Summary1. Current views of anthropogenic environments emphasize the extreme novelty of urban and industrial ecosystems. Proponents of reconciliation ecology argue that we need to use such habitats to conserve biodiversity, given the inadequacy of natural reserve systems. 2. Some of the harshest anthropogenic ecosystems may be able to support indigenous biodiversity due to their structural or functional resemblance to natural ecosystems, habitats, or microsites that may be present in the region but not part of the historic ecosystem on a particular site. Here we review recent work that evaluates similarities between urban and industrial ecosystems and natural analogues, and explore the potential for these in reconciliation ecology. 3. We find that artificial habitats represent a gradient of ecological novelty which may be independent of the degree of human influence. While hard-surfaced habitats such as walls and quarries are the most investigated artificial analogues (of natural rock pavements and cliffs), there are many other examples spanning a range of habitats in both terrestrial and marine settings. Analogous ecosystems may be present in the region but limits to dispersal can prevent appropriate species from reaching urban or industrial sites, and small differences in abiotic conditions can sometimes prevent colonization by native biota in otherwise similar artificial habitats. We suggest that a search for habitat analogues represents an important principle to guide reconciliation ecology in urban and industrial lands. In constrast, analogous ecosystems may also support pest species that exploit the similarities between anthropogenic habitats and their ancestral habitats. 4. Synthesis and applications. Identifying analogous habitats and ecosystems could enhance biodiversity conservation and ecosystem services in anthropogenic environments. Abiotic and biotic differences between artificial analogues and natural systems can be frequently overcome by ecological engineering to make the environment more suitable for native biodiversity, and ⁄ or assisted dispersal to allow suitable native organisms to reach appropriate sites within artificial ecosystems. Altering some habitats to become less analogous may help reduce impacts of pest species in urban and industrial areas.

Ecosystem rehabilitation strategies are grounded in the concept that coexisting species fit their environments as an outcome of natural selection operating over ecological and evolutionary timescales. From this perspective, re-creation of historical environmental filters on community assembly is a necessary first step to recovering biodiversity within degraded ecosystems; however, this approach is often not feasible in severely damaged environments where extensive physiochemical changes cannot be reversed. Under such circumstances management goals may shift from restoring historical conditions to reconstructing entirely new ecosystems or replicating natural ecosystems that may be locally novel but of regional conservation importance. This latter goal may be achieved by introducing to damaged sites species already adapted to filters maintaining the degraded state, through targeting assemblages from natural ecosystems biophysically analogous to the degraded state, here termed "degraded-state analogue" (DSA) ecosystems. This hypothesis predicts that, in high-stress sites where recruitment of previous inhabitants is strongly microsite-limited, DSA species will be primarily propagule-limited; furthermore, communities invaded by DSA species should shift in structure to reflect properties associated with high-value DSA target ecosystems. We tested these predictions by experimentally sowing long-abandoned limestone quarry floors with 18 perennial grass and forb species characteristic of rare natural limestone pavements called "alvars." Alvar species established successfully under a range of microsite conditions manipulated to alter suspected constraints on colonization, including nitrogen deficiency, excessive CaCO3, and competition with weeds. Alvar species performed equivalently to seeded weed species known to thrive on quarry floors. Resident communities doubled in species richness following alvar species addition, supporting 17-20 species/0.18 m2 (95% confidence interval) and providing refuge to regionally restricted or threatened species including Iris lacustris, Solidago ptarmicoides, and Liatris cylindracea. In contrast, maximum-diversity reference plots on a pristine alvar supported 20-23 species/0.18 m2. Strong propagule limitation but weak microsite constraints on quarry colonization by alvar species combined with establishment of species-rich communities comparable to natural alvar biodiversity hot spots confirms that targeting DSA assemblages in ecosystem reconstruction can promote both efficient site colonization and ex situ biodiversity conservation within difficult-to-restore anthropogenic wastelands.

Question: Are the biophysical conditions of abandoned limestone quarry floors and natural alvars sufficiently similar to each other for alvars to be used as a model for quarry floor restoration? Location: Ontario, Canada. Methods: We measured plant species frequency and environmental and soil variables in 13 abandoned limestone quarries and used ANOVA to compare them with data previously collected from seven natural alvars. We used multivariate ordinations on the quarry floor data alone and on the combined quarry floor and alvar data to determine how plant community structure was controlled by the abiotic environment in both habitats. Results: Except for higher levels of many nutrients, the physical characteristics were similar between quarry floors and alvars. 246 plant species were found on quarry floors as compared to 283 on alvars, with 79 species in common between the two habitat types. While quarry floors supported fewer bryophytes and more exotic vascular plants compared with alvars, five alvar endemics and 24 characteristic alvar species were found to grow there. The age of the site, nutrient levels, and presence of standing water and bare rock were important factors influencing species composition in both habitats. Conclusions: Through natural revegetation alone, the abandoned quarry floors surveyed in this study have already taken on many physical and vegetation characteristics of natural alvars. This makes alvars very suitable as the restoration goal for abandoned limestone quarries.

Species interactions affect plant diversity through the net effects of competition and facilitation, with the latter more prevalent in physically stressful environments when plant cover ameliorates abiotic stress. One explanation for species loss in invader-dominated systems is a shift in the competition-facilitation balance, with competition intensifying in areas formerly structured by facilitation. We test this possibility with a 10-site prairie meta-experiment along a 500-km latitudinal stress gradient, quantifying the relationships among abiotic stress, exotic dominance, and native plant recruitment over five years. The latitudinal gradient is inversely correlated with abiotic stress, with lower latitudes more moisture- and nutrient-limited. We observed strong negative effects by invasive dominant grasses on plant establishment, but only in northern sites with lower-stress environments. At these locations, disturbance was critical for recruitment by reducing the suppressive dominant (invasive) canopy. In more stressful environments to the south, the impacts of the dominant invaders on plant establishment became facilitative, and diversity was more limited by seed availability. Disturbance prevented recruitment because seedling survival depended on a protective plant canopy, presumably because the canopy reduced temperature or moisture stress. Seed limitation was similarly prevalent in all sites. Our work confirms the importance of facilitation as an organizing process for plants in higher-stress environments, even with transformations of species composition and dominance. It also demonstrates that the mechanisms regulating diversity, including invader impacts, can vary within the same plant community depending on environmental context. Because limits on native plant recruitment are environmentally contingent, management strategies that seek to increase diversity, including invader eradication, must account for site-level variations in the balance between biotic and abiotic constraints.