory were supported 75% of the time. One of the primary predictions of the model, that species dominance varies with the ratio of resource availabilities, was supported by 13 of 16 tests, but most other predictions have been insufficiently tested. We suggest that more experimental work in a variety of natural systems is seriously needed, especially studies designed to test predictions related to resource supply and consumption rates.

Fine-scale spatial patterns in bacterial community composition and function within freshwater ponds

The extent to which non-host-associated bacterial communities exhibit small-scale biogeographic patterns in their distribution remains unclear. Our investigation of biogeography in bacterial community composition and function compared samples collected across a smaller spatial scale than most previous studies conducted in freshwater. Using a grid-based sampling design, we abstracted 100+ samples located between 3.5 and 60 m apart within each of three alpine ponds. For every sample, variability in bacterial community composition was monitored using a DNA-fingerprinting methodology (automated ribosomal intergenic spacer analysis) whereas differences in bacterial community function (that is, carbon substrate utilisation patterns) were recorded from Biolog Ecoplates. The exact spatial position and dominant physicochemical conditions (for example, pH and temperature) were simultaneously recorded for each sample location. We assessed spatial differences in bacterial community composition and function within each pond and found that, on average, community composition or function differed significantly when comparing samples located >20 m apart within any pond. Variance partitioning revealed that purely spatial variation accounted for more of the observed variability in both bacterial community composition and function (range: 24–38% and 17–39%) than the combination of purely environmental variation and spatially structured environmental variation (range: 17–32% and 15–20%). Clear spatial patterns in bacterial community composition, but not function were observed within ponds. We therefore suggest that some of the observed variation in bacterial community composition is functionally ‘redundant’. We confirm that distinct bacterial communities are present across unexpectedly small spatial scales suggesting that populations separated by distances of >20 m may be dispersal limited, even within the highly continuous environment of lentic water.

Experimental evidence that the effectiveness of conservation biological control depends on landscape complexity

Summary 1. The expansion of intensive agricultural practices is a major threat to biodiversity and to the delivery of ecosystem services on which humans depend. Local-scale conservation management strategies, such as agri-environment schemes to preserve biodiversity, have been widely adopted to reduce the negative impacts of agricultural intensification. However, it is likely that the effectiveness of these local-scale management actions depend on the structure and composition of the surrounding landscape. 2. We experimentally tested the utility of floral resource strips to improve local-scale biological control of crop pests, when placed within a gradient of moderately simple through to highly complex landscapes. 3. We found that experimental provision of floral resources enhanced parasitism rates of two globally important crop pests in moderately simple landscapes but not in highly complex ones, and this translated into reduced pest abundances and increased crop yield. 4. Synthesis and applications. Our results lend experimental support for the ‘intermediate landscape complexity hypothesis’, which predicts that local conservation management will be most effective in moderately simple agricultural landscapes, and less effective in either very simple landscapes where there is no capacity for response, or in highly complex landscapes where response potential is already saturated. This knowledge will allow more targeted and cost-effective implementation of conservation biological control programs based on an improved understanding of landscape-dependent processes, which will reduce the negative impacts of agricultural intensification.

Small‐scale species richness in forest canopy gaps: the role of niche limitation versus the size of the species pool

The form of the relationship between local species richness and the number of species in the surrounding region can be used as a test between competing theories of commu- nity structure. For 32 canopy gaps in New Zealand Nothofagzts forest, we examined the relationship between the number of vascular plant species in 0.2-m2 quadrats within the gap and the species richness of the whole gap. We found no evidence that competition for a limited number of niches placed an upper limit on the number of locally CO-occurring species. Rather, the mean number of species in quadrats within canopy gaps increased in direct proportion to gap species richness. This relationship held after we controlled for potentially con- founding factors, including variation in forest floor substrate, and gap size, age, shape and orientation. Our results suggest that even over relatively small spatial scales, local species richness can be constrained by the size of the species pool in the immediately surrounding region.

URban Biotopes of Aotearoa New Zealand (URBANZ) (I): composition and diversity of temperate urban lawns in Christchurch

Christchurch urban lawns are dominated by non-native grasses and forbs. However, we document considerable plant diversity; the total number of species encountered in our 327 sampled lawns was 127, although 80 species occurred in <2% of lawns. Seven distinct lawn communities were identified by Two-Way INdicator SPecies ANalysis using occurrence of 47 species that occurred in > 2% of lawns. Our ability to explain variation in species composition was surprisingly good and indicates intensity of lawn maintenance such as frequency of mowing, irrigation, fertiliser, and herbicide use and whether clippings are removed or not plays the major role. Species richness significantly declines with an increase in total area of contiguous lawn, leaf litter cover, the presence of grass clippings, and on loamy soil. Hence, park lawns with coarser management had lower species richness than residential lawns. Native species were more prevalent in well tended residential lawns, where more frequent mowing and removal of clippings or litter build-up diminishes shoot competition or shading. There is tremendous potential for more native species in New Zealand lawns which would contribute substantially to the conservation of endangered lowland herbaceous flora.

Both species sorting and neutral processes drive assembly of bacterial communities in aquatic microcosms

A focus of ecology is to determine drivers of community assembly. Here, we investigate effects of immigration and species sorting (environmental selection) on structuring aquatic bacterial communities in both colonised and previously uncolonised environments. We used nonsterilised and presterilised water from three chemically distinct ponds to establish microcosms, which were opened for 12, 24, 48, 96 or 167 h and then closed again to allow airborne bacterial immigration and subsequent succession. Community similarity, richness, evenness and the parameters of a neutral model were investigated after 167 h. Immigration appeared to govern the assembly of communities in the presterilised water as there were no significant differences in evenness among microcosm communities containing water from each pond. Statistical estimation of neutral model parameters confirmed these findings, because the estimated immigration rate changed significantly with time of exposure to immigration. Species sorting also occurred because significant differences in community similarity (for presterilised and nonsterilised communities) and evenness (only for nonsterilised communities) were detected among microcosms containing different pond water; the magnitude of these differences was greater for communities in nonsterilised microcosms. Our study provides evidence for both processes being important during the colonisation of aquatic environments and presents a novel way to apply the neutral model.

Increased stem density and competition may diminish the positive effects of warming at alpine treeline

The most widespread response to global warming among alpine treeline ecotones is not an upward shift, but an increase in tree density. However, the impact of increasing density on interactions among trees at treeline is not well understood. Here, we test if treeline densification induced by climatic warming leads to increasing intraspecific competition. We mapped and measured the size and age of Smith fir trees growing in two treelines located in the southeastern Tibetan Plateau. We used spatial point-pattern and codispersion analyses to describe the spatial association and covariation among seedlings, juveniles, and adults grouped in 30-yr age classes from the 1860s to the present. Effects of competition on tree height and regeneration were inferred from bivariate mark-correlations. Since the 1950s, a rapid densification occurred at both sites in response to climatic warming. Competition between adults and juveniles or seedlings at small scales intensified as density increased. Encroachment negatively affected height growth and further reduced recruitment around mature trees. We infer that tree recruitment at the studied treelines was more cold-limited prior to 1950 and shifted to a less temperature-constrained regime in response to climatic warming. Therefore, the ongoing densification and encroachment of alpine treelines could alter the way climate drives their transitions toward subalpine forests.

Bacteria as Emerging Indicators of Soil Condition

ABSTRACT Bacterial communities are important for the health and productivity of soil ecosystems and have great potential as novel indicators of environmental perturbations. To assess how they are affected by anthropogenic activity and to determine their ability to provide alternative metrics of environmental health, we sought to define which soil variables bacteria respond to across multiple soil types and land uses. We determined, through 16S rRNA gene amplicon sequencing, the composition of bacterial communities in soil samples from 110 natural or human-impacted sites, located up to 300 km apart. Overall, soil bacterial communities varied more in response to changing soil environments than in response to changes in climate or increasing geographic distance. We identified strong correlations between the relative abundances of members of Pirellulaceae and soil pH, members of Gaiellaceae and carbon-to-nitrogen ratios, members of Bradyrhizobium and the levels of Olsen P (a measure of plant available phosphorus), and members of Chitinophagaceae and aluminum concentrations. These relationships between specific soil attributes and individual soil taxa not only highlight ecological characteristics of these organisms but also demonstrate the ability of key bacterial taxonomic groups to reflect the impact of specific anthropogenic activities, even in comparisons of samples across large geographic areas and diverse soil types. Overall, we provide strong evidence that there is scope to use relative taxon abundances as biological indicators of soil condition. IMPORTANCE The impact of land use change and management on soil microbial community composition remains poorly understood. Therefore, we explored the relationship between a wide range of soil factors and soil bacterial community composition. We included variables related to anthropogenic activity and collected samples across a large spatial scale to interrogate the complex relationships between various bacterial community attributes and soil condition. We provide evidence of strong relationships between individual taxa and specific soil attributes even across large spatial scales and soil and land use types. Collectively, we were able to demonstrate the largely untapped potential of microorganisms to indicate the condition of soil and thereby influence the way that we monitor the effects of anthropogenic activity on soil ecosystems into the future.

The founder space race: a response to Waters et al.

We welcome the attempt by Waters et al. [1], in the spirit of the journal that they are publishing in, to address scales not typically dealt with by ecology (too long) or evolution (too short). The basic scenario that they address [i.e., the expansion of the range of a taxon (species, population, or genotype) into empty niche space], is a nonequilibrium situation in which many ecological processes occur. However, the timescales they refer to range from very short (a few generations) to very long (thousands to millions of generations).

Phylogenetic congruence of lichenised fungi and algae is affected by spatial scale and taxonomic diversity

The role of species’ interactions in structuring biological communities remains unclear. Mutualistic symbioses, involving close positive interactions between two distinct organismal lineages, provide an excellent means to explore the roles of both evolutionary and ecological processes in determining how positive interactions affect community structure. In this study, we investigate patterns of co-diversification between fungi and algae for a range of New Zealand lichens at the community, genus, and species levels and explore explanations for possible patterns related to spatial scale and pattern, taxonomic diversity of the lichens considered, and the level sampling replication. We assembled six independent datasets to compare patterns in phylogenetic congruence with varied spatial extent of sampling, taxonomic diversity and level of specimen replication. For each dataset, we used the DNA sequences from the ITS regions of both the fungal and algal genomes from lichen specimens to produce genetic distance matrices. Phylogenetic congruence between fungi and algae was quantified using distance-based redundancy analysis and we used geographic distance matrices in Moran’s eigenvector mapping and variance partitioning to evaluate the effects of spatial variation on the quantification of phylogenetic congruence. Phylogenetic congruence was highly significant for all datasets and a large proportion of variance in both algal and fungal genetic distances was explained by partner genetic variation. Spatial variables, primarily at large and intermediate scales, were also important for explaining genetic diversity patterns in all datasets. Interestingly, spatial structuring was stronger for fungal than algal genetic variation. As the spatial extent of the samples increased, so too did the proportion of explained variation that was shared between the spatial variables and the partners’ genetic variation. Different lichen taxa showed some variation in their phylogenetic congruence and spatial genetic patterns and where greater sample replication was used, the amount of variation explained by partner genetic variation increased. Our results suggest that the phylogenetic congruence pattern, at least at small spatial scales, is likely due to reciprocal co-adaptation or co-dispersal. However, the detection of these patterns varies among different lichen taxa, across spatial scales and with different levels of sample replication. This work provides insight into the complexities faced in determining how evolutionary and ecological processes may interact to generate diversity in symbiotic association patterns at the population and community levels. Further, it highlights the critical importance of considering sample replication, taxonomic diversity and spatial scale in designing studies of co-diversification.