PhD Student, York University
Positive interactions between plants support desert biodiversity in a changing climat
The habitable area of a plant species is typically defined by their environmental requirements, but this ignores interactions from other species. In deserts, positive interactions within plant communities are important drivers of biodiversity. Thus, models of suitable habitat for desert plant species should include these potential positive interactions. We tested the hypothesis that shrubs increase the geographical distribution of desert annuals because through microclimate modification they can provide suitable environmental conditions. We used the database Global Biodiversity Information Facility to construct MaxEnt species distribution models for annuals with and without reported benefactor species in the deserts of California. We contrasted probabilities of occurrence in climate models with and without reported benefactor shrub species for annuals reported as beneficiaries (i.e. beneficiary) relative to those that have not been reported as facilitated (i.e. unreported). To determine associations with shrubs and annual plant species, we also compared density estimates for each the beneficiary and unreported plant groups. We found the inclusion of shrubs into models significantly improved model predictability and suitable area for all beneficiary species. However, the inclusion of shrubs did not affect models for the unreported plant species. This suggests that shrubs provide climatic conditions that match the habitat characteristics of the beneficiary species, but not the unreported species. We also found that shrubs strongly correlated with beneficiary annual species and slightly correlated with the unreported annual species. Therefore, shrubs and beneficiaries species are geographically associated and could be dependent on positive interactions for their occurrence. We highlight the importance of including facilitation in modeling climate scenarios that currently neglect biotic interactions. Including positive interactions in species distribution modeling is still a relatively novel concept that can enhance the predictability of climate models that estimate species loss.
Abstract: Interactions among species determine local-scale diversity, but local interactions are thought to have minor effects at larger scales. However, quantitative comparisons of the importance of biotic interactions relative to other drivers are rarely made at larger scales. Using a data set spanning 78 sites and five continents, we assessed the relative importance of biotic interactions and climate in determining plant diversity in alpine ecosystems dominated by nurse-plant cushion species. Climate variables related with water balance showed the highest correlation with richness at the global scale. Strikingly, although the effect of cushion species on diversity was lower than that of climate, its contribution was still substantial. In particular, cushion species enhanced species richness more in systems with inherently impoverished local diversity. Nurse species appear to act as a 'safety net' sustaining diversity under harsh conditions, demonstrating that climate and species interactions should be integrated when predicting future biodiversity effects of climate change.
Pub.: 19 Nov '13, Pinned: 15 Jun '17
Abstract: Understanding the processes determining species range limits is central to predicting species distributions under climate change. Projected future ranges are extrapolated from distribution models based on climate layers, and few models incorporate the effects of biotic interactions on species' distributions. Here, we show that a positive species interaction ameliorates abiotic stress, and has a profound effect on a species' range limits. Combining field surveys of 92 populations, 10 common garden experiments throughout the range, species distribution models and greenhouse experiments, we show that mutualistic fungal endophytes ameliorate drought stress and broaden the geographic range of their native grass host Bromus laevipes by thousands of square kilometres (~ 20% larger) into drier habitats. Range differentiation between fungal-associated and fungal-free grasses was comparable to species-level range divergence of congeners, indicating large impacts on range limits. Positive biotic interactions may be underappreciated in determining species' ranges and species' responses to future climates across large geographic scales.
Pub.: 24 Jul '14, Pinned: 15 Jun '17
Abstract: In dryland systems, shrubs often increase the productivity, abundance, and diversity of understory plants, however these positive interactions can also scale to other trophic levels. The facilitative effect of Larrea tridentata was hypothesized to extend beyond plants to the local insect community in the Mojave Desert. Pan traps were placed under shrubs and in open microsites to test the following predictions: 1) shrub-annual facilitation complexes increase insect abundance, richness, and diversity; and 2) specific insect families or functional groups respond to the shrub-annual facilitation complex more strongly due to increased resources. Abundance, richness, and diversity of the insect communities associated with shrubs were significantly greater in shrub compared to open microsites. The families Sphecidae, Formicidae, Bradynobaenidae and Lauxaniidae were positively associated with shrubs while Scarabaeidae was associated with open microsites. However, there was no difference in the relative abundances of major functional groups, suggesting that the primary pollinators for this ecosystem are not sensitive to differences in floral resources at this scale. This study demonstrates that shrubs facilitate local insect communities and supports the hypothesis that plant-plant facilitation can extend to other trophic levels. Management of desert shrubs is thus an effective means to enhance many components of insect biodiversity.
Pub.: 27 Jun '16, Pinned: 05 Jun '17
Abstract: Plant facilitative interactions enhance co-occurrence between distant relatives, partly due to limited overlap in resource requirements. We propose a different mechanism for the coexistence of distant relatives based on positive interactions of nutrient sharing. Nutrients move between plants following source-sink gradients driven by plant traits that allow these gradients to establish. Specifically, nitrogen (N) concentration gradients can arise from variation in leaf N content across plants species. As many ecologically relevant traits, we hypothesize that leaf N content is phylogenetically conserved and can result in N gradients promoting N transfer among distant relatives. In a Mexican desert community governed by facilitation, we labelled nurse plants (Mimosa luisiana) with (15) N and measured its transfer to 14 other species in the community, spanning the range of phylogenetic distances to the nurse plant. Nurses established steeper N source-sink gradients with distant relatives, increasing (15) N transfer towards these species. Nutrient sharing may provide long-term benefits to facilitated plants and may be an overlooked mechanism maintaining coexistence and increasing the phylogenetic diversity of plant communities. This article is protected by copyright. All rights reserved.
Pub.: 12 Feb '17, Pinned: 05 Jun '17
Abstract: Studies on the role of species interactions in community dynamics and diversity have mostly focused on competition and predator-prey interactions, but the possible role of positive interactions between species, i.e. facilitation, is increasingly recognised. A type of facilitation that received little attention is the one that arises indirectly via pathways of direct trophic and competitive interactions. Here we show that in pelagic food webs the positive effects from such ‘emergent’ facilitation can be sufficiently strong to dominate over direct negative effects, prevent competitive exclusion, promote co-existence and preserve biodiversity. We carried out a press perturbation experiment using a pelagic algae-ciliate food web model whose realism is based on extensive observations on the algae-ciliate community in Lake Constance. The model incorporated trait gradients regarding algal edibility and growth rate and ciliate selectivity and prey attack rate as commonly observed in pelagic food webs. Results of the press perturbation exercise showed that some ciliate groups did not persist alone, or only at very low biomass values, while they reached realistic biomass values in the presence of competing groups of ciliates. The mechanism behind this facilitation is that grazing by less selective ciliates protected the more edible and preferred prey for the selective ciliates. We argue that such emergent facilitation, and the positive consequences for co-existence and biodiversity, is likely to occur in real pelagic food webs. In this way, the present study revealed a potentially important mechanism in the preservation of biological diversity in pelagic food webs.
Pub.: 07 Feb '17, Pinned: 05 Jun '17
Abstract: Facilitation cascades occur when multiple foundation species in a community are involved in a hierarchy of positive interactions, and consist of a primary facilitator which positively affects secondary facilitators, each supporting a suit of dependent species. There is no theoretical limit to the number of levels in a facilitation cascade, yet the existence of more than two has rarely been examined. We manipulated biogenic substrate produced by a primary facilitator (cockle shells) and a secondary facilitator (barnacles and their empty tests) in a space-limited subtidal community to test the hypothesis that solitary ascidians would be the third-level facilitator. In the field, most ascidians were found on barnacles, and most barnacles occupied cockle shells. To produce this pattern, barnacles could nurse ascidians (a longer 'facilitation chain') or outcompete them from cockle shells (a shorter chain). Experimental results clearly supported the nursing hypothesis providing evidence for a facilitation cascade with three hierarchical levels of foundation species. Our findings confirm that like predation and competition, positive interspecific interactions nest into multi-tier hierarchies with numerous levels. While the number of foundation species should increase community stability and resilience as it increases diversity and reduces environmental stress, facilitation chain length may have the opposite effect.
Pub.: 24 Mar '17, Pinned: 05 Jun '17
Abstract: Research on plant–animal interactions has been focused on direct consumer interactions (i.e. plants as resources), but non-trophic interactions including providing shelter or interference with movement can also affect the fine-scale distribution of animals. In particular, non-trophic interactions that are positive could support threatened animal populations. Positive interactions have been used in the restoration of plant communities, but have not yet been extended to the management of animal habitat. In this study, we tested the hypothesis that non-trophic interactions influence the occurrence of an endangered lizard species in an arid shrub-annual system. At a location known to have a population of blunt-nosed leopard lizards (Gambelia sila), we geotagged 700 shrubs, measured shrub morphometric traits, collected biomass samples, and surveyed for lizard presence using scat detection dogs over two years. Relative to 2014, in 2013 plant productivity was high and lizard scats were found more frequently in areas with low invasive grass cover (i.e. residual dry matter, RDM). In 2014, plant productivity was low because of an extreme drought year, and lizard scats were more frequently observed under shrub canopies, particularly those with relatively dense cover. These findings support the novel theory that positive non-trophic interactions are a critical form of plant–animal interactions in addition to consumption. Dominant shrubs can act as a foundation species by functioning as a basal node in structuring both plant and animal communities through a network of interactions. Managing dominant plants, in addition to habitat, is therefore important for conserving animal species in arid ecosystems.
Pub.: 17 Jan '17, Pinned: 05 Jun '17