Caitlin D. Kuempel

The emergent role of small-bodied herbivores in pre-empting phase shifts on degraded coral reefs

Natural and anthropogenic stressors can cause phase shifts from coral-dominated to algal-dominated states. In the Caribbean, over-fishing of large herbivorous fish and disease among the long-spined urchin, Diadema, have facilitated algal growth on degraded reefs. We found that diminutive species of urchin and parrotfish, which escaped die-offs and fishing pressure, can achieve abundances comparable to total herbivore biomass on healthier, protected reefs, and exert sufficient grazing function to pre-empt macroalgal dominance following mass coral mortality. Grazing was highest on the most degraded reefs, and was driven by small herbivores that made up >93% of the average herbivore biomass (per m2). We suggest that previously marginal species can achieve a degree of functional redundancy, and that their compensatory herbivory may play an important role in ecosystem resilience. Management strategies should consider the potential role of these additional herbivore functional groups in safeguarding natural controls of algal growth in times of increased uncertainty for the world’s reefs.

Methods for calculating Protection Equality for conservation planning

Protected Areas (PAs) are a central part of biodiversity conservation strategies around the world. Today, PAs cover c15% of the Earth’s land mass and c3% of the global oceans. These numbers are expected to grow rapidly to meet the Convention on Biological Diversity’s Aichi Biodiversity target 11, which aims to see 17% and 10% of terrestrial and marine biomes protected, respectively, by 2020. This target also requires countries to ensure that PAs protect an “ecologically representative” sample of their biodiversity. At present, there is no clear definition of what desirable ecological representation looks like, or guidelines of how to standardize its assessment as the PA estate grows. We propose a systematic approach to measure ecological representation in PA networks using the Protection Equality (PE) metric, which measures how equally ecological features, such as habitats, within a country’s borders are protected. We present an R package and two Protection Equality (PE) measures; proportional to area PE, and fixed area PE, which measure the representativeness of a country’s PA network. We illustrate the PE metrics with two case studies: coral reef protection across countries and ecoregions in the Coral Triangle, and representation of ecoregions of six of the largest countries in the world. Our results provide repeatable transparency to the issue of representation in PA networks and provide a starting point for further discussion, evaluation and testing of representation metrics. They also highlight clear shortcomings in current PA networks, particularly where they are biased towards certain assemblage types or habitats. Our proposed metrics should be used to report on measuring progress towards the representation component of Aichi Target 11. The PE metrics can be used to measure the representation of any kind of ecological feature including: species, ecoregions, processes or habitats.

Operationalizing ecological connectivity in spatial conservation planning with Marxan Connect

Globally, protected areas are being established to protect biodiversity and to promote ecosystem resilience. The typical spatial conservation planning process leading to the creation of these protected areas focuses on representation and replication of ecological features, often using decision support systems such as Marxan. Unfortunately, Marxan currently requires manual input or specialised scripts to explicitly consider ecological connectivity, a property critical to metapopulation persistence and resilience. “Marxan Connect” is a new open source, open access Graphical User Interface (GUI) designed to assist conservation planners in the systematic operationalization of ecological connectivity in protected area network planning. Marxan Connect is able to incorporate estimates of demographic connectivity (e.g. derived from tracking data, dispersal models, or genetics) or structural landscape connectivity (e.g. isolation by resistance). This is accomplished by calculating metapopulation-relevant connectivity metrics (e.g. eigenvector centrality) and treating those as conservation features, or using the connectivity data as a spatial dependency amongst sites to be included in the prioritization process. Marxan Connect allows a wide group of users to incorporate directional ecological connectivity into conservation plans. The least-cost conservation solutions provided by Marxan Connect, combined with ecologically relevant post-hoc testing, are more likely to support persistent and resilient metapopulations (e.g. fish stocks) and provide better protection for biodiversity than if connectivity is ignored.

Herbivore community determines the magnitude and mechanism of nutrient effects on subtropical and tropical seagrasses

Numerous studies have examined the combined effects of nutrients (bottom-up control) and consumers (top-down control) on ecosystem structure and functioning. While it is recognized that both can have important effects, there remains a limited understanding of how their relative importance shifts across large spatial scales where consumer functional types can vary. Using seagrasses as a model ecosystem, we explore the effects of ambient variation in herbivore functional composition on the relative importance of bottom-up and top-down forcings. Distributed experiments were conducted across four Western Atlantic sites that encompassed a gradient in consumer composition. Herbivores at two subtropical sites were predominantly comprised of small crustacean invertebrates (mesograzers that consume epiphytic algae), while herbivores at two tropical sites were dominated by large macrograzers (sea urchins and herbivorous fishes that directly consume seagrass biomass). To test the relative importance of bottom-up and top-down factors at each site, we manipulated nutrient supply, mesograzer abundance, and the presence of macrograzers (mainly herbivorous fishes) in a fully factorial design over a 14-week growing season. Seagrass above-ground biomass, shoot density, canopy cover, leaf productivity and epiphyte mass were measured as indicators of habitat structure and productivity. Overall, nutrient addition elicited a strong response across sites, causing widespread loss of seagrass biomass and shoot density. However, the mechanisms driving these declines strongly varied as a function of resident herbivore identity. Seagrass loss at tropical sites was attributable to macrograzers, which increased their direct consumption of fertilized, nutrient-rich seagrass. Conversely, at the subtropical sites, nutrient loading caused seagrass declines associated with the proliferation of epiphytic algae, but only in locations where mesograzer abundance was low. Synthesis. Our results confirm that nutrient enrichment generally has negative effects on seagrasses, but that the underlying mechanisms vary and may depend upon herbivore presence and functional identity. Along a subtropical to tropical gradient, the adverse effects of nutrient loading may switch from competitive algal overgrowth to a stimulation of seagrass consumption. Thus, in the tropics, top-down and bottom-up factors can act in combination, and in the same direction, to contribute to habitat loss.