There is increasing evidence that climate warming will have both direct and indirect effects on species. Whereas the direct effects of climate warming represent the proximate physiological consequences of changing abiotic conditions, the indirect effects of climate change reflect changes mediated by at least one other interacting species. The relative importance of these two kinds of effects has been unclear, limiting our ability to generalize the response of different species to climate change. Here, we used a series of experiments to disentangle some of the key direct and indirect effects of warming on the growth of monarch butterfly caterpillars (
The global movement of pathogens is altering populations and communities through a variety of direct and indirect ecological pathways. The direct effect of a pathogen on a host is reduced survival, which can lead to decreased population densities. However, theory also suggests that increased mortality can lead to no change or even increases in the density of the host. This paradoxical result can occur in a regulated population when the pathogen’s negative effect on survival is countered by increased reproduction at the lower density. Here, we analyze data from a long-term capture–mark–recapture experiment of Trinidadian guppies (
- NSF-PAR ID:
- 10187064
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 117
- Issue:
- 36
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- p. 22580-22589
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Danaus plexippus ) and showy milkweed plants (Asclepias speciosa ) during a window of rapid growth for both species. The effects of warming differed between direct, indirect, and combined effect experiments. Warming from 26°C to 30°C directly increased the growth of both monarch larvae and milkweeds, with monarch and milkweed growth rates showing similar sensitivity to warming. However, in a subsequent experiment, we did not observe significantly increased growth when comparing caterpillars and plants reared at 27°C and 31°C, suggesting that small differences can change the direct effects of warming. When caterpillars that were maintained at laboratory temperatures were fed leaves from host plants that were exposed to warmer temperatures, warming had a negative indirect effect on larval growth rates likely mediated by decreases in milkweed leaf quality. In experiments combining direct and indirect effects, we observed a net positive effect of warming on larval growth rates. Warming had no combined effects on milkweed growth, potentially due to opposing positive direct and negative indirect effects on growth mediated via increased monarch herbivory. These results show how variability among the direct, indirect, and combined effects of even relatively simple, short‐term climatic perturbations can present challenges for predicting the broader effects of climatic warming in multispecies communities. -
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Abstract Forest removal for livestock grazing is a striking example of human‐caused state change leading to a stable, undesirable invasive grass system that is resistant to restoration efforts. Understanding which factors lead to resilience to the alternative grass state can greatly benefit managers when planning forest restoration. We address how thresholds of grass cover and seed rain might influence forest recovery in a restoration project on Hawaiʻi Island, USA. Since the 1980s, over 400,000
Acacia koa (koa) trees have been planted across degraded pasture, and invasive grasses still dominate the understory with no native woody‐plant recruitment. Between this koa/grass matrix are remnant nativeMetrosideros polymorpha (ʻōhiʻa) trees beneath which native woody plants naturally recruit. We tested whether there were threshold levels of native woody understory that accelerate recruitment under both tree species by monitoring seed rain at 40 trees (20 koa and ʻōhiʻa) with a range of native woody understory basal area (BA). We found a positive relationship between total seed rain (but not bird‐dispersed seed rain) and native woody BA and a negative relationship between native woody BA and grass cover, with no indication of threshold dynamics. We also experimentally combined grass removal levels with seed rain density (six levels) of two common understory species in plots under koa (n = 9) and remnant ʻōhiʻa (n = 9). Few seedlings emerged when no grass was removed despite adding seeds at densities two to 75 times higher than naturally occurring. However, seedling recruitment increased two to three times once at least 50% of grass was removed. Existing survey data of naturally occurring seedlings also supported a threshold of grass cover below which seedlings were able to establish. Thus, removal of all grasses is not necessary to achieve system responses: Even moderate reductions (~50%) can increase rates of native woody recruitment. The nonlinear thresholds found here highlight how incremental changes to an inhibitory factor lead to limited restoration success until a threshold is crossed. The resources needed to fully eradicate an invasive species may be unwarranted for state change, making understanding where thresholds lie of the utmost importance to prioritize resources. -
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Abstract Previous theoretical work has highlighted the potential for natural enemies to mediate the coexistence of species with similar life histories via density‐dependent effects on survivorship. For plant pathogens to play this role, they must differ in their ability to infect or induce disease in different host plant species. In tropical forests characterized by high diversity, these effects must extend to phylogenetically closely related species pairs. Mortality at the seed and seedling stage strongly influences the abundance and distribution of tropical tree species, but the host preferences and spatial distributions of fungi are rarely determined.
We examined how host species identity, relatedness and seed viability influence the composition of fungal communities associated with seeds of four co‐occurring pioneer trees (
Cecropia insignis ,C. longipes ,C. peltata andJacaranda copaia ). Seeds were buried in mesh bags in five common gardens in the understorey of a lowland tropical forest in Panama and retrieved at intervals from 1 to 30 months. A subset of the seeds in each bag was used to determine germination success. One half of each remaining seed was tested for viability; and the other half was used to culture and identify seed‐infecting fungi.Seeds were infected by fungi after burial. Although fungal communities differed in viable versus dead seeds, and across burial locations, community composition primarily varied as a function of plant species identity (30.7% of variation in community composition vs. 4.5% for viability and location together), even for congeneric
Cecropia species. Phylogenetic reconstruction showed that relatedness of fungi mostly reflected differences betweenJacaranda (Bignoniaceae) andCecropia (Urticaceae).Although the proportion of germinable seeds decreased gradually over time for all species, intraspecific variation in survival was high at the same location (e.g. ranging from 0% to 100% for
C. peltata ) suggesting variable exposure or susceptibility to seed pathogens.Synthesis . Our study provides evidence under field conditions that congeneric tree species with similar life history traits differ markedly in seed‐associated fungal communities when exposed to the same soil‐borne fungi. This is a critical first step supporting pathogen‐mediated coexistence of closely related tree species. -
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Abstract A major question in ecology is how often competing species evolve to reduce competitive interactions and facilitate coexistence. One untested route for a reduction in competitive interactions is through ontogenetic changes in the trophic niche of one or more of the interacting species. In such cases, theory predicts that two species can coexist if the weaker competitor changes its resource niche to a greater degree with increased body size than the superior competitor.
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