Browsing by Subject "Residence time"

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Alien plant fitness is limited by functional trade‐offs rather than a long‐term increase in competitive effects of native communities
(2023) Brendel, Marco R.; Schurr, Frank M.; Sheppard, Christine S.
Alien plants experience novel abiotic conditions and interactions with native communities in the introduced area. Intra‐ and interspecific selection on functional traits in the new environment may lead to increased population growth with time since introduction (residence time). However, selection regimes might differ depending on the invaded habitat. Additionally, in high‐competition habitats, a build‐up of biotic resistance of native species due to accumulation of eco‐evolutionary experience to aliens over time may limit invasion success. We tested if the effect of functional traits and the population dynamics of aliens depends on interspecific competition with native plant communities. We conducted a multi‐species experiment with 40 annual Asteraceae that differ in residence time in Germany. We followed their population growth in monocultures and in interspecific competition with an experienced native community (varying co‐existence times between focals and community). To more robustly test our findings, we used a naïve community that never co‐existed with the focals. We found that high seed mass decreased population growth in monocultures but tended to increase population growth under high interspecific competition. We found no evidence for a build‐up of competition‐mediated biotic resistance by the experienced community over time. Instead, population growth of the focal species was similarly inhibited by the experienced and naïve community. By comparing the effect of experienced and naïve communities on population dynamics over 2 years across a large set of species with a high variation in functional traits and residence time, this study advances the understanding of the long‐term dynamics of plant invasions. In our study system, population growth of alien species was not limited by an increase of competitive effects by native communities (one aspect of biotic resistance) over time. Instead, invasion success of alien plants may be limited because initial spread in low‐competition habitats requires different traits than establishment in high‐competition habitats.
From alien to native Asteraceae : how effects of climate, functional traits, and biotic interactions on population growth change with residence time
(2023) Brendel, Marco; Sheppard, Christine
Biological invasions pose a major threat to native biodiversity and even drive native species to extinction. It is thus of utmost importance to gain a better understanding of limits to population growth and spread of invasive plants. Invasion success in the introduced area is determined by the combined effects of climatic mismatches between the area of origin and the introduced area as well as biotic resistance of resident native communities. Alien plants can respond to environmental selection via changes in functional traits and thereby adapt to novel abiotic conditions. Native community species are expected to adapt to the presence of the invader by gaining eco-evolutionary experience and build-up biotic resistance over time. The aim of this thesis is to investigate interactions of alien plants with the novel abiotic and biotic environment in their introduced range over eco-evolutionary timescales. To this end, I conducted common garden experiments based on an alien-native species continuum to cover a broad range of residence times in Germany (7 to 12,000 years before present). I followed the population growth of 47 annual Asteraceae (including neophytes, archaeophytes, and natives) over two years and measured their performance in intra- and interspecific competition to answer the following questions: 1) How are effects of climatic distances between the area of origin and the introduced area as well as functional traits on population dynamics of alien plants determined by residence time? 2) How is biotic resistance of native communities towards alien plants related to residence time? 3) How are competitive outcomes between single alien and native plants shaped by residence time and serve as a predictor of range sizes? For the first question, I followed population growth of the Asteraceae in monocultures. I calculated climatic distances between the area of origin and the introduced area and measured functional traits in terms of seed mass, maximum height, and specific leaf area. Firstly, I tested whether negative effects of climatic distances on population growth weaken with residence time. Secondly, I investigated trait-demography relationships and tested if functional traits converge towards values that increase population growth. I found a strong effect of seed mass and no effects climatic distances on population growth. A strong negative relationship between seed mass and population growth resulted in directional selection of seed mass towards low values with increasing residence time. For the second question, I measured population growth of the Asteraceae in a Central European grassland community. I tested if competitive effects of the community on the Asteraceae increase with residence time (i.e. co-existence time with the native community). I used a second community native to North American grasslands that never co-existed with the Asteraceae to disentangle competitive effects related to eco-evolutionary experience of the native community from inherent competitive abilities of the Asteraceae. I compared trait-demography relationships in both community types with monocultures and found very similar competitive effects of both communities on the Asteraceae and thus no evidence for a build-up of competition-induced biotic resistance over time. Instead, invasion success was determined by a strong seed-mass-mediated trade-off between population growth in low- vs. high-competition. For the third question, I tested if the response of biomass and seed production of native targets to competition with alien and native neighbours depends on residence time. I tested if competitive effects differ between invasion status groups and explain species’ range sizes in Germany. I generally did not find a higher tolerance of native Asteraceae to competition of neighbouring aliens and natives with increasing residence time. Both established neophytes and natives showed similar competitive abilities and species’ range sizes were not influenced by competitive effects. The detected trait-demography relationships and related directional selection as a mechanism of adaptation to novel abiotic conditions improve the understanding of constraints on population growth and spread of invaders. The lack of interspecific competitive superiority as a determinant of range sizes might indicate that other mechanisms are more important for invasion success. The functional trade-off between population growth in low vs. high competition reveals that invaders that are likely to escape this trade-off should be of highest management concern. By the combination of experimental macroecology with approaches of functional and community ecology used in my study, I strongly advanced the understanding of mechanisms of limits to population growth and spread of alien plants and provide a fundamental basis for future research in invasion ecology.