Estimation of metapopulation colonization rates from disturbance history and occurrence‐pattern dataFabritius, H.; Singer, A.; Pennanen, J.; Snäll, T.
doi: 10.1002/ecy.2814pmid: 31290140
Occurrence patterns of many sessile species in dynamic landscapes are not in equilibrium due to their slow rates of metapopulation colonization and extinction. Colonization–extinction data enable the estimation of colonization rates for such species, but collecting the necessary data may require long waiting times between sampling years. Methods for estimating colonization rates of nonequilibrium metapopulations from single occurrence‐pattern data have so far relied on additional data on patch ages and on past patch connectivities. We present an approach where metapopulation colonization rates are estimated from occurrence‐pattern data and from disturbance history data that inform of past patch dynamics and that can be collected together with occurrence‐pattern data. We estimated parameter values regulating patch and metapopulation dynamics by simulating patch network and metapopulation histories that result in present‐like patch network configurations and metapopulation occurrence patterns. We tested our approach using occurrence‐pattern data of the epiphytic lichen Lobaria pulmonaria in Fennoscandian forests, and fire‐scar data that inform of the 400‐yr history of fires and host tree dynamics in the same landscapes. The estimated model parameters were similar to estimates obtained using colonization–extinction data. The projected L. pulmonaria occupancy into the future also agreed with the respective projections that were made using the model estimated from colonization–extinction data. Our approach accelerates the estimation of metapopulation colonization rates for sessile species that are not in metapopulation equilibrium with the current landscape structure.
When the “selfish herd” becomes the “frozen herd”: spatial dynamics and population persistence in a colonial seabirdMcDowall, Philip S.; Lynch, Heather J.
doi: 10.1002/ecy.2823pmid: 31310664
Aggregations are common in ecological systems at a range of scales and may be driven by exogenous constraints such as environmental heterogeneity and resource availability or by “self‐organizing” interactions among individuals. One mechanism leading to self‐organized animal aggregations is captured by Hamilton's “selfish herd” hypothesis, which suggests that aggregations may be driven by an individual's effort to minimize their risk of predation by surrounding themselves with conspecifics. We demonstrate that aggregations observed in Adélie Penguin (Pygoscelis adeliae) colonies are a convolution of both self‐organized dynamics and external forcing arising from landscape terrain. In fluid, highly mobile aggregations, individuals are constantly moving in response to changing environmental conditions, the locations of predators, or the movements of conspecifics. However, when the ability to rearrange is limited and spatial reconfiguration occurs on slower time scales than changes in population size, systems may become trapped in suboptimal arrangements. We use simulated annealing to demonstrate that Adélie Penguin colonies are frozen in suboptimal spatial arrangements, and employ an individual‐based modeling approach to demonstrate that this suboptimal spatial configuration is driven by a convolution of nest site fidelity and stochastic events at the level of individual nests. The resulting spatial dynamics are responsible for a hysteretic response to long‐term changes in abundance. We find that declining abundance leads to fragmentation even in a homogeneous environment, which has population‐level consequences for reproductive success because predation is biased towards colony edges. Strong edge effects from heterogeneous predation coupled with fragmentation in response to population declines create a positive feedback cycle that can accelerate population decline. This work provides a mechanistic understanding of complex spatial structuring in penguin colonies, provides a link between current spatial patterning and past dynamics, and suggests the possibility of critical collapse in seabird populations.
Plant structural complexity mediates trade‐off in direct and indirect plant defense by birdsNell, Colleen S.; Mooney, Kailen A.
doi: 10.1002/ecy.2853pmid: 31351007
Direct and indirect defenses are predicted to trade‐off due to costs associated with redundancy in plant defense, but the factors mediating a plant's position along this trade‐off axis are unknown. We conducted a bird exclusion experiment of nine sympatric shrub species to assess convergent associations among direct defense, indirect defense from birds, and shrub structural complexity, a trait predicted to influence bird foraging. We found high variation in defense; direct resistance varied four‐fold, with indirect defense ranging from a 59% reduction to a 32% increase in herbivore density. These resistance strategies traded off and were mediated by plant structure; high complexity was associated with weaker indirect defense from birds, strong direct defense, and more predatory arthropods. Our findings suggest that species with growth forms that inhibit bird foraging invest more in direct defense and may provide refuge for arthropod predators. Accordingly, we provide evidence for a potentially widespread mechanism underlying the evolution of plant defenses.
Life history traits predict colonization and extinction lags of desert plant species since the Last Glacial MaximumButterfield, Bradley J.; Holmgren, Camille A.; Anderson, R. Scott; Betancourt, Julio L.
doi: 10.1002/ecy.2817pmid: 31291688
Variation in life‐history strategies can affect metapopulation dynamics and consequently the composition and diversity of communities. However, data sets that allow for the full range of species turnover from colonization to extinction over relevant time periods are limited. The late Quaternary record provides unique opportunities to explore the traits that may have influenced interspecific variation in responses to past climate warming, in particular the rate at which species colonized newly suitable habitat or went locally extinct from degrading habitat. We controlled for differences in species climate niches in order to predict expected colonization and extinction sequences recorded in packrat middens from 15 localities in the Mohave, Sonoran, and Chihuahuan deserts of North America. After accounting for temperature niche differences, we tested the hypotheses that dispersal syndrome (none, wind, vertebrate), growth form (herb, shrub, tree) and seed mass mediated variation in postglacial colonization lags among species, whereas clonality (clonal, non‐clonal), growth form, and seed mass affected extinction lags. Growth form and dispersal syndrome interactively affected colonization lags, where herbaceous species lacking long‐distance dispersal mechanisms exhibited lags that exceeded those of woody, wind or vertebrate‐dispersed species by an average of 2,000–5,000 yr. Growth form and seed mass interactively affected extinction lags, with very small‐seeded shrubs persisting for 4,000–8,000 yr longer than other functional groups. Taller, vertebrate‐dispersed plants have been shown in other studies to disperse farther than shorter plants without specialized dispersal mechanisms. We found that variation along this axis of dispersal syndromes resulted in dramatic differences in colonization rates in response to past climate change. Very small seeded shrubs may have a unique combination of long vegetative and seed bank lifetimes that may allow them to persist for long periods despite declines in habitat condition. This study indicates that readily measurable traits may help predict which species will be more or less sensitive to future climate change, and inform interventions that can stabilize and promote at‐risk populations.
Stoichiometry and daily rhythms: experimental evidence shows nutrient limitation decouples N uptake from photosynthesisChamberlin, Catherine A.; Bernhardt, Emily S.; Rosi, Emma J.; Heffernan, James B.
doi: 10.1002/ecy.2822pmid: 31310322
Diel variability in nutrient concentrations is common but not universal in aquatic ecosystems. Theoretical models of photoautotrophic systems attribute the absence of diel uptake variation to nutrient scarcity, such that diel variability in nutrient uptake disappears as nutrients become limiting. We tested this prediction in a mesocosm experiment, by exposing benthic algal communities to a range of nitrogen (N) and phosphorus concentrations and recording the rates of uptake during both day and night. We found that higher concentrations of N produced diel variability in uptake and that the difference between the day and night total mass uptakes approximately equaled N demand for observed primary production as seen in other studies. At lower concentrations of N, uptake rates during the day and night were indistinguishable. These results are the first empirical evidence to imply that diel nitrate patterns in streams and rivers indicate a release from N limitation and offer a new way to assess nutrient limitation.
Kelp beds and their local effects on seawater chemistry, productivity, and microbial communitiesPfister, Catherine A.; Altabet, Mark A.; Weigel, Brooke L.
doi: 10.1002/ecy.2798pmid: 31233610
Kelp forests are known as key habitats for species diversity and macroalgal productivity; however, we know little about how these biogenic habitats interact with seawater chemistry and phototroph productivity in the water column. We examined kelp forest functions at three locales along the Olympic Peninsula of Washington state by quantifying carbonate chemistry, nutrient concentrations, phytoplankton productivity, and seawater microbial communities inside and outside of kelp beds dominated by the canopy kelp species Nereocystis luetkeana and Macrocystis pyrifera. Kelp beds locally increased the pH, oxygen, and aragonite saturation state of the seawater, but lowered seawater inorganic carbon content and total alkalinity. Although kelp beds depleted nitrate and phosphorus concentrations, ammonium and dissolved organic carbon (DOC) concentrations were enhanced. Kelp beds also decreased chlorophyll concentrations and carbon fixed by phytoplankton, although kelp carbon fixation more than compensated for any difference in phytoplankton production. Kelp beds entrained distinct microbial communities, with higher taxonomic and phylogenetic diversity compared to seawater outside of the kelp bed. Kelp forests thus had significant effects on seawater chemistry, productivity and the microbial assemblages in their proximity. Thereby, the diversity of pathways for carbon and nitrogen cycling was also enhanced. Overall, these observations suggest that the contribution of kelp forests to nearshore carbon and nitrogen cycling is greater than previously documented.